PERCHLORATE IN PLEISTOCENE AND HOLOCENE GROUNDWATER IN NORTH-CENTRAL NEW MEXICO

Perchlorate (ClO₄^-) concentrations of 0.12 to 1.8 mg/L were measured in 42 aerobic groundwater samples with ¹⁴C ages of 0 to 28 ka from remote parts of the Middle Rio Grande Basin in north-central New Mexico. Because the water samples are mostly pre-anthropogenic in age and there are no industrial sources in the study area, a natural source of the ClO₄^- is suspected. Samples that were not affected by saline water leakage along faults or sulfide mineral oxidation have concentrations of Br^-, Cl^-, and SO₄^2- that are similar to those of modern bulk atmospheric deposition with evapotranspiration (ET) factors of about 7 to 60. The estimated ET values were highest in low-altitude Holocene recharge, and lowest in high-altitude Pleistocene 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 ‰) similar to those of modern bulk atmospheric N deposition. The δ¹⁸O values of the NO₃^- (-3 to -1 ‰) 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 to 1.8 mg/L) with a nearly constant ClO₄^-/Cl^- mole ratio of 1.4 x 10^-4. If atmospheric in origin, the ClO₄^-/Cl^- ratio indicates that the ClO₄^- concentration in bulk atmospheric deposition during the late Holocene was 0.093 ± 0.005 mg/L in north-central NM. Apparently, ClO₄^- concentrations as high as 5-6 mg/L are possible in pre-anthropogenic groundwater in parts of the southwest US where ET approaches a factor of 60. Higher ClO₄^- concentrations in uncontaminated groundwater could occur in recharge beneath arid areas where ET is greater than 60, where long-term accumulations of atmospheric salts are leached suddenly from dry soils, or where other (non-atmospheric) natural sources of ClO₄^- exist. 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^- may also have depleted ClO₄^- to varying degrees prior to recharge.