In-Situ Arsenic Removal During Groundwater Recharge Through Unsaturated Alluvium in the Antelope Valley

Arsenic in excess of the maximum contaminant level of 10 micrograms per liter (mg/L) is common in deeper aquifers throughout much of the southwestern United States. A field-scale experiment is being done in Antelope Valley, California, in cooperation with United States Geological Survey and Antelope Valley East-Kern Water Agency, to determine if high-arsenic groundwater pumped from deeper aquifers can be infiltrated through the 300 ft thick unsaturated zone at the site to remove arsenic. The infiltrated water would serve as a source of recharge to shallow aquifers where it could later be pumped for public supply. If successful, arsenic in water infiltrated from a half-acre pond would be sorbed on iron, manganese, and alumina oxides on the surfaces of mineral grains in the unsaturated zone. These naturally occurring oxides are similar to those used in commercial resins. The two-year experiment is designed to treat and recharge 1,600 ac-ft of groundwater having an arsenic concentration of about 30 mg/L to drinking water levels. However, the unsaturated zone at the site may contain enough oxides to treat 50,000 acre-ft of high-arsenic water infiltrated from the same pond from 50 mg/L to less than the drinking water standard of 10 mg/L. The cost of treatment is expected to be the cost of pumping the water, about $150 per ac-ft. This compares favorably with the cost of treating high-arsenic groundwater using commercial resins, about $600 to $800 per ac-ft. If successful, the approach could provide recharge to the increasingly overdrafted shallow aquifer while restoring beneficial use to the deeper aquifer. Monitoring of unsaturated-zone water quality during the experiment is intended to ensure little or no arsenic reaches the water table if sorption of arsenic does not occur as expected during the field experiment.