An assessment of hydrologic factors that influence contaminant concentrations determined from domestic well-water samples

Abstract

Open borehole wells in fractured crystalline rock are commonly used for domestic water supply. Concentrations of contaminants present in the well water, typically collected from the tap after some degree of purging, are weighted average concentrations. In this study, factors which influence sample concentrations collected from domestic wells are evaluated through the use of conceptual models and confirmatory field tests. Focus is on the extent of drawdown during sampling, distance between the pump inlet and contaminated fractures, relative fracture transmissivity, fracture discharge rate into the borehole, and the hydraulic head distribution of the fractures. Simplified spreadsheet models were developed to assess the relation between these factors and sample concentrations. The models show that in addition to fracture transmissivities, drawdown in relation to fracture head plays a significant role in how much water recharges a well from each fracture, yet it is rarely measured when sampling domestic wells. To evaluate the extent of drawdown that may occur under typical household usage, water levels were monitored in three domestic wells. Pumping tests were conducted in three wells to measure changes in concentration as a function of pumping duration. Downhole profiling was conducted to examine how pumping induces changes to water quality within the wellbore. The results indicate that these factors can have a strong influence on contaminant concentrations in samples collected from these wells. As such, the weighted averaging factors should be considered in sample collection, in data interpretation, in assessing exposure risk and in mapping contaminant distributions.