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.