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WATER QUALITY SITE ASSESSMENT FOR SOILS AND G ROUND WATER �RA po F7 This paper will describe how a site-wide water quality site assessment should be con- ducted to evaluate the impact of soil contaminants on ground water quality. The first step is to establish background conditions for both soils and ground water. The purpose of establishing background concentrations is to accurately assess whether detec- tion of a constituent represents man-made contamination or a naturally occurring con- centration. Assessment of background may also be necessary in determining cleanup levels that are protective of the quality of waters of the State. If a constituent is detected at a statistically significant concentration above background, then it is considered a con- stituent of concern for water quality and remediation may be necessary to protect benefi- cial uses and to restore and protect native water quality, as feasible. A site-wide assess- ment of background levels should be conducted in accordance with the regulations of Article 5 of Chapter 15, Division 3, Title 23 of the California Code of Regulations (CCR). The stratigraphy of the unsaturated and saturated zones beneath the facility should be well defined by the discharger. Hydrogeologic zones must be clearly defined. This is im- portant in order to delineate contaminant transport pathways and water resources that have or may be affected by site contaminants. Background levels need to be established for all distinct lithologies above and below the water table. In urban and industrial areas it is often difficult and sometimes impossible to establish true background concentrations for shallow soils above the water table, due to extensive past waste management activities. Off-site, undisturbed areas may need to be sampled. It will be necessary to establish site-wide background levels for soils based on extractable (soluble) concentrations, determined by a variation of the Title 22 CCR Waste Extraction Test (WET). By using extractable concentrations, the assessment deter- mines what quantities of unsaturated zone contaminants and constituents are mobile and present a threat to water quality. First, the process for establishing background concentrations in ground water and select- ing appropriate water quality limits will be discussed. Second, the process for establish- ing extractable background concentrations in soils will be described. Finally, this paper will describe how to convert soil extract data into a form for comparison with back- ground ground water concentrations and water quality limits. Site-wide Background Assessment of Ground Water Zones 1. Select a minimum of four ground water sampling sites that,based on available in- formation, are out of the influence of past waste management activities. These should be upgradient of existing and potential site contaminants. Off-site sampling may be necessary if unimpacted areas cannot be found on-site.