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Dames & Moore, 1999 - USDA Forest Service

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7.1.2.2 Statistical Analysis of Data<br />

For each data set, summary statistics were calculated for use in the risk assessment. These summary<br />

statistics included:<br />

'Number of samples<br />

Number of detections<br />

Minimum detected concentration<br />

Maximum detected concentration<br />

Range of detection limits<br />

Distribution (normal, lognormal, or neither)<br />

Arithmetic mean<br />

Arithmetic standard deviation<br />

Median<br />

Estimate of 95 percent upper confidence limit (UCL) concentration based on appropriate<br />

distribution, if available<br />

Statistical calculations were conducted using the Ecology MTCAStat V2.1 Excel Macro. Tables presenting<br />

the summary statistics are included as Tables 7.1-A through 7.1-K.<br />

7.1.2.3 Evaluation of Background Data<br />

Twenty background samples were collected in 1998 to characterize area background for soil at the Site.<br />

These area background levels are detailed in Table 7.1-1. In addition, Table 7.1-1 shows natural<br />

background concentrations for the Yakima Basin which were obtained from Ecology (1994), and<br />

Washington State background concentrations obtained from Dragun (1991).<br />

Table 7.1-1 also shows area background concentrations of total metals calculated for surface water. These<br />

background data were calculated using Ecology MTCAStat V2.1 Excel Macro, and were used in the risk<br />

assessment to evaluate whether detected constituents in surface water were site-related. Upgradient data<br />

(assumed to represent background or non-Holden Mine influenced areas) were available for sediment,<br />

seeps, .fish, and groundwater; however, there were an insufficient number of samples to calculate area<br />

background values.<br />

Section 5 describes the statistical derivation of area background concentrations for the site.<br />

7.1.2.4 Process for Selection of Indicator Hazardous Substances<br />

1-<br />

IHSs were selected for evaluation in this Baseline Risk Assessment based on a screening process consistent<br />

with Ecology Model Toxics Control Act (MTCA) guidance. IHSs are defined by Ecology as those<br />

hazardous substances that can be used to define site cleanup requirements. Hazardous substances not<br />

selected as IHSs should contribute a small percentage of the overall threat to human health and the<br />

G:\~phuU)O5kpomUlo1dcn-2\ni74~doc 7-6<br />

17693-005011Nuly 27.39995:16 PMDRAFT FINAL R1 REPORT

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