HomeMy WebLinkAboutApproved Perry Cr TMDL 20100518
Total Maximum Daily Load to Address
Impaired Biological Integrity
in the Perry Creek Watershed, Neuse River Basin
April 2010
EPA Approval on May 13, 2010
North Carolina Department of Environment and Natural Resources
Division of Water Quality
Perry Creek TMDL Page 2
Table of Contents
SUMMARY .................................................................................................................... 3
IMPLEMENTATION SUMMARY ............................................................................... 3
INTRODUCTION .......................................................................................................... 3
APPLICABLE SURFACE WATER QUALITY STANDARDS AND RULES ........... 5
WATERSHED DESCRIPTION ..................................................................................... 6
POLLUTANT SOURCE ASSESSMENT ...................................................................... 8
STRESSOR IDENTIFICATION .................................................................................... 8
WATER QUALITY TARGET ..................................................................................... 10
SEASONAL VARIATION AND CRITICAL CONDITIONS .................................... 11
WASTELOAD ALLOCATION (WLA) AND LOAD ALLOCATION (LA) ............. 12
TMDL IMPLEMENTATION ...................................................................................... 13
IMPLEMENTATION RESOURCES ........................................................................... 14
REFERENCES ............................................................................................................. 16
Appendix 1 Summary data from all benthos sites in the Perry Creek
Watershed, from 1995 to present.
Perry Creek TMDL Page 3
SUMMARY
Based on a weight of evidence analysis for the Perry Creek watershed, the two most
important factors contributing to impairment are scour and habitat degradation. These
stressors and other indicator parameters are associated with the high levels of
development (impervious areas) in the Perry Creek watershed.
A TMDL must address stressors believed to be contributing to the impairment. Where
the major cause of impairment is stormwater runoff, the use of surrogate indicators
expressed as quantitative targets is appropriate in TMDL development. Because of
stormwater-associated pollutants and the effects on the system‟s hydrology, these targets
are used as surrogates to estimate stormwater pollutant load reductions needed to meet
water quality standards.
IMPLEMENTATION SUMMARY
The goal of this TMDL is to achieve water quality standards, in this case, a benthic
macroinvertebrate community bioclassification of Not Impaired, Good-Fair, or better.
Achievement of this water quality standard may be met by implementing management
practices designed to mitigate the effects of stormwater runoff. Eliminating impervious
cover (IC) is not necessary to reach the TMDL target reductions. Aquatic life (biological
community) will be the measure of TMDL success.
When the TMDL is implemented, stressors (scour and habitat degradation, for example)
will be reduced.
INTRODUCTION
Section 303(d)(1)(C) of the federal Clean Water Act (CWA) and the U.S. Environmental
Protection Agency‟s (EPA) implementing regulations direct each State to develop a Total
Maximum Daily Load (TMDL) for each impaired segment on the Section 303(d) list,
taking into account seasonal variations and a protective margin of safety (MOS) to
account for uncertainty. Traditionally, a TMDL reflects the total pollutant loading of the
impairing substance a waterbody can receive and still meet water quality standards.
The purpose of this report is to establish a TMDL to address the aquatic life impairments
in the Perry Creek watershed. The goal is to provide the basis for improving the
watershed ecosystem through implementation of stormwater best management practices
such that the beneficial uses of the waterbodies are restored. Upon approval by EPA, this
TMDL becomes part of the NC Division of Water Quality (DWQ) Neuse River
Basinwide Water Quality Plan.
In 2002, EPA provided clarifications of existing regulatory requirements for establishing
wasteload allocations (WLAs) for stormwater discharges in TMDLs (EPA Memorandum
2002). Specific key points of the memorandum include:
EPA expects that most Water Quality Based-Effluent Limits (WQBELs) for
NPDES-regulated municipal and small construction stormwater discharges will
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be in the form of Best Management Practices, and that numeric limits will be
used only in rare instances.
When a non-numeric WQBEL is imposed, the permit‟s administrative record,
including the fact sheet when one is required, needs to support that the BMPs are
expected to be sufficient to implement WLA in the TMDL.
It may be reasonable to express allocations for NPDES-regulated stormwater
discharges from multiple point sources as a single categorical wasteload
allocation when data and information are insufficient to assign each source or
outfall individual WLAs.
EPA expects TMDL authorities to make separate allocations to NPDES-regulated
stormwater discharges (in the form of WLAs) and unregulated stormwater (in the
form of load allocations). EPA recognizes that these allocations might be fairly
rudimentary because of data limitations and variability in the system.
This TMDL applies to the entire Perry Creek watershed, which includes the assessment
units listed in Table 1.
Table 1. Perry Creek Watershed Assessment Units and Descriptions
Stream Name
Assessment
Unit Number
(AU) *
Sampling
Location Description Current
Bioclassification
Perry Cr 27-25-(2) SR 2006 From dam at Greshams
Lake to Neuse River. Fair 2005
Perry Cr 27-25-(1) Hunters
Way
From source to dam at
Greshams Lake Not Rated 2004
Unnamed
Tributary Perry
Cr
27-25-3-(2) SR 3514 From dam at Camp
Durant to Perry Creek Not Rated 2004
Unnamed
Tributary Perry
Cr
27-25-3-(1) N/A From source to dam at
Camp Durant N/A
* See Figure 1 for spatial reference.
Perry Creek at SR 2006 has been on the NC 303(d) list of impaired waters since 1998.
Perry Creek at Hunters Way was sampled approximately 500 meters downstream of an
impoundment. Because of the sampling site‟s close proximity to an impoundment, a
bioclassification of Not Rated was given for this location. This segment [AU#27-25-(1)]
will be moved to Category 4c in a subsequent Integrated Report because of the impact of
the impoundment. EPA‟s 4c category is defined as the non-attainment of any applicable
water quality standard which is the result of pollution and is not caused by a pollutant.
