RELATION OF ARSENIC, IRON, AND MANGANESE CONCENTRATIONS IN GROUND WATER TO BEDROCK GEOLOGY AND LAND USE IN EASTERN NEW ENGLAND
American Geophysical Union (AGU)
Spring Meeting, June 1-4, 1999, Boston, Mass.
Hydrology, session H42F-05
Joseph D. Ayotte, U.S. Geological Survey
Martha G. Nielsen, U.S. Geological Survey
Gilpin R. Robinson, Jr., U.S. Geological Survey
Data from 804 public-supply wells in bedrock of eastern New England, analyzed as part of a USGS National Water Quality Assessment study, indicate that arsenic concentrations in water exceeded 0.005 mg/L in 20 percent of the wells and exceeded 0.01 mg/L in water from 13 percent. This has implications for Safe Drinking Water Act compliance under the lower arsenic standard proposals being examined by USEPA. Data for the study are compliance monitoring analyses obtained from the Maine Department of Human Services, New Hampshire Department of Environmental Services, Massachusetts Department of Environmental Protection and Rhode Island Department of Health. The data are limited in that concentrations of arsenic must meet the drinking water standard of 0.05 mg/L.
Despite this limitation, the concentration of arsenic in bedrock well water is significantly different among six major bedrock lithochemical groups, which are calcareous metasedimentary, sulfidic metasedimentary, undifferentiated metasedimentary, clastic sediments, mafic igneous, and felsic igneous. Multiple comparison tests showed the mean rank concentration of arsenic was significantly higher in water from the calcareous metasedimentary group than in water from the other bedrock groups (p=0.0001). The sulfidic metasedimentary group had the lowest mean rank arsenic concentrations. Iron and manganese concentrations were significantly higher in water from the sulfidic metasedimentary group than in water from the other metasedimentary groups. Spearman's rank correlation coefficients computed for arsenic and iron in water from all lithochemical groups are weak and are only slightly stronger between arsenic and manganese; neither is significant. Iron and manganese correlations are strong, however, for water from all three metasedimentary groups, and these correlations are significant (p=0.01) and positive. Thus, the iron and manganese data were not strong predictors of arsenic concentration. These relations suggest that simple dissolution of arsenic-bearing iron phases (sulfides, hydroxides, arsenates) may not control arsenic in bedrock groundwater, although interactions between precipitation and sorption processes could decouple arsenic and iron. Statistical correlations linking the calcareous metasedimentary group with arsenic may reflect correlations with arsenic sources, solubility controls, land-use characteristics, or vulnerability to contamination.
Relations between arsenic concentration and three land uses (urban, agricultural, and forest, derived from early 1970's high-altitude photography) were tested to determine if an anthropogenic source of arsenic was apparent. The mean rank arsenic concentration was significantly higher in water from wells in agricultural land than in water from wells in forested areas; results for wells in urban land were not significantly different from those from wells in the other two land uses (p=0.013). Relations are, however, weaker for the land-use association than for the lithochemical association. The relation between arsenic concentration and agricultural land may result, in part, from agricultural land being present predominantly on particular geologic substrates. The data examined to date indicate that geology is an apparent predictor of arsenic occurrence and distribution in bedrock ground water.
U.S. Geological Survey
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