Ch. 10: Chapter 13, Metallic Mineral Exploration and Mining Certifications, including but not limited to the standards for metallic mineral exploration in that chapter
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section.
A site’s phosphorus export must
be calculated and compared to
the Project Phosphorus Budget
(PPB) to determine the extent of
phosphorus reductions needed
and the best method for
achieving these reductions.
PPE = Project Phosphorus Export = amount of phosphorus that
will reach the lake from a new development.
Pre-treatment PPE = Raw Phosphorus that the new
development will create.
Post-treatment PPE = Phosphorus that will be discharged after
treatment by all stormwater management treatment practices.
For all project development and subdivisions projects where the
land use and impact has been determined and lot development
will be restricted, the pre-treatment export is estimated by:
1. Dividing the project into various land use types (i.e. parking,
roads, roofs, lawns by soil type),
2. Determining the area within each land use in acres and to
the second decimal place,
3. Multiplying that area by the appropriate phosphorus export
factor from Table 3.1 or Table 3.2 as is appropriate, and
4. Summing the resulting phosphorus exports to get the Pre-
PPE.
Use the first four columns in Worksheet 2 in Appendix D to
calculate the Pre-PPE.
The amount of phosphorus exported
from a project site will depend on the
land use and soil type, with greatest
exports from impervious surfaces as
the one shown here.
Table 3.1 gives pre-treatment phosphorus export for new commercial/industrial development and for
roads in residential subdivision projects. Table 3.2 should be used for lots in residential subdivisions
unless the dimensions and locations of buildings, driveways and lawns have been pre-determined,
specifically restricted, and all construction and landscaping on the lot will be done by the developer, in
which case the High Export Option from Table 3.1 may be used.
Table 3.2 gives pre-treatment phosphorus export for single family residential lots. In most cases, the
specific area of the development on a lot (houses, garages, driveways, lawns) within the subdivision is
usually not known
mined,
specifically restricted, and all construction and landscaping on the lot will be done by the developer, in
which case the High Export Option from Table 3.1 may be used.
Table 3.2 gives pre-treatment phosphorus export for single family residential lots. In most cases, the
specific area of the development on a lot (houses, garages, driveways, lawns) within the subdivision is
usually not known. Table 3.2 must be used to determine the export from each lot unless the developer:
•
Has pre-determined the area of each land use on each lot
•
Will be constructing the buildings and driveways and landscaping the lots, and
•
Will be restricting any further expansion of these land uses. A deed restriction will be required for
each lot that incorporates an area restriction.
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The Low Export Option factors may be selected for commercial/industrial development and roads if all of
the following are incorporated in the project:
•
A deed restriction prohibiting the use of fertilizers containing phosphorus except when establishing
new turf or vegetation on bare soil will be established for all lots. It is recommended that the use of
fertilizer containing phosphorus always be limited.
•
All roads, driveways and parking areas are paved.
•
All ditches and drainage ways are designed, constructed and maintained as stable vegetated swales
in accordance with the specifications in Volume III, Chapter 9, or as riprapped swales where required
by steep slopes. The algal available phosphorus export for ditches and swales is based on soil type
and is the same as the export from a lawn.
If all of these elements are not clearly and permanently incorporated in the project design, use the High
Export Option factors of Table 3.1
getated swales
in accordance with the specifications in Volume III, Chapter 9, or as riprapped swales where required
by steep slopes. The algal available phosphorus export for ditches and swales is based on soil type
and is the same as the export from a lawn.
If all of these elements are not clearly and permanently incorporated in the project design, use the High
Export Option factors of Table 3.1.
Table 3.1
Algal Available Phosphorus Export (pre-treatment) for Commercial Development and
Subdivisions
Low Export Option
High Export Option
Land Use
Hydrologic
Soil Group
P Fertilizers restricted,
roads and drives paved
and constructed with
stable swales (lb/acre/yr)
No restrictions on fertilizer
use, road surface or ditch
design and construction
(lb/acre/yr)
Landscaped Areas,
Lawns & Ditches
A
0.1
0.2
B
0.2
0.4
C
0.3
0.6
D
0.4
0.8
Roads/Driveways
N/A
1.25
1.75
Parking
N/A
1.25
1.25
Roofs/Other
N/A
0.5
0.5
Riprap/crushed rock
N/A
0.3
0.6
- 6 -
Table 3.2
Algal Available Phosphorus Export from Single Family Residential Lots
(pre-treatment)
Hydrologic Soil
Group
With Area Restrictions
Without Area Restrictions
Cleared Area < 12,000 sq ft
Driveway/Park < 1,750 sq ft
(lb/lot/year)
No Restriction on cleared area or
driveway/parking area
(lb/lot/year)
w/ 75%
drive/park
area to buffer
w/o 75%
drive/park
area to buffer
w/ 75%
drive/park
area to buffer
w/o 75% drive/park
area to buffer
A
0.09
0.14
0.12
0.18
B
0.12
0.17
0.17
0.24
C
0.15
0.20
0.22
0.29
D
1.08
0.23
0.27
0.34
Note: Driveways and parking are considered to be draining directly to a buffer if the flow path to the buffer is 50 feet or less
and if the runoff reaches the buffer in well distributed overland flow.
Note: phosphorus export values in this table assume a driveway of 150 feet in length, or less. If driveways will likely exceed
150 feet, the excess driveway length should be considered a road and its export calculated using Worksheet 2 and Table 3.1
raining directly to a buffer if the flow path to the buffer is 50 feet or less
and if the runoff reaches the buffer in well distributed overland flow.
Note: phosphorus export values in this table assume a driveway of 150 feet in length, or less. If driveways will likely exceed
150 feet, the excess driveway length should be considered a road and its export calculated using Worksheet 2 and Table 3.1.
Example 2: Pre-treatment PPE Calculation for Commercial Development
Problem: 'Good Intention Business Mall' is proposing a business mall on 6 acres. The development
will consist of 3.5 acres of paved parking; a 0.5 acre paved access road, 1.5 acres of buildings and
0.5 acre of lawn. A deed restriction is proposed, prohibiting the use of fertilizers containing
phosphorus. Soils on the site are classified as Hydrologic Soil Group C. Calculate the pre-treatment
PPE for the proposed project.
Solution: Use Table 3.1 and Worksheet 2 to calculate the pre-treatment Algal Average Phosphorus
Export.
Worksheet 2
Pre-PPE Calculations
Project Name:
Good Intention Business Mall
Development Type:
Commercial
Land Surface Type of Lot
#(s) with description
Acres or
# of lots
Export
Coefficient
from Table 3.1
Pre-treatment
Algal Av. P Export
(lbs P/year)
Parking (pavement)
3.50
1.25
4.375
Access Road (low export)
0.50
1.25
0.625
Buildings
1.50
0.50
0.750
Lawn
0.50
0.30
0.150
Pre-PPE
(lbs P/year)
5.900
Based on these calculations, the pre-treatment PPE is 5.9 lbs P/year.
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Example 3: Pre-PPE Calculation for Subdivision Development
Problem: The 'Homesweet Home Subdivision' in Example 1 involves the development of 12 lots and
0.5 acres of road. Four lots will be constructed on HSG B soils with no restrictions on cleared area or
driveway/ parking area. Eight of the lots will be constructed on HSG C soils and will have restrictions
to minimize site clearing to <12,000 square feet each and to minimize driveway/parking areas to
<1,750 square feet
esweet Home Subdivision' in Example 1 involves the development of 12 lots and
0.5 acres of road. Four lots will be constructed on HSG B soils with no restrictions on cleared area or
driveway/ parking area. Eight of the lots will be constructed on HSG C soils and will have restrictions
to minimize site clearing to <12,000 square feet each and to minimize driveway/parking areas to
<1,750 square feet. Six of the restricted lots will not direct 75% of the driveway and parking area
runoff to a buffer. Two of the restricted lots will direct the stormwater runoff from 75% or more of the
driveways and parking areas to a buffer. Driveways from two of the lots will exceed 150 feet in length,
with an anticipated 0.056 acres of driveway over the 150 length. The proposed road will be paved.
Calculate the pre-treatment PPE for the proposed subdivision.
Solution: Use Tables 3.1 and 3.2 and Worksheet 2 to calculate the pre-treatment Algal Average
Phosphorus Export.
Worksheet 2
Pre-PPE Calculations
Project Name:
Homesweet Home
Development Type:
Residential
Land Surface Type of Lot
#(s) with description
Acres or
# of lots
Export
Coefficient
from Table 3.1
Pre-treatment
Algal Av. P Export
(lbs P/year)
Lots 1-4 (HSG B) no
restriction, w/o 75% to buffer
4
0.24
0.96
Lots 5-10 (HSG C)
<12,000 sqft clearing,
w/o 75% to buffer
6
0.20
1.2
Lots 11 & 12 (HSG C)
<12,000 sqft clearing,
w/ 75% to buffer
2
0.15
0.30
Lots 2 & 3 driveway access
> 150 feet
0.056
1.25
0.07
Subdivision Road
(low export)
0.50
1.25
0.625
Pre-PPE
(lbs P/year)
3.155
Since two of the driveways exceeded 150 feet in length, the excess driveway length was considered a
road and its export calculated as such.
Based on these calculations, the Pre-treatment PPE is 3.155 lbs P/year.
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3.1 - Redevelopment or Expansion of Existing Uses
Phosphorus export need not be estimated for any land uses that were in existence prior to 1997 (prior to
1980 for projects that require a Site Location of Development Act (SLODA) Permit from the DEP)
riveway length was considered a
road and its export calculated as such.
Based on these calculations, the Pre-treatment PPE is 3.155 lbs P/year.
- 8 -
3.1 - Redevelopment or Expansion of Existing Uses
Phosphorus export need not be estimated for any land uses that were in existence prior to 1997 (prior to
1980 for projects that require a Site Location of Development Act (SLODA) Permit from the DEP). For any
proposed project that will be built within a parcel having existing development (built before 1997 or 1980
for SLODA projects) that will be enlarged, upgraded or expanded, the phosphorus export should only be
estimated for the net increase. This would apply to the redevelopment or expansion of any existing
buildings, parking, roads and lawns. Any existing development or land disturbance that was created after
1997 (1980 for SLODA Projects) must be included as a new phosphorus export. This includes logging
roads, new access roads, and all other projects created from an undisturbed condition that did not require
a phosphorus design and permit at the time of construction.
