H42C-01 10:20h
Modeling of Recent Chemical Mobility in Weathering Profile Through U-Th Disequilibria: Example of the Kaya Lateritic Profiles.
Determination and dating of recent mobility in weathering profiles is important to constrain their steps of formation and evolution but also to recover the presently active processes that control the chemical fluxes carried by rivers from continents to ocean. Recent studies (1-2) have shown the interest of comparing variations of trace elements and U-series disequilibria in weathering profiles to constrain the nature and the age of recent element mobility in weathering profiles. We pursue this approach and propose a simple modelling of U series disequilibria, using a Monte Carlo inversion approach, to assess the time scale of U mobility in weathering profiles. This approach is exemplified in the case of an old lateritic profile, the Kaya toposequence, in Burkina Faso. It relies on a comparison of U-Th data in both the isochron (230Th/232Th) vs. (238U/232Th) diagram, and the concordia (234U/238U) vs. (230Th/238U) diagram. The method consists in finding, for a given data point, a set of mobility parameters, that provide the same age in the two above diagram. Application of the method to the Kaya lateritic toposequence underscores that the recent chemical mobility in the profile are mainly controlled by the iron-cap dismantling, and would vary with time possibly in response to climatic variations. 1 Dequincey et al. (2002), GCA, 1197-1210; 2 Chabaux et al.(2003), CR Geoscience 335, 1219-1231.
H42C-02 10:35h
Use of Stable-isotope and Radioisotope Data to Assess Ground-water Leakage Between the D and N Aquifers in the Black Mesa Area, Arizona
Ground-water leakage between the D aquifer and underlying N aquifer has been assessed, in part, by using isotopic data to calculate ground-water age, characterize general climatic conditions during ground-water recharge, delineate areas where ground-water mixing and leakage is occurring, and determine sources of sulfate. The hydrogeologic characteristics of the Carmel Formation, which is predominantly a confining unit between the D and N aquifers throughout the study area, and the relation of these characteristics to ground-water leakage are being described in an ongoing study. The Navajo Nation and the Hopi Tribe in the Black Mesa area depend on the N aquifer to supply water for domestic, agricultural, municipal, and industrial needs, and are concerned that pumping of ground water from the N aquifer could induce leakage from the D aquifer and degrade the water quality in the N aquifer. Adjusted 14C ages for ground water in the D aquifer range from 4,000 to 33,000 years. Values of 18O and D (hydrogen-2) indicate that most ground-water recharge occurred when the climate was cooler and more humid than at present. Detectable amounts of tritium in water from a few wells imply that localized recharge has occurred in some areas in recent time. Ratios of strontium-87 to strontium-86 in ground-water samples from the D and N aquifers in the southern part of Black Mesa support the hypothesis of leakage between the D aquifer into the N aquifer. Strontium isotope ratios in the N aquifer in the northern part of Black Mesa are larger than ratios in the southern part; statistical means are -0.14 % and -2.49%, respectively. Ratios in the N aquifer in the southern part of Black Mesa, however, are similar to ratios in the D aquifer in the same area; statistical means are -2.74 % and -2.49 %, respectively. The similarity of ground-water ages in the D aquifer to ages in the N aquifer suggests that leakage has been occurring for thousands of years. Elevated sulfur concentrations in the D aquifer occur from gypsum stringers and lignite seams in the rock units of the Dakota Sandstone. Sulfur-34 data indicate that sulfate reduction occurs when ground water comes in contact with lignite seams in the Dakota Sandstone, a rock unit of the D aquifer. Consequently, sulfur isotope values can be used to determine areas where ground water may be in contact with lignite seams and be used to identify the source of elevated sulfate concentrations in the water. Preliminary results from the ongoing study of the Carmel Formation indicate a relation between the thickness of the Carmel Formation and leakage indicated by 87Sr/86Sr data. Additional characterization of the thickness, lateral extent, and lithology of the Carmel Formation from the ongoing study will provide an improved understanding of the geographic extent of leakage in the Black Mesa area.