The Unnamed Tributary (UT) Perry Creek at SR 3514 is considered impaired for
biological integrity but is not listed on the 303(d) list due to the Not Rated
bioclassification. A bioclassification of Not Rated can be assigned to streams which have
a drainage area size less than three square miles and would have received a Fair or Poor
Perry Creek TMDL Page 5
bioclassification using the benthic macroinvertebrate criteria derived for larger streams
(NCDWQ 2003). The UT Perry Creek (AU#27-25-3-(1)) benthic macroinvertebrate
community has not been assessed.
APPLICABLE SURFACE WATER QUALITY STANDARDS AND RULES
TMDLs are established to achieve and maintain water quality standards. A water quality
standard is the combination of a designated use for a particular body of water and the
water quality criteria designed to protect that use. Examples of designated uses include
aquatic life survival and propagation, swimming, drinking water supply, and shellfish
harvesting. Water quality criteria consist of narrative statements and numeric values
designed to protect the designated uses. Criteria may differ among waters with different
designated uses.
The surface water classifications for the Perry Creek watershed include Class C and Class
B uses. All waters in North Carolina have the base classification of “C.” Class C waters
are protected for aquatic life propagation and biological integrity (including fishing and
fish), wildlife, secondary recreation, agriculture and other uses suitable for Class C.
There are no restrictions on watershed development or types of discharges associated
with Class C (15A NCAC 02B.0211, 2007).
In addition, the Perry Creek segment AU# 27-25-(1) from source to Greshams Lake dam
and the UT Perry Creek segment AU# 27-25-3-(1), from source to dam at Camp Durant
are protected as Class B for primary recreation. Class B uses are primary recreational
activities that include swimming, skin diving, water skiing, and similar uses involving
human body contact with water where such activities take place in an organized manner
or on a frequent basis.
Since the Perry Creek watershed is in the Neuse River Basin, waters have the
supplemental water quality classification of Nutrient Sensitive Waters (NSW). This
supplemental classification is intended for waters needing additional nutrient
management due to their experiencing or being subject to excessive growth of
microscopic or macroscopic vegetation. In general, management strategies for point and
nonpoint source pollution control require control of nutrients (nitrogen and/or phosphorus
usually) (15A NCAC 02B.0233, 2007). The Neuse River Basin-Nutrient Sensitive
Waters Management Strategy: Basinwide Stormwater Requirements are designed for
local governments to implement their own stormwater management plan (15A NCAC
02B.0235, 2007). This requirement has been in effect since August 1, 1998.
This TMDL addresses Perry Creek watershed, which has impaired biological integrity.
Impairment for biological integrity is based on a narrative standard that pertains to the
aquatic life use designation. Biological integrity means “the ability of an aquatic
ecosystem to support and maintain a balanced and indigenous community of organisms
having species composition, diversity, population densities and functional organization
similar to that of reference conditions” (15A NCAC 02B.0202, 2007).
DWQ‟s criterion for assessing aquatic life as impaired is a biological community at a
benthic macroinvertebrate or fish sampling site with a bioclassification of Poor, Fair or
Severe Stress. The criterion for assessing aquatic life as supporting is a bioclassification
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of Good-Fair, Good, Excellent, Not Impaired, Natural or Moderate Stress at a biological
community sampling site.
Biological impairments to the Perry Creek watershed were identified using bioassessment
protocols outlined in the North Carolina‟s Standard Operating Procedure for Benthic
Macroinvertebrates (NCDWQ 2006). Perry Creek at SR 2006 has carried a benthic
macroinvertebrate bioclassification of Fair since 1995. The supporting data analysis that
resulted in the listing for impaired biological integrity is located in Appendix 1.
Habitat evaluation is not currently linked to the North Carolina Piedmont
bioclassification ratings so there are no habitat thresholds or breakpoints distinguishing
“condition groups” (e.g., Excellent, Good, Good-Fair, Fair, or Poor); higher overall
scores simply represent better aquatic habitat than lower overall scores. To reduce the
subjectivity of visual interpretations of the habitat components, the DWQ assessment
form provides definitions of various conditions and the associated score (NCDWQ 2006).
The DWQ methodology (2006) for evaluating instream aquatic habitat focuses on eight
key components that affect the availability and suitability of habitat. The components are
rated individually and the summation of the scores ranges from 0 to 100, with 100
reflecting the highest quality habitat. Habitat scores for the sites in the Perry Creek
watershed are provided in Appendix 1. The eight components and their relative weight to
the overall score are:
• Channel modification (5)
• Instream habitat types (20)
• Bottom substrate (15)
• Pool variety (10)
• Riffle habitats (16)
• Bank stability and vegetation (14)
• Light penetration (10)
• Riparian vegetative zone width (10)
WATERSHED DESCRIPTION
The Perry Creek watershed is located in northeast Raleigh, Wake County, in the Neuse
River Basin. The mainstem of Perry Creek flows generally east toward its confluence
with the Neuse River. This watershed is within the USGS 12-digit hydrologic unit (HU)
030202010704, which is approximately 31.6 square miles in area. For purposes of this
TMDL, the Perry Creek watershed was extracted from the 12-digit HU. It comprises a
smaller drainage area of 11 square miles (Figure 1). The TMDL watershed is defined as
Perry Creek from source to SR 2006 (Perry Creek Road) and all tributaries draining to
this portion of Perry Creek (Figure 1).
The TMDL watershed lies within the City of Raleigh‟s jurisdiction. Two major
highways (US 1 and I-540) transect this TMDL watershed. The predominant land cover
is developed with extensive residential subdivisions and urban office parks/commercial
areas. Impervious surfaces (areas such as roof tops, roads and parking lots that prevent
infiltration of precipitation into the soil) cover approximately 17% of this watershed.
Significant impacts to stream biota can generally be expected with this degree of
unmitigated impervious cover (Schueler, 1994).
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The watershed is located in the Northern Outer Piedmont ecoregion (45f; Griffith et al.
