Arsenic in North Carolina: public health implications

Abstract

Arsenic is a known human carcinogen and relevant environmental contaminant in drinking water systems. We set out to comprehensively examine statewide arsenic trends and identify areas of public health concern. Specifically, arsenic trends in North Carolina private wells were evaluated over an eleven-year period using the North Carolina Department of Health and Human Services database for private domestic well waters. We geocoded over 63,000 domestic well measurements by applying a novel geocoding algorithm and error validation scheme. Arsenic measurements and geographical coordinates for database entries were mapped using Geographic Information System techniques. Furthermore, we employed a Bayesian Maximum Entropy (BME) geostatistical framework, which accounts for geocoding error to better estimate arsenic values across the state and identify trends for unmonitored locations. Of the approximately 63,000 monitored wells, 7712 showed detectable arsenic concentrations that ranged between 1 and 806μg/L. Additionally, 1436 well samples exceeded the EPA drinking water standard. We reveal counties of concern and demonstrate a historical pattern of elevated arsenic in some counties, particularly those located along the Carolina terrane (Carolina slate belt). We analyzed these data in the context of populations using private well water and identify counties for targeted monitoring, such as Stanly and Union Counties. By spatiotemporally mapping these data, our BME estimate revealed arsenic trends at unmonitored locations within counties and better predicted well concentrations when compared to the classical kriging method. This study reveals relevant information on the location of arsenic-contaminated private domestic wells in North Carolina and indicates potential areas at increased risk for adverse health outcomes.

Figure 1

The top thirty-five counties that exceed the EPA standard (10 μg/L). Counties are ranked by the percent of wells that exceed the EPA standard and are represented in a heat map. Counties with percent of wells exceeding the statewide 2.25% appear in red-scale, while those below the statewide percent appear in blue-scale. Counties with no information available appear in white.

Figure 2

Geocoded arsenic concentrations in 2009. (A) Samples exceeding the EPA standard are shown in black. Well locations of samples below the standard appear in gray. (B) County averages are displayed in grayscale and the number of arsenic analyses in 2009 appear within each county. *No wells were sampled in Chowan County in 2009. (C) A classical kriging method estimated arsenic distribution across the state at unmonitored locations. (D) The Bayesian Maximum Entropy framework estimated arsenic distribution across the state at unmonitored locations.