Months-long spike in aqueous arsenic following domestic well installation and disinfection: Short- and long-term drinking water quality implications.

Exposure to high concentration geogenic arsenic via groundwater is a worldwide health concern. Well installation introduces oxic drilling fluids and hypochlorite (a strong oxidant) for disinfection, thus inducing geochemical disequilibrium. Well installation causes changes in geochemistry lasting 12 + months, as illustrated in a recent study of 250 new domestic wells in Minnesota, north-central United States. One study well had extremely high initial arsenic (1550 µg/L) that substantially decreased after 15 months (5.2 µg/L). The drilling and development of the study well were typical and ordinary; nothing observable indicated the very high initial arsenic concentration. We hypothesized that oxidation of arsenic-containing sulfides (which lowers pH) combined with low pH dissolution of arsenic-bearing Fe (oxyhydr)oxides caused the very high arsenic concentration. Geochemical equilibrium considerations and modeling supported our hypothesis. Groundwater equilibrium redox conditions are poised at the Fe(III)(s)/Fe(II)(aq) stability boundary, indicating arsenic-bearing Fe (oxyhydr)oxide mineral sensitivity to pH and redox changes. Changing groundwater geochemistry can have negative implications for home water treatment (e.g., reduced arsenic removal efficiency, iron fouling), which can lead to ongoing but unrecognized hazard of arsenic exposure from domestic well water. Our results may inform arsenic mobilization processes and geochemical sensitivity in similarly complex aquifers in Southeast Asia and elsewhere.

A mass balance approach to investigate arsenic cycling in a petroleum plume.

Natural attenuation of organic contaminants in groundwater can give rise to a series of complex biogeochemical reactions that release secondary contaminants to groundwater. In a crude oil contaminated aquifer, biodegradation of petroleum hydrocarbons is coupled with the reduction of ferric iron (Fe(III)) hydroxides in aquifer sediments. As a result, naturally occurring arsenic (As) adsorbed to Fe(III) hydroxides in the aquifer sediment is mobilized from sediment into groundwater. However, Fe(III) in sediment of other zones of the aquifer has the capacity to attenuate dissolved As via resorption. In order to better evaluate how long-term biodegradation coupled with Fe-reduction and As mobilization can redistribute As mass in contaminated aquifer, we quantified mass partitioning of Fe and As in the aquifer based on field observation data. Results show that Fe and As are spatially correlated in both groundwater and aquifer sediments. Mass partitioning calculations demonstrate that 99.9% of Fe and 99.5% of As are associated with aquifer sediment. The sediments act as both sources and sinks for As, depending on the redox conditions in the aquifer. Calculations reveal that at least 78% of the original As in sediment near the oil has been mobilized into groundwater over the 35-year lifespan of the plume. However, the calculations also show that only a small percentage of As (∼0.5%) remains in groundwater, due to resorption onto sediment. At the leading edge of the plume, where groundwater is suboxic, sediments sequester Fe and As, causing As to accumulate to concentrations 5.6 times greater than background concentrations. Current As sinks can serve as future sources of As as the plume evolves over time. The mass balance approach used in this study can be applied to As cycling in other aquifers where groundwater As results from biodegradation of an organic carbon point source coupled with Fe reduction.

The role of alluvial aquifer sediments in attenuating a dissolved arsenic plume.

In a crude-oil-contaminated sandy aquifer at the Bemidji site in northern Minnesota, biodegradation of petroleum hydrocarbons has resulted in release of naturally occurring As to groundwater under Fe-reducing conditions. This study used chemical extractions of aquifer sediments collected in 1993 and 2011-2014 to evaluate the relationship between Fe and As in different redox zones (oxic, methanogenic, Fe-reducing, anoxic-suboxic transition) of the contaminated aquifer over a twenty-year period. Results show that 1) the aquifer has the capacity to naturally attenuate the plume of dissolved As, primarily through sorption; 2) Fe and As are linearly correlated in sediment across all redox zones, and a regression analysis between Fe and As reasonably predicted As concentrations in sediment from 1993 using only Fe concentrations; 3) an As-rich "iron curtain," associated with the anoxic-suboxic transition zone, migrated 30m downgradient between 1993 and 2013 as a result of the hydrocarbon plume evolution; and 4) silt lenses in the aquifer preferentially sequester dissolved As, though As is remobilized into groundwater from sediment after reducing conditions are established. Using results of this study coupled with historical data, we develop a conceptual model which summarizes the natural attenuation of As and Fe over time and space that can be applied to other sites that experience As mobilization due to an influx of bioavailable organic matter.

Arsenic Cycling in Hydrocarbon Plumes: Secondary Effects of Natural Attenuation.

Monitored natural attenuation is widely applied as a remediation strategy at hydrocarbon spill sites. Natural attenuation relies on biodegradation of hydrocarbons coupled with reduction of electron acceptors, including solid phase ferric iron (Fe(III)). Because arsenic (As) adsorbs to Fe-hydroxides, a potential secondary effect of natural attenuation of hydrocarbons coupled with Fe(III) reduction is a release of naturally occurring As to groundwater. At a crude-oil-contaminated aquifer near Bemidji, Minnesota, anaerobic biodegradation of hydrocarbons coupled to Fe(III) reduction has been well documented. We collected groundwater samples at the site annually from 2009 to 2013 to examine if As is released to groundwater and, if so, to document relationships between As and Fe inside and outside of the dissolved hydrocarbon plume. Arsenic concentrations in groundwater in the plume reached 230 µg/L, whereas groundwater outside the plume contained less than 5 µg/L As. Combined with previous data from the Bemidji site, our results suggest that (1) naturally occurring As is associated with Fe-hydroxides present in the glacially derived aquifer sediments; (2) introduction of hydrocarbons results in reduction of Fe-hydroxides, releasing As and Fe to groundwater; (3) at the leading edge of the plume, As and Fe are removed from groundwater and retained on sediments; and (4) downgradient from the plume, patterns of As and Fe in groundwater are similar to background. We develop a conceptual model of secondary As release due to natural attenuation of hydrocarbons that can be applied to other sites where an influx of biodegradable organic carbon promotes Fe(III) reduction.

Rates of As and Trace-Element Mobilization Caused by Fe Reduction in Mixed BTEX-Ethanol Experimental Plumes.

Biodegradation of organic matter, including petroleum-based fuels and biofuels, can create undesired secondary water-quality effects. Trace elements, especially arsenic (As), have strong adsorption affinities for Fe(III) (oxyhydr)-oxides and can be released to groundwater during Fe-reducing biodegradation. We investigated the mobilization of naturally occurring As, cobalt (Co), chromium (Cr), and nickel (Ni) from wetland sediments caused by the introduction of benzene, toluene, ethylbenzene, and xylenes (BTEX) and ethanol mixtures under iron- and nitrate-reducing conditions, using in situ push-pull tests. When BTEX alone was added, results showed simultaneous onset and similar rates of Fe reduction and As mobilization. In the presence of ethanol, the maximum rates of As release and Fe reduction were higher, the time to onset of reaction was decreased, and the rates occurred in multiple stages that reflected additional processes. The concentration of As increased from <1 µg/L to a maximum of 99 µg/L, exceeding the 10 µg/L limit for drinking water. Mobilization of Co, Cr, and Ni was observed in association with ethanol biodegradation but not with BTEX. These results demonstrate the potential for trace-element contamination of drinking water during biodegradation and highlight the importance of monitoring trace elements at natural and enhanced attenuation sites.