Abstract: A Case Study of Cost Effective, Geophysical Ground Water and Plume Mapping

CREWDSON, ROBERT

Surface-based resistivity surveys can easily map two important hydrologic parameters: 1 ) the depth to ground water and ground water salinity. Depths can be used to determine ground waterflow direction and gradient, and spatial variation in salinity can be used to map aquifer zones of seawater invasion, formation water plumes, injected brine migration, etc. During 1995-1996, the Department of Geology at CSUB used a 66-sounding, Schlumberger resistivity survey to evaluate a 3200 ft long, groundwater impact plume presumably from a large source of septic wastewater in an alluvial basin which drains toward Lake Isabella, near Bakersfield, CA. Based on resistivity and confirmed by monitoring wells we made the following findings: 1) background water quality is about 300 mg/1 TDS; 2) residential septic wastewater constitutes a distributed source which adds 20 mg/l TDS to the groundwater; 3) the large, wastewater source adds 1,200 mg/l TDS to the groundwater, which decreases by 50% with every 300-400 ft of downgradient flow; 4) ground water impacts from two, previously unknown, wastewater sources were identified directly downgradient from the source under study for which the client is not responsible; 5) the number and placement of wells were optimized for plume monitoring and corrective action verification. Based on the above findings, the responsible party saved over $1.5 million in unnecessary monitoring well and remedial costs. CSUB can perform 15-20 resistivity soundings for the cost of a single monitoring well and we attribute the success of the evaluation to the detail provided by the large number of data points.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California