Sources, transport, and fate of arsenic in ground water

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The main goal of this study is to characterize the sources, transport and fate of arsenic in groundwater systems in New Hampshire which are known to contain high levels of arsenic. We will accomplish this goal by studying each of the four geographic regions within New Hampshire where high arsenic levels were identified by us in a regional survey that was completed as a pilot project. This will include regions where arsenic may be derived from either geologic or human-derived sources. In our recent study (Peters et al., 1999) we measured arsenic concentrations in 992 drinking water samples collected from randomly selected households in New Hampshire. Arsenic was found to be highly elevated in drilled bedrock wells, with approximately 35% of these wells exceeding 2 g/L, which is the lowest value being considered by the USEPA for the new maximum contaminant level standard. We identified four geographic regions with high arsenic levels, all of which are in the southern half of the state. The largest cluster (Bow, NH) was studied in more detail and found to have highly elevated arsenic in bedrock, which was in an easily leachable form. In the study proposed here, we wish to closely investigate each of the four clusters with respect to the hydrology and geochemistry of the groundwater systems. We will use 1) basic hydrologic techniques to map the regional groundwater flow patterns, 2) electron microprobe x- ray mapping to determine arsenic carrier phases in rocks and soils, 3) column experiments to measure the chemical conditions under which arsenic is solubilized from the bedrock and soils, 4) species-specific chemical analyses of waters to characterize in detail the redox conditions of the groundwater system and the chemical mechanisms responsible for arsenic mobility, and 5) column experiments to measure the retardatioin properties of bedrock and soils. For the purposes of illustrating our approach to this study, we will focus in this proposal on two field sites where we have done some preliminary work and which may represent the end-member cases of geologically-derived versus human-derived arsenic sources. Results from this work will further our general knowledge of the sources and chemistry of arsenic in the aquifers and mine waste areas. Our study will concentrate on a previously under-emphasized arsenic contamination problem in crystalline (i.e., granitic) geological teranes. Our work will provide information that will aid in making more informed decisions with respect to the management and remediation of arsenic- contaminated crystalline bedrock regions in new England and elsewhere.