NC0039586_Other Agency Documents_20210903 (2)

Duke Shearon Harris Nuclear Plant (NC0039586) Mixing Zone and WER Summary for the
Copper Compliance Schedule
March 17, 2021
Introduction
Special Condition A. (9) of the HNP permit requires the facility to achieve compliance with the final
effluent limits for zinc and copper at outfall 006 as specified in in the effluent limitations and monitoring
requirements for Special Condition A. (6) by September 30, 2021. As part of that compliance schedule,
Duke also had to develop a corrective action plan (CAP) with annual benchmarks in the years leading up
to September 30, 2021. The CAP identified anti -corrosion chemicals as the main contributor to elevated
zinc levels in the effluent. After changing processes and discontinuing the use of zinc based corrosion
inhibitors, zinc levels have dropped dramatically and compliance with the final effluent limits has been
achieved. Sources of copper were also evaluated in the CAP. No copper compounds are added to the
process and the source of copper was determined to be the concentration of copper levels in the lake
(-1 ug/L) from evaporation in the cooling water with minor contributions from leaching from existing
equipment.
To achieve compliance with copper limits, Duke conducted a water -effects ratio (WER) study for copper
in 2019 (Corrective Action Plan for Copper and Zinc for Harris Nuclear Plant NPDES Permit- Year 3
Activities Report, August 19, 2019). Dissolved copper standards in 15A NCAC 2B allows for the use of a
WER when deriving hardness -based dissolved copper standards. The WER study proposed a WER = 5 for
use in the standards calculation. However, the WER study was conducted at 100% effluent with an
average effluent hardness of 43.8 mg/L. The Division requested, per EPA guidance for WER evaluations
in lakes, to do a mixing zone analysis (MZA) in conjunction with another WER study which incorporates
effluent mixed with site water at a ratio to be determined by the near -field dilution from the mixing
zone in order to derive a WER that reflects mixing of the effluent with the receiving water. This would
factor in all the variables beyond hardness that affect copper toxicity (e.g. dissolved organic carbon, pH,
cations/anions that form copper complexes, etc.) inherent in the combined effluent and receiving water.
Duke completed the MZA using CORMIX simulations in 2020 (Corrective Action Plan for Copper and Zinc
for Harris Nuclear Plant NPDES Permit- Year 4 Activities Report, August 19, 2020). The MZA evaluated
four seasonal conditions, with the summer season determined to be the most limiting condition due to
the presence of the summer thermocline which affects the buoyant thermal discharge. Duke submitted
a revised report in February 2021 along with a permit modification application requesting the
incorporation of the MZA in calculating copper and zinc limits and withdrawal of the WER study. The
CORMIX study proposes a dilution of 15.3:1 at a radial distance of 177 ft and plume width of 278 ft. This
value is the dilution found by applying the EPA criteria for sizing acute mixing zones found in EPA's
Technical Support Document for Water -quality Based Toxics Control (TSD) section 4.3.2.

Table 1. Summary of Proposed Copper Limits from the WER and CORMIX studies.
Proposed Limits Cu
Hardness (mg/L)
Monthly Average (ug/L)
Daily Maximum (ug/L)
WER
43.6
63.6
88.8
CORMIX
26.2
166.1
166.1
Discussion & Suggestions
While the acute mixing zone size in the MZA is correctly sized according to the most limiting criteria
from the TSD, the TSD also cautions about applying mixing zones when the effluent may attract biota
and specifically mentions temperature as a possible attractive force. In the MZA, the summer condition
is most limiting (with respect to dilution) because of the summer thermocline inhibiting the buoyancy of
the discharge, resulting in plume contact with a thermal stratified layer. Temperature profiles of the lake
showed no stratified layer during colder seasons. There is concern that a large acute mixing zone as
proposed may attract biota, especially in the winter months. The MZA is still useful for understanding
plume dynamics, future WER studies, and potential 316(a) variances.
When a WER is granted, a study must be performed every permit cycle to maintain the WER for the next
permit. Because a WER is site specific, this value may change permit to permit to reflect changes in the
effluent and receiving water characteristics as well as updates to methodology. Given that the existing
WER study is based on effluent characteristics only, several options are provided for the current permit
modification request and compliance schedule:
1. Use the proposed WER study value to derive limits for the renewal/modification and require the
WER study for the next permit cycle to incorporate site water per EPA guidance and the
Division's prior request.
2. Reissue the permit with an extended compliance schedule for copper to conduct another WER
study as listed above with modification of the permit after. With the work done so far in
achieving compliance with zinc, it is reasonable to extend the compliance deadline.
3. Use the proposed WER study value to derive limits for the renewal/modification but use the
lower combined hardness which reflects the low hardness of the receiving water. At hardness of
25 mg/L and WER = 5 the MA = 39.43, and DM =52.36. These values could still trigger limit
violations.