2002). Upland soils of the watershed consist of a variety of soil associations (Cawthorn,
1970), corresponding to two major geologic belts running in a north-south direction
through the study area. The western portion of the study area (encompassing the
headwaters of Perry Creek to U.S. 1) has predominant soils of the Cecil-Appling
associations, derived primarily from crystalline materials (mostly granite, gneiss and
shist) and mudstone. Soils along the eastern portion of the study area (from U.S. 1 to the
confluence of the Neuse River). are largely of the Wehadkee series. These are nearly
level poorly drained soils formed in sandy alluvium and are common along streams in
Wake County.
Impoundments in the watershed include Greshams Lake on Perry Creek and a lake at
Camp Durant on an unnamed tributary (UT) to Perry Creek. Smaller impoundments are
also present: two above Greshams Lake on Perry Creek, one above the Lake at Camp
Durant, and two in the upper watershed of an unnamed tributary originating near U.S. 1.
Figure 1. Perry Creek TMDL watershed
Perry Creek TMDL Page 8
POLLUTANT SOURCE ASSESSMENT
This TMDL report largely draws its information from two stressor studies conducted by
DWQ in 2004 for the Perry Creek watershed. These efforts analyzed the causes of
impairment by measuring various water chemistry, sediment toxicity, physical and
biological parameters (NCDWQ 2004a, NCDWQ 2004b).
The NCDWQ Environmental Sciences Section conducted two stressor studies in 2004.
(NCDWQ 2004a, NCDWQ 2004b). Stressor studies are specialized studies that attempt
to identify specific factors leading to degraded water quality conditions. The assessment
included field measurements and water chemistry data identifying potential causes of
impairment. Major watershed activities and sources of pollution contributing to those
causes such as stream bank erosion and stormwater runoff from urban areas were
identified.
In addition, DWQ‟s Biological Assessment unit conducted a stressor study in May 2004.
This effort assessed benthic macroinvertebrates, habitat characteristics, chemical and
physical data to analyze specific stressors identified as causes of impairment to the
aquatic community. This study confirmed potential sources of stressors for TMDL
development and determined the current status of the impairment. The supporting data
analysis that resulted in the listing for impaired biological integrity is located in
Appendix 1.
Conclusions of the studies show aquatic organisms in the Perry Creek watershed are
heavily impacted by multiple stressors associated with high levels of development
(impervious areas).
The relative contribution of these stressors cannot be clearly differentiated based on the
available data. For example, increases in stream flow associated with development
during storm events result in bank erosion and scour. The stream bank erosion and
sedimentation associated with these events contributes to habitat degradation that would
be associated with biological impairment. Additionally, nutrient enrichment associated
with the residential and commercial development around Greshams Lake, possible
leaking sewer lines, and the golf course and residential areas present in the watershed
may also contribute to biological impairment by causing algal activity and lowered
dissolved oxygen levels. A number of impoundments in the watershed were identified as
hindering downstream macroinvertebrate recolonization. No single pollutant or group of
specific pollutants has been identified as the cause of impairment in the watershed.
Significant contributors to impairment of Perry Creek and UT Perry Creek include the
following NPDES Stormwater Phase I permit holders: City of Raleigh (NCS000245) and
NC Department of Transportation (NCDOT) (NCS000250).
STRESSOR IDENTIFICATION
Studies conducted by the Environmental Sciences Section of DWQ in the Perry Creek
watershed in 2004 included a detailed evaluation of the causes of impairment. The
following paragraphs summarize the findings of that analysis. These studies included the
following primary tasks:
Identification of potential “candidate” causes of impairment in the watershed
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Collection of additional data
Characterization of the causes of impairment using a “strength of evidence” approach
The strength of evidence evaluation included analysis of whether candidate stressors
were primary causes of impairment, secondary causes of impairment, part of the
cumulative cause of impairment, a contributing stressor, a potential cause or contributor,
or an unlikely cause or contributor. Other sources of evidence evaluated included benthic
macroinvertebrate community data, habitat and riparian area assessment, chemistry,
toxicity data, current watershed activities, land uses, and pollutant sources.
The following candidate causes were not determined to be significant or primary causes
of impairment in Perry Creek and therefore are not addressed by this TMDL:
Toxicity (resulting from residential and commercial development). Water chemistry
data, sediment chemistry and bioassay data, watershed characteristics, and benthic
community data were utilized in the evaluation of toxicity as a cause of impairment.
Although toxic impacts are very episodic and difficult to identify, data from benthic
mentum deformity tests did not show toxic conditions. Water column samples indicated
elevated values for aluminum, iron, and manganese but this is not unusual due to the
presence of these metals in local soil types. Sediment metals samples found no values
above the hazardous waste site screening values used by the USEPA (NCDWQ 2004a).
Hydromodification – Low Dissolved Oxygen.
Although there are areas with poor habitat (including deep uniform stream channels, little
habitat diversity, rip-rap, minimal stream bottom diversity, and very low or no flow
velocities), dissolved oxygen levels are not viewed as a primary limiting factor for
benthos. The watershed is not impaired for dissolved oxygen.
Organic and nutrient enrichment. Benthic community data and water quality monitoring
data were utilized to evaluate organic and nutrient enrichment as a potential cause of
stress on the biological community. The watershed is not impaired for chlorophyll-a or
dissolved oxygen. Although it was difficult to distinguish between the impacts of organic
and nutrient enrichment, based on algal activity there is some enrichment associated with
urbanization such as commercial and residential development, possible leaking sewer
lines and the golf course (NCDWQ 2004a). The control of untreated stormwater runoff
to this watershed should provide benefits to the aquatic community. The nutrient rules
for the Neuse River Basin are applied in the Perry Creek watershed.
