Background Soil Dataset Eval_20161215

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Technical Memorandum
Date:
Tuesday, December 13, 2016
Project:
Dan River Steam Station
To:
Kim Witt and Melonie Martin
From:
Jacob Ruffing
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 June 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 during seven well gauging events conducted between June 2015 and
September 2016; and
• Soil sample collected from a location not impacted by periodic flooding.
A total of 56 soil samples were collected at Dan River during CSA activities. Of these 56 samples, 25
samples satisfy the four criteria listed above and are considered viable samples for use in statistical
calculations. These samples were collected from the following locations on-site: BG -1 D, BG -5D, GWA-
2D, GWA-3D, GWA-6S, GWA-9D, GWA-11 D, SB -1, SB -2, and SB -3. Note that viable samples collected
from borings near the waste boundary or down -gradient of ash management areas were collected at least
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, cobalt, 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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