NC0085812_Speculative Limits_20210510

Union County Public Works
500 N. Main Street
r =} Suite 600
County
Monroe. NC 28112
T. 704-296-4212
www.unioncountync.gov nn
May 3,2021
MAY 1 0 2021
Mr.Danny Smith,Director DIVISION OF WATER RESOURCES
Dept.of Environmental Quality _DIRECTOR'S OFFICE
Division of Water Resources
1611 Mail Service Center
Raleigh,NC 27699-1611
Subject: Request for Speculative Limits for the Proposed Union County
q P P
Lower Crooked Creek Water Reclamation Facility
Dear Mr.Smith,
I am writing you today to request your consideration,under 15A NCAC 02B.0404 section (b),for
providing speculative limits including seasonal limitations for biochemical oxygen demand (BOD) and
ammonia nitrogen.The rule specifies that the request should be sent to the Director with justification
for such limitations.This letter and the attached technical memorandum from Tetra Tech are intended
to provide that justification.
Union County submitted its original request to the Division of Water Resources (DWR) for speculative
limits for a proposed new discharge to Lower Crooked Creek on January 5,2021.The request followed
years of investment in stream monitoring studies,water quality model calibration and corroboration,
and model applications that were performed after dialog with DWR staff.The County has invested
more than$300,000 on these efforts.This month DWR staff informed the County and its consultants
that DWR plans to apply an interpretation of 15 NCAC 02B.0206 (d)(1)that would negate the findings
of the DWR-approved monitoring plan,model calibration report,and application analyses in favor of a
for a sin le BUDS limit of 5 mg/L.Such a decision would mean that the County prescription g g/ has wasted
5 years of its time and a significant amount of money conducting studies that DWR recommended the
County perform to determine scientifically based limitations.
Section (a) of 2B.0206 does allow for the use of more complex modeling techniques to be used and the
County in good faith sponsored such studies.The studies revealed that Crooked Creek could assimilate
approximately twice as much BUD as the County is requesting and still have a significant margin of
safety regarding the dissolved oxygen standard instream.The County proposes meeting limits of 5
mg/L BOD5 in the summer and 10 mg/L in the winter based on the approved model's scientific
predictions.Your approval of this request for a winter seasonal limit of 10 mg/L BOD5 would be in line
with the vast majority of municipal NPDES permits that include a 5 mg/L BOD5 limit,and it would
avoid even greater additional expense to the County to design a facility to try to comply with a 5 mg/L
BOD5 limit in the winter that the science indicates is not required to protect water quality in the
receiving water.
?i.1a' o
H CAS
L

Please see the attached updated technical memorandum justifying this request.Note that Section 3 of
the memo provides the rationale for interpreting the state's water quality rules that was requested by
staff during our meeting this week.We would be glad to discuss this further with you and your staff
and hope that we can come to a mutually agreed upon resolution to this matter.
Sincerely,
Aubrey Lofton,P.E.
Planning&Resource Management Director
Enclosure
CC: Hyong Yi-Public Works Administrator
John Shutak-Engineering Division Manager
Andy Neff-Water and Wastewater Operations Director
Kent Lackey-Black&Veatch
Steve Tedder-Tedderfarm Consulting
Trevor Clements-Tetra Tech
Julie Gryzb- Deputy Director,Division of Water Resources
Corey Basinger-DWR Mooresville Regional Water Quality Supervisor
yr _1 Page 2
C pQ '

t •
ThTETRA TECH One Park Drive,Suite 200•PO Box 14409
V Research Triangle Park,NC 27709
Tel [phone]•Fax [phone]
MEMORANDUM
To: Aubrey Lofton, Union County Date: April 27, 2021 Revised
Andrew Neff, Union County
Cc: Kent Lackey, Black &Veatch Subject: Basis for Speculative Limits for
the Proposed Lower Crooked
Creek Water Reclamation Facility
From: Trevor Clements, Hillary Yonce
1.0 INTRODUCTION
This technical modeling memorandum is intended to support the Union County plan to construct a new
wastewater treatment facility that would discharge its effluent to the lower portion of Crooked Creek.