The following candidate causes were determined to be significant causes of impairment
in Perry Creek:
Hydromodification and associated scour due to storm flows (resulting from
development). The Neuse River Basinwide Water Quality Plan (NCDWQ 2009) listed
habitat degradation from urban runoff as a potential cause for impairment based on
biological data. Available data (benthic macroinvertebrate, habitat and riparian area
assessments, stream observations and watershed characteristics) were reviewed to assess
stormflow scour as a cause of impairment in the watershed. Scour associated with rapid
Perry Creek TMDL Page 10
and significant increases in stream flow was found to be the most pervasive stressor in
the watershed. The stream bank erosion and sedimentation associated with these events
contribute to habitat degradation associated with biological impairment. The benthic
macroinvertebrate data lacked specific indicator taxa but rather exhibited highly tolerant
benthic communities, suggesting considerable impacts from urban/suburban pressures.
Hydromodification (resulting from dams). Impacts from dams include the following:
Prevention of downstream colonization of aquatic populations
Lower water levels below dams
Change in temperature and dissolved oxygen
Change in food type
Although it is difficult to isolate these impacts from those of lower baseflows associated
with urbanization, there is evidence that the lowered water levels below dams are an
important stressor to the biological communities in the Perry Creek watershed. For this
reason, Perry Creek at Hunters Way will be moved to Category 4c, impairment due to
hydromodification. This TMDL does not address this stressor.
Based on a weight of evidence analysis, the two most important factors are scour and
hydromodification (dams); the impacts of nutrient enrichment and habitat degradation are
more localized. Although toxicity was not viewed as a primary cause of impairment,
combined with other causes of impairment, it may contribute to the cumulative effect.
All of the stressors and indicator parameters discussed above are associated with the high
levels of development in the Perry Creek watershed.
WATER QUALITY TARGET
Extensive national and state level research shows strong correlations between impairment
and watershed development. Stormwater runoff from impervious surfaces can carry a
complex array of potential pollutants that can impact the aquatic community. Because of
the uncertainty in identifying specific pollutants in urbanized stormwater runoff,
difficulties arise in quantifying the real target (biological integrity) in a TMDL.
A TMDL must address stressors believed to be contributing to the impairment. Where
the major cause of impairment is stormwater runoff, the use of surrogate indicators
expressed as quantitative targets is appropriate in TMDL development. Because of
stormwater-associated pollutants and the effects on the system‟s hydrology, these targets
are used as surrogates to estimate stormwater pollutant load reductions needed to meet
water quality standards.
Research conducted by The Center for Watershed Protection (CWP) indicated that
variability in stream quality indicator data is usually dampened when impervious cover
(IC) exceeds 10%, which presumably reflects the stronger influence of stormwater runoff
on stream quality indicators. In particular, the chance that a stream quality indicator will
attain a high quality score is sharply diminished at higher IC levels. This trend becomes
pronounced within the 10 to 25% IC range and almost inevitable when watershed IC
exceeds 25%. This pattern suggests that IC is a more robust and reliable indicator of
overall stream quality beyond the 10% IC threshold (CWP 2003).
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According to „Estimating and Projecting Impervious Cover in the Southeastern United
States‟ (USEPA 2005), degraded benthic community sites are evident as impervious area
increases. Specifically, among sites in North Carolina with a total impervious area
greater than 10%, 62% were degraded. In contrast, 90% of sites with less than 10% IC
were not degraded.
Based on the above findings, the surrogate TMDL target for Perry Creek is 9% IC. This
target incorporates a 1% IC margin of safety. North Carolina data in the USEPA study
(2005) indicate that 10% of sites with <10% impervious cover could remain degraded.
The Perry Creek TMDL target is expected to protect all waters, however, because the
measure of TMDL success is based upon water quality standards for benthic
macroinvertebrate data and not upon a quantified actual reduction in IC. Specifically, the
goal of this target is to achieve a benthic macroinvertebrate community bioclassification
of Good-Fair, Not Impaired or better. Achievement of this water quality standard may be
met by implementing management practices designed to mitigate the effects of
stormwater runoff. Because IC is a surrogate measure, eliminating IC is not necessary in
reaching the TMDL target reductions. Measuring the aquatic life (biological community)
directly will be the method for assessing attainment of the TMDL goal. “TMDLs may be
established using a pollutant-by-pollutant or biomonitoring approach.” (40 CFR
130.7(c)(1)(i)) This approach was first used in North Carolina for the Swift Creek
TMDL (NCDWQ 2009), approved by USEPA in 2009.
SEASONAL VARIATION AND CRITICAL CONDITIONS
Regulations require that a TMDL be established with consideration of seasonal
variations. Stormwater flows occur throughout the year, with different environmental
effects at both low and high flows. Critical conditions for aquatic life protection are not
limited to flow conditions or time of year. Benefits realized from impervious cover
mitigation occur in all seasons because stormwater management will be implemented to
meet the IC target and will reduce adverse impacts (pollutant loading and damaging
flows) for the full spectrum of storms throughout the year.
MARGIN OF SAFETY
A Margin of Safety (MOS) is required as part of a TMDL in recognition of many
uncertainties in the understanding and simulation of water quality in natural systems. For
example, knowledge is incomplete regarding the exact nature and magnitude of pollutant
loads from various sources and the specific impacts of those pollutants on the chemical
and biological quality of complex, natural water bodies. The MOS is intended to account
for such uncertainties in a manner that is conservative from the standpoint of
environmental protection.
A 1% IC Margin of Safety (MOS) was subtracted from the surrogate TMDL target to
account for uncertainty in the analysis, resulting in a combined WLA and LA target of
9%. The goal of the TMDL is to reduce impacts from stormwater on the aquatic life in
the Perry Creek watershed.
Perry Creek TMDL Page 12
WASTELOAD ALLOCATION (WLA) AND LOAD ALLOCATION (LA)
In the Perry Creek watershed there are no continuous NPDES wastewater discharges,
individual industrial stormwater discharges, or permitted animal operations. There are
two intermittent NPDES permitted dischargers in the Perry Creek watershed. The City of
Raleigh (NCS000245) and the NC Department of Transportation (NCS000250) are
regulated under Phase I NPDES Stormwater permits. The WLA portion of this TMDL
applies to these Phase I NPDES permits. This TMDL applies to designated and future
MS4 permits that are subject to the NPDES program.
