BOREHOLE FLOW MODIFICATION: DEVELOPING A SIMPLE SOLUTION TO A COMPLEX DRINKING WATER COMPLIANCE PROBLEM

Arsenic, uranium, and radium are naturally occurring contaminants in ground water and are widespread in many New England bedrock aquifers. New England community drinking water supply systems that are non-compliant with the drinking water Maximum Contaminant Levels (MCLs) for these contaminants often have concentrations slightly above the MCL. For example, the arsenic MCL is 10 parts per billion (ppb), and most systems with an arsenic problem have concentrations between 10 and 20 ppb. The goal of a project currently underway is to develop cost-effective methods to improve the quality of water produced by an affected well by evaluating whether various transmissive fractures or fracture zones contributing water to a borehole may produce variable concentrations of contaminants (e.g. arsenic, uranium, radium). The methods to be developed will include a process for characterizing the vertical distribution of naturally occurring contaminants entering a borehole and various methods for mitigating the effects of contaminated water inflow, while preserving the water-producing integrity of the well. Methods for mitigating the effects of contaminated water inflow may include sealing fractures, hydro-fracturing, and/or drilling. Strategies for reducing contaminant inflow through pump cycling will also be explored. Contamination mitigation at one well resulted in a decrease in the arsenic concentration of the bulk discharge water, implying that compliance with the arsenic standard via borehole modification in lieu of treatment may be a feasible alternative for public water supply managers. Findings from ongoing and recently completed work will be presented. This is a collaborative project between New Hampshire Dept. of Environmental Services, U.S. Environmental Protection Agency Region 1, and the U.S. Geological Survey.