Laura Toran

Interpretation of sulfur and oxygen isotopes in biological and abiological sulfide oxidation

Sulfur and oxygen isotope ratios in sulfate cannot distinguish unambiguously between biological and abiological mechanisms of sulfide oxidation because similar isotope signatures have been observed or predicted for different mechanisms. However, field and laboratory data on sulfur and oxygen isotopes have demonstrated the importance of understanding environmental factors to limit the set of reasonable mechanisms. In this paper, we review sulfur and oxygen isotope compositions reported in both field and laboratory studies of sulfide oxidation. We develop a new classification scheme for sulfide oxidation based on whether the electron transfer involves incorporation of oxygen from H2O or O2 in the sulfate. Four mechanisms for H2O-oxygen incorporation and two for O2-oxygen incorporation are presented. We also point out several environmental factors such as the production of intermediate sulfoxyanions or the presence of certain Thiobacillus species that may determine whether sulfur is fractionated during oxidation. More information on environmental factors is needed.

Colloid transport through fractured and unfractured laboratory sand columns

We examined the effects of artificial fractures on colloidal transport in laboratory sand columns. Fractures were simulated by inserting small (0.2- or 1-mm-diam) tubes of controlled length and density into a sand column with dimensions 5-cm-diam by 65-cm-length. We injected 1-μm-diam fluorescent microspheres, bacteria, and colloidal organic matter into columns with tubes and columns without tubes. Breakthrough curves were analyzed using an equilibrium transport model to obtain values for percent retention, retardation, and dispersivity. Fractures significantly increased the amount of microsphere transport; 80–99% of the spheres were retained in experiments with tubes inserted compared to 99–100% without tubes. Bacteria used in this particular experiment had a lower percent retention (increased transport), 73% with tubes and 93% retention without tubes. Colloidal organic matter had 34% retention with tubes and 55% retention without tubes. Microspheres travelled faster than a conservative salt tracer (retardation factor < 1), probably caused by exclusion of travel at slower velocities in the matrix. This effect was enhanced when tubes were present. In some experiments, the tubes created multiple flow paths manifest in irregular rather than smooth breakthrough curves. There was no measurable difference in dispersion for the “fractured” versus the “unfractured” system The retention coefficient was determined by sectioning the column at the end of the experiment and counting the number of microspheres retained in sections along the column. The exponential retention rate for spheres was on the order of 0.1 cm−1, with no measurable variation between sand columns with and without tubes. This retention coefficient is needed for models to predict colloid transport. In addition to providing parameter values for models, these observations have implications for monitoring colloid transport in natural systems. It is necessary to consider the possibility of travel faster than a conservative tracer, irregular breakthrough curves, and irreversible retention when sampling for colloids.

Stream Bottom Resistivity Tomography to Map Ground Water Discharge

This study investigates the effectiveness of direct current electrical resistivity as a tool for assessing ground water/surface water interactions within streams. This research has shown that patterns of ground water discharge can be mapped at the meter scale, which is important for understanding stream water quality and ecosystem function. Underwater electrical resistivity surveys along a 107-m stream section within the Burd Run Watershed in South Central Pennsylvania identified three resistivity layers: a resistive (100 to 400 Omega m) surface layer corresponding to the streambed sediments, a conductive (20 to 100 Omega m) middle layer corresponding to residual clay sediments, and a resistive (100 to 450 Omega m) bottom layer corresponding to the carbonate bedrock. Tile probing to determine the depth to the bedrock and resistivity test box analysis of augered sediment samples confirmed these interpretations of the resistivity data. Ground water seeps occurred where the resistivity data showed that the residual clays were thinnest and bedrock was closest to the streambed. Plotting the difference in resistivity between two surveys, one conducted during low-stage and the other during high-stage stream conditions, showed changes in the conductivity of the pore fluids saturating the sediments. Under high-stream stage conditions, the top layer showed increased resistivity values for sections with surface water infiltration but showed nearly constant resistivity in sections with ground water seeps. This was expressed as difference values less than 50 Omega m in the area of the seeps and greater than 50 Omega m change for the streambed sediments saturated by surface water. Thus, electrical resistivity aided in characterizing ground water discharge zones by detecting variations in subsurface resistivity under high- and low-stream stage conditions as well as mapping subsurface heterogeneities that promote these exchanges.

