NC0026441_Report_19910516

MPDES DOCUMENT SCANNING COVER SHEET
NPDES Permit:
NCO026441
Siler City WWTP
Document Type:
Permit Issuance
Wasteload Allocation
Authorization to Construct (AtC)
Permit Modification
Engineering Alternatives Analysis
201 Facilities Plan
Instream Assessment (SOC)
-Rocky-River-subbasin nutrient budget
Permit History
Date Range:
Document Date:
May 16, 1991
THIS DOCUMENT 15 PRINTED ON REUSE PAPER - IGNORE ANY CONTENT ON THE
REVERSE SIDE
I

State of North Carolina
Department of Environment, Health, and Natural Resources
Division of Environmental Management
512 North Salisbury Street • Raleigh, North Carolina 27604
James G. Martin, Governor George T. Everett, Ph.D.
William W. Cobey, Jr., Secretary Director
May 16, 1991
Mr. Jim Greenfield
WLA Coordinator
U.S. EPA, Water Division
Region IV
345 Courtland Street
Atlanta, GA 30365
Subject: Rocky River QUAL2E and nutrient budget analyses
Cape Fear River Basin
Chatham County
Dear Mr. Greenfield:
Enclosed, please find copies of the documents entitled "Town of Siler City WLA Modeling
Analysis" and "Rocky River Subbasin Nutrient Budget (Chatham County)" for your review. You
should note that the Division of Environmental Management (DEM) is planning on collecting AGPT
samples in Loves Creek and the Rocky River this summer to validate the nutrient analysis, and the
limits imposed on the Siler City WWTP.
If you have questions or comments concerning either document, please do not hesitate to can Mike
Scoville or me at [phone].
Attachments
J. r or Clements, hief
W to Quality Section
Pollution Prevention Pays
P.O. Box 29535, Raleigh, North Carolina 27626-0535 Telephone [phone]
An Equal Opportunity Affirmative Action Employer

ROCKY RIVER SUBBASIN
NUTRIENT BUDGET
(CHATHAM COUNTY)
NORTH CAROLINA DEPARTMENT OF ENVIRONMENT, HEALTH AND
NATURAL RESOURCES
DIVISION OF ENVIRONMENTAL MANAGEMENT
WATER QUALITY SECTION
MAY. 1991

INTRODUCTION
Nitrogen and phosphorus are important nutrients with respect to aquatic plant
growth. Elevated nutrient levels are often associated with noxious algal blooms in
aquatic systems. Numerous blooms have been observed in the lower reaches of the
Rocky River in Chatham County during the last two years. According to the North
Carolina Department of Environment, Health and Natural Resources (NCDEHNR) Raleigh
Regional Office staff, the majority of the algal blooms occurred during the summer
of 1990.
The Town of Siler City WWTP discharges into Love's Creek, a tributary to the
Rocky River (Figure 1). Samples collected on July 26, 1990 by the Raleigh Region-
al Office in the Rocky River, downstream of the Love's Creek confluence, indicated
high concentrations of chlorophyll A (44.0 ug/1). Also, nutrient analyses revealed
elevated concentrations of total phosphorus (0.49 mg/1) and total nitrogen (5.9
mg/1) at this sampling location.
Comprehensive sampling of the Rocky River was performed on August 3, 1990, by
the Environmental Siences Branch (Appendix 1). Results indicated that high
concentrations of total phosphorus occurred both upstream (0.13 mg/1) and down-
stream (0.88 mg/1) of the Love's Creek confluence. However, high levels of total
nitrogen (8.2 mg/1) were found only below the confluence with Love's Creek.
In 1988 the Town of Siler City requested an expansion for its wastewater
treatment plant to a wasteloow of 4.0 MGD from 1.8 MGD. A wasteload allocation
(WLA) was performed for this expansion and permit limits were recommended based on
a Level-B modeling analysis. However, due to public interest in this issue, a
Level-C analysis was conducted to verify the previously recommended permit limita-
tions at the expanded flow. The Level-C modeling analysis is documented in a
memorandum from Mike Scoville dated November 1, 1990 (Appendix 2). The revised WLA
resulted in more stringent permit limitations including a summer period total
phosphorus limitation of 0.5 mg/1 as per NCAC 15A 2B .0211 (b)(3)(A), which states
"the Commission or its designee may prohibit or limit any discharge of waste into
surface waters if, in the opinion of the Director, the surface waters experience or
the discharge would result in growths of microscopic vegetation such that the
standard established pursuant to this Rule would be violated or the intended best
usage of the waters would be impaired."
This document develops a nutrient budget for total phosphorus and total
nitrogen in the Rocky River subbasin. Nutrient loads are grouped into point source
and non -point source categories. The analysis demonstrates a need to limit the
total phosphorus input to the system from Siler City WWTP to 0.5 mg/l.
ANALYSIS
Non -point Source Loading Calculation
Annual non -point loading estimates were calculated using land use information
(U.S. Department of Agriculture, Soil Conservation Service, and Forest Service,
1985) and estimates of areal nutrient export:

