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' C-SHADRIFS.6/WPADD.2 05/17/90 2.0 EXTENDED GROUNDWATER MONITORING Arsenic is present in the groundwater and soils underlying SHAD. In terms of arsenic, the SHAD soils are natural soil assemblages ' statistically indistinguishable from offpost soils that were collected adjacent to the post. Arsenic concentrations in SHAD soil range between 0.7 part per million (ppm) and 109 ppm, with an average value between 5 and 6 ppm. Arsenic concentrations in groundwater have been measured in excess of the U.S. Environmental Protection Agency Maximum Contaminant Level (EPA MCL) of 50 parts per billion (ppb) in the groundwater at 35 onpost monitor wells. As an example, the average arsenic concentration at monitor well 407A through the 5-year period of the groundwater sampling program is 489 ppb. Analysis of a limited number (10 borings) of soil samples collected on 5-foot (ft) depth intervals indicates that arsenic concentrations are higher in the aquifer materials (approximately 35 ft) than in the near- surface soils. Since the groundwater is more or less in equilibrium with the materials composing the aquifer, these aquifer materials at SHAD must be the source of arsenic in the groundwater. However, the ' presence of arsenic-enriched material within the aquifer may not be the only cause of the elevated arsenic concentrations in the groundwater. ' One of three explanations is likely to account for the elevated arsenic concentrations in groundwater on and near SHAD: ' 1. The offsite or onsite use of arsenic-enriched materials (i.e. , rodenticide) may have been an anthropogenic cause of contamination. 2. Arsenic is a component of the naturally occurring minerals which appear in the subsurface at SHAD. The geochemical Icharacteristics of the groundwater that result from the presence of the volatile organic compound (VOC) contamination i 2 1 i