Assimilative capacity of the receiving water was assessed using an existing QUAL2K model that was
calibrated and validated (Tetra Tech, 2019a), with development and documentation reviewed and
approved by the North Carolina Division of Water Resources (DWR, 2019). Union County first applied the
Crooked Creek QUAL2K model to support the County's request for interim effluent limits for the Grassy
Branch facility (NPDES Permit No. NC0085812) under a Special Order by Consent(SOC) along with
modified final permit limits to reflect plant improvements (Tetra Tech, 2019b). The County has further
directed that the model be applied to develop a speculative wasteload allocation for a proposed new
facility to be located approximately 12 miles downstream of the existing Crooked Creek#2 wastewater
treatment plant(WWTP) discharge and less than a mile downstream of the existing Grassy Branch
WWTP discharge on Crooked Creek (Figure 1), referred to as the Lower Crooked Creek Water
Reclamation Facility (LCCWRF). The facility is to be built in stages, with the first stage to be permitted at
4.6 million gallons per day (MGD) and the second stage to be permitted at 8.2 MGD and that would
include eliminating the existing Crooked Creek#2 WWTP and the Grassy Branch WWTP incorporating
those wasteflows with the expanded LCCWRF. The proposed LCCWRF outfall location is approximately
5.5 miles upstream of the confluence with Rocky River in a portion of Crooked Creek observed to flow
more freely and exhibit dissolved oxygen concentrations above the state water quality standard thereby
offering assimilative capacity (Tetra Tech, 2019a).
lb TETRA TECH 1

Memorandum—Basis for Speculative Limits for Proposed LCCWRF April 27, 2021
i. f Stanly
o County
w /
0gces t •¢oc
t.
Mecklenburg Grassy Branch
I c,
County
Union
County
Hemby Acres `
° Crooked Creek#2 4,6s
...,.. .� p Proposed Lower
"°`� ''' Crooked Creek WRF
North Fork Crooked Creek ° ik.
`` Go
06
l O
thaRan Yr. (,
.,
_ c°-'.
9 Legend
Y b h
u 4�r A Proposed Outfall
i; _: 41' c 3 Existing Outfall
ci
• itiq' Stream/River
>> Model Extent
o
Crooked Creek Watershed N 0 0 75 1 5 3 =I Watershed Boundary
la TETRA TECH ^ oKibmeters
NAD•4b]SLLLPI.0�.K:J, a,�,,.FPS 321,0 Fu: N o c 7s i s 3 County Boundary
•iw m�,.::.nz m.�zo"rw,: Miles '-
---
Figure 1. Crooked Creek Watershed with Existing and Proposed Facility Outfall Locations.
2.0 QUAL2K MODEL APPLICATION
North Carolina Water Quality Regulations (15A NCAC 02B .0206) specify that water quality standards
related to oxygen-consuming wastes be protected using the minimum average flow for a period of seven
consecutive days that has an average recurrence of once in ten years (7Q10 flow). NC regulations (15A
NCAC 02B .0404) also provide for seasonal variation for the discharge of oxygen-consuming wastes, with
the summer period defined as April through October and winter period as November through March.
Additionally, all existing permitted wasteload allocations (WLAs) must be accounted for to evaluate
available assimilative capacity for a speculative WLA for the proposed LCCWRF.
Set-up of the calibrated Crooked Creek QUAL2K model for evaluating impacts under seasonal critical
conditions for a speculative WLA is documented below.
lb TETRA TECH 2

Memorandum- Basis for Speculative Limits for Proposed LCCWRF April 27, 2021
2.1 SIMULATING CRITICAL CONDITIONS
2.1.1 Low Flow Statistics
The U.S. Geological Survey (USGS) provided 7Q10 estimates for multiple locations in Crooked Creek
based on a watershed drainage area relationship of 0.001 cubic feet per square mile (cfsm) derived from
the nearby Richardson Creek and Crooked Creek monitoring data (USGS, September 2019 via email
correspondence with Curtis Weaver). The USGS winter 7Q10 estimate was one order of magnitude
greater, at 0.01 cfsm. The 7Q10 flow estimates at Highway 601 and NC Highway 218 are 0.037 cfs and
0.044 cfs respectively for summer, and 0.371 cfs and 0.444 cfs respectively for winter. Applying this
drainage-area based 7Q10 relationship, flow was calculated at the model boundary inputs for the
Crooked Creek QUAL2K model (Table 1). Based on the tributary inflows and the two instream estimates
provided by USGS, a simple flow balance equation was used to estimate the amount of flow entering the
stream via diffuse baseflow (Figure 2).