For example, if a proposed subdivision is served by an existing 2000 foot gravel road, which was built
before 1997, and will be upgraded and expanded from a width of 14 feet to a width, with shoulders, of 24
feet, the phosphorus export should only be estimated for the net increase in road area, or 10 feet x 2000
feet = 20,000 square feet.
3.2 - Reduction of Phosphorus Export
Most projects will generate more phosphorus than the project's phosphorus budget (PPB) will allow. In
order to meet the budget, the excess phosphorus export must be reduced. Comparison of the pre-
treatment PPE with the PPB will determine how much export will need to be reduced. This section
describes options for reducing phosphorus export and how to estimate phosphorus export after treatment.
There are two basic options for reducing long term phosphorus export.
Option 1
udget (PPB) will allow. In
order to meet the budget, the excess phosphorus export must be reduced. Comparison of the pre-
treatment PPE with the PPB will determine how much export will need to be reduced. This section
describes options for reducing phosphorus export and how to estimate phosphorus export after treatment.
There are two basic options for reducing long term phosphorus export.
Option 1. Redesign to Reduce Phosphorus Export: The first option is to redesign the project so that
initial phosphorus export is minimized. This can be accomplished by:
•
Limiting the size or intensity of the project (i.e. reducing the number of lots, the length of roads, the
size of a parking area),
•
Locating the developed portion of the project on the best soils and shallowest slopes, and
•
Incorporating such measures as clearing restrictions and limitations on the use of phosphorus
fertilizers.
These reductions in phosphorus export will be reflected in the calculation of pre-treatment PPE described
in the previous section.
Option 2. Implement Best Management Practices (BMPs): The second option for reducing a project's
stormwater phosphorus export is to incorporate stormwater best management practices (BMPs) to
remove phosphorus from the stormwater before it leaves the site. Some examples of BMPs are vegetated
buffer areas, wet ponds, soil filters and infiltration beds. Volume III presents detailed design standards for
a number of commonly used BMPs.
All BMPs are not created equal. Some BMPs do a better job of removing phosphorus from stormwater
than others. Also, within a given type of BMP, differing designs or locations may result in different levels
of effectiveness is removing phosphorus. For example, a broad, wooded buffer on permeable soils with a
shallow slope will retain much more phosphorus than a narrow, field buffer on tight soils and steeper
slopes. For stormwater treatment ponds, such as wetponds, the size of the pond relative to its
contributing watershed, its depth, and its shape determine its effectiveness
ult in different levels
of effectiveness is removing phosphorus. For example, a broad, wooded buffer on permeable soils with a
shallow slope will retain much more phosphorus than a narrow, field buffer on tight soils and steeper
slopes. For stormwater treatment ponds, such as wetponds, the size of the pond relative to its
contributing watershed, its depth, and its shape determine its effectiveness.
In this Volume, a BMP's effectiveness in treating stormwater runoff is described in terms of a "treatment
factor". The Treatment Factor (TF) indicates the fraction of stormwater phosphorus that will pass through
the BMP and not be retained. For example, if a wooded buffer was projected to retain 60% and
discharges 40% of the inflow phosphorus, it would have a treatment factor of 0.4 and a removal efficiency
of 0.6. A simple way of estimating treatment factors for a variety of BMPs based on an adjustment of the
standard sizing specifications for BMPs described in Volume III of this manual is presented in Chapter 4
of this volume.
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When planning the project, the project designer should look for opportunities to locate the most effective
BMPs (those with the highest removal efficiency) to collect runoff from the portions of the project which
produce the most phosphorus export (i.e. roads, parking areas, driveways, house lots). If the project site
is large enough, the preferred BMP is a natural wooded buffer area located immediately downhill of the
stormwater source area. Buffers are preferred because they are natural and they require little, if any,
maintenance (just don't cut the trees or disturb the ground cover). The critical element in siting buffers is
to insure that the stormwater runoff enters the buffer in overland, non-channelized flow that will not
concentrate into a channelized flow within the buffer. By comparison, other BMPs require site specific
design and careful construction as well as regular inspection and maintenance
f any,
maintenance (just don't cut the trees or disturb the ground cover). The critical element in siting buffers is
to insure that the stormwater runoff enters the buffer in overland, non-channelized flow that will not
concentrate into a channelized flow within the buffer. By comparison, other BMPs require site specific
design and careful construction as well as regular inspection and maintenance.
RE = Removal efficiency = The fraction of stormwater phosphorus that will be removed by the
BMP. The higher the removal efficiency, the more effective it will be at retaining phosphorus from
reaching the resource.
TF= Treatment factor = (1.0 - RE) = The fraction of the stormwater phosphorus that will pass
through a BMP and not be retained. The lower the treatment factor, the more effective the BMP.
When phosphorus from a project draining to a BMP is multiplied by that BMP's Treatment factor
(1.0 - RE), the resulting product is the amount of phosphorus that, after treatment, will still be
exported to the lake.
3.3 - Estimating Project Phosphorus Export after Treatment (Post-PPE)
In order to determine if the BMPs incorporated into the project are adequate to meet the PPB
(Project Phosphorus Budget), the pre-treatment PPE must be revised to reflect the treatment capabilities
of those BMPs. This is accomplished by multiplying the phosphorus export from each source area (i.e. a
parking lot, a house lot, a segment of road) by the treatment factor (1.0 - RE) of the BMP to which it
drains. See Section 4.1 BMP Rules of Thumb, if the individual source areas (or subcatchments) drain
cumulatively to more than one BMP. The export values for all source areas, both treated and untreated,
are then added together to get the total phosphorus export for the project using Worksheet 2.
For large projects or projects where the natural topography divides the site drainage into a number of
sub-drainage areas, this process may not be as straight forward. For instance, often the entire length of a
road will not drain to the same BMP
rt values for all source areas, both treated and untreated,
are then added together to get the total phosphorus export for the project using Worksheet 2.
For large projects or projects where the natural topography divides the site drainage into a number of
sub-drainage areas, this process may not be as straight forward. For instance, often the entire length of a
road will not drain to the same BMP. The local topography will result in one segment of road being treated
by one BMP, other segments by other BMPs, and still other segments receiving no treatment at all. In the
case of a crowned road, the uphill side of the road might drain to a road ditch that flows to a wet pond,
while sheet runoff from the downhill side of the road may drain to a wooded buffer, which for part of the
road length is 75 feet wide, meadow and on shallow slopes and for the remaining length is only 50 feet
wide, wooded on steep slopes. Since all three BMPs, the wetpond and the two buffers, have different
treatment factors, it is necessary to break the road surface area into three discreet subcatchments based
on the BMP(s) to which each road segment drains to calculate treated phosphorus export from the road.
This is true not only for roads but for all other types of development as well.
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Summarize the project's phosphorus export and treatment as follows:
1. Show, on a topographic site plan, all the BMPs that will be incorporated into the project.
2. Delineate all subcatchments for which each BMP is providing treatment.
3. Identify all portions of the developed area (i.e. buildings, road segments, parking, lawns, house
lots) which are being treated by a given BMP or combination of BMPs on Worksheet 2.
4. List on the worksheet each subcatchment and export area with the pre-treatment phosphorus
export factor from Table 3.1 or Table 3.2 and the treatment factor(s) for the BMP(s) to which the
area drains. If the runoff from a subcatchment receives no treatment, enter 1.0 in the treatment
factor column.
5
awns, house
lots) which are being treated by a given BMP or combination of BMPs on Worksheet 2.
4. List on the worksheet each subcatchment and export area with the pre-treatment phosphorus
export factor from Table 3.1 or Table 3.2 and the treatment factor(s) for the BMP(s) to which the
area drains. If the runoff from a subcatchment receives no treatment, enter 1.0 in the treatment
factor column.
5. BMP treatment factors and removal efficiencies can be calculated from Chapter 4 of this volume
and Volume III. Enter these values on the worksheet.
6. Multiply each export area with its export factor and treatment factor to obtain the post-treatment
phosphorus export value. For areas receiving no treatment, use a treatment factor of 1.0.
7. See “Section 4.1. BMP Rules of Thumb”, if the source area drains to more than one BMP.
8. Sum the export from all treated and untreated areas to obtain the total post- treatment
phosphorus export (Post-PPE) from the project.
3.4 - Evaluating Project Overall Phosphorus Export
For an acceptable site development, the Post- PPE needs to be smaller than the PPB for the parcel. The
calculations can be summarized in Worksheet 4. If the resulting project phosphorus export (Post-PPE) is
less than or equal to the project phosphorus budget (PPB) from Worksheet 1 than the project meets its
budget. If not, further reductions in stormwater phosphorus are required. Credits for mitigation of existing
sources may be another option for reducing the net project phosphorus export (Post-PPE) or paying a
compensation fee may be an option (See Chapter 5, Credits for Mitigation and Compensation Fee for
guidance).
Using Worksheet 4, summarize the Net Project Phosphorus Export as follows:
1. Bring in the Project Phosphorus Budget (PPB) from Worksheet 1
2. Bring in any Mitigation Credits from Worksheet 3
3. Bring in the total Pre-Treatment Phosphorus Export (Pre-PPE) as calculated on
Worksheet 2
4. Bring in the total Post-Treatment Phosphorus Export (Post-PPE) as calculated on
Worksheet 2
5
Fee for
guidance).
Using Worksheet 4, summarize the Net Project Phosphorus Export as follows:
1. Bring in the Project Phosphorus Budget (PPB) from Worksheet 1
2. Bring in any Mitigation Credits from Worksheet 3
3. Bring in the total Pre-Treatment Phosphorus Export (Pre-PPE) as calculated on
Worksheet 2
4. Bring in the total Post-Treatment Phosphorus Export (Post-PPE) as calculated on
Worksheet 2
5. If the Post PPE is less than or equal to the site's PPB, the project meets its
phosphorus budget.
6. If the Post-PPE is larger than the site's PPB but the Post-PPE is less than or equal to
0.4 times the Pre-PPE, then paying a compensation fee may be an option in certain
lake watersheds. That list is available in Appendix F.
7. If the Post-PPE is larger than the site's PPB and the Post-PPE is more than 0.4 times
the Pre-PPE, then more phosphorus treatment needs to be provided or less
development must occur.
