Web Release Date: February 14,
Perchlorate in Pleistocene and Holocene Groundwater in North-Central New Mexico
U.S. Geological Survey, 432 National Center, Reston, Virginia 20192
Received for review September 1, 2005
Revised manuscript received December 19, 2005
Accepted January 5, 2006
Abstract:
Groundwater from remote parts of the Middle Rio Grande
Basin in north-central New Mexico has perchlorate
(ClO4-) concentrations of 0.12-1.8 
g/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 SO42- 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 SO42-/Cl-. Samples thought to have recharged under the most
arid conditions in the Holocene have relatively high NO3-/Cl- ratios 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-/Cl- ratios have
relatively high ClO4- concentrations (1.0-1.8 g/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 g/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-/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 g/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.
Download the full text: PDF | HTML