Li = Ai * Ri
Where Li = total phosphorus or nitrogen loading (lbs/yr)
Ai = area (acres)
Ri = export coefficient (lbs/ac/yr) for phosphorus or nitrogen
and
i = land use
The total phosphorus export coefficient estimates used in this analysis were
derived from North Carolina Division of Environmental Management (NCDEM) guidance
(Appendix 3). The total nitrogen export coefficient estimates for agricultural,
urban and forest land were obtained from NCDEM Nutrient Management in the Lower
Neuse Basin (1988). The atmospheric deposition export coefficient (applied to
surface area of lakes) for total nitrogen was obtained from The Role of Acid Rain
by the Environmental Defense Fund (1988).
The results of the loading calculations are presented in Tables 1 and 2 and
presented graphically in Figures 2 and 3.
Note that although 23.1% of the basin is comprised of agricultural and pasture
land, 59.2 % of non -point phosphorus and 51.3% of non -point nitrogen annual loading
are attributed to these two land use types. According to U.S. Department of
Agriculture, Soil Conservation Service and Forest Service (1985), the area of
commercial forest land is decreasing. The conversion of forest land to cropland,
urban land and water reduces the forest land base by about 2,000 acres each year,
but is partly offset by tree planting on idle agricultural lands. This practice
should stabilize the area of commercial forest land until 1995. The 1985 study
further states that "cropland is the largest single contributor to the erosion and
sedimentation problem in the area."
Point -Source Loading Calculation
Available data from September, 1989, to August, 1990, were retrieved from
NCDEM's compliance monitoring system for all permitted discharges required to
monitor their effluents in the subbasin. There are twenty one permitted discharg-
ers in the subbasin (Figure 1). Two facilities are schools with average flow rates
of 0.0025 MGD and 0.0020 MGD, respectively. One facility is a rest home with an
average flow rate of 0.0816 MGD. Seventeen facilities are single family residences
with permitted flow rates in the range of 0.0004 to 0.0006 MGD. The largest
observed flow during this period corresponded to Siler City WWTP with an average
flow of 1.8558 MGD. It should be noted that DEM has previously determined that the
Siler City DMR's flow data are inaccurate; actual discharge rates are greater than
those reported.
Phosphorus and nitrogen data were sufficient to develop plant -specific load
estimates for Siler City WWTP. For facilities with no available total phosphorus
and total nitrogen data, an average total phosphorus concentration of 3.0 mg/1 and
an average total nitrogen concentration of 15.0 mg/l were assumed to calculate load
estimates. These are total phosphorus and total nitrogen values commonly observed
in secondary treatment domestic effluent in North Carolina.
The results of the point load calculations are presented in Tables 3 and 4.
The total phosphorus loading attributed to point -sources is estimated to be
11,609.73 lbs/year while the total nitrogen loading originated from point -sources
is estimated at 115,307.17 lbs/year.

Siler/Ci
Nutrient Loading
Point Sources
O_Siler
City WWTP
Bonlee
O2 _Elementary
School
O3
Hill Forest
_Res1
Home
OChatham
Central
—High
School
ODeniece
Orgeron
_Resident
O'Conner
ORobert
_Residence
ORonald
Phillips
_Residence
John W. Smith
O$
_Residence
Kathleen Hundly
O
_Residence
James Scott
10
_Residence
Swanie Cristle
G> >
_Residence
FIGURE 1.
ROCKY RIVER SUBBASIN AND ITS
NUTRIENT LOADING POINT SOURCES
11
64
/c— Statute Niles
5 0 5 10
Receiving
Nutrient Loading
Receiving
Stream
Point Sources
Stream
2
Elizabeth Headen
Loves Creek
t
_Residence
Bear Creek
13
Mark Rogers
UT Bear Creek
_Residence
Harlands Creek
Frank Clark
Bear Creek
14
_Residence
UT Greenbrier Creek
Dwayne Chavis
UT Bear Creek
t 5
_Residence
UT Greenbrier Creek
Barry Park
URocky River
16T
_Residence
UT Hans Creek
Kevin S. Cieciorka
ULoves Creek
»T
_Residence
UT Rocky Creek
David Hinton
UT Bear Creek
O18
_Residence
Bear Creek
J. Thomas Peace
URocky River
19T
_Residence
Rocky River
Jonnie Webster
Rocky River
_Residence
Bear Creek
Leonard Gunter
UT Bear Creek
21
_Residence
Pones Creek
UT Bear Creek

Table 1.
Table 2
Rocky River Watershed Non -Point Source Annual
Phosphorus Loading
LAND USE I
AREA
IEXP. COEFF. I
TP LOAD -
[
(acres)
1(#/ac./yr.) I
(#/year)
Cropland I
10,462
1 1.80 1
18,831.6
Pasture I
27,172
1 0.49 1
13,314.3
Fallow Land I
1,772
1 0.18 1
319.0
Urban Land I
6,662
1 1.15 1
7,661.3
Forest Land 1
116,536
1 0.12 1
13,984.3
Lakes I
288
I 0.58 1
167.0
-
------------
TOTAL I
------------
162,892
162,892
------------
1 I
54,277.5
Note: Urban land includes rural residences and
churches, farmsteads, roads, mines, recreation
areas and critical areas
Rocky River Watershed Non -Point Source Annual
Nitrogen Loading
LAND USE I
AREA
IEXP. COEFF. I
TN LOAD
I
(acres)
1(#/ac./yr.)
(#/year)
Agricultural[
39,406
1 5.60 1
220,673.6
Urban 1
6,662
1 4.7 1
31,311.4
Forest Land I
116,536
I 1.5 1
174,804.0
Lakes
288
I 12.05 1
3,471.0
TOTAL I
162,892
1 I
430,260.0
Note: Agricultural land includes cropland, pasture
and fallow land
Urban land includes rural residences and
churches, farmsteads, roads, mines, recreation
areas and critical areas

