No description available.

Cart Lake, located 1.6 km south of the town of Cobalt, ON, contains elevated concentrations of As (0.95 to 18.05 mg/L Ast) resulting from uncontained mine tailings deposited during a silver mining boom from 1910 to 1983. Aqueous geochemical and investigations indicate tailings are sulfate-affected but subject to extensive carbonate buffering, with pH ranging from 6.7 to 8.6, Eh ranging from 46 to 210 mV, and alkalinity ranging from 32 to 201 mg/L HCO3-. The dominant cation species are Ca (4.9 to 192.0 mg/L), K (1.1 to 13.5 mg/L), Mg (1.2 to 8.7 mg/L), and Na (2.1 to 16.9 mg/L) and the dominant anionic species are alkalinity, As (0.003 to 18.1 mg/L), Cl- (0.3 to 16.8 mg/L), PO4 (0.04. to 9.6 mg/L), and SO42- (6.8 to 456.1 mg/L). Concentrations of minor and trace cations such as Fe, Co, and Ni are present at the g/L scale. Mineralogical investigations suggest As is mobilized from primary reactive sulfide and arsenide minerals such as arsenopyrite, skutterudite, and safflorite/löllingite/ rammelsbergite solid solutions. Primary mineral alteration products including Ca-Fe arsenates and erythrite are found in these tailings, resulting from direct alteration from primary minerals and precipitation from solution respectively. Secondary Ca-Fe arsenates and Co-Arsenates attenuate As, Co, and Fe by absorption and coprecipitation throughout the tailings profile, influenced by the presence of alteration rims around reactive mineral grains in the vadose zone. Field analysis of cation speciation and geochemical modelling of elemental equilibrium concentrations with weathering primary mineral surfaces would further inform conclusions regarding the relative contributions of each mineral to total As attenuation.

An often overlooked connection between karst groundwater systems and surface water is spring flow reversal, the flow of river water into karst springs caused by changes in hydraulic gradient. Karst aquifers are subject to the intrusion of river water when the hydraulic head of a base level river is higher than the hydraulic head of a base level spring. When this occurs, the flow out of the spring reverses, allowing river water to enter base level conduits. River water thus becomes a source of recharge into karst basins, transporting both valuable nutrients and harmful contaminants into karst aquifers. The rapid recharge of meteoric water, brief groundwater residence times, and the interconnection of surface and subsurface waters through a variety of karst features necessitates studying groundwater and surface water in karst landscapes as a unified system. This study examines the influence of spring flow reversal on cave dissolution in a telogenetic karst aquifer in Mammoth Cave, Kentucky. Spring flow reversals in Mammoth Cave National Park (MCNP) were first recorded nearly one-hundred years ago, but a high-resolution study measuring the effects of spring flow reversals on dissolution in MCNP, or any other telogenetic karst system, had not been conducted until recently. In this study, high-resolution data were collected for pH, SpC, temperature, and stage, as well as weekly samples for major ion concentrations, alkalinity, and carbon isotopes, from June 2018 to December 2018. Surface water and groundwater data were used to quantify the complex hydrologic processes associated with the spring flow reversals, including seasonal changes in karst geochemistry and dissolution taking place between the Green River, River Styx Spring, and Echo River Spring. Data show distinct changes in geochemical parameters as flow reversals occur, with temperature being the principal indicator of flow direction change. During this study, all ten stable reverse flows coincided with increased discharge from the Green River Dam. The predominant drivers of dissolution in the River Styx and Echo River karst basins are storm events and seasonal changes in the hydrologic regime, rather than seasonal CO2 production, normal baseflow conditions, or stable reverse flow events. Estimated dissolution rates generally show that stable reverse flows contribute no more to dissolution than normal baseflow conditions – the highest amount of dissolution during a single stable reverse flow was only 0.003 mm. This is contrary to flow reversal studies in an eogenetic karst system in Florida, which estimated 3.4 mm of wall retreat during a single spring flow reversal. These contrasting results are likely due to significant differences in pH of river water, matrix porosity of the bedrock, basin morphology, and flow conditions.

