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tC-SHADRIFS.6/WPADD.3 05/17/90 1 have increased the solubility of the arsenic-bearing minerals, resulting in arsenic concentrations in excess of the EPA MCL. 3. The arsenic concentrations in the groundwater beneath SHAD may be the result of naturally occurring mineral assemblages in contact with groundwater which reflects conditions unique to the San Joaquin Valley. Identifying the contamination mechanism will have important implications for the FS that follows the RI performed at SHAD. For instance, the volume of groundwater to be treated and the treatment process itself will be more accurately modeled after the investigation is complete. A computerized database currently is being compiled to help analyze the groundwater chemistry data that have been collected at SHAD over the past 5 years. The analysis is incomplete, but it appears that different mechanisms are acting on the transport of arsenic in the groundwater than act on VOCs. For instance, plumes of VOCs have been mapped, and the migration path can be predicted. In addition, the incidence of the ' VOC plume with downgradient wells can be calculated with accuracy. However, arsenic data suggest that excessive levels of dissolved arsenic occur only at certain wells, and at those wells the arsenic ' concentrations in groundwater have fluctuated between less than 50 ppb and more than 500 ppb in less than 3 months (e.g. , monitor well 407A) . Arsenic moves between suspended and dissolved species over very short horizontal distances (less than 25 ft) in less than 3 months. This conclusion affects remediation decisions because the behavior of arsenic will require treatment on a much closer interval than if arsenic were advectively transported. Water quality data collected elsewhere in the San Joaquin Valley, specifically at the Tulare Lake Drainage District 275 miles south of 3