Chromium Contamination in Leon Valley Mexico: Insights from Chromium Stable Isotopes

Chromium (Cr) is a contaminant commonly found in surface and groundwater. In nature Cr commonly occurs in two valences: Cr(VI) is highly mobile, toxic and carcinogenic, while Cr(III) is less mobile. Cr was first detected in groundwater in the Leon Valley region (located in central Mexico) in 1975 (Rodriguez and Armienta, 1995). Previous work proposed an anthropogenic origin for a high concentration plume near the industrial Buenavista (BV), while the larger sub ppb plume located near San Juan de Otates (SJO) is assumed to be caused by weathering of the SJO pyroxenite. Cr stable isotopes have been shown to be useful in monitoring reduction and can also be used to infer redox and transport conditions in natural settings and the goals of our research are: 1) use stable isotope values (δ53Cr) to monitor Cr behavior in the Leon Valley, 2) identify sources (industrial vs. weathering of ultramafic rocks). Since 1975, the highly contaminated BV area is showing a decrease in groundwater Cr(VI) concentrations. Our sampling in 2007 show Cr(VI) concentrations decreasing from 0.005mg/l to 121mg/l to 0.002mg/l to 95.1mg/l. However, the Cr waste piles still exit and Cr(VI) concentrations from leachate collection ponds range from 1.2g/l to 6.8g/l. Isotopic values obtained from leaching experiments performed on the waste pile samples show enriched δ53Cr values (0.76‰ to 3.25‰) either indicating varying reduction or that the waste is fractionated during the ore processing. δ53Cr values for groundwater range from 0.33‰ to 0.46‰, indicating minimal reduction and lack of available reducing agents. The lack of variation of isotopic values seen in the groundwater plume is also consistent with the results of previous studies showing insignificant fractionation due to sorption within the plume (Villalobos-Aragon et al., 2008). The unfractionated Cr(VI) in groundwater vs. those of the waste piles, considered the contamination source, suggests that either 1) the Cr in the waste pile was not fractionated previously, but it is now - result of "weathering", 2) an unknown process is depleting the Cr isotope signature, and/or 3) indicates the presence of other Cr sources. Weathering of ultramafics in the SJO area resulted in Cr(VI) concentrations in surface waters from 0.011mg/l to 0.014mg/l, and a slight increase in groundwater concentrations, (0.010mg/l to 0.016 mg/l) in the downstream direction. Our latest sampling round in 2008 showed Cr(VI) below detection limits. From the results, it is clear that monitoring of natural attenuation is not a remediation technology that can be used in the contaminated BV area, instead pump and treat or other active remediation technologies are required. Over the past couple of years the natural plume has been below EPA limits for groundwater and apparently natural reducing agents are helping remove Cr(VI) from solution. As with the contaminated area, Cr isotopes will once again be useful in determining if natural attenuation is still prevalent.