NC0085812_Crooked Creek Model_20200527 (2)

Strickland, Bev
From: Sadler, Mary <[e-mail]>
Sent: Wednesday, May 27, 2020 12:17 PM
To: Behm, Pamela
Cc: Andy Neff ([e-mail]); Scheller, Roberto
Subject: [External] Qua12K Modeling for Crooked Creek, Union County
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Hi Pam,
I hope you and your family are doing well during this difficult time! It's been a few months since I have had the
opportunity to talk with you.
We have a received a request from the Mooresville Regional Office regarding the QUAL2K modeling in Crooked Creek to
support Union County's Special Order of Consent (SOC) application for the Grassy Branch WRF. To refresh your memory
a bit, Tetra Tech dusted off the baseline wasteload allocation model for Crook Creek last summer and added a separate
analysis to address the Grassy Branch WRF increase in flow. We met with you and your staff in early October 2019 to
discuss the model, and Tetra Tech addressed all of your comments in subsequent technical documentation. The SOC
package for Grassy Branch contains the updated/revised QUAL2K documentation.
At the request of the Mooresville office, can you please write a letter of approval for use of the Crooked Creek QUAL2K
model to determine the impact to Crooked Creek for the County's SOC request?
Thank you! Please let me know if you have any questions.
Mary

INTEROFFICE MEMORANDUM
TO: DWR Complex Permitting Branch
FROM: DWR Modeling and Assessment Branch
SUBJECT: Review of Crooked Creek QUAL2K Model Calibration and Corroboration
DATE: September XX, 2019
CC:
The Division of Water Resources (DWR) Modeling and Assessment Branch has completed
review of the Crooked Creek QUAL2K modeling study. Union County Public Works is currently in
the process of submitting an SOC application for the Grassy Branch WWTP located 6.7 miles
upstream of Crooked Creek's confluence with Rocky River (Figure 1). Also, a new wastewater
treatment facility that would discharge to Crooked Creek in the vicinity of Highway 601 was
proposed before, although no official application has been submitted as of yet. Data collected
from three field trips in the summer of 2016 were used for the calibration and corroboration of
the model, along with other sources (DMR, AMS, etc.). The work is generally of high quality.
Most of the parameter choices are backed up by the data or otherwise justified. No
inconsistency was found between the report and model contents.
The reported simulation results suggest, and are supported by field measurements, that
dissolved oxygen may be under severe depletion around and upstream of Beaver Dams area
(Figure 1). It appears safe to assume that there is no or little assimilative capacity left from the
headwaters of North Fork Crooked Creek (upstream of the South Fork confluence) down to the
Highway 601 crossing. Dissolved oxygen in Crooked Creek starts to recover from the Highway
601 area and becomes around 6 mg/L when discharged to Rocky River. Water quality conditior
at the end of Crooked Creek, and its potential impact downstream of Rocky River confluence,
will need to be carefully considered in the subsequent scenario analyses.
Again, these simulations are based on surveys made in the summer of 2016. Critical condition
was not yet assessed in this work, and no model run was made under the permitted (as
opposed to actual) discharges. Simulated flow at the Crooked Creek -Rocky River confluence
was about 4 cfs. Subtracting point source contributions, the natural flow might have been
around 2.5 cfs at the time of field surveys. It represents a low -flow condition, though still higher
than the critical 7Q10 condition (about 0.2 cfs, or as low as 0 cfs, as estimated by USGS).
Also as a reminder, the Complex Permitting Branch asked to include NCO088838 (Radiator
Specialty Company) in the model, a groundwater remediation system permitted at 0.09 MGD
discharging to an unnamed tributary of South Fork Crooked Creek.

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Crooked Creek QUALM Dissolved Oxygen Simulations
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—Calibration Run (8/24/2016)
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—Corroboration Run (9/14/2016)
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Figure 1. Crooked Creek dissolved oxygen simulation results. Wastewater discharges used in the
simulations were obtained from DMR data: Hemby Acres WWTP at 0.09 MGD (permitted at 0.3
MGD); Crooked Creek #2 WWTP at 0.83 MGD (calibration run) or 0.87 MGD (corroboration run)
(permitted at 1.9 MGD); Grassy Branch WWTP at 0.04 MGD (permitted at 0.05 MGD).
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