VADOSE ZONE TRANSPORT OF PERCHLORATE: A CASE STUDY FROM A SEMIARID CANYON IN NEW MEXICO

There has been little study of perchlorate transport in the vadose zone despite the fact that vadose-zone processes can be critical controls on perchlorate fluxes to groundwater. We have collected detailed depth profiles of vadose-zone perchlorate from 25 boreholes along a semiarid canyon in New Mexico. These profiles represent releases of perchlorate from a water treatment plant at Los Alamos National Laboratory that occurred over a nearly forty year period starting in the early 1960s. Although the perchlorate is anthropogenic, this study provides a proxy for the kind of behaviors that may occur during transport of natural perchlorate. In this canyon system, transport is multidimensional; mainly downward and longitudinal along the canyon length. Surface flow is largely ephemeral and there is alluvial groundwater (largely effluent-supported) present along part of the canyon. Profiles show that much of the perchlorate is still within the upper 100 m of a nearly 300-m thick vadose zone. Perchlorate has been observed in a perched intermediate aquifer (approximately 197 m deep) and in the regional aquifer (approximately 294 m deep). Inventory estimates, however, suggest that most of the perchlorate still resides above the perched intermediate aquifer. Longitudinal canyon transport has been significant (down canyon extent is approximately 4000 m). Perchlorate distributions and concentrations are highly correlated with nitrate (a co-contaminant), and we see no evidence of perchlorate degradation. Stochastic transport simulations suggest that peak perchlorate concentrations may reach the regional aquifer beneath the canyon in about 50 to 200 years.