Table 1. Estimated 7Q10 flow tabulated for boundary conditions of Crooked Creek.
Drainage Area ` Summer"7Ql0 Minter 7Q10
Boundary Condition (mi2) Flow(cfs) Flow(cfs)
Headwater 7.4 0.007 0.074
South Fork Crooked Creek (SF CC) tributary 18.4 0.018 0.184
Grassy Branch tributary 3.8 0.004 0.038
Diffuse Flow 1: Headwaters to Highway 601 N/A 0.011 0.113
Diffuse Flow 2: Highway 601 to NC Highway 218 N/A 0.004 0.035
Diffuse Flow 3: NC Highway 218 to Outlet N/A 0.006 0.059
Headwater Inflow
Summer 7010:0.007 cfs
Winter 7Q10.0.074 cfs
Diffuse Inflow 1
Summer 7010:0.011 cfs
SF CC Tributary Winter 7Q10:0.113 cfs
Summer 7Q10:0.018 cfs
Winter 7Q10:0.184 cfs
Highway 601 instream flow
Summer 7Q10:0.037 cfs
Winter 7Q10:0.371 cfs
Grassy Branch Tributary Diffuse Inflow 2
Summer 7Q10:0.004 cfs - Summer 7Q10:0.004 cfs
Winter 7Q10:0.038 cfs Winter 7Q10:0.035 cfs
NC Highway 218 instream flow
Summer 7010:0.044 cfs
Winter 7Q10:0.444 cfs
Diffuse Inflow 3
Outlet instream flow Summer 7410:0.006 cfs
Summer 7Q10:0.050 cfs Winter 7Q10:0.059 cfs
Winter 7Q10:0.503 cfs
Figure 2. Crooked Creek QUAL2K model 7Q10 flow balance schematic diagram.
TETRA TECH 3

Memorandum— Basis for Speculative Limits for Proposed LCCWRF April 27, 2021
2.1.2 Modified Seasonal Inputs
The summer and winter periods are identified (per 15A NCAC 02B .0404) in the existing permit as April 1
to October 31, and November 1 to March 31 respectively. The seasonal critical conditions simulation for
summer and winter involved the following key difference relative to the calibration model:
• Modification of simulation date based on warmest summer month or warmest winter month for
water temperature, and associated meteorological inputs modified based on new simulation date
• Modification of boundary conditions (headwaters and tributaries) based on:
o Flows to represent critical seasonal 7Q10 conditions instream
o Water temperature to represent critically warm summer or winter conditions
o Dissolved oxygen (DO) concentrations to represent median DO saturation observed
during critically warm summer or winter conditions
• Diffuse inflow conditions were parameterized identically to the headwater boundary conditions
All other model inputs were held constant from the calibration model for the summer critical conditions
simulation. For the winter critical condition simulation, stream shade conditions were decreased by half
from 70 percent to 35 percent relative to summer conditions to simulate the impact of assumed winter
leaf-fall.
The warmest summer water temperatures were found to occur in the month of July based on instream
water quality data sampling conducted by the Yadkin Pee Dee River Basin Association (YPDRBA) at four
sites along Crooked Creek. To parameterize the boundary conditions (headwater, diffuse flow, and
tributary inflow), a statistical analysis was conducted on observed instream data measured immediately
upstream of the Hemby Acres WWTP. This upstream location is the only water quality sampling site in the
basin which is not influenced by an upstream effluent discharge. The 75th percentile water temperature of
all measurements at this location (2014—2019) during the month of July was 25.0 °C. The median DO
saturation observed during all July measurements of both temperature and DO at this location was 58
percent. Applying 58 percent DO saturation to the water temperature of 25.0°C results in a boundary
condition DO concentration of 4.8 mg/L applied to the headwaters, diffuse, and tributary inflows.
QUAL2K requires assignment of a simulation date to support meteorological conditions. The 75th
percentile water temperature of 25.0°C is similar to the average water temperatures observed in July
2015, so the summer critical condition simulation date was selected as July 15, 2015. Meteorological
inputs for hourly air and dew point temperatures were pulled from this new simulation date from the same
gage as was used for the calibration and corroboration model setup (KNCUNION2 at Campobello Drive).
Average air and dew point temperatures on July 15, 2015 are 29.9 °C (85.8°F) and 19.3 °C (66.7°F)
respectively.
