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4.4 ENGINEERING CONSIDERATIONS Laboratory testing demonstrated that both Fe(II) and Cascade° could decrease the concentration of Cr(VI) in groundwater, both in the presence and absence of site soil Several points or observations were made that should be considered when evaluating the feasibility of each reductant for full-scale application 1 Both reductants decreased the concentration of Cr(VI) in groundwater, but the effectiveness of each was inhibited by soil This effect was greater for Cascade° than for Fe(u) 2 An appropriate dose of Fe(II) solution in the field is probably between about 100 mg/L and 500 mg/L Fe(II), though PRIMA Environmental strongly recommends that this suggestion be reviewed by qualified personnel and confirmed by a field pilot test before beginning full-scale treatment This suggestion is based on the observations that i) a dose of 100 mg/L Fe(ED decreased the concentration of Cr(VI) to below the detection lima of 1 µg/L when the soil to liquid ratio was 1 5 I' (COC Removal test) and u) when the soil to liquid ration was 1 2 (SRD test) and the initial Fe(II) was 500 mg/L, 3% of the Fe(H) remained after 14 days, suggesting that Fe(11) can persist for several days in the sub-surface 3 Laboratory testing provided a first approximation of the amount of Cascade° that would be needed to reduce the concentration of Cr(VI), but an actual dose should be determined from a field pilot test Specifically, in the laboratory, a dose of 0 8 mUL MW-3 groundwater reduced Cr(VI) by 98% in the presence of soil 4 Treatment with Fe(II) affected more secondary water quality parameters than did treatment with Cascade° However, it is beyond PRIMA Environmental's Scope of Work to assess whether the changes were "significant" with respect to this site It should also be emphasized that the magnitude of the changes may be different ' in the field due to differences between field and laboratory conditions Thus, the results presented in this study should be used as a guide to help determine which parameters, if any, could potentially be of concern 5 The amount of acid needed to decrease the pH of site tap water to pH 3 (in order to prepare Fe(II) solution) is about 300 mL 98% H-)SO4/1000 gallons of water 6 Site soil can neutralize about 100 mmoles H+/kg soil and about 10 mmol OH-/kg soil Acid must be neutralized if Fe(II) is used, while base must be neutralized if Cascade' is used • PRIMA Environmentali 15 Eval of Reductants .January 19, 2005 ATC-Diamond Walnut