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' C-SHADRIFS.6/WPADD.11 05/17/90 ' For the purpose of this discussion it is assumed that the arsenic geochemistry at any area of elevated arsenic concentration is appropriate for all occurrences at SHAD. Although this may not be true, a detailed understanding at one site of known contamination will provide a model for assessing the others. For the purpose of this work plan, the data from well 407A will be used for modeling because arsenic concentrations there usually vary within a relatively constant and high ' range of 401 to 588 ppb. Therefore, geochemical modeling has a high probability of success. Because no geological conditions are known at this well that are unusual compared to the rest of the depot, an arsenic model here will probably relate to other, if not all, wells with high ' arsenic at SHAD. The range of arsenic concentrations that has been recorded from ' well 407A implies that one or more solid phases, present either in the aquifer or adjacent to it, are acting as the local arsenic source and controlling the arsenic concentration in the water. Considering the dynamics of the groundwater system at SHAD, it is reasonable to assume ' that the solid phases are not in absolute equilibrium with the water. However, because the arsenic concentrations are consistently high at well 407A, it may be that equilibrium is approached at this location. It is probable that dissolution of arsenic-bearing materials involves ' two separate solid phases, an initial source mineral and a secondary mineral phase that buffers the arsenic concentration in the groundwater. The exact chemical identity of these solid phases is essential to understanding the problem and will provide a mechanism for interpreting the situation at other well locations with lower concentrations, where ' either different minerals are buffering the arsenic activity or the movement of groundwater dilutes the arsenic. 11