COMPARISON OF WASTE SITES AND GEOLOGY TO THE OCCURRENCE OF ARSENIC IN GROUNDWATER: THREE EXAMPLES FROM NEW HAMPSHIRE

Regional classification of lithogeochemical characteristics of the bedrock have proven useful in mapping regional variations in groundwater arsenic concentrations in New Hampshire. Variably calcareous metasedimentary rocks have been shown to have a higher percentage of wells with elevated (above the 10 µg/l EPA maximum contaminant level (MCL)) dissolved arsenic concentrations in groundwater of New England. The two primary in situ sources are likely solid phase arsenic associated with oxhydroxide and (or) sulfide minerals in the bedrock. If primarily the former, reducing conditions in groundwater, which is often associated with waste disposal of anthropogenic organic contaminants, can exacerbate the dissolution of in situ arsenic from oxy hydroxide surfaces and induce locally elevated dissolved arsenic. Other processes are known to affect arsenic desorption and may play a role in arsenic mobility such as pH and the local variability in biogeochemical conditions. Dissolved arsenic concentrations from three waste sites were examined to assess the impact of bedrock geology and waste leachate on arsenic occurrence in groundwater. The three waste sites can be broadly grouped into two major lithogeochemical types, granitic vs. variably calcareous metasedimentary rocks, and three types of waste: solvent disposal, temporary chemical waste dump, and landfill. The occurrence of dissolved arsenic varied based on the relation between leachate type, reducing conditions, and bedrock type.