Buck Background Soil Dataset Evaluation - December 2016

F)2
Technical Memorandum
Date: Tuesday, December 13, 2016
Project: Buck Steam Station
To: Sean DeNeale
From: Chad Hearn
Subject: Quantification of Valid Background Soil Samples for Statistical Calculations
The purpose of this memorandum is to provide the basis for evaluation of soil samples proposed for use
in determining site-specific background concentrations using statistical methods. The soil samples
evaluated herein were principally collected during Comprehensive Site Assessment (CSA) fieldwork
conducted between March and May 2015. The locations of soil samples evaluated for use in background
statistical calculations are shown on Figure 1.
Viability of samples for use in statistical calculations was evaluated using the following criteria:
• Soil sample is collected from a location that is not impacted by coal combustion residuals or coal -
associated materials;
• Soil sample is collected from a location that is not impacted by other potential anthropogenic
sources of constituents;
• Soil sample is collected from the unsaturated zone, above the maximum groundwater elevation
measured in shallow wells between July 2015 and September 2016; and
• Soil sample collected from a location not impacted by periodic flooding.
A total of 61 soil samples were collected at Buck during CSA activities. Of the 61 samples, 18 samples
satisfy the 4 criteria above and are considered viable samples for use in statistical calculations. These
samples were collected from the following locations on-site: BG -1 D located beyond the compliance
boundary southwest of the active ash basin; BG -2D located beyond the compliance boundary
south/southwest of the inactive ash basin; BG-3BRU located beyond the compliance boundary
south/southeast of the active ash basin; GWA-1 D located near the compliance boundary south of the
active ash basin; GWA-6BRU located near the compliance boundary west/northwest of the Cell 3
Secondary Pond; GWA-7D located beyond the compliance boundary west of the Cell 3 Secondary Pond;
GWA-9D located north of the active ash basin; GWA-10D located near the compliance boundary west of
the active ash basin; GWA-11 D located near the compliance boundary west of the active ash basin; and
GWA-12S and GWA-22D located near the compliance boundary southwest of the Combined Cycle Plant.
Note that viable samples collected from borings near the waste boundary or down -gradient of ash
management areas were collected a minimum of one foot above the maximum measured groundwater
elevation and are assumed to have not been impacted by groundwater migrating from ash sources.
Soil sampling data used in this evaluation, including the range of groundwater elevations for shallow wells
in each well cluster and analytical results per well are provided in Table 1. Note that laboratory reporting
limit for antimony, arsenic, selenium, and thallium exceeded the North Carolina Protection of
Groundwater (POG) Preliminary Soil Remediation Goals, as revised in October 2016. Revised laboratory
reports that will provide the non -detect values and method detection limits are in the process of being
obtained from the lab.

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