Calcium stable isotope ratios are a relatively new tool that biogeochemists can use to investigate the biogeochemical cycle of calcium in terrestrial ecosystems, having seen widespread application only in the past 15 years. To advance the application of calcium isotopes in biogeoscience research, I conducted three investigations focused on interpreting calcium isotope ratios in streamwater and in the cation exchange pool of forest soils. In the first study, we observe a shift toward lower 44Ca/40Ca ratios in streamwater draining a New Hampshire watershed after an experimental clearcutting event. Isotope ratio measurements of ecosystem calcium pools indicate that enhanced leaching of the soil exchangeable pool produced the observed shift in 44Ca/40Ca ratios. A trend towards decreased 44Ca/40Ca ratios in soils in the years following the harvesting indicates that calcium leached from the soil exchangeable reservoir was likely replaced by calcium released by the decay of belowground biomass, maintaining pre-harvest levels of exchangeable calcium even in the face of a significant ecosystem disturbance. In a second study, we observed significant differences in the 44Ca/40Ca of the soil exchange pool between two neighboring tropical watersheds, although 44Ca/40Ca of calcium inputs (bedrock and atmospheric deposition) at the two sites were indistinguishable. Further, both sites had higher 44Ca/40Ca ratios compared with external inputs, a relatively rare observation globally. We propose that hurricane disturbance best explains the high 44Ca/40Ca at each site, and that the difference in 44Ca/40Ca between the two sites can be accounted for by the magnitude of disturbance at each site. Finally, a synthesis of our new data with previously published results shows that globally, soil exchangeable 44Ca/40Ca ratios can be higher, lower or equal to external inputs. Modeling work indicates that in addition to isotopic fractionation, the balance in fluxes between vegetation and soil is critical in determining how soil exchangeable 44Ca/40Ca ratios vary relative to external inputs. When plant uptake and return to the soil are equal, soil and external inputs 44Ca/40Ca are equal, while high soil 44Ca/40Ca ratios develop when uptake exceeds return. Soil develops low 44Ca/40Ca when biomass obtains calcium from sources other than the exchangeable reservoir.

The traditional paradigm declares tetravalent uranium to be immobile under reducing conditions – an assumption widely employed for nuclear waste management strategies. In contrast, experiments presented here demonstrate this assumption, although valid for low temperatures, can be erroneous for high temperature natural systems. This project focuses on the ability of sulfate-bearing solutions to transport uranium at reduced conditions and elevated temperatures, identifies the new species U(OH)2SO4, derives thermodynamic constants necessary for modeling, and expands the quantifiable range of U4+ mobility to more neutral pH conditions. The data obtained enable more accurate assessment of uranium mobility by updating the existing uranium thermodynamic databases and is applicable to uranium fluid transport in oreorming systems and nuclear waste repositories.

This dissertation combines laboratory experiments and analysis of field samples to examine tungsten (W) geochemistry. Data from low ionic strength experimental solutions at room temperature containing between 0.01 M to 0.0002 M total sulfide and 0.0027 M - 0.0001 M tungstate were analyzed using UV/VIS spectrophotometry. Stability constants have been determined for the formation of mono-thiotungstate log K01= 3.43 ± 0.61, di-thiotungstate log K12 = 3.02 ± 0.61, tri-thiotungstate log K23 = 2.82 ± 0.02, and we estimated the tetra-thiotungstate log K34 ~ 2.34. Analysis of W, Mo, Mn, and Fe concentrations in estuarine surface and pore waters and sediments captured environmental samples from oxic and sulfidic conditions. Both surface waters and sediments demonstrated a positive correlation between W and Fe. Unlike Mo, which was depleted in sulfidic salt marsh pore waters, W was enriched in all pore waters in comparison to overlying waters. Thermodynamic modeling of W and Mo thioanion species in sulfidic pore water samples predicts ≤ 50% of tungstate (WO42-) forms thiotungstate species and complete conversion of molybdate (MoO42-) to tetrathiomolybdate (MoS42-). Unlike tetrathiomolydate that is known to be more particle reactive than molybdate, increases in dissolved W coincide with increases in dissolved sulfide in pore waters, suggesting thiotungstates are less particle reactive than thiomolybdates at circum-neutral pH. Finally, sediment analysis suggests sequestration of W is dependent on surface water salinity in the intermediate marsh sediments, and long-term W entrapment occurs in sulfidic salt marsh sediments. / acase@tulane.edu