Modeling of radionuclide and heavy metal sorption around low- and high- pH waste disposal sites at Oak Ridge, Tennessee

Modeling of mineral precipitation and metal sorption reactions using MINTEQA2 and the Fe oxyhydroxide diffuse-layer model has provided insights into geochemical processes governing contaminant migration from low-level radioactive waste disposal sites at the U.S. Department of Energys Oak Ridge National Laboratory and Y-12 Plant at Oak Ridge, Tennessee. Both acidic and basic nuclear-fuel reprocessing wastes, locally mixed with decontamination solvents, were disposed of in unlined trenches and lagoons. Model results show that as wastes move toward neutral pH due to reactions with surrounding soils and saprolite, mineral precipitation and sorption can limit the solubility of heavy metals and radionuclides. However, observed contaminant levels in monitoring wells indicate that at least locally, wastes are moving in faults and fractures and are not retarded by sorption reactions along such flow paths. Model results also support previous studies indicating that organic complexing agents used in decontamination procedures can enhance radionuclide and heavy metal solubility when mixed with nuclear fuel reprocessing wastes. However, complex interactions between metal-organic complexes and mineral surfaces and natural organic matter, biodegradation, and fracture flow complicate the interpretation of contaminant mobility.

Sulfate contamination in groundwater from a carbonate-hosted mine

Abstract Sulfide oxidation in a carbonate environment produces groundwater contamination with high sulfate making the water unsuitable for drinking supplies. The zinc-lead mines near Shullsburg, Wisconsin are located in the Galena-Platteville Formation, a carbonate aquifer that was dewatered during mining. Sulfate levels have reached as high as 40 nmol/l in some local wells and eleven wells were abandoned. Geochemical modeling of chemical reactions and isotope effects using the USGS computer program PHREEQE showed the importance of dolomite, calcite, CO 2 , and siderite or iron hydroxide in controlling the water chemistry. The decrease in sulfate levels with time indicated that dilution by incoming recharge water was an ongoing process. The results of carbon isotope reaction modeling are consistent with dilution of contaminated water. The evidence for localization of contamination and dilution means that area farmers have seen the worst of the contamination. The mechanism of contamination was further examined by microbiological sampling and sulfur isotope determinations, which indicated that bacteria of the Thiobacillus species that thrive under neutral pH conditions may have catalyzed sulfide oxidation. Research into the chemical evolution of contamination in this environment not only explains how sulfide oxidation causes contamination despite buffering by carbonate rocks, but also suggests how oxidation is initiated in the case of acid mine drainage.

Clastic sediment transport and storage in fluviokarst aquifers: an essential component of karst hydrogeology

Carbonate aquifers with well-developed conduit systems carry a flux of clastic sediment as an intrinsic aspect of the functioning of the aquifer. Sources of clastic sediments include sediments carried by sinking streams, soil washdown from the epikarst, plug injection by sinkhole piping failures, residual insoluble material from the dissolution of the limestone, and sediment backflooded from surface streams. The conduit system acts as a mixing chamber where the injected materials are sorted and rearranged. Information on the sediments and their transport processes can be obtained by investigating the source areas, by inspection of cave sediments, and by monitoring clastic sediment discharged from springs as a function of flow conditions. The engine that drives the sediment transport system is storm recharge in the ground water basin drained by the conduit system. Fine-grained clastics move during ordinary storms and can be captured easily at springs, but movement of coarser materials requires high-intensity, therefore infrequent, storms so that most of the sediment flux is episodic with long periods of storage interspersed with short periods of movement. Fluid mechanics provides the basis for calculations of both bedload and suspended load components. However, these calculations become complex because of the need to take into account discharge-dependent shifts from pipe flow to open channel flow and the effect of irregularities in conduit morphology and blockages due to breakdown and other barriers.

