Bioremediation of an Arsenic-Contaminated Site, Using Sulfate-Reducing Bacteria, Bay County, Florida
Type of DegreeMaster's Thesis
Geology and Geography
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In this study an in-situ bioremediation field demonstration project using sulfate-reducing bacteria was conducted at an As-contaminated site, in Bay County, Florida. The background levels of As ranged from 0.13 to 0.34 mg/L which is above the USEPA drinking water standard of 0.01 mg/L . Prior to the bioremediation experiment, groundwater was mildly reducing and slightly acidic. A mixture of water, molasses, ferrous sulfate and agricultural grade-fertilizer was gravity fed to the groundwater through two injection wells. Solid samples of aquifer material were collected from four drilled holes and pre-and post-injection groundwater geochemistry were measured. Field measurements showed that oxidation-reduction potential (ORP) decreased to around -200 mV and pH increased to around 8 in the four injection and monitoring wells the first week after the injections that led to establishment of sulfatereduction conditions. ORP and pH values are returning back to background levels three month after the injections. One week after the injections, As concentration increased in the injection wells and monitoring wells, due to the effects of oxidation and resuspension. However since the second week after the injections, As concentrations have decreased as a result of sulfate-reducing conditions that were established at the site. Three month after the injections, the As concentration in the filtered water samples of two of the monitoring wells have decreased to below the EPA drinking water standard of 0.01 mg/L . The XRD and XRF analyses of solid samples, which precipitated from groundwater and were collected at the bottom of the wells, confirm the presence of As in iron sulfides that appears to be arsenianpyrite. Decreased concentration of dissolved As in the groundwater and presence of As in the form of arsenian-pyrite in the sediments suggest that the As is being adsorbed on to iron sulfide minerals that are being formed. The reduction of sulfate to sulfide, which reacts with ferrous iron in the groundwater appears to have caused the observed As decrease. This As decrease occurred at the same time as the formation of iron sulfide that precipitated at the bottom of the wells. Groundwater flow and solute transport modeling shows that the iron concentration that had been increased up to 483 mg/L in the groundwater after the injections, will decrease to 2.6 mg/L after one year, due to mixing with the groundwater, advective transport and hydrodynamic dispersion, and it will move in a north-west direction. Combination of data from groundwater geochemistry and solid samples analyses are encouraging and indicate that the sorption and co-precipitation of As on biogenic pyrite is happening in the groundwater. Monitoring and sampling of the amended groundwater, at the site, is going to continue for one year to evaluate the efficiency of this bioremediation method over a longer time period.