Perchlorate in Pleistocene and Holocene Groundwater in North-Central New Mexico

Groundwater from remote parts of the Middle Rio Grande Basin in north-central New Mexico has perchlorate (ClO4 -) concentrations of 0.12−1.8 μg/L. Because the water samples are mostly preanthropogenic in age (0−28 000 years) and there are no industrial sources in the study area, a natural source of the ClO4 - is likely. Most of the samples have Br-, Cl-, and SO4 2- concentrations that are similar to those of modern bulk atmospheric deposition with evapotranspiration (ET) factors of about 7−40. Most of the ET values for Pleistocene recharge were nearly twice that for Holocene recharge. The NO3 -/Cl- and ClO4 -/Cl- ratios are more variable than those of Br-/Cl- or SO4 2-/Cl-. Samples thought to have recharged under the most arid conditions in the Holocene have relatively high NO3 -/Cl- ratios and low δ15N values (+1 per mil (‰)) similar to those of modern bulk atmospheric N deposition. The δ18O values of the NO3 - (−4 to 0 ‰) indicate that atmospheric NO3 - was not transmitted directly to the groundwater but may have been cycled in the soils before infiltrating. Samples with nearly atmospheric NO3 -/Cl- ratios have relatively high ClO4 - concentrations (1.0−1.8 μg/L) with a nearly constant ClO4 -/Cl- mole ratio of (1.4 ± 0.1) × 10-4, which would be consistent with an average ClO4 - concentration of 0.093 ± 0.005 μg/L in bulk atmospheric deposition during the late Holocene in north-central NM. Samples thought to have recharged under wetter conditions have higher δ15N values (+3 to +8 ‰), lower NO3 -/Cl- ratios, and lower ClO4 -/Cl- ratios than the ones most likely to preserve an atmospheric signal. Processes in the soils that may have depleted atmospherically derived NO3 - also may have depleted ClO4 - to varying degrees prior to recharge. If these interpretations are correct, then ClO4 - concentrations of atmospheric origin as high as 4 μg/L are possible in preanthropogenic groundwater in parts of the Southwest where ET approaches a factor of 40. Higher ClO4 - concentrations in uncontaminated groundwater could occur in recharge beneath arid areas where ET is greater than 40, where long-term accumulations of atmospheric salts are leached suddenly from dry soils, or where other (nonatmospheric) natural sources of ClO4 - exist.