H31C-0512
Fate and Reactive Transport of Chromium in Leon Valley, Guanajuato, Mexico Using Stable Isotopes
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.
H31C-0513
Iron Isotope Composition of River Water During Estuarine Mixing: Case of North River (Massachusetts, USA)
Iron has recently been regarded as a regulator of ocean productivity and global climate change due to its key role as a major micronutrient in biological processes. To improve our understanding of the oceanic iron cycle, we need to better characterize iron sources and determine the processes that control the fractionation of Fe isotopes between continental run-off and the ocean. Recent studies (e.g. Fantle and DePaolo, 2004 and Bergquist and Boyle, 2006) have demonstrated that rivers present an isotopically light Fe source to the oceans. Since the input of dissolved iron from river water is generally controlled by flocculation processes occurring during estuarine mixing, it is important to investigate potential fractionation of Fe-isotopes during this process. In this study, we investigated the influence of the flocculation of Fe-rich colloids on the iron isotopic composition of estuarine waters. In October 2006, 16 samples were collected along a salinity gradient from the fresh water to the ocean in the North River Estuary (MA, USA). Samples were filtered at 0.22μm and the two fractions (dissolved and particles) were analyzed for iron isotopic composition using high-resolution MC-ICP-MS after chemical purification. Dissolved iron results show positive δ56Fe values (with an average of 0.43‰) relative to the IRMM- 14 standard and do not display any relationships with salinity or percentage of Fe colloid flocculation. These results suggest that riverine Fe-isotope composition are not affected by flocculation processes and yield a fractionation factor of less than 0.1‰. The iron isotopic composition of the particles suspended in fresh water is characterized by more negative δ56Fe values than for dissolve Fe and correlate with the percentage of Fe flocculation. δ56Fe values increased from -0.09‰ at no flocculation to ~0.1‰ at the flocculation maximum which reflect mixing effects between river-borne particles and newly precipitated colloids. Since the process of flocculation produce minimal Fe-isotope fractionation in the dissolve Fe pool, we suggest that the iron isotopic composition of fresh water is preserved during estuarine mixing and that the value of the global riverine source into the ocean can be assimilated to the fresh water values. However, this study also suggests that δ56Fe composition of rivers is not unique and may be characterized by more positive δ56Fe values (up to 0.3‰) relative to the crust than previously reported.
H31C-0514
Stable Isotopes of Selenium and Common Elements (N, O, S) Reveal Redox Controls on the Mobility of a Naturally Occurring Source of Selenium
Historically, an extensive wetland known as the "Swamp of the Frogs" occupied the central, low-lying part of the San Diego Creek Watershed in Orange County, California. The swamp acted as a sink for the redox sensitive trace element selenium. Toward the end of the nineteenth century the swamp was drained to make way for the expansion of ranching and agriculture, resulting in a shift in prevailing redox conditions. High levels selenium have been reported in the shallow groundwaters associated with the historic swamp often causing surface water concentrations to exceed the EPA chronic criterion of 5 ug/L. Nitrate in groundwater often exceeds 10 mg/L NO3-N due to leaching of fertilizer used on citrus crops. Published research has shown that nitrate can act to oxidize reduced forms of sulfur and selenium. Stable isotopes of selenium, N and O in nitrate and S in sulfate are used to investigate the link between denitrification and the oxidation of reduced forms of sulfur and selenium along two groundwater flowpaths within the historic swamp. The "El Modena" flowpath begins roughly at the northern boundary of the historic swamp. Along this flowpath selenium increases from an upgradient value of 12 ug/L to a downgradient value of 80 ug/L while nitrate varies from 16 to 28 mg/L NO3-N. Isotopes of N and O in nitrate indicate a denitrification trend along the flowpath as delta 15N [NO3-] increases from 6.8 per mille to 19.0 per mille and delta 18O [NO3-] increases from 6.8 per mille to 14.0 per mille. The behavior of sulfur isotopes varies inversely with that of nitrogen with delta 34S [SO4 2-] decreasing from 0.1 per mille to -17.2 per mille. This, coupled with a slight enrichment in delta 13C-DIC, suggests that denitrification is coupled to the oxidation of metal sulfides disseminated in soils. Selenium isotopes (del 82/76Se [SeO4 2-]) did not grow isotopically depleted as expected, but varied from about 1.0 per mille at the outer boundary of the historic marsh, decreasing to about -1.0 per mille as groundwater moves slightly inside the marsh interior, increasing to about 2.3 per mille toward the end of the flowpath, deep within the historic marsh. The relative availability of sulfur and selenium may explain the observed pattern in selenium isotopes. The "Valencia" flowpath begins roughly at the southeast boundary of the historic swamp. Selenium increases from an upgradient value of 42 ug/L to a downgradient value of 141 ug/L while nitrate varies from 13 to 18 mg/L NO3-N. Groundwaters flowing along this flowpath did not show a change in isotope ratios. Neither delta 34S [SO4 2-] nor delta 15N [NO3-] and delta 18O [NO3-] vary by more than 1 per mille, which is small enough to discount a role of redox processes. Anomalously high concentrations of silica along this flowpath may point to a possible explanation for this result.