Critical winter conditions for water temperature were estimated for boundary conditions using the period
of record of instream YPDRBA water quality data. On average, the warmest winter water temperatures
occur in the month of November. Water temperature inputs for boundary conditions (headwaters,
tributaries, and diffuse inflow)were developed based on the 75th percentile of all observed water
temperature results in the period of record for the instream water quality sampling site located
immediately upstream of Hemby Acres WWTP. The result of this analysis is 13.4 °C, which was applied
to all winter critical condition boundary inputs. The median DO saturation observed during all November
measurements of both temperature and DO was 67 percent. Applying 67 percent DO saturation to the
water temperature of 13.4 °C results in a boundary condition DO concentration of 7.0 mg/L.
TETRA TECH 4

Memorandum — Basis for Speculative Limits for Proposed LCCWRF April 27, 2021
Based on the critical warm water temperature analysis the month of November, the simulation date was
selected to be the first of November. The simulate date was selected to be November 1, 2015 as the
summer critical condition was also chosen for the year 2015.
Meteorological inputs for hourly air and dew point temperatures were pulled from station KNCUNION2.
Average air and dew point temperatures on November 1, 2015 are 15.5°C (59.9 °F) and 13.8°C (56.9
°F) respectively.
2.2 PERMITTED DISCHARGE ASSUMPTIONS
There are three permitted wastewater treatment facility outfalls located along Crooked Creek modeled
explicitly: Hemby Acres WWTP which is operated by Carolina Water Services Inc., and Crooked Creek#2
(CC2) and Grassy Branch WWTPs which are both operated by Union County. For model application
scenarios, inputs were based on permitted effluent limits. Calibration model inputs were held constant for
non-permitted constituents (e.g. inorganic and organic phosphorus)for these simulations.
Existing permit limits for the three outfalls along Crooked Creek vary seasonally and by facility for
constituents of DO, 5-day biochemical oxygen demand (BOD5), total suspended solids (TSS) and
ammonia (NH3) (Table 2). Proposed permit limits associated with the new LCCWRF were approximated
first to be equal to those for CC2, and final proposed speculative permit limits are suggested based on
modeling results.
As in the calibration and corroboration model setup, TSS is simulated conservatively as inorganic
suspended solids since organic solids are captured through the simulation of BOD5 as ultimate labile
carbonaceous BOD (CBODfast).
For the seasonal simulations, the water temperature associated with each effluent outfall was developed
using the average observed July or November water temperature for 2015. Summer water temperature
inputs for Hemby Acres, CC2, and Grassy Branch were set to 25.9°C, 26.3°C, and 25.7°C respectively.
Winter water temperatures inputs for the three were set to 14.4 °C, 18.2 °C, and 15.9 °C respectively.
Water temperatures associated with LCCWRF were set identical to CC2.
Table 2. Existing point source permit limits for water treatment facilities along Crooked Creek.
NPDES ID Facility Season Flow BOD5 NH3 DO TSS
(MGD) (mg/L) (mg/L) (mgiL) (mg/L)
Hemby Summer 9 3
NC0035041 Acres 0.3 • 5 30
Winter 15 8
Crooked Summer 5 2
NC0069841 Creek#2 1.9 >_ 6 30
Winter 10 4
NC0085812 Grassy Summer 5 1
0.12 6 30
A Branch
Winter 10 2
A:note that from a recently approved SOC analysis,Grassy Branch VWVfP increased its flow from 0.05 MGD to 0.12 MGD.
Associated water chemistry limits are also based on final limits associated with the SOC.
Note that there is one other permitted discharge for groundwater remediation located near the
headwaters of the South Fork Crooked Creek. This permittee (NPDES ID NC0088838) for the Radiator
Specialty Company has a maximum permitted discharge limit of 0.09 MGD and monthly water quality
lb TETRA TECH 5

Memorandum—Basis for Speculative Limits for Proposed LCCWRF April 27, 2021
limits for the effluent are associated with TSS (30 mg/L), with additional daily maximum limits for a
number of pollutants such as tetrachloroethene, vinyl chloride, and dioxane. Although this discharge is
located far upstream along the South Fork Crooked Creek, the point source was included explicitly in the
model at the outlet of South Fork Crooked Creek into the mainstem at permit limits for flow and TSS.
