NC0030210_Quality Modeling Approach_20180323

Brown AND .
Caldwell • Transmittal Cover Sheet
309 E Morehead Street
Suite 160
Charlotte, North Carolina 28202
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To: Juli A Grzyb Date: 3/26/18 Transmittal No.: 2
NPDES Complex Permitting Project No.: 151553 Task No.:
1617 Mail Service Center Raleigh, Project Title: Permitting of the Expansion of Mallard Creek
NC 27699 WRF
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1 Tech Memo no 1 for project 151553
Remarks.
RECEIVE®/®ENR/DWR
—22018
WaterAPR Resources
Permitting Section
cc: Prepared by: Kristin Smith
Title: Receptionist
Tiansmittial Julie Gizyb

Brown AND .
Caldwell j Technical Memorandum
309 East Morehead Street,Suite 160
Charlotte, NC 28202
Phone [phone]
Fax:[phone]
Prepared for: Charlotte Water
Project Title: Permitting for the Expansion of Mallard Creek WRF
Project No.: 151553
Technical Memorandum No. 1
Subject: Mallard Creek Water Quality Modeling Approach
Date: March 23, 2018
To: Irene (Tesha) Okioga, Project Manager, Charlotte Water
From: George Anipsitakis, Brown and Caldwell
Copy to:
zxj(
Prepared by.
Clifton F. Bell, PE, PG
,,,,/ RECEIVED/DENPJDWR
APR -22018
Reviewed by: Water Resources
Paul Leonard, CFS Permitting Section
Limitations:
This document was prepared solely for Charlotte Water(CLTWater)in accordance with professional standards at the time the services were
performed and in accordance with the contract between CLTWater and Brown and Caldwell dated July 24,2017 This document is governed by the
specific scope of work authorized by CLTWater,it is not intended to be relied upon by any other party except for regulatory authorities contemplated
by the scope of work We have relied on information or instructions provided by CLTWater and other parties and,unless otherwise expressly
indicated,have made no independent investigation as to the validity,completeness,or accuracy of such information

Mallard Creek Water Quality Modeling Approach
Seal s)n 10 Execurthe SLgmrrn&.Iff
To keep pace with growth, Charlotte Water(CLTWater) is currently planning to expand MCWRF initially to a
treatment capacity of 14.9 and eventually to 16.0 MGD. For this expansion, CLTWater will submit a
speculative limits request letter to DEQ within 2018. In preliminary discussions of this topic, DEQ indicated
that CLTWater should perform water quality modeling of oxygen-demanding substances in the receiving
stream to determine if an expansion should prompt changes to the facility's existing CBOD5 and ammonia
limits.
As a first step, DEQ requested that CLTWater submit a document that describes the proposed modeling
approach for DEQ review and approval prior to submitting the speculative limits request letter.This technical
memorandum describes CLTWater's proposed approach to modeling the effects of an expanded MCWRF
discharge on dissolved oxygen concentrations in Mallard Creek and the Rocky River.
CLTWater proposes a two-phased modeling approach. In the first phase,the existing 1993 QUAL2E-UNCAS
model will be adjusted with updated information, and used to explore the sensitivity of dissolved oxygen (DO)
to increased flows from MCWRF.The second phase would be conducted if the first phase indicated that DO
would be sensitive to the expansion, and would involve recalibration of the updated model.
Sect i 2 ackg
The Mallard Creek Water Reclamation Facility(MCWRF)treats wastewater from a growing area in the
northern part of Mecklenburg County.The plant is currently permitted to treat a maximum month flow(MMF)
of 12 MGD under NPDES Permit NC0030210; which became effective on January 1, 2014 and will expire
(unless administratively continued) on November 30, 2018. MCWRF discharges to Mallard Creek in the
Yadkin-Pee Dee basin, which discharges to the Rocky River about 2.8 stream miles downstream of the
outfall (Figure 1) MCWRF's NPDES permit includes seasonal limits for 5-day carbonaceous biological oxygen
demand (CBOD5) and ammonia (Table 1)
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An understanding of the previous modeling efforts—and the basis of MCWRF's existing CBOD5 and ammonia
limits—is important for choosing an approach for modeling the proposed expansion. The North Carolina
Department of Environment, Health, and Natural Resources (NC DEHR) originally developed a water quality
model of Mallard Creek and the Rocky River in the early 1990s (NC DEHR, 1993) (the 1993 QUAL2E-UNCAS
Model). Regional utility departments expanded this model in the early 2000s to explore effects of a
proposed regional wastewater treatment facility on the Rocky River(CH2M Hill, 2001) (the 2001 QUAL2E-
UNCAS Model). Both models are discussed in subsections below, with a focus on the predicted impacts of
MCWRF on receiving water DO.
