By Joseph D. Ayotte, Martha G. Nielsen, Gilpin R. Robinson, Jr., and Richard B. Moore
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In a study of arsenic concentrations in public-supply wells in the New England Coastal Basins, concentrations at or above 0.005 mg/L (milligrams per liter) were detected in more samples of water from wells completed in bedrock (25 percent of all samples) than in water from wells completed in stratified drift (7.5 percent of all samples). Iron and manganese were detected (at concentrations of 0.05 and 0.03 mg/L, respectively) at approximately the same frequency in water from wells in both types of aquifers.
Concentrations of arsenic in public-supply wells drilled in bedrock (in the National Water-Quality Assessment Program New England Coastal Basins study unit) vary with the bedrock lithology. Broad groups of lithogeochemical units generalized from bedrock lithologic units shown on state geologic maps were used in the statistical analyses. Concentrations of arsenic in water from public-supply wells in metasedimentary bedrock units that contain slightly to moderately calcareous and calcsilicate rocks (lithogeochemical group Mc) were significantly higher than the concentrations in five other groups of bedrock units in the study unit. Arsenic was detected, at or above 0.005 mg/L, in water from 44 percent of the wells in the lithogeochemical group M c and in water from less than 28 percent of wells in the five other groups. Additionally, arsenic concentrations in ground water were the lowest in the metasedimentary rocks that are characterized as variably sulfidic (group Ms ). Generally, concentrations of arsenic were low in water from bedrock wells in the felsic igneous rocks (group If ) though locally some bedrock wells in granitic rocks are known to have ground water with high arsenic concentrations, especially in New Hampshire.
The concentrations of arsenic in ground water also correlate with land-use data; significantly higher concentrations are found in areas identified as agricultural land use than in undeveloped areas. There is, however, more agricultural land in areas overlying the metasedimentary rocks of lithogeochemical groups Mc and the minimally-deformed clastic sediments of group Mmd than in areas overlying other lithogeochemical groups. This correlation complicates the interpretation of sources of arsenic to ground water in bedrock. A test of this association revealed that relations between arsenic concentrations and the metasedimentary rocks of group Mc are not weakened when data associated with agricultural land use is removed; the reverse is true, however, if the data associated with the group Mc are removed from the analysis.
The occurrence and variability of arsenic in water from bedrock supply wells could be related to several factors. These include (1) the distribution and chemical form of arsenic in soils and rocks that are part of the ground-water-flow system, (2) the characteristics that influence the solubility and transport of arsenic in ground water, (3) the differing degrees of vulnerability of ground-water supplies to surface contamination, and (4) the spatial associations between land use, geology, and ground-water-flow patterns. Strong relations between agricultural land use and the metasedimentary rocks of group Mc complicate the interpretation of arsenic source to water in these bedrock aquifers. This is due in part to the past use of arsenical pesticides; additionally, few whole-rock geochemical data are available for the rock types in the lithogeochemical groups of aquifers that contain ground water with elevated concentrations of arsenic. Without such data, identifying specific bedrock types as arsenic sources is not possible. In southern Maine and south-central New Hampshire, and in northern Massachusetts, the few available whole-rock analyses suggest, at least for these local areas, a connection between known bedrock chemistry and ground-water arsenic levels.
Although the lithogeochemical group and land-use category variables individually describe much of the variance in the concentrations of arsenic in ground water, the lithogeochemical relation is statistically stronger than the land-use relation. Low concentrations of arsenic in water from bedrock public-supply wells are associated with the metasedimentary rocks of group Ms (characterized as variably sulfidic). This association could reflect a variety of factors and suggests that simple dissolution of arsenic-bearing iron phases, such as sulfides, may not explain concentrations of arsenic in water in this bedrock aquifer group. Whole-rock geochemical data and more complete water-chemistry data, as well as studies of historical variation of arsenic concentrations (time-line studies), and site-specific studies, will be critical in addressing the arsenic source issue.
Purpose and Scope
Health and Regulatory Concerns Related to Arsenic, Iron, and Manganese in Ground Water
Description of the Study Area
Study Design and Methods
Lithogeochemical Reclassification of Bedrock Units and Regional-Scale Approach
Sources of Ground-Water Data and Well Selection
Relation of Arsenic, Iron, and Manganese in Ground Water to Aquifer Type, Lithogeochemistry, and Land Use
Occurrence of Arsenic, Iron, and Manganese by Aquifer Type
Occurrence of Arsenic, Iron, and Manganese by Lithogeochemical Group
Anomalous Concentrations of Arsenic by Bedrock Geologic Unit
Relation of Arsenic in Ground Water to Land Use and Lithology
Possible Sources of, and Controls on, Arsenic in Ground Water
Arsenic in Water from Public-Supply Wells and Future Drinking-Water Standards
Summary and Conclusions
Appendix 1: Geologic, Land-Use, and Chemical Data for Bedrock Wells in the New England Coastal Basins Study Unit
Appendix 2: Chemical Data for Surficial Wells in the New England Coastal Basins Study Unit
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The full report is 5,304KB.
The report is also available in sections in pdf for a faster download:
Cover, Title Page, and Foreword--61KB
Table of Contents--29KB
Abstract and Introduction--233KB
Description of Studies--267KB
Study Design and Methods--1,207KB
Relation of Arsenic, Iron, and Manganese, in Ground Water to Aquifer Type, Lithogeochemistry and Land Use--522KB
Part 2 of Relation of Arsenic, Iron, and Manganese, in Ground Water To Aquifer Type, Lithogeochemistry and Land Use--2,435KB
Summary and Conclusions--13KB
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Last modified: Thursday, July 31 2008, 01:07:52 PM