Model parameterization for temperature and DO were set equal to those of the South Fork Crooked
Creek tributary.
3.0 MODEL SCENARIO AND RESULTS
Per 15 NCAC 02B.0404, model scenarios were chosen to represent seasonal differences (summer and
winter)for the two different flow conditions: 4.6 MGD for the near-term, and an expanded flow of 8.2 MGD
in the future with CC2 and Grassy Branch facilities incorporated with LCCWRF (Table 3). This is a
commonly applied procedure for almost every municipal discharge permit for oxygen-demanding wastes.
This memo can be used to justify the County's written request for seasonal effluent limits required under
02B.0404. Note that part c of 02b.0204 states:
"For the purpose of determining seasonal effluent limitations, the year shall consist of a
summer and a winter discharge period. The summer period shall begin April 1 and
extend through October 31. The winter period shall begin November 1 and extend
through March 31. The summer oxygen-consuming wasteload allocation shall be
developed using the flow criteria specified in 15A NCAC 02B .0206. The winter oxygen-
consuming wasteload allocation shall not exceed two times the summer oxygen-
consuming wasteload limitations nor shall it be less restrictive than minimum treatment
requirements."
The summer and winter scenarios apply the flow estimates provided by the U.S. Geological Survey as
specified under 15 NCAC 02B.0206 (e) as required.
As per required WLA guidelines, all existing dischargers were modeling with inputs representing
permitted limits for flow and water chemistry to assess the assimilative capacity of Crooked Creek at the
most critical conditions. Two sets of winter limits are shown in Table 3 for the proposed LCCWRF
discharge; the first represents the current interpretation by North Carolina Division of Water Resources
(DWR) staff of 15 NCAC 02B.0206 (d)(1) with additional details found in the State's Zero Flow Policy
(Attachment A), and the second represents the recommended winter limits based on demonstration of
assimilative capacity and positive 7Q10 flow under winter conditions.
Table 3. Proposed speculative permit limits for the Lower Crooked Creek Water Reclamation Facility.
LCCWRF (Scenario#)Scenario Description BOD5 (mg/L) NH3(mg/L) DO (mg/L)
Flow(MGD)
(1) Summer 5 1.0 6.0
4.6 (2)Winter— DWR policy interpretation 5 2.0 6.0
(3)Winter—speculative modeled limit' 10 2.0 6.0
(4) Summer 5 1.0 6.0
8.2 (5)Winter— DWR policy interpretation 5 1.9 6.0
(6) Winter—speculative modeled limit' 10 1.9 6.0
' Requested speculative limit based on field calibrated and validated model application
TETRA TECH 6

Memorandum—Basis for Speculative Limits for Proposed LCCWRF April 27, 2021
DWR staff have indicated that 15 NCAC 2B.0206(d)(1) that a BOD5 effluent limit of 5 mg/L should also
apply in the winter as there is no explicit mention of winter limits for new or expanding discharges in the
policy (personal correspondence with David Hill, 2/9/2021 and confirmed by other DEQ staff during a
TEAMS call on 4/26/2021). It is recommended that DWR reconsider this interpretation with respect to
Union County's request for speculative limits in Crooked Creek for the following reasons.
1) The winter 7Q10 at the proposed discharge point is almost 0.5 cfs which is a magnitude of order
above the threshold for the distinguishing a positive 7Q10 flow. This is not an ephemeral drainage
channel, rather it is a named stream with approximately 45 square miles of drainage area at this
point. Use of winter conditions is directly allowed for under 15 NCAC 2B.0404 and routinely
applied by DWR to municipal discharges of oxygen-demanding wastes.
2) 15 NCAC 2B.0206(a) clearly states that:
"More complex modeling techniques may also be used to set effluent limitations directly based on
frequency and duration criteria published by the U.S. Environmental Protection Agency, available
free of charge at http://water.epa.gov/scitech/swguidance/standards/criteria/current/index.cfm and
incorporated by reference, including subsequent amendments and editions, and the Commission
or its designee has determined, on a case-by-case basis, that the techniques will protect the
designated uses of receiving waters."