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i
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s
ice'
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gel C,
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Table 3. Siler City WWTP Nutrient Loading Input
from September 1989 to August 1990
DATE I
FLOW I
TP I
TN
ITP LOAD
ITP LOAD
I TP LOAD
ITN LOAD
1
(MGD) I
(mg/1) I
(mg/1)
1(#/day)
1(#/day)
I (#/mo)
I (#/mo)
09-09-89 1
1.6 1
0.9 1
21.0
1 11.7
1 273.0
I 351.0
1 8,190.0
10-12-89 1
1.4 1
1.8 1
25.0
1 20.6
1 286.5
1 638.6
1 8,881.5
11-28-89 1
1.8 1
0.4 1
12.7
1 6.0
1 190.0
1 180.0
1 5,700.0
12-20-89 1
1.7 1
0.7 1
16.3
1 10.1
1 236.1
1 313.1
1 7,319.1
01-10-90 1
2.2 1
3.4 1
22.4
1 61.0
1 401.7
1 1,891.0
1 12,452.7
02-14-90 1
2.1 1
0.9 1
14.1
1 15.4
1 244.4
1 431.2
1 6,843.2
03-07-90 1
2.0 1
0.6 1
19.6
1 10.0
1 325.5
1 310.0
1 10,090.5
04-16-90 1
2.2 1
1.5 1
18.8
1 28.0
1 351.0
1 840.0
1 10,530.0
05-03-90 1
2.3 1
4.4 1
32.0
1 83.0
1 603.4
1 2,573.0
1 18,705.4
06-14-90 1
1.9 1
2.3 1
17.7
1 36.0
1 284.3
1 1,080.0
1 8,529.0
07-13-90 1
1.7 1
1.8 1
9.8
1 25.0
1 136.0
1 775.0
1 4,216.0
08-01-90
1 1.5 1
3.5 1
24.5
1 43.7
1 306.1
1 1,354.7
1 9,489.1
* AVERAGE
I 1.9 1
1.8 1
19.5
1 29.2
1 303.2
1
I
TOTAL
I I
I
1
1
110,737.6
1110,946.5
Average flow for Siler City WWTP represents the mean flow value of
twelve daily flow rates concomitant with twelve 24-hr composite sample
TP and TN concentrations reported from September 1989 to August 1990

Table 4. Rocky River Subbasin point -source Nutrient Loading
Input at Effluent Average Flow from September 1989
to August 1990
FACILITY
I FLOW RATE I
(MGD) I
Siler City WWTP
I I
I 1.8558 I
Bonlee School
I 0.0025 1
Hill Forest Rest Home
I 0.0816 1
Chatham Central High
I 0.0020 1
Orgeron SFR
I 0.0004 1
O'Conner SFR
1 0.0006 I
Phillips SFR
I 0.0004 I
Smith SFR
I 0.0006 1
Hundley SFR
I 0.0004 I
Scott SFR
I 0.0004 I
Cristle SFR
I 0.0006 I
Headen SFR
I 0.0006 1
Rodgers SFR
I 0.0006 1
Clark SFR
I 0.0006 1
Chavis SFR
I 0.0006 1
Parker SFR
I 0.0006 1
Cieciorka SFR
I 0.0006 1
Hinton SFR
I 0.0006 1
Peace SFR
I 0.0006 1
Webster SFR
1 0.0006 1
Gunter SFR
I 0.0006 1
TP LOAD I
TN LOAD
(lbs/year) I
(lbs/year)
I
10,737.60 1
110,946.50
22.83 1
114.15
745.20 1
3,725.98
18.26 1
91.32
3.65 1
18.26
5.48
1 27.40
3.65
1 18.26
5.48
1 27.40
3.65
1 18.26
3.65
1 18.26
5.48
1 27.40
5.48
1 27.40
5.48
1 27.40
5.48
1 27.40
5.48
1 27.40
5.48
1 27.40
5.48
1 27.40
5.48
1 27.40
5.48
1 27.40
5.48
1 27.40
5.48
1 27.40
TOTAL POINT SOURCE NUTRIENT LOAD = 11,609.73 115,307.17
Average flow for Siler City WWTP represents the mean flow
value of twelve daily flow rates concomitant with twelve
24-hr composite sample TP and TN concentrations reported from
September 1989 to August 1990 (See Table 3.)
Average flow for Bonlee School, Hill Forest Rest Home and
Chatham Central High are for period September 1989 to August
1990 as reported in self -monitoring data
Single family residence (SFR) flow values are design flow
Facilities for which no site specific TP and TN data were
available, an average TP concentration of 3.0 mg/l and
An averaae TN concentration of 15.0 m/1 were assumed

Total Nutrient Budgets
The annual nutrient budgets for total phosphorus and total nitrogen are
depicted in Figures 4 and 5, respectively. The results indicate that non -point
sources are the largest contributors of nutrients in the Rocky River watershed on
an annual basis.
However, the above results do not account for seasonality. They represent the
total nutrient load per year but they do not describe the nutrient loads associated
with summer low flow conditions when the algal blooms have been documented in the
system. The most significant non -point loading occurs after rainfall events which
produce relatively high stream flow conditions. Therefore, in order to account for
seasonality, a method had to be designed to translate average non -point source
loads into low flow loads present during critical conditions. To accomplish this
task, the annual loads were input to the average river flow (224 cfs) to arrive at
an average instream concentration. Using this average concentration as a best
estimate, mass loads were subsequently calculated for several low flow conditions.
are:
The expected average instream nutrient concentrations from non -point sources
Total Phosphorus: 0.12 mg/1
Total Nitrogen: 0.98 mg/1
These nutrient concentrations correspond to non -point source mass loads of 0.45
lbs/day total phosphorus and 3.7 lbs/day total nitrogen at the S7Q10 flow of 0.7
cfs. Other expected nutrient loads from non -point sources, based on the above
concentrations, are presented for different river flow regimes in Figures 6 and 7.
Note that the estimated 0.12 mg/l total phosphorus concentration compares favorably
to total phosphorus samples collected by the Town of Siler City during August,
1989, in the Rocky River upstream from the Love's Creek confluence (0.16 mg/1) and
samples collected by NCDEM on August 3, 1990, in the same general area (0.13 mg/1).
The estimated 0.98 mg/1 total nitrogen concentration compares to 1.74 mg/1 and 0.93
mg/l total nitrogen from samples collected by the Town of Siler City during August,
1989, and by NCDEM on August 3, 1990, respectively.
The average total phosphorus and total nitrogen daily loads from Siler City
WWTP have also been plotted in figures 6 and 7 to compare the expected nutrient
load from non -point sources under different river flow regimes to the documented
average nutrient load from Siler City.
Algebraic techniques were employed to determine the equations of the non -point
source nutrient load lines in Figures 6 and 7.
The results indicate the equation of the line in Figure 6 is:
TP = (0.65 * riverflow) + 0.01
and, the equation of the line in figure 7 is:
TN = (5.27 * riverflow) + 0.03

Figure 4. Sources of Total Phosphorus Load in the Rocky River
Subbasin per Year
® Point Source Load 9 Non -point Source Load .

