HR: 0800h
AN: H31C-0512    [Abstracts]
TI: Fate and Reactive Transport of Chromium in Leon Valley, Guanajuato, Mexico Using Stable Isotopes
AU: * Villalobos-Arag\'{o}n, A
EM: avillalobos4@miners.utep.edu
AF: University of Texas at El Paso, Department of Geological Sciences, 500 W. University Ave., El Paso, TX 79968, United States
AU: Ellis, A S
EM: aellis@utep.edu
AF: University of Texas at El Paso, Department of Geological Sciences, 500 W. University Ave., El Paso, TX 79968, United States
AU: Armienta, M A
EM: victoria@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Instituto de Geofisica, Circuito Exterior, Ciudad Universitaria, Mexico, DF 04510, Mexico
AU: Morton, O
EM: omorton@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Instituto de Geofisica, Circuito Exterior, Ciudad Universitaria, Mexico, DF 04510, Mexico
AU: Johnson, T M
EM: tmjohnsn@uiuc.edu
AF: University of Illinois at Urbana-Champaign, Department of Geology, 245 Natural History Building 1301 West Green Street, Urbana, IL 61801-2939, United States
AB: Chromium (Cr) is commonly found in the environment due to both anthropogenic and natural sources. Cr(VI) is highly mobile, toxic and carcinogenic, while Cr (III) is less mobile. Reduction of Cr (VI) to Cr (III) is a remediation technique frequently proposed, so monitoring it becomes essential in remediation efforts. It has been previously proposed that Cr stable isotopes may be useful in differentiating Cr(VI) reduction from other transport processes. The goals of this study, at a chromate production facility in Le\'{o}n México are: 1) use the stable isotope values (δ⁵³Cr), to monitor Cr transport, 2) verify sources of contamination, and test the hypothesis that it is both anthropogenic and natural, and 3) monitor isotopic fractionation during transport of Cr in sediment columns, to better understand and model the processes taking place during transport. Le\'{o}n is located in central México, and is one of its most important industrial centers. Several studies have been performed in the Le\'{o}n Valley, since Cr was first detected in its groundwater in 1975. It has been proposed that the high concentration plume near the factory is anthropogenic, while the large sub ppb plume is caused by weathering from ultramafic rocks. Sediment samples were collected from two waste piles in the chromate factory, water was sampled from wells and piezometers near the factory and surface water from the Juan de Otates Dam area near ultramafic rock outcrops. To complement the field study, two types of experiments were performed: 1) the 1-D column flow experiments with sand, goethite coated sands and sand-magnetite mixtures, and 2) leaching experiments of sediments from the two waste piles. Cr(VI) concentrations for the surface water range from 0.009mg/L to 0.015mg/L, and for groundwater from 0.008 mg/L to 121 mg/L. Surface collection ponds for waste pile leachates range from 1.2 to 3.8 g/L. The δ⁵³Cr values for groundwater range from 0.33‰ to 0.46‰ near the factory indicating minimal reduction in the area. The large amounts of Cr have probably exhausted natural reductants. Leaching experiment samples have enriched δ⁵³Cr values (0.76‰ to 3.25‰) indicating either varying reduction or that the waste is fractionated during the ore processing. The difference in δ⁵³Cr values in groundwater and those coming from the waste piles, previously considered to be the source of the contamination, suggests the presence of other Cr sources. The 1-D columns experiments conducted have δ⁵³Cr values that fall within our precision indicating that the isotopes are not fractionated during non- reactive transport processes, including sorption and the lack of variation in the groundwater plume are consistent with the results. Ongoing work includes analyzing the larger Cr plume hypothesized to be of natural origin and rivers draining the ultramafic rocks.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
SC: Hydrology [H]
MN: 2007 Fall Meeting