In 2016, DWR staff directed Union County to develop a complex Monitoring and Modeling Plan
which was subsequently approved by the agency. DWR then approved the calibrated model in
2019, followed by approval of the calibrated model application analyses in 2019 for the Grassy
Branch SOC process and in 2020 for the Crooked Creek LCCWRF facility planning. In this case,
based on the direction of DWR, the County sponsored complex monitoring and modeling which
demonstrates that the receiving stream at and below the requested point of discharge could
assimilate up to 20 mg/L of BOD5 during the winter, more than twice the recommended level of
10 mg/L. The County has invested significant resources (hundreds of thousands of dollars and six
years of time) to respond to DEQ directives to evaluate assimilative capacity in Crooked Creek. If
DWR had planned to use it's current interpretation of 2B.0206, why did it not tell them in 2016 to
avoid the loss of planning time and significant cost of monitoring and modeling that the agency
directed the County to perform if it wanted to request speculative limits?
3) The agency's Zero Flow Policy that accompanies 2B.0206 does include seasonal limits for
existing discharges. If the lack of a seasonal limit for new and expanding discharges is intentional
on the part of DWR, we respectfully request that DWR reconsider that policy, at least for this case
and similar situations using 2B.0404 and the provision for complex modeling to inform decisions
as allowed under 2b.0206. There are significant cost implications for designing a facility to meet 5
mg/L of BOD5 in the winter. In this case, the winter 7Q10 flow is clearly not zero (almost a
magnitude of order above the cutoff)and a calibrated and validated model demonstrates that
there is more than enough assimilative capacity to support a winter limit of 10 mg/L with a
significant margin of safety.
3.1 INSTREAM DISSOLVED OXYGEN RESULTS
Results for both summer and winter seasonal scenarios indicate that there is assimilative capacity in
Crooked Creek for both 4.6 and 8.2 MGD flow discharge limits (Table 4, Figure 3,Figure 4). DO
9
concentrations are predicted to be low in the upper portion of Crooked Creek under the most critical
1b TETRA TECH 7
1

r—
Memorandum — Basis for Speculative Limits for Proposed LCCWRF April 27, 2021
seasonal conditions due to extreme low flow and physical channel configuration, however the proposed
LCCWRF outfall is far downstream of these locations and improves minimum instream DO conditions.
The minimum DO concentration downstream of the proposed LCCWRF outfall simulated for both 4.6 and
8.2 MGD flow limits is predicted to remain above the instream water quality standard (WQS) of 5.0 mg/L
I DO with additional margin of safety during summer and winter based on the speculative limits proposed in
Table 4.
Table 4. Crooked Creek QUAL2K model scenario results for summer and winter critical conditions.
Scenario Scenario Description DO minimum downstream of
LCCWRF(mg/L)
1 Summer critical conditions, LCCWRF at 4.6 MGD 5.8
Winter critical conditions, LCCWRF at 4.6 MGD—
2 6.2
DWR policy interpretation
3 Winter critical conditions, LCCWRF at 4.6 MGD— 6.2
speculative modeled limit
Summer critical conditions, LCCWRF at 8.2 MGD
4 5.5
with CC2 and Grassy Br. WWTPs incorporated
Winter critical conditions, LCCWRF at 8.2 MGD
5 with CC2 and Grassy Br. WWTPs incorporated— 5.7
DWR policy interpretation
Winter critical conditions, LCCWRF at 8.2 MGD
6 with CC2 and Grassy Br. WWTPs incorporated— 5.7
speculative modeled limit
10
Grassy
Hemby CC#2 SFCC Trib
WWII)WWII) Beaver
9
WWTP
Trib Dams Grassy LCCWRF
WWII 8
iv 1 1 1 l�
6 E
f
- - 5
0
4 a°,
1 3
2
L.,../..--- „ 1
0
30 25 20 15 10 5 0
Distance from outlet(km)
----WQS 5.0 mg/I -Summer Critical,LCCWRF @4.6MGD
-Winter Critical,LCC 4.6 MGD(DWR policy interpretation) Winter Critical,LCC 4.6 MGD(modeled speculative limits)
TETRA TECH 8

Memorandum— Basis for Speculative Limits for Proposed LCCWRF April 27, 2021
Figure 3. Crooked Creek QUAL2K model results for the three 4.6 MGD flow condition scenarios.
