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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 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 /Cland ClO4 /Cl­ 2-/ ratios are more variable than those of Br-/Clor SO4 Cl-. Samples thought to have recharged under the most arid conditions in the Holocene have relatively high NO3 / Clratios and low 15N values (+1 per mil (‰)) similar to those of modern bulk atmospheric N deposition. The 18O 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 /Clratios have relatively high ClO4 concentrations (1.0-1.8 μg/L) with a nearly constant ClO4 /Clmole 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 15N values (+3 to +8 ‰), lower NO3 /Cl­ ratios, and lower ClO4 /Clratios 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.

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