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C-SHADRIFS.6/WPADD.4 <br /> 05/17/90 <br /> SHAD, suggest that a similar investigation of water quality at SHAD will <br /> provide helpful information about the behavior of arsenic. In the <br /> Tulare Lake study (Fujii, 1988) , groundwater samples were collected once <br /> a month for 1 year from a continuously pumping well. Concentrations of <br /> 40 chemical species were reported in both suspended and dissolved <br /> fractions for each sample collected. Arsenic concentrations from that <br /> Isingle well were reported to vary from 64 to 190 ppb. <br /> Two important conclusions were reached about the nature of arsenic in <br /> groundwater in the Tulare Lake area: <br /> 1. Arsenic fluctuates between both suspended and dissolved <br /> species. Sometimes the suspended fraction predominates, and <br /> other times dissolved arsenic is the major form. It is not <br /> clear what controls the ratio of dissolved arsenic to suspended <br /> arsenic. Neither is the nature of the suspended fraction <br /> clear. This is probably the single most important issue in <br /> understanding why arsenic exceeds the EPA MCLS. <br /> 2. ESE's analysis of the Tulare Lake data suggests a correlation <br /> between the concentration of arsenic and other <br /> dissolved/suspended species in the groundwater. However, only <br /> 12 data points are available to consider, and this is not <br /> enough to determine the correlation with certainty. <br /> During one previous sampling effort at SHAD, both dissolved and <br /> suspended arsenic were measured in groundwater. The total arsenic <br /> species behaved similarly to the arsenic at Tulare Lake, fluctuating <br /> between dissolved and suspended. In addition to arsenic, VOCs were the <br /> only other contaminant measured during that sampling effort. Without <br /> any data to compare with arsenic concentrations, the geochemical <br /> behavior of the arsenic as a response to the total system geochemistry <br /> cannot be modeled. <br /> 4 <br />