High Recharge at Semi-Arid Site Explains Wide-Spread Perchlorate in Groundwater with a Deep Water Table

Perchlorate, a highly water soluble compound, was detected in groundwater near Rialto, California beneath much of a 160-acre area where the depth to groundwater is about 250 to 400 feet below ground surface. Mean annual precipitation is about 20 inches. The soils are permeable alluvial gravels with sparse vegetation. Assumptions that diffuse recharge is about 0.5 to 5 percent of mean annual precipitation, an assumption used in regional groundwater studies, cannot explain the depth of perchlorate migration through the vadose zone. Model studies and field data show that even under non-ponded conditions, perchlorate can migrate to much greater depths than otherwise expected. This behavior is attributable to localized focused recharge, the permeability of the alluvium, and the absence of vegetation during site operations. The high local recharge, 20 to more than 50 percent of mean annual precipitation in places, also explains why shallow soil sampling did not detect significant concentrations of perchlorate in some areas.