35
30
25
20
TP (#/day)
15
10
5
0
Figure 6. Total Phosphorus (TP) Concentrations Originating from Non
point Sources at Different River Flow Regimes
0.7 10 20 30 40 50
Flow (cfs)
—TP load from non -point sources at Annual average TP load from Siler City
different river flow regimes

TN Load (#/day)
350
300
250
200
150
100
50
0
Figure 7. Total Nitrogen (TN) Concentrations Originating from Non
point Sources at Different River Flow Regimes
0.7 10 20 30 40 50 60
Flow (cfs)
— TN load
from
non -point sources at
Annual average TN load from Siler City
different
river
flow regimes

The above equations were used to determine the coordinate where the line
representing the total nutrient load from non -point sources at different flow
regimes intersects the line representing the average nutrient load from Siler City.
This X-coordinate of this point represents the river flow necessary to produce a
total nutrient mass load equivalent to the total nutrient mass load from the Siler
City effluent.
DISCUSSION
Using the nutrient loading relationships depicted in Figures 6 and 7, the
following conclusions may be drawn:
A) Total Phosphorus (Figure 6)
1) A river flow of approximately 45 cfs is necessary to produce a total
phosphorus mass load equivalent to the total phosphorus mass load from Siler City
WWTP (29 lbs/day).
2) During summer low flow conditions (S7Q10 = 0.7 cfs) the expected non -point
source contribution of total phosphorus is less than 0.45 lbs/dav (1.5 % of total
TP) while the documented total phosphorus contribution from Siler City WWTP is 29
lbs/day (98.5 % of total TP); and,
3) During river flow periods of less than 45 cfs, the Siler City WWTP total
phosphorus input exceeds the total phosphorus input from non -point sources.
B) Total Nitrogen (Figure 7)
1) A river flow of approximately 58 cfs is necessary to produce a total
nitrogen mass load equivalent to the total nitrogen mass load from Siler City WWTP
(304 lbs/day);
2) During summer low flow conditions (i.e. S7Q10 = 0.7 cfs) the expected
non -point source contribution of total nitrogen is less than 4 lbs/day (1.2 % of
total TN) while the documented total nitrogen contribution from Siler City WWTP is
304 lbs/day (98.6 % of total TN); and,
3) During river flow periods of less than 58 cfs, the Siler City WWTP total
nitrogen input exceeds the total nitrogen input from non -point sources.
Furthermore, samples of total phosphorus taken by the Town of Siler City during
August, 1989, in the Rocky River upstream from the Love's Creek confluence indicate
an average concentration of 0.16 mg/l. Total phosphorus samples collected in the
same general area by NCDEM's Environmental Sciences Branch on August 3, 1990,
indicated a concentration of 0.13 mg/l. U.S.G.S. gauging station No. 0210166029 in
the Rocky River, near Cruthfields Crossroads (upstream from the confluence of
Love's Creek), indicated an average flow of 1.57 cfs during August, 1989 and an
average flow of 0.58 cfs in August, 1990 (i.e. 0.50 cfs on August 3, 1990).
Therefore, according to these limited data, while river flow conditions approximate
S7Q10 flow conditions (i.e. 0.7 cfs), the river background total phosphorus
concentration (i.e. attributed to non -point sources) approximates 0.16 mg/l.
However, the monthly average of total phosphorus reported by Siler City WWTP was
0.80 mg/l and 3.5 mg/l in August, 1989 and August, 1990 respectively. These total
phosphorus concentrations are five and twenty-two times the documented background