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Example 4: Post-PPE Calculation for Subdivision Development
Problem: The 'Homesweet Home Subdivision' project described in Example 1 and 3 is proposing to
treat a portion of the stormwater runoff from the subdivision through the use of buffers. Lots 1-4 will
not receive any treatment. Lots 5 through 12, the excess driveway lengths from Lots 2 and 3, and the
access road is super-elevated and will be directed to a downgradient buffer sized in accordance with
Chapter 5 of Volume III of this manual. Calculate the post-treatment PPE for the proposed
subdivision.
Solution: The pre-treatment PPE was calculated in Example 3. A forested buffer treating stormwater
runoff that meets the standard sizing as provided in Chapter 5 of Volume III will achieve a Removal
Efficiency of 0.6. Thus, the corresponding Treatment Factor is 0.4, which should be entered into
Worksheet 2 as shown below. Note that 1.0 has been entered into the treatment factor column for
lots 1 through 4 for which no BMPS are providing stormwater treatment
A forested buffer treating stormwater
runoff that meets the standard sizing as provided in Chapter 5 of Volume III will achieve a Removal
Efficiency of 0.6. Thus, the corresponding Treatment Factor is 0.4, which should be entered into
Worksheet 2 as shown below. Note that 1.0 has been entered into the treatment factor column for
lots 1 through 4 for which no BMPS are providing stormwater treatment.
Worksheet 2
Pre-PPE and Post-PPE Calculations
Project Name:
Homesweet Home Subdivision
Development Type: Residential
Sheet #
Land
Surface
Type of
Lot #(s)
with
description
Acres
or #
of
lots
Export
Coefficient
from
Table 3.1
Table 3.2
Pre-treatment
Algal Av.
P Export
(lbs P/year)
Treatment
Factor for
BMP(s)
from
Chapter 6
Post-
treatment
Algal Av.
P Export
(lbs P/year)
Description
of BMPs
Lots 1-4 (HSG
B) no
No
restriction,
4.00
0.24
0.96
1.0
0.96
treatment
w/o 75% to
provided
buffer
Lots 5-10
(HSG C)
<12,000 sqft
clearing,
6.00
0.20
1,2
0.4
0.48
75 ft forest
buffer
w/o 75% to
buffer
Lots 11 & 12
(HSG C)
<12,000 sqft
clearing,
2.00
0.15
0.30
0.4
0.012
75 ft forest
buffer
w/ 75% to
buffer
Lots 2 & 3
driveway
access > 150
feet
0.056
1.25
0.07
0.4
0.028
55 ft
roadside
forest buffer
Access Road
(low export)
0.50
1.25
0.625
0.4
0.25
Pre-PPE
(lbs P/year)
3.155
Post-PPE
(lbs P/year)
1.838
Based on these calculations, the Post-treatment PPE is 1.838 lbs P/year.
- 12 -
Example 5: Project Phosphorus Export Summary for Subdivision Development
Problem: Summarize the Project Phosphorus Export for the 'Homesweet Home Subdivision' project
described in Example 1, 3 and 4. Determine whether the project as proposed meets its phosphorus
budget.
Solution: Use Worksheet 4 to summarize the Project Phosphorus Export as shown below
st-treatment PPE is 1.838 lbs P/year.
- 12 -
Example 5: Project Phosphorus Export Summary for Subdivision Development
Problem: Summarize the Project Phosphorus Export for the 'Homesweet Home Subdivision' project
described in Example 1, 3 and 4. Determine whether the project as proposed meets its phosphorus
budget.
Solution: Use Worksheet 4 to summarize the Project Phosphorus Export as shown below.
Worksheet 4
Project Phosphorus Export Summary
Summarizing the project’s algal available phosphorus export (PPE)
Project name: Homesweet Home Subdivision
Project Phosphorus Budget
PPB
1.995
lbs P/year
Mitigation Credit - Source Elimination Credit
SEC
0.00
lbs P/year
Source Treatment Credit
STC
0.00
lbs P/year
Total Phosphorus Mitigation Credit (SWC+STC)
TMC
0.00
lbs P/year
Total Pre-treatment Phosphorus Export Worksheet 2
Pre-PPE
3.155
lbs P/year
Total Post-treatment Phosphorus Export Worksheet 2
Post-PPE
1.838
lbs P/year
Project Phosphorus Export (Post-PPE - TMC)
PPE
1.838
lbs P/year
Since the calculated PPE of 1.838 lbs P/year is less than the PPB of 1.995 lbs P/year, the
project meets its phosphorus budget and no further treatment or reduction actions are
necessary.
- 13 -
IMPORTANT:
Best Management Practices (BMPs)
must be designed to meet the
required phosphorus reductions
based on the Project Phosphorus
Budget (PPB). The lowest
maintenance BMP that meets the
PPB should be selected.
Chapter 4 - Treatment Factors for Phosphorus BMPs
This chapter presents the treatment factors that the Department recommends using for the design of a
variety of phosphorus control BMPs.
4.1 - Rules of Thumb for BMP Selection
Consider these rules before selecting a BMP:
1. Given a choice, select the lowest maintenance BMP
that will provide the required phosphorus removal.
For example, natural wooded buffers require much less
long term inspection and maintenance than most other
BMPs, so, if space and topography allow, buffers are
the preferred BMP.
2
ety of phosphorus control BMPs.
4.1 - Rules of Thumb for BMP Selection
Consider these rules before selecting a BMP:
1. Given a choice, select the lowest maintenance BMP
that will provide the required phosphorus removal.
For example, natural wooded buffers require much less
long term inspection and maintenance than most other
BMPs, so, if space and topography allow, buffers are
the preferred BMP.
2. The buffer and source area (lawn, parking lot, etc.)
should be laid out for runoff to pass in non-
channelized sheet flow from the source area and be
evenly distributed across the face of the buffer. If
this design is not possible, runoff may first be
concentrated and then redistributed into the buffer using
a level spreader or ditch turnout, but care must be taken
not to hydrologically overload the buffer and to maintain
the level spreader (see design standards for level lip
spreaders in Volume III, Chapter 9).
4.2 - Common BMPs and their Standard Sizing
Volume III of this manual, BMP Technical Design Manual,
presents the standard designs for the most commonly used
BMPs. These standard BMP designs are sized to provide
retention of approximately 60% of the annual stormwater
phosphorus export, and thus would have a treatment factor
Natural wooded buffers are preferred
of 0.40. The standard sizing of these BMPs is as follows:
over other BMPs due to their low
•
Wetponds: Standard sizing for wet ponds requires a
maintenance.
storage volume below the permanent pool elevation of at
least 2.0 inch of runoff times the subcatchment's impervious area plus 0.8 inch of runoff times the
subcatchment's non-impervious developed area. The pond must have a mean depth of at least three
feet, and a length to width ratio of 2:1 or greater. See Chapter 4 of Volume III.
•
Underdrained vegetated soil filter or other approved filter: Standard sizing for filters requires
storage of a runoff volume equal to 1.0 inch times the subcatchment's impervious area plus 0.4 inch
times the subcatchment's non-impervious developed area
veloped area. The pond must have a mean depth of at least three
feet, and a length to width ratio of 2:1 or greater. See Chapter 4 of Volume III.
•
Underdrained vegetated soil filter or other approved filter: Standard sizing for filters requires
storage of a runoff volume equal to 1.0 inch times the subcatchment's impervious area plus 0.4 inch
times the subcatchment's non-impervious developed area. Filter examples are underdrained
vegetated soil filters having a single outlet with a diameter no greater than eight inches, or proprietary
filter systems approved by the department. See Chapter 7 of Volume III.
•
Infiltration Systems: Standard sizing for infiltration systems requires storage of a runoff volume
equal to 1.0 inch times the subcatchment's impervious area plus 0.4 inch times the subcatchment's
non-impervious developed area. Pre-treatment of stormwater must occur prior to discharge to the
infiltration area. See Chapter 6 of Volume III.
•
Vegetated Buffers: Standard sizing of flow path lengths for buffers depends on the type of buffer, the
soil type and slope of the buffer, and the nature and extent of land use in the contributing watershed
for phosphorus export reduction. Standard sizing for a given buffer type in a given landscape and
development setting can be determined using the tables found in Chapter 5, of Volume III.
- 14 -
4.3 - Determining BMP Phosphorus Treatment Factors
In designing the stormwater management system, the treatment factor of the selected BMPs for the
project can be adjusted based on their sizing. A BMP's treatment factor may be reduced or enhanced by
modifying either the volume of stormwater runoff it will store and treat or the length of flow path through a
buffer. The two formulas presented below provide a way of (1) adjusting the standard sizing of BMPs
described in Volume III and in Section 4.2 of this volume to provide a desired treatment factor, or (2)
determining the treatment factor of a given size BMP
tor may be reduced or enhanced by
modifying either the volume of stormwater runoff it will store and treat or the length of flow path through a
buffer. The two formulas presented below provide a way of (1) adjusting the standard sizing of BMPs
described in Volume III and in Section 4.2 of this volume to provide a desired treatment factor, or (2)
determining the treatment factor of a given size BMP.
Wetponds, underdrained soil filters, infiltration systems may be sized to provide various degrees of
treatment by adjusting the storage and treatment volume. In the case of buffers, the flow path length may
be adjusted (Volume III, Chapter 5). There is, however, a point at which further increase in sizing of a
BMP is not likely to significantly improve the BMP's ability to retain phosphorus. This limit is expressed as
a minimum treatment factor for each type of BMP. Treatment factors may not be adjusted below this
minimum. Minimum treatment factors are discussed later in this chapter and are presented in Table 4.1.
(1) If you know the treatment factor that is needed and can adjust your BMP design to meet
it, use the following equation to determine either the volume of runoff that must be
stored and treated or the necessary buffer flow path length to provide a required
treatment factor:
BMPTF = 0.4*(BMPST /TF)
Where:
TF
= Desired treatment factor (1-Removal Efficiency of the BMP)
BMPST = Standard sizing for the BMP, as described in Section 4.2
BMPTF = Required sizing to achieve the desired treatment factor
BMP sizing is based on either volume of runoff stored and treated or buffer flow path length.
Example 6: Alternative BMP Sizing
Problem: 5.0 pounds per year of phosphorus is created from the runoff of a four acre parking lot.
The owner wishes to design a wetpond to achieve a desired maximum phosphorus export from the
parking lot of 1.8 pounds/year to meet the project’s phosphorus budget for that area
sizing is based on either volume of runoff stored and treated or buffer flow path length.