Stormwater discharges are highly variable in frequency and duration. It is reasonable to
express allocations for NPDES-regulated stormwater discharges from multiple point
sources as a single categorical wasteload allocation when data and information are
insufficient to assign each source or outfall individual WLAs (EPA Memorandum 2002).
This TMDL applies the surrogate 9% IC target to the stormwater drainage area affecting
both regulated and non-regulated sources in this watershed.
To calculate the equivalent of % IC reductions required to achieve the TMDL target:
Equivalent of percent IC reduction = [(IC Current Condition – surrogate IC Target)/IC
Current Condition)] x 100
where surrogate IC Target = 9%
The equivalent of percent IC reduction (Table 2) was calculated for the Perry Creek
TMDL watershed (Figure 1). The equivalent IC reduction is 47% for the TMDL
watershed (Table 2). This TMDL watershed was extracted from the USGS 12-digit
hydrologic unit (HU) 030202010704 and is approximately eleven square miles in area.
The current condition (Table 2) was calculated from analysis of existing percent
imperviousness using the National Land Cover Dataset (2001), supplemented by the
Watershed Assessment Model for North Carolina (Pate 2009). The current condition for
the Perry Creek TMDL watershed is 17% impervious.
Table 2. TMDL Targets, Surrogate Targets, and Equivalent Percent Reductions for
Perry Creek TMDL Watershed
* Equivalent of %IC reduction means actions that mitigate the adverse impacts of stormwater, including
but not limited to reducing pollutant loading and reducing the volume of storm runoff. Such actions could
include disconnecting IC, installing infiltration basins, eliminating illicit discharges, etc.
Achieving the equivalent %IC reduction will require mitigation of the adverse impacts of
stormwater, including but not limited to reducing pollutant loading and reducing the
volume of storm runoff. Such actions could include disconnecting IC, installing
TMDL Target
Bioclassification
Percent Impervious Cover
Equivalent % Reduction * TMDL
Surrogate
Target
WLA
and
LA
MOS Current
Condition
Good-Fair or
better 10% 9% 1% 17%
47% Equivalent of % IC reduction
accomplished by improved stormwater
management.
Perry Creek TMDL Page 13
infiltration basins, eliminating illicit discharges, etc. The TMDL target bioclassification
of Good-Fair or better will be measured at the benthic compliance points (Figure 1).
TMDL IMPLEMENTATION
EPA is not required to, and does not, approve TMDL implementation plans. This section
is intended to provide some initial assistance for implementing this TMDL.
This TMDL applies to designated and future MS4 permits that are subject to the NPDES
program. The linkage of the TMDL with the NPDES Stormwater Phase I permits will
constitute a significant portion of the implementation. The goal of this TMDL is to
reduce the effects of stormwater impacts to the receiving streams so that water quality
standards for biological integrity are met. Attainment of such a standard is achieved
when a benthic macroinvertebrate community sample receives a bioclassification of Not
Impaired, Good-Fair or better. Compliance will be measured at the benthic sampling
sites at Perry Cr at SR 2006 and UT Perry Cr at SR 3514 (Figure 1).
Implementation for this TMDL will best be accomplished through incorporating an
adaptive management strategy for stormwater runoff. Such a strategy should include one
or more of the following for new and existing development:
Installing engineering BMPs to reduce the impacts of stormwater runoff from
impervious areas.
Minimizing additional disturbance to maintain existing natural buffering capacity
Disconnecting impervious cover from the surface waterbodies to reduce peak
flows and volumes of stormwater runoff.
Reducing impervious cover.
Adopting land use ordinances that require or allow Low Impact Development
(LID) techniques or other non-structural best management practices.
Detecting and eliminating illicit discharges.
Developing an educational component and outreach program.
Affected entities may propose alternative measures that meet the intent of the TMDL.
Stormwater impacts include erosion and damage to instream aquatic habitat, a complex
mix of pollutant loading, and lack of infiltration to provide stable base flow to streams.
When the TMDL is implemented, stressors affecting aquatic life will be reduced or not
delivered to the waterbody in the first place.
The waterbodies draining this watershed are located in urbanized areas that are subject to
the requirements of North Carolina‟s NPDES Phase I Stormwater permit. Several efforts
for addressing stormwater runoff are currently in place for the City of Raleigh under their
permit. The city must develop, implement and enforce a Stormwater Management Plan
and ordinances approved by the NC Department of Environment and Natural Resources
(NCDENR). The plan includes educational and regulatory initiatives to ensure sound
development. Ordinances approved by NCDENR and adopted by Raleigh address
requirements for new development, illicit discharges detection and elimination,
watershed protection, and sediment and erosion control. The City of Raleigh was issued
a NPDES Stormwater Permit, effective June 1, 2007. Under the NPDES permit,
stormwater runoff from new development that disturbs greater than or equal to one acre,
including projects less than one acre that are part of a larger common plan of
Perry Creek TMDL Page 14
development or sale, must be controlled and treated in accordance with the conditions of
the permit and the Stormwater Management Plan. The permit and Stormwater
Management Plan mandate:
1. A public education and outreach program on the impacts of stormwater
discharges and how to reduce pollutants in stormwater runoff.
2. A public involvement and participation program.
3. A program to detect and eliminate illicit discharges within the jurisdictional
area.
4. A program to reduce pollutants in any stormwater runoff from construction
activities resulting from a land disturbance of greater than or equal to one
acre.
5. A program to address post-construction stormwater runoff from new
development that cumulatively disturb greater than or equal to one acre,
including projects less than one acre that are part of a larger common plan of
development or sale; and
6. A pollution prevention/good housekeeping program for municipal operations
that addresses operation and maintenance, including a training component, to
prevent or reduce pollutant runoff from those operations.