concentrations for total phosphorus.
Similarly, samples of total nitrogen taken by the Town of Siler City during
August, 1989 in the Rocky River, upstream from the Love's Creek confluence,
indicate a monthly average of 1.74 mg/l. Total nitrogen samples collected by NCDEM
on August 3, 1990, indicated a concentration of 0.93 mg/l. Therefore, these data
indicate that while river flow conditions approximate S7Q10 flow conditions, the
river background total nitrogen concentration (i.e. attributed to non -point
sources) approximates 1.74 mg/l. However, the monthly average of total nitrogen
reported by Siler City WWTP was 21 mg/1 and 24.5 mg/1 in August, 1989 and August,
1990 respectively. These total nitrogen concentrations are twelve and fourteen
times the documented background concentrations for total nitrogen.
Although the total nitrogen input from Siler City discharge is significant, by
reducing total phosphorus concentrations from the Siler City discharge the system
can be driven to a phosphorus limited state. Data collected by the Town of Siler
City in August, 1989, in the lower reaches of the Rocky River, indicate that the
average total nitrogen concentration is 4.3 mg/1 while the total phosphorus
concentration is 0.26 mg/l. These results indicate that the N/P ratio in the lower
reaches of the Rocky River is more than 16:1. It is widely accepted that when the
ratio of N/P is less than 10:1, nitrogen controls the growth of algae and for N/P
greater than 10:1, phosphorus controls the growth (Thomann and Mueller 1987).
Therefore, by reducing the available phosphorus for plant growth it is expected
that the frequency and the severity of algal blooms will be reduced. However,
since variability in plant stoichiometry results in variation of the accepted N/P
limiting criteria NCDEM will conduct Algal Growth Potential Tests (AGPT's) in the
system. AGPT results will be used to verify that controlling phosphorus input in
this system will counteract the eutrophication experienced in the lower reaches of
the Rocky River.
It should be noted that the Siler City Reservoir (located upstream from the
confluence with Love's Creek and downstream from the U.S.G.S. gaging station No.
0210166029) regulates the flow in the Rocky River. NCDEM field staff has reported
that release from the reservoir during summer months is almost non-existent, so the
instream waste concentration is expected to approach 100% in the lower reaches of
the Rocky River. Thus, almost all nutrient input to the system would originate
from the Siler City WWTP effluent.
SUMMARY AND RECOMMENDATIONS
Based on land use of the Rocky River watershed and export coefficients for
total phosphorus and total nitrogen, non -point sources contribute 82.4 % of the
total phosphorus and 79.9 % of the total nitrogen input in the subbasin annually.
Although 23.1 % of the basin is agricultural and pasture land, 59.2 % of non -point
phosphorus and 51.3 % of non -point nitrogen loading are annually attributed to
these two particular land uses (Figures 2 and 3). However, data indicate that
non -point sources are relatively unimportant during the critical low flow periods
when eutrophication problems are exhibited in the Rocky River.
Algal blooms that have occurred in the lower reaches of the Rocky River are
correlated to summer months when the river flows approximate summer 7Q10 flow
conditions. The Siler City mass loads of total phosphorus and total nitrogen
dominate the total nutrient contribution to the system (i.e. nutrient inputs
originating from point sources and non -point sources) during summer low flow
periods (Figures 6 and 7).

Siler City is the largest contributor of total phosphorus (92.5 %) and total
nitrogen (96.2 %) originating from point sources. Limiting total phosphorus
concentrations in the Siler City NPDES permit to reflect the best available
treatment technology (i.e. a total phosphorus limitation of 0.5 mg/1) at a waste -
flow of 4.0 MGD will result in a total phosphorus mass load of 16.7 lbs/day as
opposed to the current total phosphorus mass load of 29 lbs/day at a "wasteflow rate
of approximately 1.9 MGD. This represents a reduction of 42.5 9 from the current
total phosphorus load from the Siler City discharge which would greatly reduce the
probability of algal blooms during the summer months. Note that even at the
reduced total phosphorus mass load, the Siler City total phosphorus concentration
of 0.5 mg/1 is above the documented background levels for this nutrient in Love's
Creek (i.e. 0.09 mg/1) and the Rocky River (i.e. 0.16 mg/1).
Based on N/P ratio analysis, phosphorus is best target nutrient to control
the growth of algae in this system. The system can be driven to a phosphorus
limited state by reducing total phosphorus concentrations from the Siler City
discharge.

REFERENCES
Environmental Defense Fund, The Role of Acid Rain. New York:Environmental Defense
Fund, 1988.
North Carolina Division of Environmental Management, "Nutrient Management in the
Lower Neuse Basin," Raleigh, 1988. (Typewritten)
Thomann, Robert V., and Mueller, John A. Principles of Surface Water Quality
Modeling and Control. New York: Harper and Row, Publishers, 1987.
U.S. Department of Agriculture, Soil and Conservation Service and Forest Service,
Deep River Erosion Study. Raleigh: USDA -Soil Conservation Service, 1985.

APPENDIX 1.
DIVISION OF ENVIRONMENTAL MANAGEMENT
October 2, 1990
MEMORANDUM
TO: Ken Eagleson
THROUGH: Jimmie Overton
FROM:. Karen Lynch 41)
RE: Rocky River, Chatham County
R-9
The Rocky River was sampled on August 3 at the request of the
-Raleigh .Regional Office after complaints of algal blooms. Sampling
sites were chosen above and below the confluence with Love's Creek
into which Siler City's wastewater treatment plant discharges.
Laboratory and phytoplankton analyses are complete and are
included in the attached table. The table includes chlorophyll -a and
nutrient data along with phytoplankton biovolume and density
estimates. Also attached is a map marking station locations.
Nutrient analyses revealed elevations in total phosphorus at the
four sites sampled, while high levels of total nitrogen and
ammonia/ammonium were found at Rocky-2, below the confluence
with Love's Creek.
In addition, high chlorophyll -a concentrations were detected
above and below Love's Creek at stations, Rocky-1 and Rocky-2.
Quantitative phytoplankton counts were conducted on algal samples
collected from Rocky-1 and Rocky-2. Moderate phytoplankton
densities were found at both stations. Because of the slow -flowing
nature of the Rocky River during the summer, free-floating
phytoplankton populations were allowed to build up more than
would be expected in a faster flowing creek. Algal biovolumes
ranged from 1,250 to 1662 mm3/m3 while densities were 5,590 and
6,594 units/ml below and above Love's Creek, respectively.
Typically biovolumes greater than 5,000 mm3/m3, densities higher
than 10,000 units/ml or chlorophyll -a concentrations approaching 40
ug/I (the state standard) constitute 'algal bloom' conditions

At the upper site, Rocky-1, algal classes were dominated by
chlorophytes (green algae), chrysophytes (golden -brown algae) and
cryptcphytes (cryptomonads). Downstream at Rocky-2,
chlorophytes, chrysophytes and cyanophytes (blue-green algae)
dominated the sample. The filamentous blue-green, Oscillatoria
chlorina, which comprised 29% of the biovolume is a pollution
tolerant algal species.
cc: Trish MacPherson
Steve Mitchell
Tim Donnelly, RRO
Ron Ferrell, RRO