Example 6: Alternative BMP Sizing
Problem: 5.0 pounds per year of phosphorus is created from the runoff of a four acre parking lot.
The owner wishes to design a wetpond to achieve a desired maximum phosphorus export from the
parking lot of 1.8 pounds/year to meet the project’s phosphorus budget for that area. What size
pond will be needed to achieve this export?
Solution: The desired treatment factor is calculated as:
TF = Desired Export/Existing Export = 1.8/5.0 = 0.36 or
Using the Volume III sizing criteria, 60% removal efficiency can be achieved with a wetpond, or a
0.4 TF. This requires a minimum permanent pool volume of 2.0 inches times the impervious area
(parking lot). Since this project requires a 36% TF (64% removal efficiency), the permanent pool
volume must be larger by an equally proportional amount. Thus, the wetpond can be sized as
follows:
BMPTF = 0.4*(BMPST/TF)
= 0.4*(2.0/0.36)
= 2.22 inch of runoff for the impervious area
The permanent pool of the pond must be sized for 2.22 inches of runoff over the impervious area.
If the pond is discharging to a stream before reaching the lake, then the channel protection volume
needs to be provided per Volume III.
- 15 -
(2) If, on the other hand, you know the size of a given BMP, and need to determine its
treatment factor, use the following formula:
TF = 0.4 BMPST / BMPTF
Where:
TF
= Treatment Factor (1-Removal Efficiency of the BMP) of the BMP
BMPST = Standard sizing for the BMP as described in Section 4.2
BMPTF = Actual sizing of the given BMP
BMP sizing is based on either volume of runoff stored and treated or buffer flow path length.
Example 7: Alternative Treatment Factor and Minimum Treatment Factor
Problem: The available downgradient meadow buffer for a one acre parking lot has 150 feet of
sheet flow on a 6% slope and on HSG C soil (sandy loam)
rd sizing for the BMP as described in Section 4.2
BMPTF = Actual sizing of the given BMP
BMP sizing is based on either volume of runoff stored and treated or buffer flow path length.
Example 7: Alternative Treatment Factor and Minimum Treatment Factor
Problem: The available downgradient meadow buffer for a one acre parking lot has 150 feet of
sheet flow on a 6% slope and on HSG C soil (sandy loam). What would be the allowable
treatment factor for this buffer?
Solution: According to the buffer table, “Buffer Flow Path Length Downgradient of Residential,
Largely Pervious or Small Impervious Areas” (Chapter 5 in Volume III, the required forested
buffer length on sandy loam C soils to adequately treat stormwater runoff is 100 feet. Thus, the
treatment factor for the available buffer on this project is:
TF = 0.4 BMPST/BMPTF
= 0.4 (100/150)
= 0.27
However, according to Table 4.1, the minimum treatment factor that may be used for a forested
buffer on C sandy loam soil is 0.3. Therefore, the treatment factor is limited to 0.3.
4.4 - Minimum Treatment Factors
There is a limit to the amount of phosphorus removal a BMP can accomplish, no matter how large one
makes it. The physical, chemical, and biological processes that a BMP relies on to remove pollutants
have limitations and making a BMP larger generally does not change the limits on the effectiveness of
these processes. It only means that the BMP will treat a larger fraction of the runoff generated by
infrequent storms. Hence, upper limits have been placed for BMP sizing. Table 4.1 presents the minimum
treatment factors that are allowed for selected BMPs. BMPs may be enlarged from their standard sizing
criteria as outlined in Volume III to reduce their treatment factors to a point, but treatment factors may not
be reduced below the minimums in Table 4.1.
4.5 - Multiple BMPs Placed in Series
If multiple BMPs are being used in series, make sure that the last BMP in the series has the ability to
remove the types of pollutants which are likely to reach it
s may be enlarged from their standard sizing
criteria as outlined in Volume III to reduce their treatment factors to a point, but treatment factors may not
be reduced below the minimums in Table 4.1.
4.5 - Multiple BMPs Placed in Series
If multiple BMPs are being used in series, make sure that the last BMP in the series has the ability to
remove the types of pollutants which are likely to reach it. For example, treatment credit should not be
taken for a filter located downstream of an efficient forested buffer. The filter will not add significant
additional treatment because the buffer will most likely have already removed all the fractions of
stormwater phosphorus that the filter could.
If multiple BMPs are being used appropriately in series, the net treatment factor for the series of BMPs is
the product of the lowest (most efficient) treatment factor of the individual BMPs in the series and the
square root of the treatment factor(s) of the less effective BMP(s). For example, if a small wetpond with a
treatment factor of 0.6 drained to an engineered infiltration area with a treatment factor of 0.4, the net
treatment factor for the two BMPs would be 0.4 x (0.6)1/2 = 0.31.
- 16 -
Table 4.1
Minimum Treatment Factors for Selected BMPs
BMPs
Treatment Factor
(1-RE)
Wetponds
Single Pond
0.3
Two ponds in series (per Volume III, Chapter 4)
0.25
Three ponds in series (per Volume III, Chapter 4)
0.2
Underdrained Soil Filters and Other Approved Filters
On sand, loamy sand or sandy loam
0.15
All other filters (including filters with an impermeable liner)
0.25
Infiltration
All infiltration BMPs
0.1
Vegetated Buffers *
Hydrologic Soil Group (and Texture)
Forest
Meadow
A or B
0.15
0.2
C (sandy loam or loamy sand)
0.2
0.3
C (silt loam, clay loam or silty clay loam)
0.3
0.4
D (non-wetland)
0.4
N/A
The maximum allowed flow path length in a buffer is 150 feet unless the runoff is redistributed by a midcourse stone
bermed level lip spreader
0.25
Infiltration
All infiltration BMPs
0.1
Vegetated Buffers *
Hydrologic Soil Group (and Texture)
Forest
Meadow
A or B
0.15
0.2
C (sandy loam or loamy sand)
0.2
0.3
C (silt loam, clay loam or silty clay loam)
0.3
0.4
D (non-wetland)
0.4
N/A
The maximum allowed flow path length in a buffer is 150 feet unless the runoff is redistributed by a midcourse stone
bermed level lip spreader.
RE= Removal Efficiency
N/A = not applicable
Example 8: Maximum Treatment
Problem: Using the minimum treatment factors in Table 4.1, calculate the maximum amount of
runoff that can be treated using an underdrained filter that has less than 2 feet of separation
between the bottom of the system and the restrictive layer and that is treating runoff from an
impervious area.
Solution: For this situation, the minimum treatment factor from Table 4.1 is 0.25. Standard BMP
sizing calls for treatment of 1.0 inch of runoff over the impervious surface. The maximum runoff
that can be treated in such a filter is calculated as follows:
BMPTF = 0.4*(BMPST/TF)
= 0.4*(1.0/0.25)
= 1.6 inch of runoff
The filter bed can only be expanded by area and not depth.
- 17 -
Chapter 5 - Credits for Mitigation and Compensation
Fees
Phosphorus reduction
The objective of this project planning process is to limit increases in
can also be achieved
phosphorus loading to the lake resulting from development. The
through mitigation
methods discussed thus far have focused on limiting the scope of the
measures that focus on
development or reducing its phosphorus export by incorporating
eliminating or reducing
BMPs. Phosphorus load to the lake can also be limited by reducing
phosphorus loads from
export from long standing, existing sources of phosphorus, a trade off
existing sources.
usually referred to as mitigation.
5.1 - Types of Mitigation
Mitigation can take a number of forms. It can involve the elimination of an historical source or a reduction
of the source, usually by treatment with BMPs
educing
BMPs. Phosphorus load to the lake can also be limited by reducing
phosphorus loads from
export from long standing, existing sources of phosphorus, a trade off
existing sources.
usually referred to as mitigation.
5.1 - Types of Mitigation
Mitigation can take a number of forms. It can involve the elimination of an historical source or a reduction
of the source, usually by treatment with BMPs.
Phosphorus loads can also be limited by reducing export from existing sources of phosphorus
on a site, a trade-off usually referred to as mitigation. Mitigation credits can be achieved
through two means:
•
Elimination of existing phosphorus sources (e.g., elimination of an old gravel road so it can
revert back to a forested condition); or
•
Reduction of existing phosphorus sources through treatment (e.g., diverting stormwater
flows from an existing road to a treatment device for phosphorus removal).
However, the following must also be true:
•
To be considered an existing source, it must be in existence prior to 1980.
Elimination: A wood lot being developed as a subdivision can provide a good example of elimination.
An old, gravel road passes through the wood lot. It will not, however, be used by the developer, who
plans to eliminate the old road and construct a new road in a different location to access the new house
lots. The old road has been exporting phosphorus to the lake for the last 50 years.
Elimination of the road and return to a forested condition will result in a reduction of phosphorus loading
to the lake. This reduction in historical phosphorus export can be used to balance, or mitigate, some of
the project's new phosphorus export. In this chapter we will discuss various ways of mitigating new
phosphorus export by reducing or eliminating historical phosphorus sources, and how credit for this
mitigation can be applied to a project's phosphorus budget.
Reduction by Treatment: Suppose the developer actually wants to use the old road in his
subdivision scheme
to balance, or mitigate, some of
the project's new phosphorus export. In this chapter we will discuss various ways of mitigating new
phosphorus export by reducing or eliminating historical phosphorus sources, and how credit for this
mitigation can be applied to a project's phosphorus budget.
Reduction by Treatment: Suppose the developer actually wants to use the old road in his
subdivision scheme. In order to reduce phosphorus export from other parts of his development he is
constructing a wet pond BMP down gradient of the development. The old road, which formerly drained
directly to the lake, now drains first to the wet pond, which has a treatment factor of 0.40. This means that
only 40% of the old road's phosphorus export is now reaching the lake, with the remainder retained in the
wet pond. Phosphorus export from the road has now been reduced by 60%. This reduction in
phosphorus, called a credit, can now be subtracted from the phosphorus export associated with newly
developed portions of the project.
- 18 -
Another variation on this example illustrates an even more common situation. Suppose the old gravel
road is not wide enough to meet current town standards for subdivision roads and it must be widened
from 12 feet to 24 feet. This expansion of road width means that half of the 24 foot road must be
considered new in terms of phosphorus export. Its export, as modified by the wet pond's treatment factor,
would be included in the list of phosphorus exports in Worksheet 2. Stormwater from the half of the road
which can be considered old, however, is now being treated and the resulting reduction in phosphorus
export is a credit that can be subtracted from the project's total phosphorus export.
