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A potential method is investigated for remediation of metal-contaminated groundwater by in-situ emplacement of an adsorptive coating on the aquifer matrix. The coating is emplaced by sequentially injecting solutes that react as they mix in the aquifer to form a sparingly soluble solid with a high metal-adsorption capacity. Dissolved metals are removed passively as groundwater flows through the treated aquifer. The potential effectiveness of this method was demonstrated by sequentially injecting first ferrous sulfate and then oxygen solutions into a column of unconsolidated sand, producing a coating of hydrous ferric oxide (HFO) as the unretarded oxygen reacted with Fe(II) bound to the sand. The HFO delayed the breakthrough of Cr(VI) and As(V) by 8 and 30 pore volumes, respectively, relative to the unamended material. Attenuation of solutes by the unamended sand was reaction-rate limited, but coupled transport/equilibrium geochemical modeling matched well with the increased metal attenuation by the coating. Potential advantages of this method include the following: (1) coatings are emplaced preferentially in highconductivity zones, reducing problems caused by aquifer heterogeneity; (2) surface disturbance is minimal; (3) regeneration of the coating is straightforward; (4) no hazardous material is generated; and (5) existing geochemical models can help extrapolate to larger scales.

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Synthetic basaltic ground water was spiked with different ratios of high-purity solium selenate and sodium selenite, to yield ratios of aqueous SeO42to SeO 42from 2100/1 to 1/44000, at temperatures of 25°C and 75°C. The apparent 2h of the solutions was measured by means of a platinum electrode. The time series Eh data were extrapolated to infinite time using the method of Muller [1,2]. Results show no relationship between the analyzed ratios of Se(VI)/Se(IV) in solution and the observed+Eh plues at the platinum electrode. In contrast, acidic solutions of Fe /Fe show nearly perfect Nernstian behavior and rapid electron transfer at the platinum electrode. Based on the results of this study, it is probably invalid to use Eh measurements as input to any equilibrium computer model for purposes of predicting the behavior of a pure selenium system.