Influence of Fracture Truncation on Dispersion: A Dual Permeability Model

Abstract Simulations with a dual permeability model show that a few discrete fractures can have a major influence on plume geometry. A limited number of truncated fractures (1–4) within a permeable matrix can create nearly circular plumes, with about the same degree of spreading in the direction transverse to the average hydraulic gradient as in the longitudinal direction. By comparison, continuous fractures in the direction of flow tend to produce elongated plumes, similar to those typically seen in granular materials. Both types of plumes have been observed in tracer experiments in fractured porous media on the Oak Ridge Reservation. Understanding the influence of fracture geometry is important in planning field characterization and subsequent remediation in fractured porous media.

Delineating a road-salt plume in lakebed sediments using electrical resistivity, piezometers, and seepage meters at Mirror Lake, New Hampshire, U.S.A

Electrical-resistivity surveys, seepage meter measurements, and drive-point piezometers have been used to characterize chloride-enriched groundwater in lakebed sediments of Mirror Lake, New Hampshire, U.S.A. A combination of bottom-cable and floating-cable electrical-resistivity surveys identified a conductive zone (<100 ohm-m) overlying resistive bedrock (<1000 ohm-m) beneath the lake. Shallow pore-water samples from piezometers in lakebed sediments have chloride concentrations of 200–1800 μeq∕liter , and lake water has a chloride concentration of 104 μeq∕liter . The extent of the plume was estimated and mapped using resistivity and water-sample data. The plume ( 20×35 m wide and at least 3 m thick) extends nearly the full length and width of a small inlet, overlying the top of a basin formed by the bedrock. It would not have been possible to mapthe plume’s shape without the resistivity surveys because wells provided only limited coverage. Seepage meters were installed approximately 40 m from the mouth o...

Sensitivity analysis of solute transport in fractured porous media

We conducted a sensitivity analysis using a numerical model of fractured porous media to find out how porous media and fracture parameters affect solute transport. The 2-dimensional, saturated fracture flow and transport code FRACTRAN was used to conduct the simulations. Seven parameters were considered: matrix hydraulic conductivity, matrix porosity, retardation of the matrix, gradient of the flow field, fracture retardation, fracture aperture, and fracture probability (which incorporates fracture spacing and fracture length). A Latin-hypercube design was used to select a matrix of parameter values that minimized correlations among the design parameters. The results were summarized by examining least square fit of the relationship between the seven parameters and 50% breakthrough time and spread of the breakthrough curves, with nearly identical results. The matrix parameters, in particular hydraulic conductivity and porosity, had the greatest effect. As expected, fracture probability was nearly equivalent in importance. These results indicate that field characterization in fractured porous media should not only emphasize fracture location, which strongly influences directions of contaminant transport, but also matrix properties, which have a major influence on contaminant residence times and breakthrough concentrations.

Characterizing lakebed seepage and geologic heterogeneity using resistivity imaging and temperature measurements

The contribution of groundwater-surface water exchange to lake budgets is poorly understood and depends in part on lakebed heterogeneities. These heterogeneities are difficult to characterize using traditional point sampling methods. The goal of this project was to use electrical resistivity to identify potential zones of groundwater discharge and recharge, providing focus for point measurements. Multiple resistivity surveys were conducted at Lake Lacawac, a small, glacially-formed lake in northeastern Pennsylvania. Two types of resistivity surveys were conducted. In a continuous resistivity profile, a multi-electrode cable was towed parallel to shore to look for spatial variability in resistivity around the lake. Two parallel to shore surveys were conducted, an inner and outer loop, to help characterize the lateral extent of sediment types. The results of these surveys suggested lithology changes both along the shoreline and with distance from shore. Follow-up resistivity data were collected using cables laid along the lake bottom perpendicular to the shoreline to look for finer scale zonation that affects seepage as a function of distance from shore. Follow-up seepage measurements showed that seepage rates are very low, which is consistent with the resistivity data from which we concluded that most of Lake Lacawac is lined with glacial clay and that the lake is essentially perched above the groundwater flow system, with just minor amounts of seepage in a few locations where fingers of sandy sediments extend a short distance from the shore. Discontinuities in these patches of transmissive sediments can result in reversals in the direction of seepage at nearby locations. We conclude that towed resistivity is useful as a rapid reconnaissance tool for mapping geologic heterogeneity. The results can be used to guide the more time-consuming but higher-resolution, lake bottom resistivity measurements, which in turn can guide the placement of seepage meters.