FIELD-SCALE BIOREMEDIATION OF ARSENIC-CONTAMINATED GROUNDWATER USING SULFATE REDUCING BACTERIA: GEOCHEMICAL MONITORING AND HYDROLOGICAL MODELING

Arsenic is a common metalloid contaminant found in sediments and groundwater from both natural and anthropogenic sources. Research shows that the metabolic effects of Fe- and Mn-reducing bacteria are responsible for releasing arsenic into alluvial aquifers. It has also been proposed that arsenic may be immobilized as sulfide solids under sulfate-reducing conditions. This research investigates arsenic sequestration in iron sulfide, at field scale, as an emerging remediation technology in contaminated groundwater. An unconfined sandy aquifer at an industrial site, in northwest Florida, contains high concentration (> hundreds of ppb) of arsenic (As) after long-time herbicide and pesticide use. In this pilot experiment, 3,000 gallons of solution, mixed with ferrous sulfate, molasses, and fertilizer, were injected into a heavily contaminated zone to stimulate sulfate reducing bacteria (SRB) growth. Numerical models MODFLOW/MT3D and Geochemist’s Workbench were used to model the transport of injected chemicals and geochemical consequence in the aquifer.

The mean Eh values for groundwater prior to injection was about +46 mV. The Eh values fell rapidly below -150 mV in one week after the injection and the reducing conditions were maintained over 100 days. Initially the concentrations of iron, arsenic, and sulfate increased significantly a couple weeks after injection, indicating that the injection may cause re-suspension and oxidation of pre-existing sulfide solids. Over time, the concentrations of arsenic in close-by monitoring wells fell from initial levels of 280-430 ppb to less than 20 ppb within 100 days. We observe similar decreasing trends for dissolved iron, sulfate, and phosphate. XRD, XRF, SEM-EDAX, and ICP-MS analysis confirmed the precipitation of iron sulfides as aggregates of arsenian pyrite microcrystals. Our field experiments and laboratory analysis demonstrated that indigenous SRB were capable of anaerobically catalyzing bacterial sulfate reduction, effectively removing arsenic from contaminated groundwater through co-precipitation and adsorption on arsenian pyrite biominerals. The monitoring efforts will continue for one year to assess whether the high surface areas of sulfide biominerals have the capacity to adsorb arsenic over time after the bacterial activity has slowed.