SPATIAL ASSOCIATIONS BETWEEN ARSENIC IN GROUNDWATER, SEDIMENTS, BEDROCK, AND AGRICULTURAL LANDUSE IN NEW ENGLAND
Gilpin R. Robinson, Jr. 1 and Joseph D. Ayotte 2
U.S. Geological Survey,
Reston, Va. 20912 1 and Pembroke, NH 03275 2

Analytical data for arsenic concentrations in public-supply bedrock-groundwater wells, stream-sediment samples, and unmineralized bedrock samples from Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont provide a means to (1) map the occurrence and distribution of arsenic in the surface environment throughout New England and (2) measure the spatial associations between these data and other information on geologic units and land-use features in the region. Interpolation gridding of the bedrock groundwater, stream sediment, and rock chemistry databases provides spatial coverage over 73, 71, and 31 percent of the New England area, respectively. Significant sources of arsenic to groundwaters and sediments include both natural weathering of rocks and anthropogenic applications of arsenical pesticides that were commonly used on apple, blueberry, and potato crops during the early twentieth century in New England. Agricultural census and landuse/land cover data were used to develop a spatial dataset portraying the intensity of cultivation of apple, blueberry, and potato crops in New England. Ten percent of the public-supply bedrock wells in our New England database exceed the new EPA arsenic standard of 10 mg/L. Twenty percent of the wells exceed an arsenic value of 5 mg/L; the distribution area of these high arsenic wells in the interpolation grid occupies 27% of the New England region where data are available. Equivalent statistics for stream sediments and rocks indicate that 20% of their arsenic values exceed 7.25 and 6.5 ppm and occupy 19 and 27 percent of the area of data coverage, respectively. Calcpelite, felsic volcanic, and sulfidic schist rock groups have higher background arsenic values than all other rock groups in New England. Kappa statistics (Bonham-Carter, 1994, chapter 8), measuring spatial agreement at the 80th percentile level, indicate that the distribution of arsenic in bedrock groundwater wells has a strong positive correlation with (1) the Coastal Maine and the Central Maine-New Hampshire geologic provinces, (2) calcpelite bedrock in the Central Maine-New Hampshire geologic province, (3) volcanic and sulfidic schist bedrock in the Coastal Maine geologic province, and (4) stream sediment and rock geochemistry. Bedrock wells have a weak positive correlation with past agricultural landuse. Stream sediments, which integrate both natural and anthropogenic sources, have a strong positive correlation with groundwater chemistry, geologic provinces, and rock chemistry and a weak positive correlation with past agricultural landuse. Although spatial correlation is not sufficient to demonstrate cause-and-effect, the spatial statistics favor rock-based arsenic as the dominant source of arsenic in stream sediments and groundwaters. The distribution of bedrock geology features at the geologic province and lithology group level closely match the areas of elevated arsenic in both groundwater and stream sediments.