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 micro/L. Because the water samples are mostly preanthropogenic in age (0-28000 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 N03-/Cl- and CIO-/Cl-ratios are more variable than those of Br-/Cl- or S04(2-)/Cl-. Samples thought to have recharged under the most arid conditions in the Holocene have relatively high N03-/Cl- ratios and low delta 15N values (+1 per mil (% per thousand)) similar to those of modern bulk atmospheric N deposition. The delta 18O values of the N03- (-4 to 0% per thousand) indicate that atmospheric N03- was not transmitted directly to the groundwater but may have been cycled in the soils before infiltrating. Samples with nearly atmospheric N03-/CI- ratios have relatively high Cl04- concentrations (1.0-1.8 ug/L) with a nearly constant Cl04-/CI- mole ratio of (1.4 +/- 0.1) x 10(-4), which would be consistent with an average Cl04-concentration of 0.093 0.005 ,ug/L in bulk atmospheric deposition during the late Holocene in north-central NM. Samples thought to have recharged under wetter conditions have higher delta 15N values (+3 to +8 % per thousando), 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 microg/L are possible in preanthropogenic groundwater in parts of the Southwest where ET approaches a factor of 40. Higher Cl04- 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.
Plummer
L Niel
LN
U.S. Geological Survey, 432 National Center, Reston, Virginia 20192, USA. nplummer@usgs.gov
Böhlke
John Karl
JK
Doughten
Michael W
MW
D018436:Ancient Lands; D001965:Bromides / Q000032:analysis; D002712:Chlorides / Q000032:analysis; D004784:Environmental Monitoring; D005618:Fresh Water / Q000032:analysis*; D055593:Geological Phenomena; D005844:Geology; D009516:New Mexico; D009566:Nitrates / Q000032:analysis; D009584:Nitrogen / Q000032:analysis; D009587:Nitrogen Isotopes / Q000032:analysis; D010103:Oxygen Isotopes / Q000032:analysis; D010163:Paleontology; D010472:Perchlorates / Q000032:analysis*; D017670:Sodium Compounds / Q000032:analysis*; D013431:Sulfates / Q000032:analysis; D013997:Time Factors; D014877:Water Pollution, Chemical / Q000032:analysis*; D014881:Water Supply
D001965:Bromides; D002712:Chlorides; D009566:Nitrates; D009587:Nitrogen Isotopes; D010103:Oxygen Isotopes; D010472:Perchlorates; D017670:Sodium Compounds; D013431:Sulfates; C031068:sodium perchlorate; D009584:Nitrogen