ROCKY RIVER NUTRIENT, CHLOROPHYLL AND PHYi-OPL /VJKTO.N DATA
_
BIOVOLUME
DENSITY
CHL-A
TOTALI
NH3
I TOTAL
NITROGEN
PF LOSPHOR
STATION
JDATE
mm3/m3
units/ml
u /I
m /I
m
/I
m
/I
-------------------------------------------------------------
-
ROCKY-1
900803
1662
6594
32
0.93
O.t4
0.13
ROCKY-2
900803
1250
5590
44
8.2
0.42
0.88
ROCKY-3
900803
---NOT SAMPLED---
1
0.6
0.05
0.23
ROCKY-5
900803
---NOT. SAMPLED---
<1
0.68
0.08
0.23

ROCKY RIVER, CHATHAM COUNTY
B AUGUST 3, 1990
WI
� JY
_ JL
iau
!r
W! lin i.r T, W ' Q � =1X �� 1 1 12Y , • '3!r!. MY � •!n �;:
� � JlM rJ lW
i
,r ♦ ROCKY-1
vII `
�..Ja w M � l IAl IL'• �� ILC rLJ V���
Y 1� b r �� 11Ir � • ,y, ,
/ OCKY_2
ni
na m m uw
• • ROCKY�3
'liu J
rl. v , un ye. 1e
!lll -
ROCKY-5
-
un
nn ♦, Q'Y
•• — 1 \, rlu� _ - ,,,, � rn \ �... `Tim, •�•
! •ar r �.• u
1 v�-Y\ rs 1LLL - t

APPENDIX 2
Division of Environmental Management
November 1, 1990
Memorandum
To: Arthur Mouberry
Alan Clark
Steve Tedder
Don Safrit
Bobby Blowe
Thor: Trevor Clements
Ruth Swanek
From: Mike Scoville
Subject: Town of Siler City WLA Modeling Analysis (NPDES No. NC0026441, Chatham
County)
Summary
In 1988 the Town of Siler City requested an expansion for its wastewater treatment plant to a
wasteflow of 4.0 MGD from 1.8 MGD. At that time, a Level-B modeling analysis was performed and
permit limits recommended. Since then, a Level-C analysis was required to verify the limits necessary to
protect North Carolina's instream standards at an expanded wasteflow, due to public interest in the issue, the
inadequacies of the Level-B model application to the Rocky River, requirements of the EIS process, and
WWTP design considerations. The entire wasteload allocation analysis was re -initiated, and based on
available information and data, the following limits should apply if an NPDES permit is issued for Siler
City with a design flow of 4.0 MGD:
Summer
Winter
Floc,
MGD
4
4
BODS
m g/I
5
10
NH3-N
mg/l
1
2
DO
mg/1
6
6
TSS
mg/I
30
30
Fecal Coliform
#/100 ml
200
200
pH
SU
6-9
6-9
Oil & Grease
mg/1
30
30
MBAS
mg/I
0.5
0.5
Lem
ug/I
26
26
Chromium
ug/I
52
52
Cyanide
ug/I
5.2
5.2
Mercury
ug/I
0.012
0.012
Copper
ug/I
Monitor
Monitor
Nickel
ug/I
Monitor
Monitor
Zinc
ug/I
Monitor
Monitor
Silver
ug/I
Monitor
Monitor
Aluminum
ug/I
Monitor
Monitor
Total Residual Chlorine'
ug/I
17
17
Total Phosphorus
mg/1
0.5'"
NR
Not necessary if an alternate method of disinfection is utilized
Total Phosphorus Limit required May through September only.
The recommended limits are necessarily stringent to meet water quality criteria downstream of the
discharge. Given the documented impacts downstream, an expanded Siler City WA'TP should be designed

to meet [these limits at 4.0 MGD and at any future flows greater than 4.0 MGD; by taking appropriate
actions now, retrofitting the plant to meet more stringent limits in the future can be avoided.
Modelingof f Oxygen Consuming Wastes
A modeling analysis of Loves Creek and the Rocky River below the Siler City WWTPdischarge
was recently completed to evaluate the impact of the effluent on instream dissolved oxygen concentrations
and to determine the NPDES limits necessary to protect NC water quality standards at a wasteflow of 4.0
MGD. A QUAL2E water quality model was developed for approximately 21 miles of stream below the
plant. Hydraulic functions for the model were developed based on the results of a low flow time of travel
study performed by Black & Veatch this past August and September. Rates and kinetics were estimated
based on best professional judgement and on typical values observed in similar streams. The upstream
characteristics of Loves Creek and Rocky River, averaged from Siler City's instmam monitoring data, were
kept the same for each simulation, so any change in the model results are directly attributable to the effluent
characteristics. Four scenarios were simulated:
1. Average conditions from August 1990
2. Current permit limits for 1.8 MGD
3. Proposed permit limits for 4.0 MGD
4. No discharge
The model inputs for the upstream and various effluent conditions are shown in Table 1. While
not a formally calibrated model, the DO profile predicted by the model for average low flow conditions
provides a close fit to instream DO data collected on several separate occasions during warm, low flow
conditions of 1989 and 1990. This is shown in Figure 1. Most importantly, the predicted DO profile
accurately matches the observed DO in the impact and recovery zone; the model is appropriate for
evaluating the impacts of the wastewater on the water quality in this area. It is important to remember that
the model simulation represents mean conditions, while the data points in Figure I are instantaneous
samples.
Table 1.
QUAL2 Model Input for Siler City WWTP
Upstream
Effluent
Characteristics
Parameter
Loves C eek
Rocky River
8/90 Average
Existing
Proposed
Permit
Expansion
Flow
0.2 cfs
0.3 cfs
1.63 MGD
1.80 MGD
4.0 MGD
BOD5
mg/l
1.670
2.130
5.33
12.00
5.00
DO
mg/l
4.410
2.700
6.10
6.00
6.00
Organic N
mg/I
0.600
0.650
6.92
5.00
5.00
NH3-N
mg/1
0.325
0.575
6.90
2.00
1.00
NOx
in
0.640
0.517 1
0.65
0.65
0.65
Low flow conditions are frequently experienced in the Rocky River, often for extended periods of
time. This is due to the presence of a reservoir upstream that releases little or no water to provide
assimilative capacity. As a result, 7Q10 conditions are experienced much more often than every ten years,
and DO violations probably occur every summer. The lack of significant instream flow facilitates low
velocities and substandard DO concentrations. Non -point loading also affects the water quality to some
extent, particularly in Loves Creek above the Siler City WWTP. Since the flow characteristics of the
Rocky River also lead to extended periods of time that the stream is dominated by the effluent, a very high
quality of effluent is necessary to protect the water quality standards.
Figure 2 shows the predicted DO profiles for each of the four scenarios previously mentioned. As
can be seen, the discharge has a significant impact on the water quality for approximately five stream miles
immediately below the plant. The average existing conditions produce the most severe DO sag, although
substandard DO concentrations are predicted in each of the models in which the discharge was included. The