Problems with Estimating Credit for Mitigation: But how does one estimate the amount of
phosphorus export that is being reduced or eliminated? Phosphorus export from old, preexisting sources
can be estimated in the same way as new sources, using the export factors in Table 3.1
he resulting reduction in phosphorus
export is a credit that can be subtracted from the project's total phosphorus export.
Problems with Estimating Credit for Mitigation: But how does one estimate the amount of
phosphorus export that is being reduced or eliminated? Phosphorus export from old, preexisting sources
can be estimated in the same way as new sources, using the export factors in Table 3.1. However, in
doing so two important issues must be considered.
•
Comparability of existing and proposed phosphorus export sources: Let us
return to the road example. Suppose the old gravel road has seen only very light use as a woods
road over the last 50 years. It has started to revert with vegetation encroaching on the sides and in
the middle between the tire tracks. It is unlikely that it currently exports nearly as much phosphorus
per square foot as will the new subdivision road, which will receive comparably heavy use. So
treatment or elimination of this export will not provide equivalent mitigation for the new road which
replaces it. To avoid this problem, this evaluation process automatically cuts the estimated
phosphorus export from preexisting sources in half unless it is clearly demonstrated that the old
source is equivalent in both structure and use to the new sources being mitigated. A multiplier of 0.5
is included in Worksheet 3 for this purpose.
•
Historic drainage patterns of the phosphorus export: If the runoff from the historic
road immediately drained into a road ditch and continued, untreated, to the lake there is no problem
with taking credit for elimination or treatment of the old road. But often the runoff from old roads
sheets into the woods or fields adjacent to the road where much of its phosphorus export is retained
in a natural buffer, and never reaches the lake. In this case credit should not be taken for treatment of
the road's stormwater runoff because it was already receiving treatment, unless the new BMP has a
much better treatment factor than the historic BMP
ld road. But often the runoff from old roads
sheets into the woods or fields adjacent to the road where much of its phosphorus export is retained
in a natural buffer, and never reaches the lake. In this case credit should not be taken for treatment of
the road's stormwater runoff because it was already receiving treatment, unless the new BMP has a
much better treatment factor than the historic BMP. In that case, credit may be taken for treatment,
but only for the difference between the old and new phosphorus export. If the road is being
eliminated, credit should only be taken for an amount of export which has been reduced by the
treatment factor of the historic buffer.
Mitigation Dos and Don’ts
Don't take credit for mitigation of relatively
recent phosphorus sources. Credit should
not be allowed on sources, which were not in
existence prior to 1980.
Do halve the phosphorus export from mitigation
sources unless it is clearly demonstrated that the
old source is equivalent in both structure and use
to the new sources being mitigated.
Don't take mitigation credit for the treatment
of sources which have been historically
treated by adjacent buffers or treatment
ponds.
Do adjust the credit for any source elimination to
reflect historical treatment by buffers or ponds.
Do insure - through property owner agreements,
deed covenants and restrictions, conservation
easements, endowments and/or long term
agreements with maintenance contractors - that
eliminated sources will be allowed to revert and
remain in a forested condition and that BMPs will
be properly maintained.
- 19 -
5.2 Estimating Credits for Mitigation
Worksheet 3 may be used to calculate mitigation credits. The upper portion of the worksheet is used
to calculate credit when a pre-existing source is being eliminated. The lower portion is used when a
pre-existing, historically untreated source is being treated by new BMPs. Credits are determined as
follows:
1
and that BMPs will
be properly maintained.
- 19 -
5.2 Estimating Credits for Mitigation
Worksheet 3 may be used to calculate mitigation credits. The upper portion of the worksheet is used
to calculate credit when a pre-existing source is being eliminated. The lower portion is used when a
pre-existing, historically untreated source is being treated by new BMPs. Credits are determined as
follows:
1. List the size of the source area (in acres and to the second decimal place) and phosphorus
export coefficient from Table 3.1 for each pre-existing mitigation source in the appropriate
spaces.
2. If the source is being eliminated, enter the estimated treatment factor for historical BMPs that
provided treatment of the source or enter 1.0 if the source drains directly to the lake.
3. If the source is being treated, enter the treatment factor for the new BMP which will be treating
each mitigation source in the worksheet. Also enter the estimated treatment factor for any natural
buffers or other BMPs that historically provided treatment of the source or enter 1.0 if the source
drained directly to the lake.
4. For source elimination and treatment multiply the source area with the phosphorus export factor
and by the modifier 0.5 (unless it has been clearly demonstrated that the old source is equivalent
in both structure and use to the new sources being mitigated) to calculate the creditable pre-
treatment historical phosphorus export from each source area. The modifier can be selected as
1.0 only if it is clearly demonstrated that the old source is equivalent in both structure and use to
the new source being mitigated. Then the pre-treatment historical phosphorus export is
multiplied with any treatment factors for historical BMPs to obtain the historical phosphorus
export.
5. For each source that is being eliminated, the mitigation credit value is equivalent to the historical
phosphorus export value (from Step 3)
rated that the old source is equivalent in both structure and use to
the new source being mitigated. Then the pre-treatment historical phosphorus export is
multiplied with any treatment factors for historical BMPs to obtain the historical phosphorus
export.
5. For each source that is being eliminated, the mitigation credit value is equivalent to the historical
phosphorus export value (from Step 3). For sources being treated by new BMP(s), subtract the
treatment factor for the new BMP from 1.0 and multiply this times the source's historical
phosphorus export (from Step 3) to get the mitigation credit value for each source.
6. Sum the phosphorus export credit values for all mitigation sources to obtain the total credit value
in Worksheet 3.
5.3 On-site vs. Off-site Mitigation
Another important mitigation consideration is whether the mitigation source and, in the case of treatment,
the BMP, are located on the parcel (on-site) or elsewhere in the lake's watershed (off-site). If the
treatment source and the treatment BMP are located ”on-site”, the developer can insure - through
property owner agreements, deed covenants and restrictions, conservation easements, endowments
and/or long term agreements with maintenance contractors - that eliminated sources will be allowed to
revert and remain in a forested condition and that BMPs will be properly maintained. Essentially these
mitigation measures are treated no differently than the buffers and other BMPs incorporated in the project
to address new sources of phosphorus export.
If the mitigation measures are being implemented elsewhere in the watershed, it is a much greater
challenge, for both the developer and any regulating body which is requiring the mitigation, to insure that
the measures are maintained over the long term. In the case of elimination or reduction of off-site
mitigation sources by change of land use (i.e. conversion of road to forest), these areas can be
reasonably protected by deed restrictions and conservation easements
shed, it is a much greater
challenge, for both the developer and any regulating body which is requiring the mitigation, to insure that
the measures are maintained over the long term. In the case of elimination or reduction of off-site
mitigation sources by change of land use (i.e. conversion of road to forest), these areas can be
reasonably protected by deed restrictions and conservation easements. However, insuring long term
maintenance of offsite BMPs is much more problematic. Because of these difficulties, it is recommended
that treatment of on-site mitigation sources with “off-site” BMPs be allowed only if the property on which
the off-site mitigation is taking place and the project parcel are in common ownership, and that owner is a
permanent entity, such as a town or a school district, that is not likely to transfer ownership of either
- 20 -
parcel. Treatment of off-site mitigation sources is only allowed as described in section 3.C.2 of the
Chapter 501 Stormwater Management Compensation Fees and Mitigation Credit Rules.
5.4 Compensation Fees
The Maine Stormwater Management Law (38 MRSA § 420-D) and its accompanying regulations (DEP
Chapter 500 and 501) address some of the problems associated with off-site mitigation discussed above
by allowing an alternative known as the Compensation Fee Program. The law recognizes the difficulties a
state agency would have in tracking and insuring the maintenance of off-site BMPs used for mitigation on
a project
ormwater Management Law (38 MRSA § 420-D) and its accompanying regulations (DEP
Chapter 500 and 501) address some of the problems associated with off-site mitigation discussed above
by allowing an alternative known as the Compensation Fee Program. The law recognizes the difficulties a
state agency would have in tracking and insuring the maintenance of off-site BMPs used for mitigation on
a project. The regulations therefore allow mitigation credit only for the elimination or reduction (by land
use change) of off-site sources of phosphorus (Chapter 501, Section 3.C.1) and the limited off-site
treatment options described in Chapter 501, Section C.3.2.As an alternative to project based off-site
treatment mitigation, the Compensation Fee Program allows the department to let the developer offset a
portion of the phosphorus reduction required for the project to meet its phosphorus budget by paying a
compensation fee to an approved regional organization that performs watershed management activities.
The authorized local entity accumulates compensation fee funds in accounts for each individual lake
watershed, and uses these funds to provide long term solutions to priority chronic phosphorus sources
within the watershed.
The Compensation Fee option is only available on some lakes. Lakes for which the option is not
available include many small, relatively undeveloped lake watersheds that happen to be in a region of
high growth. In these cases there are few if any opportunities to address existing problems in the
watershed. It may also include watersheds where a large amount of restoration work has already been
performed and any problems remaining to be addressed are more expensive than what the compensation
rate can cover. Lastly, it includes areas where there are no local watershed management agencies (i.e.
soil and water conservation districts, watershed districts, etc.) interested in developing and implementing
mitigation projects
e watersheds where a large amount of restoration work has already been
performed and any problems remaining to be addressed are more expensive than what the compensation
rate can cover. Lastly, it includes areas where there are no local watershed management agencies (i.e.
soil and water conservation districts, watershed districts, etc.) interested in developing and implementing
mitigation projects. The regional organizations which the Department has approved for management of
compensation funds and implementation of compensation projects have each developed a list of lakes for
which they will accept compensation fees. Developers should not assume that the compensation fee is
an option until they have checked with the Department to be sure it is available in the lake watershed in
which the proposed development is located.
The Compensation Fee option is only available if the project incorporates onsite treatment and mitigation
measures that reduce the pre-treatment phosphorus export by at least 60%. To express this in terms of
the worksheets used to calculate project phosphorus export, the sum of post-treatment export expressed
on Worksheet 2 must be less than or equal to 0.4 times the sum of pre-treatment export expressed on
Worksheet 2 in order for a project to offset any additional phosphorus reductions through payment of a
compensation fee.
