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C-SHADRIFS.6/WPADD.9 05/17/90 described by Gilbert (1987) at the site of the 109-ppm arsenic measurement. The purpose will be to verify that the sample is part of the entire population of SHAD soils. Seven boring locations will be sampled using split-spoon samples to a total depth of 15 ft. Samples ' will represent composites collected over 0- to 5-ft, 5- to 10-ft, and 10- to 15-ft intervals. The first boring location will be immediately adjacent to the original location at which the sample with 109-ppm ' arsenic concentration was collected. The remaining six sampling locations will be chosen as shown in Figure 1. The 25-ft interval between grid lines will yield a 95-percent confidence level, assuming that the arsenic concentration is at least 5 ft in diameter. Additional soil sampling will also be done to collect material containing high concentrations of arsenic so that the specific identity of the arsenic-bearing compounds can be determined. Arsenic concentrations do not correspond with the VOC contaminant plume, ' suggesting a separate source and/or mechanism for arsenic concentrations in the groundwater. High arsenic values have been measured only in ' certain wells along the western, southern, and eastern boundaries; one well in the South Balloon Area; three wells in the North Balloon Area; and a single well in the Central Depot Area (i.e. , from restricted ' locations) . With one exception, all of the elevated arsenic measurements are restricted to wells in the A-zone aquifer. The purpose of this section is to define the sampling locations and the ' number of samples required to define the mechanisms controlling this restricted anomalous arsenic concentration in the groundwater. To accomplish that, a model incorporating the subsurface geology, the ' surface water and groundwater systems, and the solid mineral phases buffering the high arsenic concentrations in the groundwater must be ' formulated. 1 9