NC0069841_Memo_Review of Rocky River Model Senario Application_20240729

INTEROFFICE MEMORANDUM

TO: NCDWR NPDES Permitting Branch
FROM: NCDWR Modeling and Assessment Branch
SUBJECT: MAB Review of Rocky River Model Scenario Application
DATE: July 29, 2024

At the request of the NPDES Permitting Branch, the Modeling and Assessment Branch (MAB) staff reviewed the modeling report and associated files submitted to support evaluation of Rocky
River Regional WWTP (RRRWWTP) expansion. The modeling package submitted by Tetra Tech on behalf of the Water and Sewer Authority of Cabarrus County (WSACC) was initially received by
the MAB staff on April 24, 2024. Over the course of the review, numerous communications were made between the staff and Tetra Tech, which resulted in several rounds of revision and
resubmission of the work:

During the review of the initial submission, it was noted that the 7Q10 low flows for boundary conditions used in the Rocky River QUAL2K model application were different from the flow
values that had been approved by the NCDWR prior to the start of the critical condition analysis. A revised modeling package addressing this issue was submitted by Tetra Tech on June
7, 2024, along with the supplementary information detailing the flow adjustment steps to ensure the reproducibility of the work.

An inquiry was made to Tetra Tech regarding some headwater/tributary flows in the modeling files (for Long Creek headwater, Little Long Creek tributary, Little Bear Creek tributary,
and Big Bear Creek tributary) that were still seemingly inconsistent with those in the revised report. In response, Tetra Tech submitted the updated modeling report on July 8, 2024,
expanded with clarifying text and a new table.

These updates helped enhance the consistency and reproducibility of the work. The adjustment of the 7Q10 flows applied in this analysis is a two-step process of: (1) estimating the unweighted
interim flows based on the unit drainage area approach and (2) adjusting them to obtain weighted interim flows using the USGS gages as anchor points for water balancing. For future
applications of the model, details of individual flow adjustments can be best included in the supplementary information submitted by Tetra Tech (“RockyRiver_7Q10_Comparison_for_Review_V5.xlsx”).

The model was run by the staff and verified that the reported outputs can be reproduced. Predicted DO downstream of the RRRWWTP was above the water quality standards under the current
and expanded discharge scenarios. The table below lists the flow, CBOD5, NH3, and DO values used in the scenario analysis:

Parameter
Current (ATC)
Intermediate
Final

Flow
34 MGD
40 MGD
50 MGD

Summer CBOD5
10 mg/L
10 mg/L
10 mg/L

Winter CBOD5
20 mg/L
20 mg/L
20 mg/L

Summer NH3
1.6 mg/L
1.5 mg/L
1.5 mg/L

Winter NH3
3.5 mg/L
3.4 mg/L
3.2 mg/L

DO
6 mg/L
6 mg/L
6 mg/L


As it has been noted before, Rocky River is under high development pressure. A number of new and expanding discharge requests (including for the new Crooked Creek WWTP) are anticipated
that are likely to be evaluated using the Rocky River QUAL2K model. Although this analysis indicated that the proposed expansion of the RRRWWTP would not cause violation of instream
dissolved oxygen standards, it would be advisable to take these results with a grain of salt. The Rocky River monitoring and calibration studies, which led to the development of the
model, showed that Rocky River is a highly complex system that may experience periodic low DO conditions susceptible to sporadic events like stormwater slugs moving through the system.
Recognizing that a one-dimensional steady-state model like QUAL2K has limitations in evaluating complex issues, future applications of the model should consider uncertainties and give
high priorities to maintaining assimilative capacity of Rocky River.

Below are some questions for the NPDES Permitting Branch that were raised during the review:

It appears that the submission of the modeling report was the first time NPDES Permitting Branch had seen that WSACC wanted an expansion of RRRWWTP to 50 MGD. Would flow justification
be needed for this request?

Can NPDES Permitting Branch review and verify the site-specific NH3 criteria used in the report (1.2 mg/L in summer and 2.3 mg/L in winter)? According to the report, they are based on
the NCDWR’s calculation of site-specific NH3 criteria for RRRWWTP in August 2021 (citing NPDES fact sheet dated 9-29-2021).