H31C-0515
Chromium Isotopic Monitoring of HRC-Stimulated Bio-containment at the 100H Test Site, Hanford, Washington
Hexavalent Cr (Cr(VI)) groundwater contamination is a common problem in the U.S. associated with industrial activity (e.g. electroplating, tanning, paints, anti-corrosion). In the particular case of the Hanford Site, Washington, chromate was used primarily to inhibit corrosion in nuclear reactor cooling systems. During the active operation of the Hanford Site, disposal of waste water bearing chromate, and accidental releases to the vadose zone resulted in significant groundwater contamination with local concentrations near the Columbia river reaching over 1000 ppb Cr(VI). In an effort to test an effective bio-containment strategy for groundwater Cr(VI), a site was selected between the 100D and 100H reactor areas with modest concentrations (~100 ppb Cr(VI) over the past two decades). A slow-release 13C labeled polylactate amendment (HRCTM, Regenesis, Ltd.) was injected into groundwater within a sandy formation to stimulate bacterial activity in order to produce conditions that promote the reduction of dissolved Cr(VI) to insoluble Cr(III) complexes [1]. Since the injection of HRCTM in August 2004, groundwater Cr(VI) concentration has been locally below 1 ppb, and reducing conditions have been maintained to at least the present time. The isotopic composition of Cr can be fractionated during reduction from Cr(VI) to Cr(III) and so has the potential to be used as a monitor of hexavalent Cr reduction [2, 3]. This would provide a direct signature of Cr(VI) reduction, discernable from simple attenuation by dilution. In order to explore the use of Cr isotopic measurements for evaluating processes of Cr(VI) reduction, we have analyzed a series of samples in space and time for Cr isotopic composition (δ53Cr, permil deviation of sample 53Cr/52Cr from that of SRM970). Groundwater samples came from the HRC injection well, from multiple depths of three down-gradient wells, and from an up-gradient well. Samples from down-gradient wells have Cr that is isotopically fractionated relative to samples from the up-gradient well. Taken together, samples from a single sampling campaign yield an apparent fractionation of 2.2 ‰. Cr isotopic measurements of the latest samples (June, 2007) confirm continued reduction of Cr(VI) nearly three years after the introduction of HRCTM, suggesting the long-term effectiveness of this bio-stimulated containment strategy. [1] http://esd.lbl.gov/ERT/hanford100h/ [2] Ellis, AS, Johnson, TM and Bohlen, TD (2002) Science. 295:2060-2062. [3] Johnson, TM and Bohlen, TD (2004) Reviews in Mineralogy and Geochemistry, Vol. 55, p.289-317.
H31C-0516
Using Isotopic Tracers to Determine Denitrification of a Large River Swamp Basin
As the hypoxic dead zone in the Gulf of Mexico grows due to nitrogen enrichment from the Mississippi River, alternative means to reduce nutrient loads to the Gulf are evolving. One such approach is to divert river water into wetland areas, which allows for infiltration, sedimentation, and denitrification. It is, however, often uncertain how much nitrogen can be retained through these natural systems. Isotopic tracing provides a useful technique for analyzing flow pathways and residence times of nitrogen at watershed scales. This project analyzes isotopic composition of 15N and 18O of nitrate, nitrite and dissolved organic nitrogen at five locations along the 220-km Atchafalaya, the largest distributary of the Mississippi River. Changes in the isotopic ratios between these locations and over time will be investigated. This paper reports preliminary results from two sampling dates in April and May 2007. In all samples we found high concentrations of nitrate (107.6-162.4 uM), but no nitrite and very little ammonium. The d15N isotope ranged from 5.49 to 6.60 and d18O was 4.46-5.59 at all sites, indicating that some nitrate processing may have occurred in the water since this signal differs from that of pure fertilizer nitrate. Delta 15N was similar at all sites in the April sampling date, however, there was a slight trend of decreasing d15N from the upstream sites to the outlets. Although this change in the May sampling date was not significant, it may reflect the importance of higher water temperature, lower flow and increased biological activities of the swamp basin in denitrification. The preliminary results suggest that intense sampling in the summer may yield crucial insights into riverine denitrification occurring as the water moves downstream.