IBrownAND Caldwell
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Mallard Creek Modeling Approach 23 Mar 2018

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Mallard Creek Water Quality Modeling Approach
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Figure 1 - Location Map of the Mallard Creek Water Reclamation Facility.
Table 1:Existing Effluent Limitations and Monitoring Requirements
Mallard Creek Water Reclamation Facility,Outfall 001
Parameter Monthly Average Weekly Average Daily Average
Flow 12 MGD NA NA
CBOD,5 day(Apr.1-Oct.31) 4.2 mg/L 6.3 mg/L NA
CBOD,5 day(Nov.1-Mar.31) 8.3 mg/L 12.5 mg/L NA
Total suspended solids 30.0 mg/L 45 mg/L NA
NH3 as N(Apr.1-Oct 31) 1.0 mg/L 3.0 mg/L NA
NH3 as N(Nov.1-Mar.31) 2.0 mg/L 6.0 mg/L NA
Dissolved oxygen NA NA 6.0 mg/L
Fecal colifoml(geometric mean) 200/100 mL 400/mL NA
Temperature(deg.C) Monitor and report only
Total zinc Monitor and report only
pH Between 6.0 and 9.0 s.u.
Chronic toxicity Pass at effluent concentration of 90%
Effluent pollutant scan Monitor and report only
IBrownANaCaldwell
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Mallard Creek Modeling Approach 23 Mar 2018
1

Mallard Creek Water Quality Modeling Approach
3.1 1993 QUAL2E Model
NC DEHR's 1993 QUAL2E-UNCAS model focused on 42 river miles of the Rocky River, and simulated Mallard
Creek as a tributary. The model was used to examine the DO impacts of three municipal wastewater
treatment plants (WWTPs): Mooresville WWTP, MCWRF, and the Concord WWTP.The model was calibrated
to water quality data from the Rocky River and Mallard Creek, and was applied to derive MCWRF's limits for
CBOD5 and ammonia based on the need to attain the 5 mg/L DO criterion in Mallard Creek under 7Q10
stream flow(0.64 cfs). Under these conditions,the 12 MGD MCWRF discharge would constitute
approximately 97%of the flow in Mallard Creek.The application of the 1993 QUAL2E-UNCAS Model was
summarized in the 2009 fact sheet associated with the MCWRF NPDES permit:
"The facility's discharge takes up essentially all of the available assimilative capacity for oxygen
consuming wastes in Mallard Creek...The following set of effluent concentrations were obtained for
the Mallard Creek WWTP: DO: 6.0 mg/L, NH3: 1.0 mg/L, BOD5(4 mg/L CBOD5), and Flow: 6.0 MGD.
At expanded wasteflows,the DO sag is expected to occur in the Rocky River. Instream DO violations
were not predicted at 8 MGD or 12 MGD...Toxicants, as well as potential changes in the stream
channel as wastewater flows increase, may be the principal issues as opposed to instream DO
concentrations...Two years of[water quality monitoring] data confirms that the DO standard has not
been violated." (NCDWQ 2009)
MCWRF's current permit limits for CBOD5 and ammonia are based on these 1993 QUAL2E-UNCAS model
results. It should be noted that these limits were based on DO in Mallard Creek itself, as the DO sag in the
Rocky River was predicted to remain well above the 5 mg/L DO criterion.
3.2 2001 QUAL2E Model
For the 2001 modeling effort, the 1993 QUAL2E-UNCAS model was linked with a new model QUAL2E-UNCAS
of the lower reaches of the Rocky River(CH2M Hill, 2001).The new model reaches extended approximately
31 river miles from just upstream of the Muddy Creek WWTP to the USGS gage at Norwood.The new model
was calibrated to streamflow and water quality data from the Rocky River. Parameters for the old (upstream)
model reaches—including Mallard Creek—were not altered from the 1993 QUAL2E-UNCAS Model.
