IMPROVING WATER QUALITY DATA FROM FRACTURED BEDROCK WELLS IN CONSIDERATION OF CONCENTRATION AVERAGING

A variety of standard protocols have been developed to obtain water samples from bedrock wells to determine their potability, presence and concentration of contaminants, and to evaluate the relation of water quality to the bedrock lithochemistry. Typically, the protocols do not account for influences of concentration averaging of water from different fractures in open boreholes. This can lead to inaccurate interpretations concerning contaminant presence, migration, and risk of exposure. Factors such as the number and depth of water contributing fractures, their transmissivities and hydraulic heads, pump inlet depth and discharge rate, and drawdown achieved during pumping, all play a significant role in the measured concentrations obtained when sampling. Borehole concentration averaging models that take these factors into account were developed to examine a variety of sampling scenarios to evaluate how they impact water quality determinations. In addition, field tests were conducted at contaminated wells to verify modeling results. Based on the modeling and field testing, approaches were developed to determine the maximum concentration that might be obtained during typical household use, whether contamination arises from shallow or deeper fractures, and how to obtain more consistent sampling results. However, the study further demonstrates the need for downhole sampling of fractures to decipher contaminant sources, transport and spatial distributions.