PERCHLORATE ACCUMULATION BENEATH NATIVE VEGETATION IN ARID AND SEMI-ARID ENVIRONMENTS

Large accumulations of atmospherically derived chloride beneath the root zones of xeric vegetation, together with increasing reports of perchlorate detections in pristine aquifers of dry regions, raise the question of whether substantial amounts of perchlorate (ClO₄^-) are being stored in unsaturated zones beneath dryland soils under Holocene conditions. Continuous unsaturated-zone cores were collected from areas of undisturbed, creosote-dominated vegetation in the Mojave Desert (alluvial basin-floor setting, mean precipitation ~0.1 m/yr) and at the southwestern edge of the Colorado Plateau (synclinal sandstone basin-rim setting, mean precipitation ~0.2 m/yr). Collected material was oven dried to determine water content, soluble salts were leached with deionized water, and perchlorate concentrations in filtered leachate were determined by ion chromatography-mass spectroscopy (IC/MS and IC/MS/MS). Vertical profiles of perchlorate concentrations in pore-water have roughly the same shape as those of chloride, with prominent bulges (concentration peaks) just beneath the root zone of native plants (r² of exponential fit >0.8). Perchlorate concentrations up to 42 ppb (μg ClO₄^- /kg porewater) were measured beneath the root zone in the Mojave Desert near Amargosa Farms, Nevada. In the Colorado Plateau profile, which had higher salt concentrations, the peak perchlorate concentration in porewater was about 1200 ppb, 30 times higher than the Mojave peak. However, in both settings the mole fraction of chloride occurring as perchlorate was about two to fourteen millionths. Perchlorate inventories in the top 10 m of the unsaturated zone were ~10 mg/m² (Mojave site) and ~50 mg/m² (Colorado Plateau site). Atmospheric deposition (precipitation plus dryfall) composited over a continuous five-year period ending Nov. 2002 in the Mojave Desert had a precipitation-volume weighted average perchlorate concentration close to the IC/MS detection limit of 0.2 ppb; three subsequent samples ranged from 0.4 to 2 ppb. The perchlorate-to-chloride ratio is about 20 times higher in precipitation than in pore water, suggesting that root-zone (biotic?) processes reduce most of the incoming perchlorate to chloride prior to transport to the biologically inactive sub-root store by infrequent deep percolation events.