As documented by CH2M Hill (2001),the linked QUAL2E-UNCAS models were used to evaluation various
point source management scenarios under 7Q10 streamflow conditions. In the "fully permitted" model
scenario, MCWRF was set to a flow of 10 MGD and existing permit limits for DO, CBOD5, and ammonia.The
2001 effort was not used to alter MCWRF's permit limits. As in the 1993 QUAL2E-UNCAS Model,the MCWRF
was not predicted to cause violations of the 5 mg/L DO in the Rocky River downstream of the outfall. Under
the fully-permitted scenario, some DO violations were expected to occur on the Rocky River over 10 river
miles downstream from the Mallard Creek confluence, but these were attributed to tributaries with low DO
entering the Rocky River rather than point sources.As stated by CH2M Hill (2001):
"The location [of the minimum DO sag] is not near any WWTP and appears to be in the areas of
Hamby Branch.Some tributaries...contribute to drops in DO levels..."
One significant finding of the 2001 modeling work was that the 1993 QUAL2E-UNCAS model probably
overestimated CBOD decay and ammonia oxidation rates, and a sensitivity analysis indicated that this
probably caused it to underestimate DO in the receiving stream by as much as 0.5 mg/L:
"A review of the upper model, conducted with cooperation with DWQ staff, indicated that
CBOD decay and ammonia oxidation rates may not have been realistic in the existing
[1993] DWQ model...This[sensitivity analysis] indicates that if the CBOD decay rate is
incorrect(it appears...high for the levels of treatment provided by the major point sources),
significant error propagates to predicted instream DO concentrations." (CH2M Hill, 2001)
Brown AND Caldwell
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Mallard Creek Water Quality Modeling Approach
The 1993 QUAL2E Model used a CBOD decay rate of 0.2 day-1 in Mallard Creek, whereas CH2M Hill
calibrated the new downstream segments using CBOD decay rates in the 0.05 - 0.1 day-1 range. In addition
to being necessary for calibration of the downstream segments in the 2001 QUAL2E-UNCAS model,the
lower rates are also closer to what the scientific and engineering literature would identify as appropriate for
CBOD from modern WWTPs (Lung, 2001).
Section 4: Proposed Modeling Approach
The MCWRF currently has relatively stringent CBOD5 and ammonia concentration limits that require
advanced treatment.The review of previous model indicated that the modeling upon which the limits were
based was likely conservative due to the use of high CBOD and ammonia oxidation rates.The previous
QUAL2E-UNCAS models did not predict that MCWRF would cause DO violations in the Rocky River; rather,
limits were based on the need to protect DO in Mallard Creek itself. With this understanding, CLTWater
proposes a two-phased modeling approach. In the first phase, existing models would be adjusted with
updated information, and used to explore the sensitivity of dissolved oxygen (DO)to expanded flows from
MCWRF. The second phase would be conducted if the first phase indicated that DO would be sensitive to the
expansion, and would involve more detailed calibration of the updated model. Both proposed modeling
phases are described in subsections below.
4.1 Phase 1 - Model Update and Sensitivity Analysis
In Phase 1, CLTWater will update the 1993 QUAL2E-UNCAS model, upon which MCWRF's existing limits are
based. Depending on the availability and usability of the existing modeling files,the QUAL2E-UNCAS model
may or may not be updated to a more recent model version (either QUAL2K or QUAL2Kw). CLTWater will
update the model with current 7Q1O streamflows and permitted point source flows. BOD bottle tests will be
performed to measure the following for the MCWRF effluent:
• Ratio of CBOD5 to long-term CBOD
• CBOD decay rate
The resulting values will be evaluated to determine if the 1993 QUAL2E-UNCAS model should be revised with
respect to these CBOD-related parameters.
Water quality data for Mallard Creek will be compiled from available databases, and used to characterize the
observed range of concentrations of key constituents (DO, ammonia, CBOD) under dry weather summer
conditions. These data will be examined to verify that the revised model is reasonably conservative; i.e., it
does not overpredict the observed DO concentrations in Mallard Creek. If the revised model does overpredict
the observed DO concentrations in Mallard Creek, CLTWater will proceed directly to Phase 2, which would
involve re-calibration of the model.