In addition to the above limitations, the Compensation Fee option is not available for residential
subdivision projects unless the wooded or meadow buffers are the only BMPs used to provide the
reduction in pre-treatment phosphorus export.
Compensation Fee Calculations: Projects in lake watersheds regulated by the state under the
Stormwater Management Law may have the opportunity to use compensation fees to help meet their
project's phosphorus budget as required by that law. These projects may, however, also come under local
regulation, which may or may not recognize compensation fee payment as an alternative to on-site
reduction of phosphorus export
alculations: Projects in lake watersheds regulated by the state under the
Stormwater Management Law may have the opportunity to use compensation fees to help meet their
project's phosphorus budget as required by that law. These projects may, however, also come under local
regulation, which may or may not recognize compensation fee payment as an alternative to on-site
reduction of phosphorus export. If it is available for the lake in question, the Compensation Fee option
allows the developer to off-set the difference between the projects phosphorus export and the project’s
phosphorus budget by paying a compensation fee based on the amount, in pounds of phosphorus, of that
difference.
The current compensation rate (August 2015) is $25,000 per pound for a project that provides on-site
reduction of pre-treatment phosphorus export by 60%. The fee becomes progressively less as on-site
- 21 -
reduction of pre-treatment export is reduced, as shown in Table 5.1 below. This means that if a project's
phosphorus budget was 0.6 lb P/yr and, after application of reasonable BMPs, the project export could
only be reduced 60% to 1.0 lb P/yr, the remaining 0.4 lb reduction required to meet the project's budget
could be offset by a compensation fee payment of $10,000 (0.4 lb x $25,000 /lb). Exemptions to the
compensation fee application are stated in Chapter 501(3)(C)(3).
Table 5.1
Compensation Fee per Pound of Phosphorus Export
Percentage
Of Project Phosphorus Export
Additional Phosphorus
Compensation Fee
($/pound)- fees are prorated
60%
$25,000
75%
$12,500
100%
$0
Note:
a.
25,000 per pound fee is reduced by $500 per percentage point of project’s
phosphorus export between 60 and 75%.
b.
$12,500 per pound fee is reduced by $834 per percentage point of project’s
phosphorus export over 75%
Fee per Pound of Phosphorus Export
Percentage
Of Project Phosphorus Export
Additional Phosphorus
Compensation Fee
($/pound)- fees are prorated
60%
$25,000
75%
$12,500
100%
$0
Note:
a.
25,000 per pound fee is reduced by $500 per percentage point of project’s
phosphorus export between 60 and 75%.
b.
$12,500 per pound fee is reduced by $834 per percentage point of project’s
phosphorus export over 75%.
Example 9: Project Phosphorus Export Summary for Subdivision Development
Problem: The Project Phosphorus Export for a subdivision project called ‘Sunnydays’ is similar to the
‘Homesweet Home Subdivision’, but it can only meet its post-treatment phosphorus export for some
of the lots. Determine whether the project as proposed meets its phosphorus budget and what will
the compensation fee be.
Solution: Use Worksheet 4 to summarize the Project Phosphorus Export as shown below.
Worksheet 4
Project Phosphorus Export Summary
Summarizing the project’s algal available phosphorus export (PPE)
Project name: Sunnydays Subdivision
Project Phosphorus Budget – Worksheet 1
PPB
0.995
lbs P/year
Total Pre-treatment Phosphorus Export Worksheet 2
Pre-PPE
3.155
lbs P/year
Total Post-treatment Phosphorus Export – Worksheet 2
Post-PPE
1.213
lbs P/year
Total Phosphorus Mitigation Credit - Worksheet 3
TMC
0.0
lbs P/year
Project Phosphorus Export (Post-PPE - TMC)
PPE
1.213
lbs P/year
The calculated PPE of 2.213 lbs P/year is more than the PPB of 0.995 lbs P/year by
1.218lbP/year; and the project does not meet its phosphorus budget.
A compensation fee is available to off-set the 1.218 lb P/year difference because treatment is
buffer only, and the treatment level is more than 60% (Post-PPE/Pre-PPE).
The PPE has been reduced by 61.55% (1 - (1.213/3.155)). The fee is prorated at 1.55% less than
the fee for 60% or $25000 – 1.55($12,500/15) or $23,708 per pound.
The compensation fee for Sunnydays Home Project is $5,168 ($23,708x0.218).
- 22 -
Alternative performance standards
may be used for certain smaller,
residential projects
only, and the treatment level is more than 60% (Post-PPE/Pre-PPE).
The PPE has been reduced by 61.55% (1 - (1.213/3.155)). The fee is prorated at 1.55% less than
the fee for 60% or $25000 – 1.55($12,500/15) or $23,708 per pound.
The compensation fee for Sunnydays Home Project is $5,168 ($23,708x0.218).
- 22 -
Alternative performance standards
may be used for certain smaller,
residential projects. These generally
include specific development
restrictions and the use of Low
Impact Development (LID) practices.
Chapter 6 - Performance Standards for Smaller
Projects
6.1 - Single Family Residences and Small
Subdivisions with No New Road
There are some kinds of relatively low impact, residential
development where the level of analysis applied in the
previous sections may be inappropriate or unreasonable.
This section prescribes comparatively simple, alternative
performance standards which may be applied to: (1) new
single family residences or duplexes on existing lots which
are not part of a subdivision that has already incorporated
appropriate phosphorus controls; and (2) subdivisions of five
or fewer lots that do not involve the construction of a new
road or expansion of an existing road. New residential
developments which fall into either of these categories may
meet their phosphorus control obligations by incorporating
the phosphorus control measures listed either under Basic
Single Family Residential Lot Standards or Alternative Single
Family Residential Lot Standards below; and by maintaining
these measures over the long term.
Basic Single Family Residential (SFR) Lot Standards. The
Smaller residential development
following basic Single Family Residence Lot Standard is the
projects can use alternative
preferred way of addressing new development of individual
performance standards to meet their
residential lots or small residential subdivisions that do not
phosphorus control obligations.
include a new road. A project must meet all provisions of the
These generally involve restrictions
standard
al development
following basic Single Family Residence Lot Standard is the
projects can use alternative
preferred way of addressing new development of individual
performance standards to meet their
residential lots or small residential subdivisions that do not
phosphorus control obligations.
include a new road. A project must meet all provisions of the
These generally involve restrictions
standard. The standards for appropriate buffer design and
on disturbance, buffers and
maintenance are presented below.
impervious area or the incorporation
of Low Impact Development (LID)
Alternative Single Family Residential Lot Standards. A
techniques.
property owner or developer may choose not to meet the
Basic Single Family Residential Lot Standard due to site
constraints or design preference. In situations where the Basic Standard is not met on a project, the
project must meet the following Alternative Single Family Residential Lot Standards.
Meeting this standard may require the use of more than one LID practice on the site, due to existing site
topography and the layout of the property. For example, half of the roof may drain to the front of a building
while the other half drains to the back of the building, and the lawn and driveway/parking area drain off to
one side of the property. Drainage in each of these directions must be captured and treated using an LID
practice. The selection, size, and location of the LID practices used on a given site will depend on the size
of the area draining to each practice and the impervious area versus lawn area. While this may not
always be feasible, applicants are encouraged to maintain natural buffers to the extent possible as a
primary LID technique, which can then be augmented by other practices on the site. Guidance on how to
size each LID practice is found in section 6.2 below
used on a given site will depend on the size
of the area draining to each practice and the impervious area versus lawn area. While this may not
always be feasible, applicants are encouraged to maintain natural buffers to the extent possible as a
primary LID technique, which can then be augmented by other practices on the site. Guidance on how to
size each LID practice is found in section 6.2 below.
- 23 -
Requirements for New Single Family Lot Development
Basic SFR Lot Standard
Alternative SFR Lot Standards
Disturbance on an individual lot must be less than
Low Impact Development (LID) practices are used
15,000 square feet (including building, driveway,
and sized to treat 0.5 inches of runoff from all
walkways, lawn area, construction access,
impervious surfaces on the site, and 0.2 inches o
grading). And, no more than 7,500 square feet of
runoff from all disturbed pervious areas of the site
impervious cover is located on the property.
(lawn).*
A minimum natural vegetated buffer must be
The LID practices installed on the site are
maintained downgradient of all developed area on
maintained in perpetuity. If necessary, LID
the lot. This buffer shall be 35 feet deep if naturally
practices may be replaced with new LID practices
forested or 50 feet deep if maintained as a natural
as long as the overall site treatment standard
meadow.*
above is met.
A minimum of 25 percent of the lot area must be
maintained as undisturbed natural area.*
* If the lot or a portion of the lot is located within a watershed of
a Lake Most at Risk from New Development, an Urban Impaired
Stream, or other impaired or sensitive waterbodies as
designated by the municipality for the purposes of this standard,
a minimum of:
•
50 feet if naturally forested buffer, or 75 feet if maintained
as meadow must be maintained downgradient of all
developed area on the lot,
•
A minimum of 40% of the lot area must be maintained as
undisturbed natural area
isk from New Development, an Urban Impaired
Stream, or other impaired or sensitive waterbodies as
designated by the municipality for the purposes of this standard,
a minimum of:
•
50 feet if naturally forested buffer, or 75 feet if maintained
as meadow must be maintained downgradient of all
developed area on the lot,
•
A minimum of 40% of the lot area must be maintained as
undisturbed natural area.
If the existing land has been disturbed by prior activities, a
natural vegetated buffer and/or undisturbed natural area may be
proposed through restoration and revegetation.
* If the lot or a portion of the lot is located within watersheds of
Lakes Most at Risk from New Development or other impaired or
sensitive waterbodies as designated by the municipality for the
purposes of this standard, the project must treat one inch of
runoff from impervious surfaces and 0.4 inch from disturbed
pervious surfaces.
6.2 – Low Impact Development Practices
Low Impact Development (LID) practices can be used to capture and treat runoff from residential
rooftops, non-rooftop impervious areas such as paved driveways, patios and walkways, and maintained
lawn areas. While there are a number of practices considered to be LID practices, a lengthy discussion of
these is not practicable for this volume but can be found in Volume III of this manual. The most popular
LID practices are the following:
•
Buffers,
•
Underdrain soil filters (rain gardens and swales),
•
Infiltration practices (dry wells and infiltration trench),
•
Pervious pavements.