Grassy 10
Hemby CC#2 SFCC Trib
WWTP WWTP Beaver LCCWRF 9
Trib Dams Grassy
WWTP 8
t • 7
6 m
E
5 0
1 c
4 m
I 3
2
30 25 20 15 10 5 0
Distance from outlet(km)
----WQS 5.0 mg/I
—Summer Critical,LCC 8.2 MGD,no GB,no CC2
—Winter Critical,LCC 8.2 MGD,no GB,no CC2(DWR policy interpretation)
-- --Winter Critical,LCC8.2 MGD,no GB,no CC2(modeled speculative limits)
Figure 4. Crooked Creek QUAL2K model results for the three 8.2 MGD flow condition scenarios.
When effluent flows dominate instream conditions, there can be a concern for aquatic life relative to
ammonia toxicity. For low-flow streams, DWR has set forth a policy that ammonia toxicity is defined as
instream concentrations from ammonia exceeding 1.0 mg/L in summer, and 1.8 mg/L in winter. For all
model application scenarios, ammonia toxicity guidelines are not exceeded instream (Table 5).
Table 5. Simulated instream maximum ammonia concentration downstream of LCCWRF.
Scenario Scenario Description NH3 maximum downstream
. . of LCCWRF(mg/L)
1 Summer critical conditions, LCCWRF at 4.6 MGD 0.68
2 Winter critical conditions, LCCWRF at 4.6 MGD—DWR 1 29
policy interpretation
Winter critical conditions, LCCWRF at 4.6 MGD—
3 speculative modeled limit 1.29
Summer critical conditions, LCCWRF at 8.2 MGD with
4 CC2 and Grassy Br. WWTPs incorporated 0.95
Winter critical conditions, LCCWRF at 8.2 MGD with CC2
5 and Grassy Br. WWTPs incorporated—DWR policy 1.74
interpretation
Winter critical conditions, LCCWRF at 8.2 MGD with CC2
6 and Grassy Br. WWTPs incorporated—speculative 1.74
modeled limit
(Th.
T TETRA TECH 9

Memorandum-Basis for Speculative Limits for Proposed LCCWRF April 27, 2021
3.2 RESULTS AT CONFLUENCE WITH ROCKY RIVER
Model predictions of water quality parameters related to oxygen-demanding pollutants and nutrients at
the confluence with the Rocky River(i.e., mouth of Crooked Creek) are provided in Table 6 (interim flow
conditions), and 7 (future flow conditions).
Table 6. Crooked Creek QUAL2K model results for all scenarios at the confluence with Rocky River:
interim 4.6 MGD flow limit results.
Existing Critical Interim Critical Conditions(LCCWRF 4.6 MGD)
QUA
L2K Model Conditions
Outputs at Outlet Summer Winter(Scenario 2)- Winter(Scenario 3)-
Summer Winter (Scenario 1) DWR policy interpretation modeled speculative limit
Flow (m3/s) 0.11 0.12 0.31 0.32 0.32
Temperature(°C) 25 16 25 16 16
DO (mg/L) 6.0 7.2 6.5 7.5 7.3
CBODu (mg/L) 9.05 4.28 5.58 5.55 9.77
NH4(pg/L) 63 32 242 567 569
Organic N (pg/L) 242 197 219 327 327
NO3(pg/L) 2,924 8,260 5,178 19,527 19,467
Organic P (pg/L) 20 27 139 374 374
Inorganic P (pg/L) 219 263 2,092 1,906 1,906
Table 7. Crooked Creek QUAL2K model results for all scenarios at the confluence with Rocky River:
future 8.2 MGD flow limit results.
Existing Critical Future Critical Conditions(LCCWRF 8.2 MGD,CC2&Grassy Br.
QUAL2K Model Conditions WWTPs incorporated)
Outputs at Outlet Summer Winter(Scenario 5)- Winter(Scenario 6)
Summer Winter (Scenario
4) DWR policy interpretation modeled speculative limit
Flow (m3/s) 0.11 0.12 0.38 0.39 0.39
Temperature (°C) 25 16 26 16 16
DO (mg/L) 6.0 7.2 6.4 7.3 6.9
CBODu (mg/L) 9.05 4.28 5.18 6.70 12.90
NH4 (pg/L) 63 32 367 856 860
Organic N (pg/L) 242 197 238 406 406
NO3 (pg/L) 2,924 8,260 20,305 23,304 23,182
Organic P (pg/L) 20 27 213 549 549
Inorganic P (pg/L) 219 263 2,979 2,664 2,664
Th TETRA TECH 10

Memorandum— Basis for Speculative Limits for Proposed LCCWRF April 27, 2021
4.0 CONCLUSION
Based on the application of the approved calibrated QUAL2K model, Crooked Creek has assimilative
capacity for the proposed new facility. The recommended speculative limits including seasonal variation
of BOD5 and ammonia for the winter(Table 3) demonstrate that the DO standard can be maintained
downstream including allowing for a margin of safety (Table 4). It is recommended that the County
request in writing for speculative limits that include seasonal variation for BOD and ammonia using this
memo as justification for limits that are expected to meet all water quality standards with a substantial
margin of safety provided.
