posted on 2010-07-01, 00:00authored byW. Andrew Jackson, John Karl Böhlke, Baohua Gu, Paul B. Hatzinger, Neil C. Sturchio
Perchlorate (ClO4−) has been detected
widely in groundwater and soils of the southwestern United States.
Much of this ClO4− appears to be natural,
and it may have accumulated largely through wet and dry atmospheric
deposition. This study evaluates the isotopic composition of natural
ClO4− indigenous to the southwestern
U.S. Stable isotope ratios were measured in ClO4− (δ18O, Δ17O, δ37Cl) and associated NO3− (δ18O, Δ17O, δ15N) in groundwater
from the southern High Plains (SHP) of Texas and New Mexico and the
Middle Rio Grande Basin (MRGB) in New Mexico, from unsaturated subsoil
in the SHP, and from NO3−-rich surface
caliche deposits near Death Valley, California. The data indicate
natural ClO4− in the southwestern U.S.
has a wide range of isotopic compositions that are distinct from those
reported previously for natural ClO4− from the Atacama Desert of Chile as well as all known synthetic
ClO4−. ClO4− in Death Valley caliche has a range of high Δ17O values (+8.6 to +18.4 ‰), overlapping and extending the
Atacama range, indicating at least partial atmospheric formation via
reaction with ozone (O3). However, the Death Valley δ37Cl values (−3.1 to −0.8 ‰) and δ18O values (+2.9 to +26.1‰) are higher than those of
Atacama ClO4−. In contrast, ClO4− from western Texas and New Mexico has much lower
Δ17O (+0.3 to +1.3‰), with relatively high
δ37Cl (+3.4 to +5.1 ‰) and δ18O (+0.5 to +4.8 ‰), indicating either that this material was
not primarily generated with O3 as a reactant or that the
ClO4− was affected by postdepositional
O isotope exchange. High Δ17O values in ClO4− (Atacama and Death Valley) are associated with
high Δ17O values in NO3−, indicating that both compounds preserve characteristics of O3-related atmospheric production in hyper-arid settings, whereas
both compounds have low Δ17O values in less arid
settings. Although Δ17O variations in terrestrial
NO3− can be attributed to mixing of atmospheric
(high Δ17O) and biogenic (low Δ17O) NO3−, variations in Δ17O of terrestrial ClO4− are not readily
explained in the same way. This study provides important new constraints
for identifying natural sources of ClO4− in different environments by multicomponent isotopic characteristics,
while presenting the possibilities of divergent ClO4− formation mechanisms and(or) ClO4− isotopic exchange in biologically active environments.