LEACHABILITY OF NATURAL PERCHLORATE FROM SOIL WITHIN THE SIERRA DE LOS VALLES, NEW MEXICO

Meltwater from the 2009-2010 snowpack season was allowed to percolate through soil-core columns collected in a high mountainous, small catchment basin located in the Sierra de Los Valles, New Mexico. Leaching tests were performed to determine if naturally occurring perchlorate in soil could be solubilized by the meltwater at detectable concentrations greater than the analytical detection limit of 0.50 nM (0.05 µg/L) using liquid chromatography-mass spectrometry/mass spectrometry. Soil cores and snowpack-meltwater samples were collected at an elevation of 2835 m asl. Soil cores were collected in December 2009 and snowpack samples were collected in March 2010. The first set of cores was collected along the southwest-facing slope of the basin with the second set collected down-slope of the first set and near the drainage area. Cores collected along the slope represent approximately 57 cm of soil with 3 to 5 cm of weathered volcanics. The drainage cores represent approximately 61 cm of organic-rich soil. Samples were collected at a location that is considered to be an area of active groundwater recharge to bedrock. Historical snowpack data for the study area suggest that approximately 55 mL of meltwater passing through the core column reflects an average infiltration without evapotranspiration. Volumes of leachate obtained from the drainage and slope cores were 78 and 55 mL, respectively, and analyzed for low-level perchlorate. Analyses of major ions and trace elements were also performed on the acidic leachate samples. Natural perchlorate was detected in the drainage and slope leachate samples at 2.72 and 23.13 nM (0.27 and 2.3 µg/L), respectively. These results suggest that natural perchlorate becomes mobile as snowmelt reacts with soil within the recharge area of the mountain block and that the ubiquitous presence of natural perchlorate in the local groundwater system may be a result of this process. Other soluble oxyanions (nitrate, oxalate, phosphate, and sulfate) and trace elements (aluminum, barium, boron, iron, manganese, nickel, and zinc) associated with mineral dissolution were also detected at varying concentrations in the leachate samples.