The design and maintenance standard for Buffer are presented below, and should be applied to projects
meeting the Basic SFR Lot Standard. Design and maintenance standards for other LID practices (i.e
l. The most popular
LID practices are the following:
•
Buffers,
•
Underdrain soil filters (rain gardens and swales),
•
Infiltration practices (dry wells and infiltration trench),
•
Pervious pavements.
The design and maintenance standard for Buffer are presented below, and should be applied to projects
meeting the Basic SFR Lot Standard. Design and maintenance standards for other LID practices (i.e.
underdrain soil filters, dry wells and infiltration trenches, pervious pavements, rain barrels and cisterns,
green roofs, stormwater planters, micro bio-inlets) are described in detail in the Maine LID Guidance
Manual (September 2007), http://www.maine.gov/dep/land/watershed/materials/lid-guidance-manual.pdf,
and these should be applied to projects meeting the Alternative SFR Lot Standards.
Vegetative Buffers
Vegetative buffers are areas of dense forest or meadow vegetation located adjacent and downgradient of
developed areas that provide storage and treatment for stormwater that enters them in diffuse overland
flow. They should be designed, implemented and maintained in accordance with the following:
•
Discharge of stormwater to the buffer: It is essential that the stormwater entering the buffer not be
channelized prior to discharge into the buffer. Grading of developed areas upgradient of the buffer
must be done in a way that maintains diffuse overland flow and avoids concentration of the runoff.
- 24 -
•
Topography: The topography of a buffer area must maintain well-distributed stormwater runoff and
cannot allow stormwater runoff to concentrate as it flows across the buffer. Flow paths of runoff
through a buffer must not converge, but must be essentially parallel or diverging.
•
Vegetative cover: The vegetative cover of a buffer must be either forest or meadow. In most
instances the sizing of a buffer varies depending on vegetative cover type.
•
Forest buffer: A forest buffer must have a well distributed stand of trees with essentially complete
canopy cover, and must be maintained as such
through a buffer must not converge, but must be essentially parallel or diverging.
•
Vegetative cover: The vegetative cover of a buffer must be either forest or meadow. In most
instances the sizing of a buffer varies depending on vegetative cover type.
•
Forest buffer: A forest buffer must have a well distributed stand of trees with essentially complete
canopy cover, and must be maintained as such. A forested buffer must also have an undisturbed
layer of duff covering the mineral soil. Activities that may result in disturbance of the duff layer are
prohibited in a buffer
•
Meadow buffer: A meadow buffer must have a dense cover of grasses, or a combination of grasses
and shrubs or trees. A buffer must be maintained as a meadow with a generally tall stand of grass,
not as a lawn. It must not be mown more than twice per calendar year. If a buffer is not located on
natural soils, but is constructed on fill or reshaped slopes, a buffer surface must either be isolated
from stormwater discharge until a dense sod is established, or must be protected by a three inch
layer of erosion control mix or other wood waste material approved by the department before
stormwater is directed to it, with vegetation established using an appropriate seed mix.
•
Mixed meadow and forest buffer: If a buffer is part meadow and part forest, the required sizing of the
buffer must be determined as a weighted average, based on the percent of the buffer in meadow and
the percent in forest.
•
Deed restrictions and covenants: Areas designated as vegetated buffers must be clearly identified on
site plans and protected from disturbance by deed restrictions and covenants.
- 25 -
Appendix A - Phosphorus and Lake Water Quality
The Relationship between Phosphorus and Lake Water Quality: Lakes are biological
systems that are clearly affected by changes in water quality. They are most noticeably affected by an
increase in nutrients, particularly phosphorus
uffers must be clearly identified on
site plans and protected from disturbance by deed restrictions and covenants.
- 25 -
Appendix A - Phosphorus and Lake Water Quality
The Relationship between Phosphorus and Lake Water Quality: Lakes are biological
systems that are clearly affected by changes in water quality. They are most noticeably affected by an
increase in nutrients, particularly phosphorus. Increases in phosphorus usually result in more noticeable
changes to water quality than increases in other nutrients. Algae, which are microscopic organisms
common in lakes, need phosphorus in order to grow. Consequently, when phosphorus is abundant in lake
water, algal populations soar in number, causing a decline in water transparency. In some cases, algal
blooms may occur causing the growth of billions of algae to color the lake water green and release strong
odors as they decay.
Beyond the aesthetic impacts, algal blooms have serious impacts on a lake's biological community.
Through a complex chain of events, algal blooms lead to depletion of the lake water's oxygen supply,
usually resulting in the eventual loss of trout and salmon (cold water) fisheries. In addition, large algal
populations cause odor, taste, and treatment problems in lakes used for public water supplies.
The biological term for the process described above is eutrophication, which can manifest itself over time
as a gradual increase in the lake nutrient concentrations under natural conditions. Lake eutrophication
can be dramatically accelerated by human activities, causing the noticeable changes described above in
a relatively short period of time. Many lakes in Maine have already experienced dramatic declines in
water quality as a result of human disturbances.
How Phosphorus Gets into Lakes: Understanding how phosphorus gets into lake water requires
an understanding of where lake water comes from. Precipitation and stormwater runoff are significant
sources of water in rivers and lakes
es described above in
a relatively short period of time. Many lakes in Maine have already experienced dramatic declines in
water quality as a result of human disturbances.
How Phosphorus Gets into Lakes: Understanding how phosphorus gets into lake water requires
an understanding of where lake water comes from. Precipitation and stormwater runoff are significant
sources of water in rivers and lakes. Rain and melting snow flow downhill over the land surface into
streams and lakes or seep into the ground becoming groundwater, which also ultimately discharges to
streams and lakes.
The land area that contributes water to a particular lake is known as its watershed. Watershed boundaries
can be identified by connecting points of highest elevation around a lake and its tributaries. All rain and
snow falling within this area eventually flow by gravity in surface runoff, streams, and groundwater to the
lake, which is the lowest point in the watershed.
The quality of water in a lake depends on the condition of the land in its watershed. Phosphorus is
abundant in the environment, but in an undisturbed environment it is tightly bound up by soil and organic
matter for eventual use by plants. Natural systems conserve and recycle nutrients, water, and other
materials needed to sustain plant growth. Water is stored in depressions on the uneven forest floor and
seeps into the ground to become groundwater, thereby preventing it from running over the land surface
and exporting valuable nutrients from the system.
Land development changes the natural landscape in ways that alter the normally tight cycling of
phosphorus. The removal of vegetation, smoothing of the land surface, compaction of soils, and creation
of impervious surface combine to reduce the amount of precipitation stored and retained onsite,
dramatically increasing the amount of water running off the land as surface runoff.
These changes to the land surface and the associated increase in surface runoff dramatically increase
phosphorus export
sphorus. The removal of vegetation, smoothing of the land surface, compaction of soils, and creation
of impervious surface combine to reduce the amount of precipitation stored and retained onsite,
dramatically increasing the amount of water running off the land as surface runoff.
These changes to the land surface and the associated increase in surface runoff dramatically increase
phosphorus export. Land disturbance upsets the environment's ability to retain phosphorus.
Stormwater flowing over the land surface picks up phosphorus and transports it in soluble form or
attached to eroded soil particles. The phosphorus in stormwater comes from natural and human sources,
including eroded soil, road dust, plants, lawn fertilizer, and detergents. The smooth surfaces, closely
cropped lawns, and compacted soils common in developed areas do not retain phosphorus, and only
speed its export by generating surface runoff. The end result is more phosphorus is stormwater, and thus
more phosphorus in lakes.
- 26 -
A study in Maine has documented the elevated levels of phosphorus exported from developed land
(Dennis, 1985). In adjacent watersheds, one developed and one undisturbed, phosphorus export from the
developed watershed was up to 10 times greater than the export from the forested watershed.
Because the built watershed was developed years ago, these figures represent the permanent increase
in phosphorus export caused by alteration of the landscape. This permanent increase in the phosphorus
supply of the lake creates an equally permanent and irreversible decline in water quality.
Though in most lakes the majority of phosphorus comes from the watershed, there is another source of
phosphorus that can be very significant in some lakes. Over the centuries phosphorus rich organic
sediments have accumulated on the bottom of our lakes. In most cases, the phosphorus in these
sediments is trapped there by a blanket of iron hydroxide and or aluminum hydroxide, which makes the
sediments a sink for, rather than a source of, phosphorus
es from the watershed, there is another source of
phosphorus that can be very significant in some lakes. Over the centuries phosphorus rich organic
sediments have accumulated on the bottom of our lakes. In most cases, the phosphorus in these
sediments is trapped there by a blanket of iron hydroxide and or aluminum hydroxide, which makes the
sediments a sink for, rather than a source of, phosphorus. However, in lakes with sufficient algal
production to cause a severe loss of oxygen concentrations above the sediments, the iron hydroxide
blanket dissolves and large amounts of phosphorus may be recycled into the lake water. The "surges" of
phosphorus feed algal growth which further depletes dissolved oxygen, thus creating a vicious cycle of
very rapid, internally driven eutrophication. This process, which can be initially triggered by relatively small
increases in phosphorus input from the watershed, may drive a lake from apparently good, clear water
quality to having intense algal blooms in a matter of years. It is particularly important to limit any increases
in phosphorus input from the watershed to lakes with a high potential for sediment phosphorus recycling.
How Stormwater Phosphorus can be controlled: All land disturbance and development in a
lake's watershed increases phosphorus export to a lake. Although some increase must be accepted as
the inevitable and unavoidable effect of development, a variety of measures can substantially reduce
phosphorus export to lakes and help to preserve good water quality.
The simplest way to reduce phosphorus export is to limit clearing of vegetation and minimize the area
developed, especially road length. Beyond this, a variety of control measures are available. They
generally focus on detaining and storing stormwater where it can be treated and released or infiltrated
into the soil.
Buffer areas are naturally vegetated areas preserved downslope of developed areas
The simplest way to reduce phosphorus export is to limit clearing of vegetation and minimize the area
developed, especially road length. Beyond this, a variety of control measures are available. They
generally focus on detaining and storing stormwater where it can be treated and released or infiltrated
into the soil.
Buffer areas are naturally vegetated areas preserved downslope of developed areas. These buffers
intercept and store surface runoff, allowing it to infiltrate rather than flow off-site as surface flow.