5.0,REFERENCES.-
Tetra Tech. 2019a. Crooked Creek QUAL2K Model Development; Union County, North Carolina.
Prepared for Union County Public Works, Monroe, NC.
Tetra Tech. 2019b. Crooked Creek QUAL2K Model Application for Grassy Branch WWTP. Prepared for
Union County Public Works, Monroe, NC.
USGS, September 2019 via email correspondence with Curtis Weaver, South Atlantic Water Science
Center, Raleigh, NC.
'(� TETRA TECH 11

Memorandum— Basis for Speculative Limits for Proposed LCCWRF April 27, 2021
ATTACHMENT A - NORTH CAROLINA DIVISION OF WATER
RESOURCES ZERO FLOW POLICY (5/12/1999)
The following NPDES permitting policies apply to discharges to zero-flow streams:
• New and Expanding Discharge to Zero Flow Streams (both 7Q10 and 30Q2 = 0). Regulation 2B
.0206 disallows new discharges of oxygen consuming wastewater to streams which have no flow
under both 7Q10 and 30Q2 conditions.
• New and Expanding Discharge to Zero Flow Streams (7Q10= 0; 30Q2 >0). Regulation 2B .0206
sets effluent limitations at BOD5 = 5 mg/L, NH3-N= 2 mg/L, and DO = 6 mg/L to streams with no
7Q10 flow, but positive 30Q2 flow, unless it is determined that these limits will not protect water
quality standards.
• Existing Dischargers. Regulation 2B .0206 also states that existing dischargers to zero-flow
streams should be handled on a case-by-case basis. For many years, permits for the existing
discharges to so-called "zero-flow streams" did not contain any special requirements. However,
since 1990, three different requirements have been somewhat randomly placed in NPDES
permits. These requirements were:
1. Require an alternatives analysis be done and give summer limits of 5 mg/L (BOD5) and 1
mg/I (NH3-N) and winter limits of 10 mg/L (BOD5) and 1.8 mg/L (NH3-N) after a three-year
period, if the discharge continued.
2. Require an alternatives analysis to meet the above limits, but renew the permit without a
change in limits (i.e., no 3-year compliance schedule to meet limits) during that permit cycle.
3. Require an alternatives analysis to meet summer limits of 5 mg/L (BOD5) and 2 mg/L (NH3-
N) and winter limits of 10 mg/L (BOD5) and 4 mg/L (NH3-N)without a change in limits during
that permit cycle.
If alternative#1 was implemented in a previous permit, then these limits should be maintained with
the exception of ammonia limits which should be changed to reflect current BAT for facilities with
permitted flows of less than 1.0 MGD (2 mg/L and 4 mg/L in the summer and winter, respectively).
An alternative analysis is not necessary for these discharges.
If a facility has never been required to submit an alternatives analysis, then the permit should be
renewed with existing limits and a requirement to perform an alternatives analysis during the next
permit cycle. The permit holder will be required to send in the alternatives analysis six months prior
to the expiration date and should be notified of this requirement in the cover letter to permit
issuance.
If a facility has been required to submit an alternatives analysis, but has not done so or has
concluded that discharge is the only option, ammonia limits of 2 mg/L (summer) and 4 mg/L (winter)
should be given with a compliance schedule (when deemed necessary by regional office staff). This
schedule should not exceed three years. In lieu of these ammonia limits, the facility may choose to
sample for toxicity using a chronic WET test, pass/fail at 90% using Ceriodaphnia as the test
organism. Existing BOD5 limits should be maintained unless instream data indicates a potential
violation of the dissolved oxygen water quality standard.
If the facility does not discharge into a zero-flow stream, ammonia limitations should be checked
against the Division's Ammonia Toxicity Policy.
it TETRA TECH 12