NC0023906_Model Review Comments_20221116

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ROY COOPER
Governor
ELIZABETH S. BISER
Secretary
RICHARD E. ROGERS, JR. NORTH CAROLINA
Director Environmental Quality
November 16, 2022
Mr. Jimmy Pridgen, Water Reclamation Manager
City of Wilson
P.O. Box 10
Wilson, NC 27894-0010
Subject: DRAFT Water Quality Modeling Study Plan Review
Hominy Creek WRF
NPDES Permit No. NC0023906
Wilson County
Neuse River Basin
Dear Mr. Pridgen:
The Division of Water Resources (DWR) Modeling and Assessment Branch (MAB) reviewed the modeling
report "DRAFT Water Quality Modeling Study Plan" submitted to the NPDES Permitting Branch regarding
discharge to Contentnea Creek. The goal of the modeling analysis is to determine if the proposed expanded
wastewater treatment plant (WWTP) discharge to Contentnea Creek would cause reductions in dissolved oxygen
(DO) levels in the creek during critical conditions (e.g., warm weather, low flow, low DO) that will not exceed
0.10 mg/1. Our comments and suggestions, summarized below, focus on the proposed model for this modeling
study plan:
• The modeling plan proposes the QUAL2K model for evaluating the proposed expansion. QUAL2K
model is a steady state and one-dimensional model used for river and stream water quality modeling. The
spatial resolution is based on river reaches defined to have constant physical characteristics which
assumes vertically and laterally well -mixed channel. The model is assumed to operate at steady state
condition and the computational time step is based on the time it takes flow to move through a segment.
• This QUAL2K (and its predecessor QUAL2E) model has been applied to medium-sized rivers with small
width to depth ratios to track the fate and transport of targeted pollutants. The model has been widely
used for evaluation of wasteload allocation and speculative limit analysis in North Carolina. The model is
used for decision making where the riverine systems are one-dimensional and where the steady state
condition prevails. It should be noted that QUAL2K has been used for preliminary/screening level
analysis even when the one-dimensional and steady state assumption do not hold to get a general idea
how these systems behave. But the uncertainty for these types of analyses is very high and couldn't be
used for decision making
• For complex systems where these two conditions are not met or a sufficient explanation provided for the
use of a steady state model, dynamic models implementing time variable representation of watershed and
waterbody processes to simulate hydrology, sediment, and water quality processes should be considered.
Dynamic models explicitly consider the variability in all model inputs and evaluate the effect of varying
effluent limits on the assimilative capacity of the stream.
D_EQ3NORTH CAROLINA
nepnrmrene of Environmental Onnllly
North Carolina Department of Environmental Quality I Division of Water Resources
512 North Salisbury Street 11617 Mail Service Center I Raleigh, North Carolina 27699-1617
919.707.9000

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• The model selection step is a key step where the modeler considers key factors that represent the system
being modeled. In general, to simulate complex systems, models which allow for time -variable analysis
under varying meteorological and environmental conditions, and which can be used to investigate multi-
dimensional systems, and a variety of pollutant types should be considered. Where complex hydrology
and complex interactions between the tributary watershed and the mainstem waterbody occur
consideration of the use of a dynamic model should be addressed especially for systems that also may
exhibit complex interconnections among hydrology, geomorphology, soils, and vegetation.
Therefore, a stronger justification needs to be provided for the use of the QUAL2K model for Contentnea Creek.
This should include a detailed assessment of the waterbody/watershed to be modeled, including a review and
summary of available flow and water quality data, and the appropriateness of a steady-state model for the site in
question. Once this information is provided, we (with MAB consultation) will determine whether the QUAL2K
model proposal is suitable for this application.
Please let us know if you have any questions regarding these comments and we will schedule a meeting to
discuss. Should you have any questions, please feel free to contact Derek Denard at [phone] /
[e-mail] or Doug Dowden at [phone] / [e-mail].
Respectfully,
.-DocuSigned by:
C464531431644 F E...
Michael Montebello
NPDES Program Branch Chief
ec: NPDES Files [Laserfiche]
City of Wilson/ Jimmy Pridgen, Water Reclamation Manager [[e-mail]];
Grant Goings, City Manager [[e-mail]]
Laura Pruitt [[e-mail]]; Nick Eatmon [[e-mail]]
Hazen and Sawyer / Mary E. Sadler [[e-mail]]; David Wankmuller[[e-mail]]
Water Environment Consultants / Matt Goodrich [[e-mail]]; Jeffrey deBessonet [[e-mail]]
DWR / Michael J Montebello[[e-mail]]; Doug Dowden [[e-mail]];
Kristen Litzenberger[[e-mail]]; Derek Denard [[e-mail]];
Pam Behm [[e-mail]]; Nick Coco [[e-mail]]
RRO / Scott Vinson [[e-mail]]; Vanessa Manuel [[e-mail]]
NORTH CAROLINA
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North Carolina Department of Environmental Quality I Division of Water Resources
512 North Salisbury Street 11617 Mail Service Center I Raleigh, North Carolina 27699-1611
919.707.9000