Use of Integrated Vadose Zone Investigative Methods to Optimize ASR Site Selection

Successful site selection and design of surface spreading Aquifer Storage and Recovery (ASR) facilities are highly dependent on subsurface geologic conditions, source water quality, land availability and nearby land use.  A number of ASR sites have been selected and built with inadequate consideration to the near- and subsurface conditions, with subsequent failure to achieve design recharge rates.  The predicted effect of near- and subsurface hydrogeologic properties on recharge performance is usually based on data from point measurements of near-surface infiltration and geologic logging of vadose zone exploration boreholes.  These limited data are often insufficient to accurately estimate large-scale recharge rates and may result regulatory requirements for costly pilot projects.  The judicious use of vadose zone testing methods, however, can provide robust data that greatly improves initial estimates of groundwater recharge rates and can eliminate the need for a pilot test, resulting in significant cost savings.  The proposed approach integrates near-surface infiltration testing, exploratory drilling, geological logging, and if necessary, multiple-scale in-situ vadose zone testing.  Near surface characterization uses medium scale rapid testing methods at sufficient densities that correlate well to full-scale basin recharge rates.  These rapid tests also provide direct hydraulic conductivity measurements that can be correlated to the geologic sediments observed in boreholes.  In-situ testing in selected instrumented exploratory boreholes quantifies vadose zone hydraulic properties in much the same way that aquifer tests quantify hydraulic properties of the saturated zone.  Testing at multiple scales maximizes field-based data collection and provides a better understanding of subsurface heterogeneities and the efficiency of vadose zone sediments to transmit recharge water.  This approach results in a cost-effective investigative program and has been very successful in regulatory review.  In addition, integrated vadose zone testing is well suited for comparing relative recharge characteristics between proposed sites.