NC0023906_Model_Review _20230413

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ROY COOPER _
Governor d C
ELIZABETH S.BISERr
Secretary •E .
RICHARD E.ROGERS,JR. NORTH CAROLINA
Director Environmental Quality
April 13,2023
Mr.Jimmy Pridgen,Water Reclamation Manager
City of Wilson
P.O. Box 10
Wilson,NC 27894-0010
Subject: Water Quality Modeling Selection Review
Hominy Creek WRF-Neuse River Basin
NPDES Permit No.NCO023906
Wilson County
Dear Mr. Pridgen:
The Division of Water Resources(DWR)Modeling and Assessment Branch(MAB)reviewed the
modeling report"Technical Memorandum: Water Quality Model Selection" submitted to the NPDES
Permitting Branch regarding discharge to Contentnea Creek. The goal of the model selection review
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/l.
Overall,there were concerns with the selection of a steady state model QUAL2K for evaluation of the
City of Wilson WWTP expansion. As we noted earlier, a dynamic model may be better suited for
addressing some of the questions below and while it remains a preferable choice to address
uncertainties that could impact recommendations for speculative permit limits,we acknowledge your
plan to proceed with a QUAL2K modeling action.
However,the Division would expect when you move forward with the QUAL2K model that our
recommendations be incorporated in the final submission including increased in-stream monitoring to
account for uncertainties as described below. In addition,the final report for the QUAL2K model
should contain sections discussing these issues, including any proposed effluent limitations.
• Flow and water quality data during the 2007-2019 periods were examined near the Hookerton
station(J7450000). This site is about 50 miles downstream from the Wilson WWTP discharge point
and receives tributary flows from several swamps including the Toisnot Swamp and Nahunta
Swamp. The data show that,during some high precipitation events, sudden and large reductions in
stream DO are often observed. Flushing of swamp waters, changes in flow dynamics and
consequent impacts on stream habitats have long been a great concern, and it is not well understood
how these processes are affected by increased effluents from wastewater treatment plants,not only
near the immediate discharge location but also along the downstream paths.
• The Technical Memorandum proposes to make a number of changes to the QUAL2K model
that was used in the earlier scoping analysis, including(1)adding phosphorus variables (PO4 and
organic phosphorus) and phytoplankton processes (algal productivity and respiration)to the model,
and(2) explicitly entering diel(i.e.,24-hour)variations of the forcing variables(light,temperature,
North Carolina Department of Environmental Quality I Division of Water Resources
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etc.)to the model. These changes were deemed necessary after the examination of the Hookerton
data revealed instances of elevated chl-a levels,which will cause diurnal fluctuations of the stream
DO. This can be one way of making the best use of the quasi-steady-state models like QUAL2K
providing some,though limited,temporal considerations to the analysis. The final report should
detail how the proposed expansion would alter the hourly fluctuations of the predicted stream DO,
as well as daily averages,throughout the modeled domain.
• The Technical Memorandum proposes that all the tributary flows are added to the main stem
(Contentnea Creek)as non-point source(i.e., diffuse)flows. It is generally recommended that major
tributaries, especially those with WWTPs,are explicitly specified as point source contributions in
the model. It generally makes it easier to run sensitivity and scenario analyses, as well as to
examine the results,and make it more intuitive to connect and compare models of tributaries when
the need arises. Setting major tributaries like Toisnot Swamp,Nahunta Swamp and Little
Contentnea Creek as point source contributions would be more appropriate.
• The dates of model simulation chosen for calibration(6/8/2011 and 6/9/2011) and
corroboration(9/20/2010 and 9/24/2010) are close to each other and more than 10 years ago.
Although the 1 Oth percentile line for the monitored DO data throughout the Contentnea Creek is
around 5 mg/L(Figure 2-12,Draft Study Plan),the scoping model was calibrated when the
observed stream DO was mostly higher than 6 mg/L(Figure 3-5). More recent dates need to be
evaluated for added assurance that the model is well representing recent conditions and when
stream DO is critically low. In addition,the final report should include an evaluation of land cover
changes that have occurred since the 2011 time period to ensure that the choice of 2011 is
representative of current conditions.
• The preliminary modeling analysis (included in the earlier Draft Study Plan) suggests that
some impacts from the Wilson WWTP discharge may be observed through the downstream end of
the modeling domain(i.e.,confluence with the Neuse River). Please make sure that all the
dischargers downstream of the Wilson WWTP are included and they are most up to date and at total
permitted capacity. Also make sure to run all the expansion scenarios as applicable(including 17.3
and 20.7 mgd) and, if different winter limits are expected, include winter scenario runs when
submitting the modeling package for review.
Should you have any questions,please feel free to contact Derek Denard at(919) 707-3618/
derek.denard(a�ncdenr.gov or Doug Dowden at(919) 707-3605/dou�.dowden(c�r�ncdenr.gov.
Respectfully,
DocuSigned by, Al•h'e.''`
C454531431644FE...
Michael Montebello
NPDES Program Branch Chief
cc: 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 Udebessonet@water-eo,com]
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]]
D E Q�� North Carolina Department of Environmental Quality I Division of Water Resources
512 North Salisbury Street 1 1617 Mail Service Center I Raleigh,North Carolina 27699-1611
NORTH CAROUNA _
o,p,�„F r��,�wla,\ /./ 919.707.9000