Raleigh and Wake County are subject to the Neuse River Basin –Nutrient Sensitive
Waters Management Strategy: Basinwide Stormwater Requirements (15A NCAC
02B.0235).
Since implementation is expected to meet TMDL requirements at the compliance points
(Figure 1, benthic macroinvertebrate sampling sites), an ongoing biological monitoring
program is critical in assessing the effectiveness of the implementation efforts. DWQ
will continue monitoring the biological communities in this watershed to track TMDL
implementation and attainment of water quality standards. This will be an iterative
process to meet TMDL targets for attaining a bioclassification of Good-Fair or better at
the compliance points. This process is recognized as lengthy, possibly spanning multiple
permit cycles.
IMPLEMENTATION RESOURCES
Resources are available to assist in the implementation of this TMDL.
The Center for Watershed Protection has produced a series of Urban Subwatershed
Restoration Manuals. The manuals provide comprehensive information on watershed
restoration techniques by introducing an integrated framework for restoration and
techniques for assessing urban watersheds. The manual series can be located here:
http://www.cwp.org/PublicationStore/USRM.htm
A report prepared for The United States Environmental Protection Agency, Stormwater
TMDL Implementation Support Manual, March 2006 (Project No: 10598-001-500)
shows the impervious cover method used in developing TMDL targets. The report can
be located here: http://www.epa.gov/ne/eco/tmdl/assets/pdfs/Stormwater-TMDL-
Implementation-Support-Manual.pdf
Perry Creek TMDL Page 15
The North Carolina Division of Water Quality 2007 Stormwater BMP Manual provides
guidance for meeting stormwater regulations and designing stormwater BMPs that meet
water quality objectives. The manual can be located here:
http://h2o.enr.state.nc.us/su/bmp_updates.htm
The North Carolina State University developed a NC Low Impact Development
Technique document. The project documented changes in runoff and pollutant export
resulting from the construction of a low impact development residential subdivision.
This is located here: http://www.bae.ncsu.edu/programs/extension/wqg/.
Perry Creek TMDL Page 16
REFERENCES
Cawthorn, J.W. 1970. Soil Survey of Wake County North Carolina. USDA Soil
Conservation Service.
Center for Watershed Protection. 2003. Impacts of Impervious Cover on Aquatic Systems.
Griffith, G.E., Omernik, J.M., Comstock, J.A., Schafale, M.P., McNab, W.H., Lenat,
D.R., and MacPherson, T.F., 2002, Ecoregions of North Carolina, U.S. Environmental
Protection Agency, Corvallis, OR
North Carolina Division of Water Quality. 2006. Standard Operating Procedures For
Benthic Macroinvertebrates.
North Carolina Division of Water Quality. 2007. Fresh Surface Water Quality Standards
for Class C Waters, 15A NCAC 02B.0211.
North Carolina Division of Water Quality. 2007. Fresh Surface Water Quality Standards
for Class WS-III Waters, 15A NCAC 02B.0215.
North Carolina Division of Water Quality. 2007. Neuse River Basin –Nutrient Sensitive
Waters Management Strategy: Basinwide Stormwater Requirements, 15A NCAC
02B.0235.
North Carolina Division of Water Quality. 2009. Neuse River Basinwide Water Quality
Plan.
North Carolina Division of Water Quality. 2009. Total Maximum Daily Load for
Addressing Impaired Biological Integrity in the Headwaters of Swift Creek Watershed,
Neuse River Basin.
North Carolina Division of Water Quality, Environmental Sciences Section. 2004a.
TMDL Stressor Study Memorandums. October 15 & 27
North Carolina Division of Water Quality, Environmental Sciences Section. 2004b.
TMDL Stressor Study Memorandums. November 9
North Carolina Division of Water Quality. 2007. Stormwater Best Management Practices
Manual.
North Carolina Division of Water Quality. 2006. Draft TMDL Water Quality Recovery
Program Guidance Document.
Pate, Travis. 2009. Watershed Assessment Model for North Carolina. Master‟s Project-
University of North Carolina-Chapel Hill.
Schueler, T.R. 1994. The Importance of Imperviousness. Watershed Protection
Techniques1, 100-111.
Perry Creek TMDL Page 17
United States Environmental Protection Agency. 2005. Estimating and Projecting
Impervious Cover in the Southeastern United States. EPA/600/R-05/061.
United States Environmental Protection Agency, Memorandum. 2002. Establishing
TMDL Wasteload Allocations (WLAs) for Stormwater Sources and NPDES Permit
Requirements Based on Those WLAs.
Perry Creek TMDL Page 18
Appendix 1. Summary data for all benthos sites in the Perry Creek Watershed,
from 1995 to present.
