NC0085812_Crooked Creek Model_20201117

INTEROFFICE MEMORANDUM
TO: DWR NPDES Permitting Branch
FROM: DWR Modeling and Assessment Branch
SUBJECT: Review of Crooked Creek QUAL2K Model Application for Grassy Branch WWTP
DATE: November 16, 2020
CC:
The NC Division of Water Resources (DWR) Modeling and Assessment Branch has completed
review of the report "Crooked Creek Model Application for Grassy Branch Wastewater
Treatment Plant" produced by Tetra Tech for Union County.
In general, the report is clearly written and consistent with findings in the modeling files. One
exception is a typo in the text, on page 6: "...permit limits associated with ammonia change
from existing permit limits of 2 and 4 mg/I for summer and winter to 1 and 3 mg/I respectively",
whereas the actual new ammonia permit limit for the winter is 2 mg/I (not 3 mg/1). This winter
limit is correctly shown in Table 3 of the report though. For the purpose of aiding in the NPDES
Branch's permit writing process, below are the permit limits used in the scenario analysis:
Interim SOC limits:
Flow = 0.12 MGD
BODS = 30 mg/I
NH3 = 6 mg/I (summer) and 20 mg/I (winter)
DO = 5 mg/I
TSS = 100 mg/I
Final limits:
Flow = 0.12 MGD
BODS = 5 mg/I (summer) and 10 mg/I (winter)
NH3 = 1 mg/I (summer) and 2 mg/I (winter)
DO=6mg/I
TSS = 30 mg/I
The results of the scenario analysis indicate that the minimum DO concentration of Crooked
Creek between the outfall of Grassy Branch WWTP and the Rocky River confluence would stay
above the water quality standard of 5 mg/I under the interim and final discharge limits. In
addition to the DO impact analysis, Tetra Tech assessed the TSS discharge effects on the stream
TSS/turbidity. In our last meeting with Tetra Tech on October 1, 2019, we asked them to make

two additional assessments: (1) per NPDES branch's request, add NC0088838 (Radiator
Specialty Company) to the model as one of the dischargers and (2) investigate the sensitivity of
predicted stream DO to the assumed bottom algae conditions. Tetra Tech performed these
additional analyses and summarized their findings in the report.
During our internal meeting with the NPDES team on October 16, 2020, a question was raised
on why the discharge to a (near) zero -flow stream appeared to have only small impacts on the
stream DO. It may help to note that the simulated streamflow just upstream of the Grassy
Branch WWTP outfall is 3.6 cfs in the reported scenario analysis, which is very close to that in
the calibrated model (3.7 cfs). Scenario analyses are typically conducted under the "seasonally
critical conditions and maximum permitted discharges", and here happened to result in similar
total flows but with different sources (99% of 3.6 cfs comes from point sources in the scenario
analysis; 39% of 3.7 cfs in the calibrated model). Higher point source composition in the
scenario analysis means much more oxygen consuming materials are discharged into North
Fork Crooked Creek even if the flows are similar. However, the movement of water slows down
considerably around the beaver dams area above the Grassy Branch WWTP outfall, depleting
most of the oxygen consuming materials (and severely lowering stream DO in that area). Water
movement speeds up afterwards, and the predicted BOD concentrations do not change much
past the Grassy Branch WWTP until the stream travels about 6.7 miles and meets Rocky River.
The Modeling and Assessment Branch conducted an additional analysis to address the potential
impact of Grassy Branch WWTP expansion on Rocky River DO. The model used for this analysis
is a QUAL2K model for Rocky River, originally developed by Brown and Caldwell for Charlotte
Water to simulate the expansion of Mallard Creek WRF ("Mallard Creek Water Quality
Modeling - Update and Sensitivity Analysis"). The Modeling Branch slightly refined the model by
updating permit limits and discharge locations for the WWTPs included in the model. Potential
impacts from Grassy Branch WWTP expansion were considered by taking the flows and water
quality parameter concentrations predicted by the Crooked Creek QUAL2K model at the
downstream end of the model (Rocky River confluence) and entering them to the Rocky River
QUAL2K model as tributary inputs. The simulation results indicate that the proposed expansion
of Grassy Branch WWTP is not likely to cause a violation of dissolved oxygen standards in Rocky
River (Figure 1).

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9
8
J
E
0
6
5
4
Rocky River QUAL2K Model Predictions for Proposed Expansion
of Grassy Branch WWTP
(Summertime)
—Grassy Branch WWTP Interim Limits
3
—Grassy Branch WWTP Final Limits
� —DO Standard
0
� a F
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}
6 V G
u c m v
¢ d
80 70 60 50 40 30 20 10 0
River Mile
Rocky River QUAL2K Model Predictions for Proposed Expansion
of Grassy Branch WWTP
(Wintertime)
—Grassy Branch WWTP Interim Limits
—Grassy Branch WWTP Final Limits
—DO Standard
v d H
v
V �
c�
80 70 60 50 40 30 20 10 0
River Mile
Figure 1. Rocky River dissolved oxygen simulation results for summer (top) and winter (bottom).
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