Abstract
We examined the chemical reactions influencing dissolved concentrations, speciation, and transport of naturally occurring arsenic (As) in a shallow, sand and gravel aquifer with distinct geochemical zones resulting from land disposal of dilute sewage effluent. The principal geochemical zones were: (1) the uncontaminated zone above the sewage plume [350 μM dissolved oxygen (DO), pH 5.9]; (2) the suboxic zone (5 μM DO, pH 6.2, elevated concentrations of sewage-derived phosphate and nitrate); and (3) the anoxic zone [dissolved iron(II) 100–300 μM, pH 6.5–6.9, elevated concentrations of sewage-derived phosphate]. Sediments are comprised of greater than 90% quartz but the surfaces of quartz and other mineral grains are coated with nanometer-size iron (Fe) and aluminum (Al) oxides and/or silicates, which control the adsorption properties of the sediments. Uncontaminated groundwater with added phosphate (620 μM) was pumped into the uncontaminated zone while samples were collected 0.3 m above the injection point. Concentrations of As(V) increased from below detection (0.005 μM) to a maximum of 0.07 μM during breakthrough of phosphate at the sampling port; As(III) concentrations remained below detection. These results are consistent with the hypothesis that naturally occurring As(V) adsorbed to constituents of the coatings on grain surfaces was desorbed by phosphate in the injected groundwater. Also consistent with this hypothesis, vertical profiles of groundwater chemistry measured prior to the tracer test showed that dissolved As(V) concentrations increased along with dissolved phosphate from below detection in the uncontaminated zone to approximately 0.07 and 70 μM, respectively, in the suboxic zone. Concentrations of As(III) were below detection in both zones. The anoxic zone had approximately 0.07 μM As(V) but also had As(III) concentrations of 0.07–0.14 μM, suggesting that release of As bound to sediment grains occurred by desorption by phosphate, reductive dissolution of Fe oxides, and reduction of As(V) to As(III), which adsorbs only weakly to the Fe-oxide-depleted material in the coatings. Results of reductive extractions of the sediments suggest that As associated with the coatings was relatively uniformly distributed at approximately 1 nmol/g of sediment (equivalent to 0.075 ppm As) and comprised 20%-50% of the total As in the sediments, determined from oxidative extractions. Quartz sand aquifers provide high-quality drinking water but can become contaminated when naturally occurring arsenic bound to Fe and Al oxides or silicates on sediment surfaces is released by desorption and dissolution of Fe oxides in response to changing chemical conditions.
Highlights
The upper part of the sewage plume, referred to as the suboxic zone37 ͑below approximately 11 m to sea level Fig. 2͒, is characterized by low but measurable concentrations of dissolved oxygenapproximately 5 M, pH values near 6.2, and elevated concentrations of phosphorus and dissolved saltsthe latter indicated by specific conductance values above 100 S/cm
Chemical weathering reactions driven by oxic, mildly acidic groundwater over thousands of years resulted in the mass transfer of As from reduced forms in the interior of primary minerals of these glacial outwash sediments to AsVadsorbed onto the hydrous Fe and Al oxide and silicate particles that comprise the coatings on sediment-grain surfaces
Deeper in the sewage plume, biodegradation of organic compounds transported away from the disposal beds resulted in reductive dissolution of Fe oxides associated with the coatings; AsVwas reduced to AsIIIeither by As-respiring microorganisms or abiotic reactions
Summary
Biogeochemical reactions in aquifers can mobilize naturally occurring arsenicAsfrom sediments that are not enriched in As compared to its average abundance in crustal rocks[1,2] Aquifers with dissolved As concentrations significantly higher than 0.67 micromoles per literM, which equals 50 micrograms per literg/l, that supply large populations with drinking water have received considerable attention.[1,2,3] With the recent lowering of the maximum contaminant levelMCLfor As in drinking water in the United States to 0.13 M10 g/l, interest in understanding the fate and transport of As in potential drinking-water supplies aPresented at the ACS Division of Geochemistry Symposium, ‘‘The Impact of Nanoparticle Growth and Transformation Processes on Contaminant Geochemical Cycling,’’ New Orleans, March 2003. J. Geochemistry Vol 5, No 1, March 2004. Fox improved understanding of subsurface biogeochemistry and could provide important information for maintaining critical drinking water supplies.[12]
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