GROUNDWATER SAMPLING INFLUENCES RISK ASSESSMENT OF ARSENIC IN FRACTURED ROCK AQUIFERS

The presence of arsenic at significant concentrations in fractured bedrock wells in New England is being discovered more and more as sampling progresses. The arsenic in the groundwater has long been attributed to the bedrock; certain rock types and water quality conditions could cause arsenic to leach from naturally occurring arsenic rich minerals (although this has never been proven to be the source with any high degree of certainty). A number of possible anthropogenic sources also exist given the widespread use of arsenic in pesticides, poultry treatment, wood treatment, and as an embalming fluid. Since the summer of 2014, widespread arsenic testing in domestic well water has been conducted for Lebanon, Connecticut. To date approximately 150 domestic wells have been analyzed for arsenic and results indicate that arsenic is present under oxic and alkaline conditions. Although wells with high arsenic concentrations appear to cluster, it is unclear why adjacent houses with similar well construction characteristics do not exhibit similar arsenic concentrations. This may be attributed to concentration averaging where multiple water bearing fractures intersect the well bore resulting in dilution of arsenic bearing waters from one or more fractures. Additionally, this study demonstrated the influence of particulate filters on arsenic concentrations – accumulation of iron oxides on the filters increases the likelihood for arsenic to adsorb to the filter, thus reducing the concentration of arsenic observed. Concentration averaging and the influence of particulate filters on arsenic concentrations make mapped distributions of arsenic contamination based on domestic well samples highly uncertain. Such mapped distributions should be used with a high degree of caution, especially as the basis for risk assessment.