2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 2
Presentation Time: 1:30 PM-5:30 PM


PLUMMER, L. Niel, BÖHLKE, John Karl and DOUGHTEN, Michael W., U.S. Geological Survey, 432 National Center, Reston, VA 20192, nplummer@usgs.gov

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 SO42- 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 SO42-/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.