NC0020648_Report_20231106

Groundwater Management Associates, Inc.
GMA 4300 Sapphire Court, Suite 100
Greenville, North Carolina 27834
Telephone [phone]
www.cima-nc.com
November 6, 2023
Mr. Doug Dowden
Supervisor, NPDES Industrial Permitting Unit
NCDEQ, Division of Water Resources
1617 Mail Service Center
Raleigh, NC 27699-1617
Re: Dilution Study Plan of Effluent Mixing for Wastewater Treatment Plant Discharge
City of Washington, Beaufort County, North Carolina
Dear Mr. Dowden,
The City of Washington (the City) seeks to obtain an updated NPDES permit to increase the
discharge volume for the City's existing wastewater treatment plant (WWTP). The City plans to
expand the existing 3.65 million gallons per day (MGD) plant to 4.95 MGD treatment capacity.
Currently, the existing NPDES permit includes an outfall into the Tar River (a class C, NSW -
Saltwater) that is equipped with an effluent diffuser. The proposed increase in discharge
volume will require an updated CORMIX model to support the final optimized design and
permitting of the upgraded outfall. This project only involves the discharge of existing waste
streams and additional effluent volume. Additionally, effluent water chemistry is known, and no
pilot studies are needed to characterize the proposed discharge. CORMIX modeling will
incorporate the actual flow volumes and water quality of the wastewater to be discharged. The
City of Washington only seeks an updated NPDES permit for the increase in discharge volume.
The City does not seek to change the existing effluent limits, establishment of a Regulatory
Mixing Zone (RMZ), or changes to the current sampling/monitoring procedures.
Groundwater Management Associates, Inc. (GMA) was contracted by Rivers and Associates
(Rivers) to provide consulting on the expanded discharge and to model mixing of the proposed
discharge for the City of Washington. GMA will perform a series of conceptual surface -water
mixing simulations using the CORMIX (Version 12.0 GT) modeling program. Simulations will be
based on conservative assumptions of ambient flow, published bathymetry data, and available
water -quality data collected adjacent to the existing WWTP discharge location.
In accordance with the revised (October 2018) "Mixing Zones in North Carolina" guidance
document provided by the NCDWR, GMA has prepared this Dilution Study Plan for the Division
to review and approve prior to GMA undertaking the CORMIX modeling of the proposed

Mr. Dowden
Page 2
discharge. Below, we provide the full details of our proposed study and justification for our
approach.
• Proposed Modeling Approach of the Potential Discharge Site
The City of Washington has selected a proposed discharge site adjacent to the current
permitted discharge outfall located on the Tar/Pamlico River. The current outfall location on
the Tar River is located between the Highway 17 bypass bridge and the South Bridge Street
bridge in downtown Washington, nearest to the southern bank of the river (Figure 1). GMA
will use CORMIX (Cornell Mixing Zone Expert System) to model the degree of effluent
mixing within the Tar/Pamlico River at that location and to evaluate the dilution of the
proposed increased effluent discharge. GMA will model the proposed discharge using a
conservative, worst -case approach that is most protective of the affected surface water
body. GMA has already performed field studies to characterize the physical, hydrologic,
water -quality, and benthic biological attributes of the Tar/Pamlico River at the selected
potential discharge location. The field study results will provide the site -specific data
needed to model mixing of the proposed discharge using CORMIX.
Model runs will be based on conservative estimates of critical conditions. GMA will use the
results of the CORMIX dilution modeling to aid Washington's consulting engineer, Rivers and
Associates, Inc., in selecting an outfall configuration that will effectively mix the effluent
within a short distance of the outfall and minimize impact on the receiving water body.
To ensure that the proposed effluent will mix under environmental conditions outside of
those measured during the field study, GMA will conduct a sensitivity analysis using the
selected diffuser design that evaluates the effects on mixing resulting from large variations
in ambient conditions. The sensitivity analysis will be broad enough to cover the full range
of expected ambient flow characteristics under varying portions of the tidal cycle (e.g.
spring and neap tides) and extreme weather events. GMA will also run model simulations
that predict mixing of the effluent under varying effluent discharge velocities that might be
encountered based on probable flow -rates under consideration for the pump design. This
approach will provide confidence that effluent constituents will be diluted to below water
quality standards within an acceptable distance from the outfall.
• Justification for the Selected Model
GMA will use the surface water mixing and dispersion model, CORMIX, to evaluate mixing
characteristics at the discharge site and to support updated NPDES permitting for the
proposed expanded discharge. CORMIX is an expert rule -based system for the analysis,
prediction, and design of pollutant discharges into several types of water bodies. CORMIX is
widely used across the country, and it is specifically described in the EPA's Technical
Guidance Manual for Performing Waste Load Allocations (Book 3, Part 3, 1992). CORMIX

