HR: 1340h
AN: H53A-1203    [Abstracts]
TI: The Release of Chromium in Aquifers Underlying the Western Mojave Desert
AU: Khachikian, C S
EM: ckhachi@calstatela.edu
AF: California State University Los Angeles, Department of Civil Engineering 5151 State University Drive, Los Angeles, CA 90032 United States
AU: * Plotkin, C
EM: CPlotkin@dtsc.ca.gov
AF: California State University Los Angeles, Department of Civil Engineering 5151 State University Drive, Los Angeles, CA 90032 United States
AU: Monterrosa, A
EM: atmonterrosa@yahoo.com
AF: California State University Los Angeles, Department of Civil Engineering 5151 State University Drive, Los Angeles, CA 90032 United States
AU: Ramirez, P
EM: pramire@calstatela.edu
AF: California State University Los Angeles, Department of Geological Sciences 5151 State University Drive, Los Angeles, CA 90032 United States
AB: Recent studies suggest that chromium (Cr) in aquifers underlying the western part of the Mojave Desert may be naturally occurring, resulting from the release of this element from aquifer materials in oxic ground water with pH $>$ 8.0. In order to test this hypothesis, we studied the kinetic release of Cr from aquifer material collected from the Sheep Creek fan near Victorville, CA. Pulverized and untreated aquifer material were incubated in 15mL Falcon tubes with water at three different pH values (2, 7, and 9) for 227 days. Duplicate samples were incubated in a heating bath at temperatures 20$^{o}$C above ambient, in order to accelerate the kinetics of Cr release. Samples from each tube were collected, filtered through a 0.22 $\mu$m filter, and analyzed using a graphite furnace atomic absorption photospectrometer using EPA Method 7199. Due to the heterogeneity of the aquifer material an accurate measure of Cr release kinetics was not possible; results presented here represent equilibrium values at the end of the incubation period. For untreated aquifer material incubated at ambient temperatures, Cr was found at concentrations of 26.0 $\pm$ 8.0 ppb at pH = 2, 5.2 $\pm$ 0.6 ppb at pH=7, and 9.1 $\pm$ 0.9 ppb at pH=9. As expected, the highest value of Cr was found for the samples incubated at pH=2. Pulverization of the samples resulted in an increase in Cr release by a factor of 1.5, 2.4, and 1.6 at pH values of 2, 7, and 9, respectively. Heating the pulverized samples at pH 7 and 9 resulted in an increase in the concentration of Cr released (by 25% and 9% respectively). However, heating the pulverized sample at pH=2 resulted in an almost 95% decrease in the concentration of released Cr (from 37.8 ppb to 2.2 ppb). The reason for this decrease is not known at this point. Our results indicate that significant concentrations of Cr are released naturally in Western Mojave Desert aquifers under various geochemical conditions. Additional studies are underway to study the kinetic release of Cr in packed columns.
DE: 1803 Anthropogenic effects
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
DE: 1884 Water supply
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting