Proposed Use of Long Screened Test Wells in Environmental and Groundwater Resource Investigations

We present two case studies where long screened wells (150 meters to 300 meters deep) were used to delineate the vertical extent of total dissolved solids and arsenic in the surrounding aquifers, respectively. The cost benefits included eliminating the requirement for multiple short screened wells to characterize the same vertical extent of formation, and to characterize the problem under actual pumping and hydraulic head conditions. In the TDS study, high resolution, vertical delineation led to a significant cost reduction in the use of reverse osmosis treatment for cooling water at a power plant facility as well as reduced costs in power plant operations. In the second case, the vertical delineation provided a clear understanding about arsenic distribution in the aquifers providing groundwater to the consumer’s public water supply and enabling a decision to be made about using well modification or treatment to make the well compliant with the regulatory standard (10 ug/L). In both cases, “Intra Borehole Flow” bias was avoided by pumping the wells at a high enough rate to overcome intra well mixing. For each test, depth dependent inflow velocity measurements and water chemistry samples were collocated at each depth and then flow-weighted to determine the zonal flow and chemistry contributions. Use of the Tracer Flowmeter and Depth Dependent Sampler method developed by the USGS made it possible to collect the equivalent of samples from 30 monitoring wells in the TDS study, in only six days. In the arsenic study, the equivalent of 10 zones were obtained in two days.