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Page 3
predicts mixing and dilution of several types of effluent into various water bodies, and it
covers a majority of common discharge and environmental conditions likely to be
encountered. The model requires the user to input data about the nature of the ambient
conditions, the effluent, and the discharge configuration (diffuser design, etc.). These input
parameters are run through a series of "If" (conditions) and "Then" (conclusions) logical
steps to classify how the effluent -plume will behave. Once the flow is classified, CORMIX
utilizes integral, length -scale, and passive -diffusion approaches to simulate the
hydrodynamics of different regions of the effluent plume (Doneker and Jirka, 2012).
CORMIX simulates the hydrodynamics of both near -field and far -field mixing zones, and it
includes a tidal mode that accounts for the re -entrainment of the plume during tidal
reversal. Thus, CORMIX is well suited to model the proposed discharge to the Tar/Pamlico
River upstream of Washington.
Data Collection
During the field study, GMA deployed a Falmouth Scientific, Inc. 3-Dimensional Acoustic
Current Meter to collect high -resolution (cm/s) flow velocity data. We deployed the
instrument for 1 week of continuous monitoring to characterize the prevailing flow system
and variations with tidal fluctuations and weather. GMA documented bathymetry within the
vicinity of the proposed discharge, deployed transducers, and we collected field water -
quality measurements during both deployment and retrieval of the flow meter. Water -
quality measurements included: location, date, water depth, water temperature, dissolved
oxygen, ORP, conductivity/salinity, and pH. Water depths will be considered relative to
published historical tide monitoring from the Pamlico River Gaging Station at Washington
located approximately 0.5 miles downstream from the study site as well as other published
tide predictions of the area.
GMA used a benthic sampler to collect sediment samples and to survey the immobile
benthic community in the immediate vicinity of the outfall. We submitted benthic samples
to a materials testing laboratory for grain size analyses and to a macroinvertebrate biologist
for macroi nverteb rate species counts and identification. Field water -quality measurements
were provided to the biologist for interpretation in relation to the macrofaunal species
identified.
GMA also collected surface -water samples at the proposed discharge location for laboratory
analyses. We collected laboratory samples from the middle of the water column using a
weighted bailer. GMA collected one sample near high tide conditions and a second sample
near low tide conditions. We delivered these samples to a NC Certified laboratory for
analyses of total dissolved solids, total chloride, total copper, total manganese, total lead,
total zinc, total phosphorus, total nitrogen, ammonia nitrogen, and turbidity.

Mr. Dowden
Page 4
• Identification of Critical Conditions
In unsteady, tidal environments, the lowest dilution of an effluent occurs not at slack tide
when the ambient water velocity is zero, but just shortly after slack tide when the plume re -
entrains material remaining from the previous tidal cycle. In order to simulate mixing under
this worst -case scenario, GMA will perform all CORMIX simulations using the average water
velocity measured at 1-hour after slack tide during the field study, and we will use the tidal
mode of CORMIX to account for the reduced dilution caused by plume re -entrainment. The
determination of the site as an acceptable location for the proposed discharge will be based
on whether the model predicts that all constituents can be diluted to below their respective
surface water standards within a reasonable distance under chronic design flow conditions
of the receiving water, Tar/Pamlico River.
A reasonable potential analysis (RPA), conducted by the City's selected engineer, Rivers and
Associates, Inc., indicates that ammonia nitrogen will be the effluent constituent requiring
the most dilution to meet NCAC 02B Surface Water Standards at the proposed discharge
site. Therefore, GMA plans to model the dilution of ammonia nitrogen as the primary
mixing constituent in all CORMIX simulations. At the proposed discharge location (Stream
Index 28-(102.5)), the NCDEQ classifies the Tar/Pamlico River (HUC 03020103) as a Class
C, NSW surface water body. No water quality standards exist for ammonia nitrogen in
surface water bodies of North Carolina, however, the NCDEQ has taken steps to address
potential ammonia issues by establishing a NPDES ammonia toxicity policy which is used to
establish ammonia permit limits where appropriate. The original permit's effluent limits
were supported by a CORMIX study conducted by Black and Veatch in 1994. The instream
waste concentration (IWC)-based calculations for ammonia nitrogen for the current
discharge permit limits the effluent to a monthly average of 1.0 mg/L, and a weekly average
of 3.0 mg/L.
• Model Calibration / Validation Procedures
CORMIX does not have user -adjustable parameters outside of the site -specific physical
attributes of the modeled water body. However, as previously described, GMA will run a
sensitivity analysis that will evaluate a range of discharge velocities, discharge densities,
ambient water -body velocities, ambient water -body densities, and other environmental
parameters, such as wind speeds. By analyzing the effects of a wide range of parameters
on the mixing and dilution characteristics of the proposed discharge, we will have
confidence that the effluent will mix adequately under all conditions likely to be encountered
at the site.

Mr. Dowden
Page 5
0 Project Timeline
The field study of the existing discharge location was completed in August of 2023. GMA
plans to perform the CORMIX modeling of the proposed site and finalize a report of our
findings within the next 3-4 months.
Thank you for taking the time to review this dilution study plan. If you have any questions
regarding this plan or need any additional information, please feel free to contact me.
Best regards,
Groundwater Management Associates, Inc.
Cody J. Shell, P.G.
Project Hydrogeologist
Attachments: Figure I
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2-73
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CC: Wren Thedford — NCDWR, NPDES Permitting
Blaine Humphrey, P.E. — Rivers and Associates, Inc.
Greg Churchill, P.E. — Rivers and Associates, Inc.
James K. Holley, P.G. — GMA, Greenville
Z:\GMA\213XX - Rivers\21350 City of Washington CORMIX\Dilution Study Plan\COW Dilution Study Plan GMA
20230921.docx

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LEGEND
Current Outfall Location GMA
Proposed Discharge Location SCALE IN FEET
0 125 250 500
File: DRAWINGS/21350/
FIG 1 Outfall SITE MAP DATE: 11/6/2023
PROJECT NO. 21350 CITY OF WASHINGTON, BEAUFORT COUNTY, NC FIGURE 1