WELL DESIGN CRITERIA PROVIDING REDUCED CONCENTRATIONS OF GEOGENIC ARSENIC AND URANIUM, SAN JOAQUIN VALLEY, CALIFORNIA

The San Joaquin Valley of California contains large and prolific aquifers that produce elevated concentrations of geogenic arsenic and uranium exceeding drinking water standards. The subsurface is assessed using lithologic, geophysical and geochemical methods to avoid water producing zones that contain these elevated constituents. A spectral gamma ray tool was used in an existing municipal water supply well located in Kerman, California to assess formations contributing gamma rays produced by thorium, potassium and uranium radioactive isotopes. A uranium isotope signature excursion was identified at approximately 350 feet below grade representing elevated uranium isotope activity with eight times more energy than background activity. Four of the municipality's five wells are screened opposite this stratum producing well water with uranium concentrations over twice the drinking water standard. Zone testing was conducted at three water-saturated zones below 500 feet bgs, using a Formation Sampling Tool (FST) in temporarily sealed zones to confirm low uranium concentrations in the groundwater. The replacement wells are constructed with shorter (less than 70 feet) well screen sections, installed opposite the zones tested. The concentrations of uranium from the replacement wells are reported as less than 3 ug/l with production yields exceeding 1000 gpm. In areas known to produce elevated concentrations of arsenic (> 80 ug/l) zone testing was performed using the FST with water quality based well design successfully avoiding the hazard of municipal wells producing arsenic in concentrations above the new drinking water standard of 10 ug/l without the need for expensive treatment. Replacement wells have been designed with approximately 100 feet of well screen to depths ranging from 700 to 1000 feet, producing well water with concentrations of arsenic between 2 and 9 ug/l and production yields exceeding 1500 gpm.