In Phase 1, CLTWater will use the updated QUAL2E-UNCAS model to evaluate the sensitivity of DO in the
receiving water to expanded flows (14.9 and 16.0 MGD)from MCWRF, assuming CBOD5 and ammonia
concentration limits are unchanged. The modeling team will vary key model parameters (e.g., CBOD decay
rate and ammonia oxidation rates)within a limited range that represents a reasonable estimate of
uncertainty in those parameters. The evaluation will focus on predictions of DO concentrations in Mallard
Creek. However, the modeling team will also examine the model predictions for the Rocky River, to verify
prior findings that the MCWRF would not cause DO violations below the Mallard Creek-Rocky River
confluence.
A possible outcome of the sensitivity analysis is the determination that MCWRF's existing CBOD5 and
ammonia limits are clearly protective. This could be concluded if the revised model predicted that DO in
receiving water was predicted to be largely insensitive to the MCWRF flow rate, and if DO was predicted to
Brown.Catdwelt
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Mallard Creek Modeling Approach 23 Mar 2018

Mallard Creek Water Quality Modeling Approach
remain well above the water quality criterion under a range of CBOD/ammonia oxygenation rates. Under this
outcome,the Phase 1 model effort would be sufficient to conclude that MCWRF's existing concentration
limits could be maintained with an expansion to 14.9 or 16.0 MGD, and the Phase 2 modeling effort would
be unnecessary.
Conversely,the Phase 1 model effort might show that DO in the receiving water would be sensitive to
increased flow rates, and that violation of the 5 mg/I DO limit are within the range of predicted outcomes,
given the uncertainty in model parameters. In this case, it would be recommended to recalibrate the model
with new in-stream monitoring data, which described in Phase 2.
CLTWater will document the results of the Phase 1 modeling effort in a technical memorandum suitable for
review and discussion with DEQ.The memo will include the technical basis for the conclusion on whether
MCWRF's existing concentration limits should be carried forward into any expansion, or whether the Phase 2
model effort should be pursued.
4.2 Phase 2 - Model Recalibration
Phase 2 will only be conducted if needed based on the results of Phase 1. Under Phase 2, CLTWater will
conduct additional water quality monitoring in Mallard Creek to support re-calibration of the QUAL2E-UNCAS
model. A synoptic water quality monitoring event will be conducted on Mallard Creek under warm season,
dry weather conditions.The field team will measure field parameters (temperature, DO, pH, specific
conductance)and collect grab samples for laboratory analysis of CBOD5, ammonia nitrogen, other major
nutrient species, and chlorophyll-a.Sampling locations will include the following:
• One station on Mallard Creek upstream of the MCWRF discharge
• Up to five stations on Mallard Creek below the discharge
• The MCWRF effluent
If the Phase 1 model effort indicates that the MCWRF may cause DO violations on the Rocky River,the
monitoring effort will also include Rocky River stations.This would include at least one station on the Rocky
River upstream of the Mallard Creek confluence, and up to three stations on the Rocky River below Mallard
Creek.
The model will be calibrated to the longitudinal profile of water quality as measured during the synoptic
monitoring event. After adjustment to ensure that the flows and velocities are reasonable,the model will be
calibrated to temperature,followed by ammonia, and then to CBOD5 and DO.
After re-calibration,the updated model will be used to simulate expanded flows (14.9 and 16.0 MGD) at
MCWRF.These results will be used to determine the appropriate CBOD5 and ammonia limits for MCWRF
under the expansion scenarios,which may or may not be equal to the existing limits.
The methods and results of the Phase 2 modeling effort will be documented in a technical memorandum for
review by DEQ.
IBrown.Caldwell
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Mallard Creek Water Quality Modeling Approach
References
CH2M Hill. 2001. QUAL2E-UNCAS Application to Rocky River from Mooresville WWTP to USGS Flow Gage
near Norwood in North Carolina. Draft report prepared for Cabarrus County, Charlotte-Mecklenburg utility
Dept., and Union Co. Public Works. 37 p. plus appendices.
Lung, S. 2001. Water Quality Modeling for Wasteload Allocation and TMDLs.John Wiley&Sons, New York,
NY. 33 p.
North Carolina Department of Environment, Health, and Natural Resources (DEHNR). 1993.A QUAL2E-
UNCAS Application to the Rocky River and Mallard Creek—Cabarrus, Iredell, and Mecklenburg Counties,
North Carolina. 24 p. plus appendices.
North Carolina Division of Water Quality(NDWQ). 2009. Fact Sheet for NPDES Permit Development—Mallard
Creek Water Reclamation Facility. 9 p.
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Mallard Creek Modeling Approach 23 Mar 2018