E
0
Q
Figure I. Observed Insiream DO Concentrations and Predicted DO Profile.
10 T
0 5 10 15 20
We
model simulation with no discharge should be interpreted with caution, however, due to the uncertainty of
how the stream would behave with so little flow. The DO profile for the most part reflects only changes in
hydraulics; the oxygen -consuming parameters are absent for most of the model because of the slow
velocity. In reality some son of equilibrium would most likely be reached. The no -discharge model
provides a good indication of water quality in the upper part of the model, though, and it's results were
included in Figure 2 for comparison. In every case that was modeled, the Rocky River seemed to recover by
approximately the fifth mile, below which the effluent characteristics are expected to have little effect on
instream DO.
The existing permit limits were modeled at a now of 1.8 MGD, rather than 4.0 MGD. As can be
seen in Figure 2, under this scenario substandard DO concentrations would be expected for an extended
stretch of the stream. The results of other model runs (not shown) indicate the even lower DO
concentrations would result from a 1.8 MGD discharge with secondary treatment and from a 4.0 MGD
discharge with the existing limits. The predicted DO profile shown is included for comparison of the range
of limits necessary for the maintainence of the DO stream standard. In other words, the lower flow with the
better treatment has less impact than either of the existing permits, and it is still unacceptable from a DO
standpoint.
The Rocky River model is sensitive to organic nitrogen and ammonia nitrogen loading from the WWTP.
For example, although the average effluent BODS was less than half of the permitted amount, the DO sag
was more severe due to excessive levels of organic and ammonia nitrogen (see Table 1). In the allocation
model runs, the amount of organic nitrogen in the effluent was a critical factor in the DO sag. For the
average effluent concentration, a value of 6.92 was estimated based upon effluent values of total nitrogen
and ammonia nitrogen. For the two allocation scenarios a conservative value of 5 mg/1 was assumed.
Generally, as the removal of ammonia increases, the effluent organic nitrogen should decrease as well.
Based on other facilities' effluent data, if Siler City were meeting an NH3-N limit of 1 mg/l, their organic
nitrogen would be considerably less, probably around 2-3 mg/I. The sensitivity of the model to organic
nitrogen was tested in allocation runs for the proposed expansion by modeling effluent concentrations of I,
5 and 10 mg/l. The predicted DO profiles resulting from this analysis are shown in Figure 3. It is evident
that based on the model results, if the otter oxygen -consuming limits were met, organic nitrogen could
potentially make die difference of whether the instream DO standard is maintained. If the Siler City WWTP
were to maintain a high degree of nitrogen removal and consistently meet the proposed limits, the DO

stream standard would not be expected to be violated. This modeling analysis confirms that the limits
derived in the 1988 Level-B WLA would not adequately protect water quality in Loves Creek and the Rocky
River.
Figure 2. Rocky River Predicted DO Profiles
7
6
k,
ij
1
0
0 5 10 15 20
Mile
Figure 3. Sensitivity of Siler City WWTP Allocation Model to Organic Nitrogen
(Values are effluent organic nitrogen)
8
7
6
5
4
3
2
1
0
0
5 10 15 20
Mile

Nutrients
A total phosphorus limit was recommended to prevent over -enrichment of the natural stream
system per 15A NCAC 2B .0211 (b)(3)(A), which states "the Commission or its designee may prohibit or
limit any discharge of waste into surface waters if, in the opinion of the Director, the surface waters
experience or the discharge would result in growths of microscopic or macroscopic vegetation such that the
standards established pursuant to this Rule would be violated or the intended best usage of the waters would
be impaired." Due to the significance of Siler City's discharge on the downstream TP concentrations during
low flow periods, and based on staff observations and citizen complaints of algal growth on the substrate
and the water surface, a TP limit is appropriate. The above regulation specifically applies to slow moving
waters not designated as trout waters.
Toxics
The toxics limits were all determined according to standard Division procedure. A mass balance
was performed for each individual parameter and a limit derived which represents the maximum daily
allowable effluent concentration that will protect the instream standard. The mass balance used the plant
design flow (4.0 MGD) and the 7Q10 of Loves Creek at the point of discharge (0.25 cfs). Monitoring
requirements for metals were based on the latest pretreatment headworks analysis and priority headworks
analysis.
The total residual chlorine limit is to prevent toxicity; it is suspected that elevated chlorine
concentrations were a factor in the facility failing it's whole effluent toxicity test. The NC instream
standard for total residual chlorine is 17 ug/l. If the facility utilizes a method of disinfection other than
wastewater chormation, this limit would not be necessary, otherwise effluent dechlorination should be a
written permit condition.
The ammonia limit, while being limited for its oxygen consuming characteristic, is also limited as
a toxic substance. The recognition and documentation of ammonia toxicity led to the adoption of Division
procedure protecting the instream summer ammonia concentration to 1.0 mg/l or less. The ammonia limit
would have to be 1.0 mg/1 whether protection of the DO standard required it or not.
All major dischrgers in North Carolina are required to perform whole effluent toxicity tests, the
type of which is determined by the instream waste concentration (IWC) during 7Q10 conditions. At the
proposed 4.0 MGD wasteflow, Siler City's IWC is 96.1 %, which requires a quarterly chronic toxicity
testing at 96% using Ceriodaphnia as the test organism.