The low temperature phase behavior of the system H2O-NaCl-FeCl2 was examined using synthetic fluid inclusions. Experiments were conducted along the 5 wt% NaCl (relative to the total solution) pseudobinary, with FeCl2 concentrations varying from 2 to 33 wt%, and along the pseudobinary defined by mixing known amounts of FeCl2-4H2O with a 5 wt% NaCl solution, with final FeCl2 concentrations varying from 0 to 29 wt%. Synthetic fluid inclusions in quartz were prepared in cold-seal pressure vessels at 500 degrees C - 800 degrees C and 2 or 3 kilobars. The fO2 conditions were controlled by the Ni-NiO equilibrium curve. The liquid released from the capsule upon opening was initially colorless, but turned yellow-orange after contact with atmospheric O2. The clear color is characteristic of ferrous iron solutions, whereas the yellow-orange color is consistent with the presence of Fe3+ in solution. This color change suggested that the unopened capsules initially contained ferrous iron in solution, which oxidized to ferric iron when exposed to the atmosphere. Borisenko (1977) reported a eutectic temperature of -37 degrees C for the system H2O-NaCl-FeCl2. In this study, it was not possible to verify this temperature due to the persistence of a metastable liquid down to liquid N2 temperatures (~-196 degrees C). Final ice melting temperatures were obtained for concentrations less than 24 wt% FeCl2 and show a decrease in temperature with increase in FeCl2 concentration. For more concentrated solutions, final melting temperatures could not be obtained because the samples could not be frozen. / Master of Science

No description available.

The focus of this work is to investigate the dissolution of MW25, a non-radioactive simulant of UK high-level nuclear waste borosilicate glass, and to predict its performance in the near field of a geological repository. A single-pass flow-through (SPFT) experimental system was used to measure the forward dissolution rates of MW25. Experiments were conducted in two parts. Experiment Part 1 considers the dissolution of the waste glass in deionised water at 40 and 90 oC and circum-neutral pH. Experiment Part 2 considers the dissolution of the waste glass in simulant groundwaters, with similar compositions to groundwaters of Callovo-Oxfordian clay (lower-strength sedimentary rock (LSSR)) and Borrowdale Volcanic Group rocks (higher-strength rock (HSR)), at 40 oC and pH 7. The forward dissolution rate measured in deionised water was found to be approximately one order of magnitude higher at 90 oC than at 40 oC. A similar release was observed for Si, Mg and Al at 40 oC and 90 oC, whereas the B, Cs, Na, Li and Mo showed an order of magnitude increase when the temperature was increased from 40 to 90 oC for low q/S values. The activation energy (Ea) of the reactions shows that the dissolution process is a surface phenomenon. At 90 oC the net effect of the processes governing MW25 dissolution led to the preferential release of boron and alkali metals relative to the release of Si during the transient dissolution stage, accompanied by an increase in the concentration of silicic acid. This suggests that the solution activity of silicic acid at a higher temperature has a weak influence on the release of the mobile elements. The forward dissolution rate measured in LSSR simulant groundwater was found to be slightly higher than that measured in HSR simulant groundwater. The dissolution behaviour of MW25 in both groundwaters is consistent with its behaviour in deionised water at 40 oC, with the dissolution rates of elements increasing as flow rates were increased. However, forward dissolution rates measured in the simulant groundwaters were lower than the forward dissolution rates measured in deionised water under these experimental conditions. This is attributable to the interaction of the components of the simulant groundwaters with the glass, as revealed by post-reaction surface analyses, and a consequential lower alkalinity of the leachates collected in the experiments with simulant groundwater than in deionised water. Reactive chemical transport simulations of waste glass dissolution and radionuclide release in a hypothetical near field were conducted over a time span of a million years with GoldSim. The results showed that enclosing the waste glass in a steel canister covered by a copper canister and emplacing the waste package in a granite host rock is optimal for the long-term isolation of the radionuclides. The waste glass was found to play a significant role in the overall performance of the near field. This study features a new method for estimating the surface area of reacted glass powder more accurately than the geometric surface area estimate, which is the preferred standard method among researchers.