Infiltration systems are more sophisticated. Runoff is collected from rooftops, driveways and/or impervious
parts of a lot and then directed to surface or underground storage, similar to a subsurface disposal area,
from which wastewater infiltrates into the soil. Soils must be fairly deep, coarse, and permeable for
infiltration systems to work.
Underdrained soil filters are similar to infiltration systems in that runoff is collected and directed to
a storage depression, but the depression is vegetated with flood and drought tolerant species and is lined
with a specific soil filter media which is underlain with a pipe system to discharge the filtered runoff.
Wet ponds are generally used to treat runoff from a large area. They receive and retain stormwater from
large drainage areas, allowing sediment to settle out and dissolve phosphorus to be removed by
biological activity.
Development can proceed in lake watersheds without generating more phosphorus than the lake can
tolerate by limiting the extent of development and incorporating one or more of these phosphorus
controls. Once a lake has accepted more phosphorus than it can tolerate, there will be a noticeable
decline in water quality.
- 27 -
Appendix B - DEP’s Method for Defining Watershed
per Acre Phosphorus Allocations
The Department defines per acre phosphorus allocations (P) for lake watersheds and these are
presented in Appendix C
ent of development and incorporating one or more of these phosphorus
controls. Once a lake has accepted more phosphorus than it can tolerate, there will be a noticeable
decline in water quality.
- 27 -
Appendix B - DEP’s Method for Defining Watershed
per Acre Phosphorus Allocations
The Department defines per acre phosphorus allocations (P) for lake watersheds and these are
presented in Appendix C. The list in Appendix C is not complete, so if a per acre allocation for a town's
portion of a lake watershed is needed and it is not listed in Appendix C, request the Division of Watershed
Management to provide a per acre allocation for the desired watershed. The Department will continually
update Appendix C, both by adding new lakes to the list and by revising allocations for lakes already on
the list as new information becomes available. This Appendix describes the process the Department uses
to define watershed per acre phosphorus allocations.
Step 1. Defining the Acceptable Increase in Lake Phosphorus Concentration (C)
The first step is to determine how much the lake's phosphorus concentration could be increased without
risking a perceivable increase in its algal production or a decline in its healthy, natural fish community.
This value, the acceptable increase in lake phosphorus concentration (C), is a function of two variables:
Water Quality Category of the lake and the Level of Protection appropriate for the lake. The Department
has assigned Water Quality Categories to each lake for which sufficient water quality data is available
based on the information in the following table. If insufficient data is available, the lake is assigned a
default water quality category of Moderate Sensitive
is a function of two variables:
Water Quality Category of the lake and the Level of Protection appropriate for the lake. The Department
has assigned Water Quality Categories to each lake for which sufficient water quality data is available
based on the information in the following table. If insufficient data is available, the lake is assigned a
default water quality category of Moderate Sensitive.
Lake Water Quality Categories
Category
Conditions
Outstanding
Exceptional clarity; very low phosphorus and
chlorophyll concentrations; low risk of internal
recycling from sediments
Good
Average to better than average clarity,
phosphorus and chlorophyll; low risk of internal
recycling from bottom sediments
Moderate Sensitive
Average clarity, phosphorus and chlorophyll;
high potential for internal recycling from the
bottom sediments
Poor (Restorable)
Poor clarity; high phosphorus and chlorophyll
concentrations; supports blue green algal
blooms; good prospects for restoration
Poor (Natural)
Poor clarity; high phosphorus and chlorophyll
concentrations; supports blue green algal
blooms; poor prospects for restoration because
lake is naturally very productive
- 28 -
Step 2. Determine the Allowable Increase in Annual Phosphorus Load
The next step is to determine how much the annual phosphorus load to the lake could be increased
without risking an increase in lake phosphorus concentration greater than the acceptable increase (C)
defined in Step 1.
This is accomplished by multiplying "C" by a lake specific coefficient (F) that
estimates the amount of increase in annual phosphorus load to the lake that will
result in a 1.0 ppb lake phosphorus concentration. Where a lake has upstream
lakes draining to it, "F" represents the direct watershed's (that portion of the total
watershed that does not first pass through an upstream lake) share of this load.
Where a lake's direct watershed is located in more than one town, "F" reflects the
given town's portion of the load
l phosphorus load to the lake that will
result in a 1.0 ppb lake phosphorus concentration. Where a lake has upstream
lakes draining to it, "F" represents the direct watershed's (that portion of the total
watershed that does not first pass through an upstream lake) share of this load.
Where a lake's direct watershed is located in more than one town, "F" reflects the
given town's portion of the load. "F" is derived using a steady state solution of
Vollenweider's 1976 phosphorus loading model and is expressed in lbs/ppb/year.
Allowable increase in annual phosphorus load = F x C or FC
Step 3. Determine the Per Acres Phosphorus Budget (P)
The next and final step is to determine the per acre phosphorus budget (P, in lbsP/acre/year) by
allocating the allowable increase in annual phosphorus load (FC) over the portion of the direct watershed
This is accomplished by projecting how much of the direct watershed area is likely to be
developed (D, in acres) and dividing FC by this acreage.
P = FC / D
"D" is estimated by:
•
Determining the area available for development within the town's share of the direct
watershed by subtracting undevelopable acreage (i.e. wetlands, steep slopes, state
parks) and already developed land from direct watersheds area.
•
Projecting how much of the area available for development will be developed over time
based on:
o The general growth rate in the town or region
o The quality, density and distribution of the road network within the town's share of the
direct watershed
o Other lake specific, locally identified information
- 29 -
Appendix C - Per Acre Phosphorus Allocations for
Selected Maine Lakes
The attached spreadsheet presents per acre phosphorus allocations (P) for all the lake/town
combinations that have been calculated to this point. This Appendix will be modified on a regular basis as
additional lake/town combinations are added and allocations are amended as new information becomes
available
ed information
- 29 -
Appendix C - Per Acre Phosphorus Allocations for
Selected Maine Lakes
The attached spreadsheet presents per acre phosphorus allocations (P) for all the lake/town
combinations that have been calculated to this point. This Appendix will be modified on a regular basis as
additional lake/town combinations are added and allocations are amended as new information becomes
available. If you do not find the lake watershed/town combination you are looking for in the appendix,
contact Jeff Dennis at the DEP Division of Environmental Assessment (207-287-7847 or
jeff.dennis@maine.gov) to request an allocation.
- 30 -
Appendix D - Worksheets
The Excel spreadsheet provides four worksheets to assist in the calculations required to determine:
•
Worksheet 1, Project Phosphorus Budget (PPB)
•
Worksheet 2, Pre-treatment and Post-treatment Phosphorus Export Calculations
•
Worksheet 3, Mitigation Credits
•
Worksheet 4, Project Phosphorus Export Summary
The spreadsheets have the calculations built into them and can be used either as active excel
spreadsheets or may be printed out as worksheets for use by hand.
- 31 -
Appendix E - Alternative Method for Small
Commercial-type Developments Located within
Designated Growth Areas
It can be difficult for densely developed projects on small parcels to meet their phosphorus budgets.
Because of the density of high phosphorus producing surfaces like parking lots and lawns, the stormwater
draining these projects carries relatively large amounts of phosphorus. The small parcel size, however,
means that the phosphorus budget for the parcel will also be small. As a result, highly intensive
phosphorus control measures, which are often fairly costly, may be required for the project to meet its
phosphorus budget
gh phosphorus producing surfaces like parking lots and lawns, the stormwater
draining these projects carries relatively large amounts of phosphorus. The small parcel size, however,
means that the phosphorus budget for the parcel will also be small. As a result, highly intensive
phosphorus control measures, which are often fairly costly, may be required for the project to meet its
phosphorus budget.
In these cases it may cost less to develop outside the designated growth area where land is more readily
available for larger parcel sizes (and hence larger project phosphorus budgets) and for less intensive, and
less expensive, phosphorus control measures like natural wooded buffers. If a municipality is concerned
that the phosphorus budget will counter local planning efforts by being a disincentive for locating
development within designated growth areas, they may request that the department allow commercial
developers within their designated growth areas to use an alternative means of defining the project
phosphorus budget. This alternative is described below.
To prevent sprawl and encourage building within designated growth areas, a municipality may request
that projects with no more than 1.0 acre of impervious surfaces (building, parking, driveways, both paved
and gravel) located on a small parcel with less than 5 acres within an area specifically designated for
commercial growth in the municipality's DEP approved comprehensive plan be allowed to calculate the
site's PPB as follows:
Alternative PPB Calculation for a Small Commercial-type Development (as defined by having
less than 1 acre of impervious area and on a parcel that is less than 5 acres and located
within a designated growth area)
The alternative PPB shall be the lesser of the following:
•
Option A = PPB = PPB as calculated (using Worksheet 1) multiplied by 5, or
•
Option B = PPB = Project's proposed impervious area multiplied by 0.5 lb per acre
Example: PPB for Small Commercial Development
Problem: 'Parking for Rent' is proposing a one acre impervious p
that is less than 5 acres and located
within a designated growth area)
The alternative PPB shall be the lesser of the following:
•
Option A = PPB = PPB as calculated (using Worksheet 1) multiplied by 5, or
•
Option B = PPB = Project's proposed impervious area multiplied by 0.5 lb per acre
Example: PPB for Small Commercial Development
Problem: 'Parking for Rent' is proposing a one acre impervious parking lot on a 1.5 acre lot
within the identified growth zone of a watershed with a phosphorus allocation of 0.03
lb/acre/year. The proposed treatment is through a buffer. Calculate the Project Phosphorus
Budget.
Solution: The standard PPB would be 1.5 acre X 0.03 lb/acre/year = 0.045 lb P /year.
However, since the project is a small commercial-type development with no more than 1 acre
of impervious area and on a parcel that is less than 5 acres and is located in a municipality's
designated growth area, the alternative method for calculating the PPB may be used upon
request by the municipality. The alternative PPB calculation is the lesser of:
•
Option A. Standard PPB (as calculated on Worksheet 1) X 5 = 0.045 lb P /year X 5 =
0.225 lb P /year
•
Option B. Project's Proposed Impervious Area X 0.5 lb P /acre / year = 1 acre X 0.5 = 0.5
lb P /year
Thus, the PPB is 0.225 lb P /year as in Option A.
- 32 -
This is a copy of a public record, reproduced as it was published. It is not legal advice, and it may not be the version a court would rely on. Check the official source before you cite it.