Perry Cr
Hunters Way
5/11/2004
Perry Cr
SR 2006
7/25/1995
Perry Cr
SR 2006
12/9/1996
Perry Cr
SR 2006
7/6/2000
Perry Cr
SR 2006
5/10/2004
UT Perry Cr
SR 3514
5/10/2004
COMMUNITY
Ephemeroptera 2 5 6 6 5 6
Plecoptera 0 0 3 0 0 0
Trichoptera 3 3 2 2 4 4
Coleoptera 4 0 0 0 8 4
Odonata 5 0 0 0 7 8
Megaloptera 0 0 0 0 1 1
Diptera: Chironomidae 13 0 0 0 26 31
Misc. Diptera 5 0 0 0 4 4
Oligochaeta 4 0 0 0 3 5
Crustacea 1 0 0 0 2 1
Mollusca 3 0 0 0 2 4
Other 4 0 0 0 1 1
Total Taxa Richness 44 8 11 8 63 70
EPT Richness 5 8 11 8 9 10
Seasonal Corr
(- out of season spp)
5 8 8 8 9 10
EPT Abundance 34 30 51 50 67 77
Biotic Index 6.97 - - - 6.88 6.92
Seasonal Correction 7.17 - - - 7.08 7.12
EPT BI 6.97 5.87 5.57 5.23 6.26 5.84
Bioclassification Not Rated Fair Fair Fair Fair Not Rated
HABITAT
Stream Width 3 9 5 7 5 4
Average Depth 0.1 0.2 0.2 0.2 0.2 0.2
Flow/ Current Moderate Moderate Moderate Moderate Moderate Moderate
Bank Height 1.5 1.5 0.5
Bank Angle (º) 60 90 90
Bank Erosion None Severe Severe Moderate Moderate None
Canopy (%) 98 90 50 80 70 90
Canopy Type Deciduous Deciduous Deciduous Deciduous Deciduous Deciduous
Aufwuchs None None-
Moderate
Moderate Abundant None None
Pedostemum None None None None None None
Tribs Present? No No No No Yes No
Substrate (%)
Boulder 20 0 0 0 0 0
Rubble 30 0 0 0 0 0
Gravel 30 10 35 20 0 10
Sand 20 80 60 80 70 90
Silt 0 10 5 0 30 0
Other 0 0 0 0 0 0
Habitat Score 71 60 70 56 54 56
CHEMISTRY
Temp ºC 22 27 6 25 22 21.2
DO mg/l 6.3 6.9 -- 8.2 7.1 7.7
Cond µS/cm 116 110 66 150 148 115
pH 6.7 7 -- 7.2 6.6 6.7
LOCATION/ GENERAL
Basin (subbasin) Neuse 02 Neuse 02 Neuse 02 Neuse 02 Neuse 02 Neuse 02
County Wake Wake Wake Wake Wake Wake
Latitude 355230 355250 355250 355250 355247 355249
Longitude 783556 783250 783250 783250 783253 783309
Collection Card 9376 6887 7232 8141 9375 9374
Sample Type Full Scale EPT EPT EPT Full Scale Full Scale
Drainage Area
(approx sq mi)
3.5 9.5 9.5 9.5 9.5 2
Shaded columns denote samples collected for the 2004 Stressor Study.
ATTACHMENT A
Public Notice
A public notice was posted to the DWQ TMDL website and notice was sent to a mailing
list of interested parties.
Notice was also posted in the Raleigh NC News and Observer newspaper. The Affidavit
of Publication is provided below.
ATTACHMENT B
Public Comment
The public comment period extended from October 24, 2009 through November 27,
2009. Comments were received from four entities:
City of Raleigh
Wake County
City of Charlotte
NC Department of Transportation
These comments with NC Division of Water Quality responses are provided in the
Responsiveness Summary (below).
Meetings
In addition, the following meetings were held with the affected MS4s:
City of Raleigh on September 22, 2009
NC Department of Transportation on October 5, 2009
Another meeting was held with an interested party:
Wake County on October 30, 2009
Perry Creek TMDL Responsiveness Summary
April 2010
One commenter suggested that other surrogate measures be used to draft a TMDL for
addressing biological integrity. The Center for Watershed Protection study does not
indicate that mitigating high IC levels enough to create a net IC level of approximately
10% will necessarily return high stream quality indicator scores.
Response: There are several citations in the Federal Regulations that support the use of
surrogate measures for TMDL Development. For example, 40 CFR §130.2-(i) states that
"TMDLs can be expressed in terms of either mass per time, toxicity, or other appropriate
measure." In addition 40 CFR §130.7 (c)(1)(i) "states that TMDLs may be established
using a pollutant-by-pollutant or biomonitoring approach." We would be willing to
consider other targets, surrogate targets and non-TMDL approaches for impaired
biological integrity streams in Charlotte. The commenter did not suggest an alternative
surrogate.
One commenter suggested that local data be collected to determine conditions that result
in high stream quality indicator scores which lead to a more appropriate surrogate.
Response: The TMDL is not so prescriptive to limit local government’s collection of
additional data to develop the most effective management strategy. The goal of this
TMDL is to achieve a bioclassification of Good-Fair or better, not necessarily to achieve
the surrogate target. Percent impervious cover serves as a surrogate measure of the
complex mixture of pollutants transported by stormwater. Since the impairment cannot
be attributed to a specific pollutant, impervious cover (IC) was used as a surrogate
measure of the complex array of stressors associated with stormwater that impact
aquatic life. We would be willing to consider other targets, surrogate targets and non-
TMDL approaches.
One commenter stated that using a 9% IC target is inconsistent with minimum Post-
Construction Controls Ordinance (PCCO) rules. The State’s minimum PCCO regulations
require controls when IC meets 24%. Although not part of the TMDL, and if an IC
surrogate is ultimately used, we recommend that this discrepancy be addressed to avoid
the resulting burden that will be placed on regulated municipalities to make up the
difference through TMDL implementation.
Response: The PCCO Rules were not aimed at restoring impaired biological integrity
and do not address existing development. Local governments have the option of adopting
more restrictive ordinances.
One commenter stated that to address degraded habitat and scour, stream bank restoration
is a necessary precursor to begin biological restoration and will address other related
impairments such as turbidity. Therefore, we believe that stream stabilization and
restoration, not IC removal, is a more appropriate first step and a partial (and likely
significant) success toward biological recovery.
Response: The TMDL is not requiring IC removal nor does it propose to be the only
solution in restoring the biological community. In addition, stabilization and restoration
without mitigation of stormwater impacts would not provide a permanent solution. We
recognize that any approach at water quality improvement that requires management
changes in existing areas creates implementation challenges. That is why the TMDL
document notes the need for an iterative process that may take a long period of time for
implementation. The Division’s approach in this process will be to allow entities to
develop a recovery plan that is effective and that is also reasonable. Over time we
anticipate that management alternatives in these areas will increase/improve and we
anticipate that implementing agencies will effectively adapt their management strategies
to utilize the most effective approaches. While the TMDL confers no additional authority
to local entities for implementation, we do feel that there are authorities available that
allow local entities to select and prioritize stormwater management practices today that
can begin this process. This could include identifying public lands for stormwater
controls, looking at stormwater provisions for redevelopment or utilizing innovative
approaches to reuse or otherwise reduce stormwater runoff.
