NC0089630_Modification_20240312

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ROY COOPER
Governor
ELIZABETH S. BISER
Secretary
RICHARD E. ROGERS, JR.
Director
NORTH CAROLINA
Environmental Quality
March 12, 2024
Ms. Nicole Bartlett P.E., Senior Project Manager
Charlotte Water
5100 Brookshire Blvd.
Charlotte, NC 28216
Subject: Review of Catawba River DO Modeling Report
Stowe Regional WRRF
NPDES Permit No. NCO089630
Mecklenburg County
Catawba River Basin
Dear Ms. Bartlett:
The Division of Water Resources (DWR) Modeling and Assessment Branch reviewed the modeling files and
report submitted on December 8, 2023 by HDR, Inc on behalf of Charlotte Water. The purpose of this
modeling project was to evaluate the impact of changing the dissolved oxygen (DO) limit for Phase 11 of
Stowe Regional WWRF change from 7 mg/L to 6 mg/L. Although the original modeling plan was to use a
steady-state model QUAL2K, flow releases from the Mountain Island Dam approximately 4 miles upstream
of the discharge location were observed to be causing large diel fluctuations in streamflow. To account for the
hydrology of study area, the reported assessment was made using a dynamic version of the model
QUAL2KW.
The submitted calibration and scenario runs were reproduced and it was verified that they were consistent
with the modeling report. Model inputs for the scenario runs were examined to verify that the only inputs that
were changed were permitted DO concentration limits of the Stowe Regional WRRF from 7 mg/L to 6 mg/L.
Simulated stream DO was decreased to a varying degree as the result of changing the DO limit, and the
maximum DO decrease was 0.11 mg/L as reported. All simulated hourly stream DO values were greater than
4 mg/L with daily averages above 5 mg/L. The impact of DO limit change was mostly limited to the
immediate discharge location (reach 8). These findings appear reasonable and in support of the proposed
effluent DO limit change.
As a cautionary note, the submitted QUAL2KW model has several shortcomings and limitations that prevent
its application beyond this study's goal (testing the impact of effluent DO change for the Stowe WRRF):
• The river sampling data collected for this study indicate that vertical temperature and DO
stratification may start developing near downstream of the proposed Stowe WRRF outfall location
during the summer periods, likely due to the backwater effects from Lake Wylie. The observed
stratification became more pronounced in the downstream portions of the modeling domain. The
QUAL2KW is a one-dimensional model and limited in its ability to represent vertically stratified
waters.
0 Surface water samples collected during the summer periods in the downstream areas show signs of
DO supersaturation, suggesting elevated algal activities, large diel DO swings and significant
sediment oxygen demand. Currently only factors affecting dissolved oxygen in the model are
North Carolina Department of Environmental Quality I Division of Water Resources
512 North Salisbury Street 1 1617 Mail Service Center I Raleigh, North Carolina 27699-1617
NORTH CARCLINA
919.707.9000

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biological oxygen demand (BOD) degradation, nitrification and reaeration without algal processes
and sediment oxygen demand.
Although hourly values of temperature and water quality variables are specified for the headwater,
meteorological inputs (e.g., atmospheric temperature) have no diel variation. As a result, diel
variation in simulated DO diminishes as the water moves downstream, and the model does not appear
to be doing well in reproducing the observed pattern of DO in the downstream portion of the modeled
domain.
Despite these shortcomings, the submitted model appears appropriate for assessing the impact of DO limit
change for the Stowe WRRF, considering that the impact is mostly limited to the immediate discharge
location in the relatively upstream area. The modeling results appear to support the proposed change in the
Stowe WRRF effluent DO limit from 7 mg/L to 6 mg/L at the discharge flow of 25 MGD. The modeling
results should be interpreted with care, considering the limitations outlined earlier. It is essential not to extend
their use beyond evaluating the impact of effluent dissolved oxygen (DO) changes for the Stowe WRRF.
To proceed with the NPDES permitting process, Charlotte Water can choose to either submit a modification
request package and corresponding fee at this time to the Municipal Permitting Branch OR include the
modification request in the permit renewal package and avoid a second fee.
Please let us know if you have any questions regarding these comments and recommendations and we will
schedule a meeting to discuss.
Respectfully,
AD,oc'ulSigne'd byy:� A .' I A
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Michael Montebello
NPDES Program Branch Chief
ec: NPDES Files [Laserfiche]
Charlotte Water / Nicole Bartlett [[e-mail]]
HDR Inc. / Barry Shearin [[e-mail]]
DWR / Pam Behm [[e-mail]]; Kristin Litzenberger [[e-mail]]; Nick Coco
[[e-mail]]; Bongghi Hong [[e-mail]]; Adugna Kebede [[e-mail]]; Derek
Denard [[e-mail]]
Mooresville Regional Office / Andrew Pitner [andrew.pitner@ deq.nc.gov]
REQ5 North Carolina Department ofEnvironmental Quality I Division ofWater Resources
512 North Salisbury Street 1 1617 Mail Service Center I Raleigh, North Carolina 27699-1611
NORTH CAROLINA 919.707.9000
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