Unexpectedly Rapid Migration of Perchlorate Through a Thick Vadose Zone Under Low Rainfall Conditions

Perchlorate, a highly water soluble compound, has been detected in groundwater near Rialto, CA where the depth to groundwater is about 250 to 400 feet below ground surface. Mean annual precipitation is about 15 inches. The soils are highly permeable alluvial gravels with sparse vegetation. Assumptions that diffuse recharge is about 0.5 to 5% 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 high permeability of the alluvium, and the absence of vegetation during site operations. The high local recharge, more than 50% of mean annual precipitation in places, also explains why shallow soil sampling did not detect significant concentrations of perchlorate in some areas.