NC0004961_Final GAP Approval Letter_20150219

N,CDENR.
North Carolina Department of Environment and Natural Resources
Pat McCrory
Governor
February 19, 2015
Mr. Harry Sideris
Senior Vice -President
Environment, Health, and Safety
Duke Energy
526 South Church Street
Mail Code EC3XP
Charlotte, NC 28202
Re: Riverbend Steam Station
NPDES Permit No. NC0004961 — Gaston County, North Carolina
Conditional Approval of Revised Groundwater Assessment Work Plan
Dear Mr. Sideris:
Donald R. van der Vaart
Secretary
On December 31, 2014, the Division of Water Resources (Division) received the revised
Groundwater Assessment Plan (GAP) for the above listed facility. The revised GAP was submitted
in response to the DWR's Review of Groundwater Assessment Work Plan letter dated November 4,
2014. A review of the plan has been completed and several deficiencies or items requiring
clarification were noted. Therefore, in order to keep the site assessment activities on a timely
schedule, the Division has approved the revised GAP under the condition that the following deficient
items are addressed in the Groundwater Assessment Report:
• Comment Section 5.3 Hydrogeologic Site Characteristics:
The initial site conceptual site model (ISCM) section of the revised GAP does not provide a
clear, cohesive description of how constituents of potential concern (COPCs) may migrate
from the source(s) to the receptors through various pathways. It is acknowledged that there
is information available to develop an ICSM, but data are not presented in a manner such as
groundwater elevation maps, geologic maps, cross-sections that depict detailed site
conditions, flow diagrams, or in a tabulated format to illustrate where data gaps may exist.
Duke Energy should incorporate all existing data at the site and be prepared to collect
additional data if the Division determines that additional data gaps exist. Continued site
conceptual model development should follow guidelines similar to those presented in the
American Standards Testing Measures E1689 - 95(2014) Standard Guide for Developing
Conceptual Site Models for Contaminated Sites to direct data collection, data interpretation,
and model development efforts.
1636 Mail Service Center, Raleigh, North Carolina 27699-1636
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Riverbend Steam Station
February 19, 2015
Page 2 of 3
• Comment 7.1.3 Deep Monitoring Wells and Comment Section 7.1.4 Bedrock Monitoring
Wells:
The Division suggests installing a cluster of monitoring wells that are screened across
various flowpaths (shallow aquifer, transition zone/partially weathered bedrock and within
bedrock) near the compliance boundary at a location approximately 200 feet north of existing
monitoring well MW -9. Additional monitoring wells that are screened within the transition
zone/partially weathered rock and within bedrock are also suggested in the immediate
vicinity of existing monitoring well MW -15. These locations will provide more data adjacent
to the Catawba River for assessment of multiple flowpath transects across the site.
• Comment Section 7.2 Groundwater Sampling and Analysis:
Direction provided in the EPA Region 1 Low Stress Purging and Sampling Procedure for the
Collection of Groundwater Samples from Monitoring Wells (2010) should be followed
strictly and any deviations from the procedure must be approved by the Division and
documented accordingly. For example, samples should not be collected until pH is stabilized
within t 0.1 for three consecutive readings rather than t 0.2 written in the GAP.
Temperature and specific conductivity readings should stabilize within 3% for three
consecutive readings before samples are collected instead of 10% noted in the GAP. Also
note that if the pumping rate is so low that the flow-through-cell/chamber volume cannot be
replaced in a 5 minute interval, the time between measurements should be increased
accordingly.
• Table 10 — Groundwater, Surface Water, and Seep Parameters and Constituent Analytical
Methods:
Low level Vanadium listed as having a detection limit unit of mg/L. This is likely a
typographical error but the units should be in µg/L rather than mg/L
• Comment Section 7.2.3 Speciation of Select Inorganics and 7.3.3 Seep Samples:
The GAP text indicates that review of the Division's March 2014 seep and surface water
sampling analytical data will be incorporated into assessment plans to evaluate seep and
surface water sample locations at the facility. Locations where the Division's March 2014
seep and surface water sampling data indicated exceedances or elevated concentrations of
iron, manganese and other constituents of concern should be incorporated into the
assessment's seep/surface water sampling plans with speciation of analytical data
sufficient to support delineation and modeling efforts.
In addition, technical direction that will serve as the basis of expectations for completion of the site
assessment is provided at Attachment 1. Failure to address the deficient items stated above will
result in Duke Energy not being in compliance with the stated statutes. Per G.S. 130A -309.209(a)
(3) and (4), you must begin implementation of the revised GAP on March 1, 2015 and the
Groundwater Assessment Report is due on August 18, 2015. It is our understanding that Duke
Energy may have to obtain additional permits to facilitate installation of certain monitoring wells. In
the event permits are needed for this purpose, Duke Energy should take all steps necessary consistent
with the law to avoid delaying completion of the assessment report.
If you have any questions, please contact Bruce Parris at [phone].

Riverbend Steam Station
February 19, 2015
Page 3 of 3
Sincerely,
zn
S. Jay Zimmerman, P.G., Acting Director
Division of Water Resources
cc: WQROS — MRO
WQROS Central Files
DENR Secretary - Don van der Vaart
HDR (Attn: William Miller) 440 South Church Street, Suite 1000, Charlotte, NC 28202