M
APPENDIX 3
TABLE I
RUNOFF COEFFICIENTS
FOR NONPOINT
SOURCE PHOSPHORUS
- COEFFICIENT
RANOE IN
WHERE
'
LAND USE USED
LITERATURE
MEASURED
REFERENCE
!lb/ac/yr>
!lb/ac/yr)
CROPLAND 1.8
0.13
Ohio
T.-Pr.iri♦ o K.lft 1906
0.19-0.41
Various
OH.rnik 19T6
0.3
Various
Horner a Mar 1982
-
p_49
Wake Co.
DEN 1981
0.52
Ohio
T.-Prairie 1 Kalft 1906
0.94
Florida
Shahann. 1982
0.90
Chowan
DEN 1982
1.8
N.C.
DEN 1906
3.65-3.92
Arkansas
U.sterdahl It •1. 1961
10.7
Chown
Craig 0 Kuenzler 1983
PR5TURELRNO 0.45
0.06
Various
Horner R Mar"3982
0.06
Ohio
T.-Prairie: C Kalft 1986
0.19-0.41
Chowan
Cra19 6 Kt�eAzler. 1903
0.27
Florida
Shahan* 1902.
0.49
Wake Co.
OEM 1984'
0.98
Chowan
OEM 1982
FALLOW LAUO 0.18
0.06
V.rio�s
Horner A Mar 1982
0.18
Wake Co.
OEM 1984
0.19-0.41
Chowan
Craip a Kuenzler 1183
0.98
Chowan
DEN 1982
FORE5TLnHO 0.12
0.05
Maine
Dennis 198S
0.07
Ohio
T.-Pr.iri. 9 Kallt 1986
0.08
Various
Horner R Mar 1902
0.01
Chowan
DEN 1982
0.09
Wake Co.
OEM 1904.
0.09-0.15
Varlous
OH.rnik 39TS
0.09
Florid.
Shahan .1904;-
0.17
Arkansas
u.stir'�h;'k?!!'.1. 1981
0.5
Chowan
Craig Q Kuenzler 1983
URBAN LAND 1.15
0.02-0.16
7 houses/ac
iischlor it Ll. 1983
0.05
Various
ONernik 1976
0.28
MiHad Urban
NouotnV at al. 1985
0.2-1.3
Residential
Horner 4 Mar 1962
0.39-1.34
.85 house/Ac
Dennis 198S
0.4
.2 house/ac
Randall et al. 1970
0.80-3.6
Industrial
Horner A Mar 3982
0.89
.5 house/ac
OEM 190A
•.• •.
y O•.89
1 house/ac
.
DEN 19"
•
0.9
1 house/ac
Randall .t al. 1978
0.9-1.0
1 hous#/.c
0EENN 1901;};,=.'
1.0
2.5 houses/ac
DEfi 1984;
1.04
4 houses/ac.
09�} (l00]
1.1
B.6 houses/ac
OEI7',1904 A.
1.1-0.0`
Mimed Urban
Cl�y1lan }'al. 19T0
1.2-1.4
multi-fakily
HOrnet: a 'mar 3982
1.25
4.S houses/ac
OEM 1984 -'
1.70
20 houss/Ac
Dorned 4 Krrutzberyer 1586
1.78
MSHed Urban
DEN 198]
2.0
Residential
Shahan# 1982
• 2.23
R&D Park
OEM 1986
2.4
C.... rcia1
Horner 4 Mar 3982
2.5
C8D
Horner h Mar 1582
3.4
Mi..d Urban
Bryan 1970
3.68
CBD
DEN 190]
4.7
Mi.ed Urban
Colston 1974
LAKES O.SB
0.58
50 in. .in
OEM 1983

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Note: DEI-1 refers to the North Carol Ina Divio,nn of CnvironmentaI
Man.aoement -
Bryan, E.H. 1970. Quality of Stormwater Drainage from Urban
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Cleveland, J.G., R.H. Ramsey, and P.R. Walters- 1970.
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Colston. N.V., Jr1974. Characterization and Treatment of
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1 Craig. N.J., and E.J. Kuenzler. 1993. Land use, Nutrient Yield,
and Eutrophication in the Chowan River Basin. Water Resources
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DEM. 1982a. Chowan River Water Quality MI[nagement Plan.
Raleigh, N.C.
DEM. 19B2b. Chowan/A,.l:bemarle Action 11 Plan. Raleigh, N.C.
Water Quay n
litDiscuSSos4.of Falls of the Neuse and
DEM. 1983a. '
B. Everett Jordan Lakes. DEM 83-06. Reigh, N.C.
DEM. 1983b. Nationwide Urban _RLrtg6ff Program, Winston-Salem,
N.C. DEM Report No- 83-07. Ral'6i6h, N.C.
DEM. 1984. Assessment of Nutra.erit Runoff from Projected
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DEM. 1985a Toxic Substances in Surface Waters of the B.
Everett Jordan Lake Watershed and September 1985 Update. DEM
Report No. 85-02. Raleiah, N.C.
DEM. 1985b. Toxic Substances in Surface Waters of the Fa115NOf
the Meuse Lake Watershed. DEM Report No. HS-OH. Raleigh,
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.-k-