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Dames & Moore, 1999 - USDA Forest Service

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Frequency of Detection<br />

If a constituent was detected in less than 5 percent of the samples of a medium within an exposure area it<br />

was eliminated as an MS. This approach is consistent with USEPA risk assessment guidance (USEPA.<br />

1989) and is recommended in MTCA guidance (WAC 173-340-708[2][b][vi]).<br />

Background Concentrations<br />

In order to determine which constituents are Site-related, analytical data were compared with the<br />

statistically-derived background concentration, when available. For soil and tailings. site analytical data<br />

were compared with Site-specific area background (Section 5) or natural background concentrations for the<br />

Yakima Basin available in Natwal Background Soil Metals Concentrations in Wahineon State (Ecolo~.<br />

1994). Surface water analytical data were also compared with statisticallyderived area background<br />

concentrations as described in Section 5. If the maximum detected concentration was less than the<br />

corresponding background concentration, the constituent was eliminated as an IHS. This approach is<br />

consistent with USEPA risk assessment guidance (USEPA, 1989) and is recommended in MTCA guidance<br />

(WAC 173-340-708[2]b][iv]).<br />

Method A Levels<br />

If the maximum concentration of a constituent was less than the relevant MTCA Method A level, it was<br />

considered to contribute a small percentage of the overall risk and was eliminated as an IHS. Method A<br />

levels are generic cleanup levels which were obtained from The Model Toxics Control Act-Cleanup (WAC<br />

173-340), amended January 1996. The following MTCA Method A levels were used for each exposure<br />

medium:<br />

Surface Soil and Home-grown Produce. Method A levels for residential soil (WAC 173-<br />

340-740[2])<br />

Tailings. Method A levels for residential soil (WAC 173-340-740[2])<br />

Sediment. Method A levels for residential soil (WAC 173-340-740[2])<br />

Railroad Creek and Copper Creek Water and Fish. Method A levels for surface water<br />

(national ambient water quality criteria for ingestion of water and fish pursuant to Section<br />

304 of the Clean Water Act, as referred to in WAC 173-340-730[2][a][2])<br />

Seeps. Method A levels for groundwater (or Maximum Contaminant Levels if Method A<br />

levels not available) (WAC 173-340-720[2])<br />

1500.Level Main and Ventilator Portal Drainages. Method A levels for groundwater (or<br />

.Maximum Contaminant Levels if Method A levels not available) (WAC 173-340-720[2])<br />

Sauna Water (Copper Creek Diversion). Method A levels for groundwater (or<br />

Maximum Contaminant Levels if Method A levels not available) (WAC 173-340-720[2])<br />

Groundwater. Method A levels for groundwater (or Maximum Contaminant Levels if<br />

Method A levels not available) (WAC 173-340-720[2])<br />

Fish Tissue. No Method A levels were available for fish tissue. Instead, U.S. EPA Region<br />

111 Risk-Based Concentrations (Oct. 1998) for fish were used.<br />

G:\wpd.uUW)J~\hoIkn-2\ri\700doc 7-14<br />

1769300SO19Uuly 27.<strong>1999</strong>;5:16 P MDM FINAL RI REWRT

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