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Gotkowitz, Madeline B.; Roden, Eric E.; Schreiber, Madeline E.; Shelobolina, Evgenya S. / Mineral transformation and release of arsenic to solution under the oxidizing conditions of well disinfection
[DNR-192] (2007)

Introduction,   pp. 1-4 PDF (2.1 MB)


Page 1


INTRODUCTION
Background
       Arsenic release from naturally-occurring sources is the dominant cause of
elevated arsenic concentrations in ground water (Nordstrom 2002; Welch et al. 2000).
Geochemical mechanisms including reductive dissolution of iron (hydr)oxides, release of
sorbed arsenic from mineral surfaces, and oxidation of arsenic-bearing sulfide minerals
can release arsenic to natural waters. These reactions can be reversible, in chemical
disequilibrium, and can be abiotic or microbially mediated. These processes can
potentially occur simultaneously, complicating identification of a single mechanism of
arsenic release in a particular environment. Spatial or temporal changes in geochemistry
can transpose a geologic source of dissolved arsenic to an arsenic sink, and vice versa.
       Naturally occurring arsenic is present in many aquifers in Wisconsin. In east-
central Wisconsin, the primary source of arsenic is sulfide minerals in the St. Peter
sandstone (Schreiber et al. 2000). Reduction of arsenic-bearing iron (hydr)oxides is likely
a secondary source of arsenic to groundwater in this region where the aquifer is under
confined conditions and the groundwater is reducing (Gotkowitz et al 2004). Up to 30
percent of wells in some parts of Winnebago County have arsenic concentrations greater
than 5 tg/L. Arsenic also impacts groundwater quality in some glacially deposited sand
and gravel aquifers in Wisconsin, where the lack of oxygenated recharge along deep
flowpaths leads to reducing geochemical conditions and release of arsenic via reductive
dissolution of Fe-(hydr)oxides (Root et al. 2005). These findings are similar to other
glacial aquifers throughout the Midwest (Warner 2001; Thomas 2003; Kelly et al. 2005).
       Domestic wells completed in bedrock aquifers are often constructed with long
open intervals (lOs to 100 of m) that provide a significant volume of well bore storage
relative to typical domestic water use. Therefore, the quality of the well water can be
affected by rock-water interactions that occur over a long residence time in the borehole.
Although the potential for transport within the subsurface depends on aquifer water
chemistry, reactions that occur in the borehole environment have the potential to control
human exposure to arsenic via well water. Conditions in wells that may affect arsenic
concentrations include changes in redox related to pump action, which introduces oxygen
into well water, and the potential for the growth of microorganisms that facilitate a
variety of biogeochemical reactions (Taylor et al. 1997).
       Current Wisconsin Department of Natural Resources (DNR) guidelines for
maintenance of domestic wells in arsenic sensitive areas suggest disinfect with a low-
dose chlorine solution. This guidance is based upon anecdotal evidence suggesting that
well disinfection treatments (e.g., chlorination) to control iron oxidizing microbes may
increase concentrations of arsenic in well water (DNR 2002). In other areas of the state,
the DNR recommends in situ well disinfection with a high chlorine concentration and
longer contact time (DNR 1999; DNR 2005). At the foundation of the arsenic-sensitive
guidance is the assumption that the oxidizing strength of chlorine disinfectant results in
chemical oxidation of arsenic-rich sulfide minerals in the aquifer. However, in areas
where microbially-facilitated reduction of iron (hydr)oxides contributes arsenic to
groundwater, successfully ridding a well of iron-reducing bacteria through in situ


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