INFLUENCE OF SURFICIAL AQUIFER MATERIALS ON NATURAL CONTAMINANTS IN BEDROCK AQUIFER GROUNDWATER IN NEW ENGLAND

Concentrations of some natural contaminants in drinking water from shallow (< 200 m) wells in the fractured crystalline bedrock aquifer of New England have spatial associations with both host lithology (Flanagan and others, 2011, USGS Scientific Investigations Report 2011-5220, 104 p.) and overlying surficial geology features. The surficial associations suggest that there is a hydrochemical linkage between the surficial and bedrock aquifers and that recharge water entering bedrock and groundwater evolution in bedrock is influenced by the chemistry, mineralogy, and depositional setting of overlying surficial and glacial sediments. A number of natural contaminants typically associated with granitic source rocks, such as F, U, and Rn, occur at elevated levels in groundwater from wells in non-granitic bedrock that is overlain by glacial source sediments predominantly derived from granitic rocks. In addition, a hydrochemical linkage between the surficial and bedrock aquifers is suggested by high pH groundwaters in a variety of bedrock lithologies that are overlain by Pleistocene glaciomarine clay deposits in the eastern part of the region. Interaction of shallow recharge groundwater with the rock flour matrix in these glaciomarine clay deposits likely increases alkalinity and pH through accelerated aluminosilicate hydrolysis reactions, influencing concentrations of pH-sensitive constituents such as As and F in bedrock groundwater. Continued aluminosilicate hydrolysis reaction leads to calcite and dolomite saturation, resulting in high pH sodium-bicarbonate water types. Similar elevated groundwater pH patterns have been observed in southwestern Sweden where the crystalline bedrock aquifer is overlain by Pleistocene glaciomarine clay (Lang and Swedberg, 1990, Water, Air, and Soil Pollution, v. 49, p. 315-328).