MONTHS-LONG SPIKE IN AQUEOUS ARSENIC FOLLOWING DOMESTIC WELL INSTALLATION AND DISINFECTION: SHORT- AND LONG-TERM DRINKING WATER QUALITY IMPLICATIONS

Exposure to high concentrations of geogenic arsenic (As) 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 geochemical changes that persist for 12+ months, as illustrated in a 2019 study about temporal geochemical changes, including increasing As concentrations, in 250 new domestic wells in Minnesota, north-central U.S. One atypical study well had extremely high initial As (1,550 µg/L) that substantially decreased after 15 months (5.2 µg/L). The mud-rotary drilling and development of the study well were ordinary: nothing observable indicated the very high initial As concentration. We hypothesized that oxidation of As-containing sulfides (which lowers pH) combined with low pH dissolution of As-bearing Fe (oxyhydr)oxides caused the very high As concentration. Geochemical equilibrium considerations and modeling supported our hypothesis. Groundwater redox conditions are poised at the Fe(III)_(s)/Fe(II)_(aq) stability boundary, indicating As-bearing Fe (oxyhydr)oxide mineral sensitivity to pH and redox changes.

The geochemical measurements at our study well and the geochemical modeling results illustrate that the physical and geochemical disturbance of well drilling and disinfection can result in very high As concentrations in wells when a low concentration of Fe sulfides are present in sediment. Our study demonstrates that there is a potential risk of subsequent As release in the study region or wells in a similar geochemical setting during routine disinfection operations using bleach or other strong oxidizing agents. Changing groundwater geochemistry can also have negative implications for home water treatment (e.g., reduced As removal efficiency, iron fouling), which can lead to ongoing but unrecognized hazard of As exposure from domestic well water. Our study illustrates why water quality sampling before putting a new potable well into service is crucial for public health protection – and why resampling drinking water wells could improve long-term hazard mitigation. Our results may inform As mobilization processes and geochemical sensitivity in similarly complex aquifers in Southeast Asia and elsewhere.