One commenter stated that on page 6, the TMDL watershed area is reported to be 11
square miles (or 306,662,400 square feet). Our understanding of the TMDL is that if the
impervious cover in the watershed is reduced to 306,662,240 square feet or less, then
water quality standards will be achieved as measured by a Not Impaired, Good-Fair or
better benthic macroinvertebrate bioclassification. It is our further understanding that the
maximum allowable impervious cover in the watershed allocated to point and nonpoint
sources (WLA+LA) is equal to 27,599,616 square feet (9% of 306,662,400 s.f.) Is our
understanding of the draft Perry Creek TMDL correct and are our calculations correct? If
not, please provide clarification and corrections to our calculations.
Response: The current imperious cover is 17% and the mitigation is aimed at achieving
the equivalent of 9% through stormwater management.
One commenter suggested that more specific stressors be identified.
Response: There is a list of stressors in the TMDL since the impairment cannot be
attributed to a specific pollutant, impervious cover (IC) was used as a surrogate measure
of the complex array of stressors associated with stormwater. The TMDL report does
contain site-specific information referenced from two stressor studies conducted by DWQ
in 2004.
One commenter stated that more research is needed to determine a proven, definitive
course of action that will restore the biological integrity of streams in North Carolina
prior to TMDL implementation.
Response: The implementation plan is not a required component of the TMDL. The
TMDL language and permitting approach attempt to recognize that there may be
multiple ways for entities to meet the intent of the TMDL, not just one single approach.
Through the development of your plan in your stormwater permit, you will have the
opportunity to work with the Division to target the most appropriate approach for your
jurisdictional area.
Two commenters stated that it is not clear from the draft report which MS4 permittees
have been assigned a WLA?
Response: The WLA portion of the TMDL applies to the Phase I NPDES stormwater
permits. EPA recommends expressing the wasteload allocation in the TMDL as either a
single number for all NPDES-regulated storm water discharges, or when information allows,
as different WLAs for different identifiable categories. In this case, information was not
available to establish separate WLA numbers.
One commenter stated that Wake County has no land use regulatory authority in the
Perry Creek watershed, the TMDL is not applicable to Wake County. It is requested that
all references to unincorporated Wake County be removed from the TMDL.
Response: A TMDL must include a load allocation for all nonpoint sources. Not all of
this TMDL watershed is covered by a NPDES permit.
Two commenter asked if future permit applications for new construction will be denied
by DWQ if a proposed project increases impervious cover in the watershed? If the
answer is No, then what requirements related to this TMDL will be a condition of permit
application approval for future construction projects in the watershed that add new
impervious cover?
Response: The DWQ Stormwater Permitting Unit will work with permitees to access the
need for any potential modifications or clarifications in the permit language.
One commenter asked if in the future, will either DWQ or EPA a 9% impervious cover
limitation in the Perry Creek watershed?
Response: The goal of the TMDL is to mitigate for the effects of stormwater runoff
through stormwater management, not documenting the current or future extent of
impervious cover. The studies done in this particular watershed clearly indicate that
stormwater runoff from impervious areas is having an adverse impact on the biological
community.
One comment stated that NCDOT-owned impervious cover in this watershed is
approximately 3.2%. Assuming the accuracy of this figure, is NCDOT currently in
compliance with the draft TMDL? At what percent impervious cover would NCDOT be
out of compliance with the draft TMDL?
Response: It is not clear what the 3.2% refers to from the commenter. All MS4 permits
include a requirement to implement an approved TMDL and strongly encourage
adjoining jurisdictions to work together in this process. We also realize that individual
entities will be interested in their specific compliance and the Division anticipates that
for each permitee adherence to their final recovery plan will establish their compliance.
One commenter stated that under the “SUMMARY” heading (pg 3) - the document
summary specifies hydromodification (impoundments), nutrient and organic loading, and
low DO as primary stressors. In order to avoid distraction from the primary focus of
stormwater (and the surrogate IC) as the stressor, I would suggest removing the nutrient
and organic loading from the summary.
Response: The text has been changed for clarification.
One commenter requests that the classification of impairment and application of the
TMDL be limited to the lower portion of the watershed from US 1 to the Neuse until
additional sampling establishes an accurate and objective assessment of the remainder of
the watershed.
Response: Percent impervious cover appears to be uniform across this watershed
regardless of the type of development. The DWQ study indicated strong pressures from
suburban and urban development. In implementing the TMDL and developing the Water
Quality Recovery Program through the NPDES Stormwater permit local governments t
will be able to target the areas they believe will result in meeting the narrative water
quality standard of a Good-Fair bioclassification.
One commenter stated that under the “WATERSHED DESCRIPTION” heading (pg 6) -
reference is made to the Perry Creek watershed (study area) which appears to be an 11
square mile subset of the entire 31.6 square mile watershed. Does the TMDL apply to the
entire Perry Creek watershed or just the “study area”?
Response: The TMDL applies to the entire 11 square mile watershed. The text has been
revised for clarification.
One commenter stated that under the “POLLUTANT SOURCE ASSESMENT” heading
(pg 8) - in the 5th paragraph the document indicates low DO may be a stressor due to
nutrient enrichment and algal activity. Since there is no evidence presented to support
this hypothesis and local staff have traditionally not found low DO to be a problem in
urban streams, we would request that this assumption be removed.
Response: The text has been clarified.
One commenter stated that the document fails to discuss Load Allocation (LA) for those
portions of the watershed not subject to NPDES permits such as discharges to waters of
the state from systems not owned or operated by an NPDES permit holder (direct
discharges from private land into waters of the state) which qualify as non-point sources.
Response: This TMDL includes a Load Allocation of 9% impervious cover.