H42C-03 10:50h
Experimental Determination of Isotopic Fractionation of Chromium(III) During Oxidation by Manganese Oxides
In environmental conditions, chromium (Cr) exists in either the immobile, micronutrient trivalent form (Cr(III)) or the mobile, toxic hexavalent (Cr(VI)) form. Cr(VI) quickly reduces upon encountering Fe(II) or soil organic material (SOM). Therefore, it is often assumed that human Cr additions to terrestrial systems will impact localized areas and natural sources pose minimal threat to human or ecosystem health. However, oxidation and mobilization of Cr(III) by common manganese (Mn) oxides is less understood, especially in field settings. Moreover, Cr(VI)$'$s anionic form should enhance mobility through Fe- and SOM-poor soil and saprolite matrices. The variety of redox environments along a flowpath makes Cr source identification difficult with only concentration and speciation data. However, Cr has four stable isotopes (50, 52, 53, and 54), and characteristic fractionations during redox transformations might allow clarification of sources and flowpaths. For example, Cr(VI) reduction by a variety of reductants discriminates against heavy Cr, resulting in an increasingly heavy Cr(VI) fraction as reduction proceeds ($\alpha_{Cr(III)-Cr(VI)} \sim$ 0.996). Measurement of isotopic fractionation in other environmental Cr transformations, including oxidation, is necessary to understand Cr fate and transport. Recent estimates of isotopic fractionation between Cr aqueous species based on theoretical considerations indicate that at equilibrium $\alpha_{Cr(III)-Cr(VI)} \sim$ 0.994. To test this theoretical prediction, we are assessing the isotopic variability of aqueous Cr during oxidation of Cr(III) on MnO$_{2}$ materials such as birnessite in laboratory experiments. Initial results indicate that the isotopic composition of the product Cr(VI) ranges from -2.50 to +0.71 $\permil$ $\delta ^{53}$Cr, suggesting an important role for kinetic isotope effects during the initial oxidation process. Large fluctuations in isotopic composition continue after dissolved Cr(VI) and Cr(III) ratios stabilize and net Cr(VI) production rates are very slow. Moreover, the Cr(VI) isotopic composition fluctuates between heavy and light compositions several times over the course of reaction. Overall, however, the long term trend appears to be toward the equilibrium fractionation predicted by theory. This adds further credence to hypothesized multiple oxidation pathways existing in the system and suggests that multiple processes with off-setting fractionations are driving the system. If these results are representative of natural systems, environmental Cr samples that have been oxidized or been oxidized/reduced multiple times along a flowpath, will have isotopic compositions that vary widely, depending predominantly on sample collection time. In turn, this suggests that Cr isotopic compositions alone will not clarify Cr fate and transport, especially at larger scales (e.g., catchments), and other geochemical and hydrologic constraints will be required.
H42C-04 11:05h
Groundwater Flowpaths Using Isotopic and Geochemical Tracers at a Mine Drainage, Leadville, Colorado
Determining the natural and anthropogenic sources of solutes and contaminants is crucial for watershed mass balance modeling and water resources management. A draining tunnel was constructed in 1952 to dewater mine workings in the Leadville Mining District in Colorado. Since its construction, the physical condition of the tunnel has deteriorated significantly and resulted in blockage and pool-up of ambient groundwater. Presently a 1,000,000,000-galloon pool of contaminated mine water lies under the town of Leadville and is connected to the tunnel. Precise knowledge of groundwater flowpaths is needed to install bulkheads into the tunnel so as to decommission the tunnel without causing catastrophic changes to the mine pool that could flood Leadville with toxic water. Isotopic (O-18 and tritium of water) and geochemical tracers were used in end-member mixing analysis (EMMA) to understand sources and quantity of bedrock groundwater in the tunnel. Conservative tracers were determined by the diagnostic tools of mixing models. A principal component analysis (PCA) was performed to extract eigenvalues and eigenvectors using correlation matrix of conservative tracers. The EMMA results indicated that the tunnel drains only a small volume of mine pool water and a very large volume of bedrock groundwater under current condition.
H42C-05 11:20h
A multi-isotope approach to characterize acid mine drainage in a hardrock alpine mine, Chaffe Co,Colorado.
Here we present information from an innovative suite of stable, radiogenic, and cosmogenic isotopes to better understand groundwater flowpaths and groundwater-surface water interactions in an applied acid mine drainage system. Stable water isotopes, tritium, helium-tritium, sulfur-35, and uranium 234/238 ratios were analyzed from precipitation, groundwater wells, interior mine drainages, and surface waters at the Mary Murphy Mine in Colorado to determine hydrologic transport mechanisms responsible for contaminated zinc releases. Hydrometric measurements suggested a snowmelt-driven pulse of elevated zinc in adit outflow. However, mixing models using stable water isotopes showed a regional groundwater signal in the adit outflow. Tritium values of 11 to 13 TU showed a slight enrichment of bomb spike water compared to snow values of about 9 TU, suggesting an older water source as well. Helium/tritium ratios on a subset of groundwater wells suggested that average residence times of alluvial wells ranged from 2.5 to 8 years. The combination of stable water isotopes and sulfur-35 (half-life of 87 days), showed that zinc-rich waters within the mine derived from infiltrating snowmelt more than a year old. However, measurement of sulfur-35 using low-level scintillation counts was compromised at times by the presence of uranium. We were able to remove the uranium through wet chemistry procedures, improving the accuracy of S-35 measurements. The U234/U238 ratio shows promise in discriminating between acid mine drainage and acid rock drainage. Acid rock drainage shows an unaltered ratio of 1:1, while acid mine drainage is enriched relative to the 1:1 equilibrium ratio. The combination of cosmogenic and stable isotopes within and near the Mary Murphy Mine may provide a useful tool for studying interactions between groundwater and surfacewater in a fractured rock setting. Remediation techniques can be directed more appropriately, and cost effectively, by the characterization of flowpaths within the mine as well.
