NC0069841_Basis for Speculative Limits Tech Memo_20201208

TETRA TECH One Park Drive,suite 200•PO Box 14409
Research Triangle Park, NC 27709
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MEMORANDUM
To: Aubrey Lofton, Union County Date: December 8, 2020
Andrew Neff, Union County
Cc: Kent Lackey, Black&Veatch Subject: Basis for Speculative Limits for
John Brinkley, Black&Veatch 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).
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Memorandum— Basis for Speculative Limits for Proposed LCCWRF December 8, 2020
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TETRA TECH oKilometers
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Figure 1. Crooked Creek Watershed with Existing and Proposed Facility Outfall Locations.
2.0 QUAL2K MODELAPPLICATION
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.
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Memorandum- Basis for Speculative Limits for Proposed LCCWRF December 8, 2020
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.
0
mmw
-M. . 1W * I
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 7Q10:0.007 cfs
Winter 7Q10:0.074 cfs
Diffuse Inflow 1
Summer 7Q10:0.011 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 10 Winter 7Q10:0.035 cfs
NC Highway 218 instream flow
Summer 7Q10:0.044 cfs
Winter 7Q10:0.444 cfs
Diffuse Inflow 3
Outlet instream flow Summer 7Q10:0.006 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.
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Memorandum— Basis for Speculative Limits for Proposed LCCWRF December 8, 2020
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/I applied to the headwaters, diffuse, and tributary inflows.
QUAL2K requires assignment of a simulation date to support meteorological conditions. The 751h
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 OF) and 19.3 °C (66.7 OF)
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 751h 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/I.
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Memorandum— Basis for Speculative Limits for Proposed LCCWRF December 8, 2020
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 IF) and 13.8 °C (56.9
OF) 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 • TSS
Hemby Summer 9 3
NC0035041 0.3 >_ 5 30
Acres
Winter 15 8
Crooked Summer 5 2
NCO069841 Creek 1 1.9 >_ 6 30
#2 Winter 10 4
A Grassy Summer 5 1
NC0085812 Branch 0.12 >_ 6 30
Winter 10 2
A:note that from a recently approved SOC analysis,Grassy Branch WWTP 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
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Memorandum— Basis for Speculative Limits for Proposed LCCWRF December 8, 2020
limits for the effluent are associated with TSS (30 mg/1), 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
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). As per required WLA guidelines, all
permitted or proposed dischargers were set to permit limits for flow and water chemistry to assess the
assimilative capacity of Crooked Creek at the most critical conditions.
Table 3. Proposed speculative permit limits for the Lower Crooked Creek Water Reclamation Facility.
LCCWRF Flow(MGD) Season BOD5 NH3 D•
4.6 Summer(1) 10 1.0 6.0
Winter(2) 20 2.0 6.0
8.2 Summer(3) 6 1.0 6.0
Winter(4) 12 1.9 6.0
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
concentrations are predicted to be low in the upper portion of Crooked Creek under the most critical
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/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 scenarios results for summer and winter critical conditions.
Scenario Scenario Description • minimum downstream of
LCCWRF (mg/1)
1 Summer critical conditions, LCCWRF at 4.6 MGD 5.5
2 Winter critical conditions, LCCWRF at 4.6 MGD 5.9
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Memorandum— Basis for Speculative Limits for Proposed LCCWRF December 8, 2020
Scenario Scenario Description • minimum downstream
LCCWRF (mg/1)
Summer critical conditions, LCCWRF at 8.2 MGD
3 with CC2 and Grassy Br. WWTPs incorporated 5.4
Winter critical conditions, LCCWRF at 8.2 MGD
4 with CC2 and Grassy Br. WWTPs incorporated 5.6
10
Grassy
Hemby CC#2
SFCC Beaver Trib 9
WWTP WWTP Trib Dams Grassy LCCWRF
1 1 WWTP 1 8
6 °D
E
---------------- ----------------------- 5---------------- ------------------------------ o
4 c
v
3
2
1
0
30 25 20 15 10 5 0
Distance from outlet(km)
---- WQS 5.0 mg/I -Summer Critical,LCCWRF @4.6MGD -Summer Critical,LCCWRF @8.2MGD,CC2&GB incorporated
Figure 3. Crooked Creek QUAL2K model scenario results for the two summer flow condition scenarios.
10
Grassy
Hemby CC#2 SFCC Beaver Trib 9
WWTP WWTP Trib Grassy LCCWRF
Dams WWTP 1 8
1 1 11 7m
6 E
- -------------------------------------------------- ----------- ----------------------- 5 00
0
4 m
v
3
2
1
0
30 25 20 15 10 5 0
Distance from outlet(km)
----WQS 5.0 mg/I -Winter Critical,LCCWRF @4.6MGD -Winter Critical,LCCWRF @8.2MGD,CC2&GB incorporated
Figure 4. Crooked Creek QUAL2K model scenario results for the two winter flow condition scenarios.
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Memorandum— Basis for Speculative Limits for Proposed LCCWRF December 8, 2020
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/I in summer, and 1.8 mg/I 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.
downstream-1 IF- NH3 MaXiMUM
Scenario Scenario Description
1 Summer critical conditions, LCCWRF at 4.6 MGD 0.68
2 Winter critical conditions, LCCWRF at 4.6 MGD 1.29
Summer critical conditions, LCCWRF at 8.2 MGD with CC2
3 0.95
and Grassy Branch WWTPs incorporated
Winter critical conditions, LCCWRF at 8.2 MGD with CC2
4 and Grassy Branch WWTPs incorporated 1.74
4.0 CONCLUSION '
Based on the application of the approved calibrated QUAL2K model, Crooked Creek has assimilative
capacity for the new facility. The recommended speculative limits (Table 3) demonstrate that the DO
standard can be maintained downstream including allowing for a margin of safety (Table 4).
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Memorandum— Basis for Speculative Limits for Proposed LCCWRF December 8, 2020
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.
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