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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)

Conclusions and recommendations,   pp. 12-13 PDF (1004.1 KB)

Page 13

conditions is unlikely to lead to arsenic release because sulfides are not the source of
aqueous arsenic. In fact, effective chlorination may limit arsenic release by reducing the
numbers of iron-reducing and other anaerobic bacteria. The low-dose chlorination
treatment at the test well reduced the numbers of all microorganisms tested, but the
populations recovered within three weeks. This suggests that either fresh formation water
re-inoculated the well or that biofilm and scale in the well harbored some microbes from
the disinfectant.
        Taken together, this set of laboratory and field experiments do not provide
scientific evidence for recommendation of low-dose well chlorination in all arsenic-
impacted areas of Wisconsin. Presumably, in areas such as southeast Wisconsin, where
the source of aqueous arsenic is reductive dissolution of Fe (hydr)oxides (Root et al.
2005), imposing strongly oxidizing conditions over short time periods is unlikely to
exacerbate arsenic release because the source is not sulfide minerals. In this setting, high-
dose chlorination may be preferable because it may be more effective in ridding the well
of pathogenic and nuisance bacteria.
        In northeast Wisconsin, the St. Peter sandstone aquifer contains arsenic-bearing
sulfide minerals and arsenic-bearing iron oxides. The complexity and variability in
arsenic geochemistry and aquifer mineralogy in this region preclude a single preferred
method for well disinfection. Where the aquifer is under confined conditions, well water
has very low DO, and aqueous arsenic is relatively low (about a few tens of Lg/L), the
source of arsenic is more likely attributable to reduction of iron hydr(oxides). Under these
conditions at the test well, effectively ridding the well of Fe-reducing bacteria (though
routine pumping or in situ chlorination) improved well water quality. In earlier work,
Sonzogni et al. (2004) demonstrated that high-dose chlorination had no detrimental effect
in this setting.
        Where water levels in wells completed in the St. Peter aquifer suggest unconfined
conditions (that is, where static water levels are close to the elevation of the top of the
formation), well water is generally oxygenated, and where aqueous arsenic
concentrations are relatively high, the source of aqueous arsenic is likely oxidation of
arsenic-bearing sulfide minerals. The current guidelines for low-dose chlorination are
appropriate in this setting. The laboratory experiments reported on here demonstrate that
strongly oxidizing conditions imposed under high-dose chlorine treatment can increase
the rate of sulfide oxidation. However, the experiments also indicated that strongly
oxidizing conditions favor the formation of iron oxides and lead to complex cycling of
iron and arsenic. These experiments were limited in nature and do not provide conclusive
evidence of long-term geochemical impacts to water quality from high-dose chlorination.

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