H42C-06 11:35h
Tracer studies of transport processes in the tidal Hudson River: A comparison of SF$_{6}$ and a fluorescent dye
Fluorescent dyes have frequently been used to study transport processes such as net advection and longitudinal dispersion in rivers. Recently, it has been shown that sulfur hexafluoride (SF$_{6}$) is a viable alternative to dyes, while offering many advantages. For example, SF$_{6}$ is less expensive than dye, and has a greater dynamic range. As a result, experiments can be conducted on longer timescales and greater spatial scales. Also, because SF$_{6}$ is a gas, its loss across the air-water interface can be used to quantify the gas transfer velocity, which affects the fate of volatile and semi-volatile compounds. Here, we present results from a dual tracer release conducted in the tidal Hudson River using a fluorescent dye (Fluorescein; C$_{20}$H$_{10}$O$_{5}$Na$_{2}$) and SF$_{6}$. Fluorescein was surveyed for 5 days (until it was undetectable) and SF$_{6}$ was surveyed for 11 days. The dye resolves initial vertical mixing on the first day, and then net advection and longitudinal dispersion on subsequent days, while the SF$_{6}$ provides information on net advection and longitudinal mixing on larger spatial and longer time scales. Transport processes (net advection and longitudinal dispersion) calculated from the two methods were consistent for the first four days, and start to deviate on the fifth day when the signal to noise ratio of the dye deteriorated. Initial results indicate that for the first four days, net advection was 3.0 $\pm$ 0.1 km d$^{-1}$, and longitudinal dispersion coefficients were 24.6 $\pm$ 6.6 and 19.6 $\pm$ 2.5 m$^{2}$ s$^{-1}$ for SF$_{6}$ and dye, respectively. The SF$_{6}$ results for the longer timescale and larger special scale will be discussed in the context of similar experiments conducted in other reaches of the tidal Hudson River.
H42C-07 11:50h
Multivariate Analysis of Amargosa Desert, NV Groundwater Chemistry
Major ion and isotopic chemistry from 203 wells located in the vicinity of the Amargosa Desert, Nevada was analyzed using Principal Component Analysis (PCA). PCA is a multivariate method that allows for a better understanding of the chemical evolution of the groundwater, potential flow paths, and groundwater flow rates. The PCA of the major ion chemistry was used to reduce the number of variables that describe the groundwater chemistry from seven to three principal axes which are then rotated to better interpret the systems variation. A cluster analysis of the reduced system produced nine separate groups of wells. The derived axes and well groups are presented as biplots and overlaid on a digital elevation model of the region giving a visual picture of potential interactions and flow paths. A distinct signature of the groundwater chemistry along the extended flow path of Fortymile Wash can be observed along with some potential interaction at possible fault lines near Highway 95. The signature from Fortymile Wash is believed to represent the relict of water that infiltrated during past pluvial periods when the amount of runoff in the wash was significantly larger than during the current drier period. This hypothesis appears to be supported by 14C and 18O data which indicate that younger groundwater is found in the upper part of wash near Yucca Mountain and older groundwater is found in the lower region of the wash near Amargosa Valley. Corrected 14C dating along Fortymile Wash present ages between 8 thousand years in the upper region and 14 thousand years in the lower region. This range in ages corresponds to the end of the Pleistocene and early Holocene, marking the end of the Wisconsin glaciation and beginning of the current interglacial period. The range of the 18O data correspond to a relatively cold clime precipitation and have a similar spatial signature to the 14C data. 18O values in the upper part of Fortymile Wash correspond to the warmer climate of the cold clime range, but still cold relative to the present; whereas values found in the lower portion of the wash correspond to colder climates. Plots of 2H versus 18O compared with the Global Meteoric Water Line (GMWL), demonstrate a humid climate type of precipitation with very little surface evaporation before infiltration, which along with a cold climate, suggest snowmelt infiltration.
H42C-08 12:05h
Field experiments on impact of ethanol on natural attenuation of BTEX and MTBE in groundwater
Ethanol has replaced MTBE in gasoline in California and elsewhere, in part to reduce problems with MTBE contamination of groundwater resulting from fuel spills. We are conducting field studies of the subsurface impacts of releases of ethanol-containing gasoline. In May 2004, we began side-by-side controlled release experiments at an existing fuel spill site (a former gasoline station) at Vandenberg Air Force Base, California. In one side of the paired experiments, we have been releasing benzene, toluene, o-xylene and tracers in a controlled and steady rate into a shallow, extremely well-characterized aquifer. In the other side, we have been releasing benzene, toluene, o-xylene, tracers and ethanol into the aquifer in the same manner. Using an extremely detailed grid of monitoring wells, we are gathering information on the migration and fate of the species in the two sides, and thus on the differences caused by the presence of the ethanol on one of the sides. Results from the first several months of the experiments suggest that the preferential degradation of the ethanol causes a significant reduction in the natural attenuation of the BTX species. We are also gaining insight into the impact that the more reduced conditions caused by ethanol degradation have on the fate of MTBE. Changes in the microbial community are being monitored as a means of understanding as well as generalizing the results.