A Hydrologic and Geochemical Assessment of Potential Groundwater Contamination and Vadose Zone Attenuation at the Caselton Tailings Impoundment; Lincoln County, Nevada, USA
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Authors
Whitman, Spencer Kade
Issue Date
2023
Type
Dissertation
Language
Keywords
attenuation , geochemistry , hydrology , modeling , tailings , transport
Alternative Title
Abstract
Disposal of tailings material from the processing of lead, zinc, and silver sulfide ores as well as manganese oxide ores in the unlined Caselton Wash from 1941-1968 has created a potential source of groundwater contamination. However, the alluvial soils underlying the tailings impoundment are likely to provide significant natural attenuation of any acid mine drainage generated from the mine tailings, due to a relatively high carbonate content. The overarching goal of this project was to investigate the likelihood of possible impacts to groundwater quality from percolation of tailings-derived leachate through the Caselton Wash Tailings and the underlying alluvium. In addition, this study seeks to identify possible attenuation mechanisms for contaminants of concern which were identified in tailings leachates. To assess the likelihood of groundwater contamination, a field, laboratory, and modeling-based research program was conducted.Hydraulic properties were characterized through a combination of field infiltration measurements and laboratory characterization. Surface tailings material, as well as tailings material from subsurface borings were collected for use in laboratory experiments, and representative synthetic leachates were created in the laboratory from the collected tailings material. In addition, two monitoring well borings were drilled and one monitoring well was installed at the Caselton Wash site to determine the groundwater level and groundwater chemistry, both upgradient and downgradient of the tailings impoundment. Clean alluvial soils were collected during drilling of the upgradient well for use in laboratory column and beaker scale batch reaction experiments. The geochemical composition of sulfide tailings, Mn-oxide tailings, and alluvial soils were characterized using acid-base accounting, whole rock geochemistry analyses, X-ray diffraction, Raman spectroscopy, and scanning electron microscopy.Attenuation of contaminants in tailings leachates by the alluvial soil was assessed through bench scale laboratory batch reaction experiments, as well as a reactive column experiment. Batch reaction reconnaissance experiments were conducted prior to performance of the column experiment to assess possible attenuation mechanisms, and to inform column experiment design. Conservative transport properties of the alluvium were assessed using NaCl and deuterated water column tracer experiments, while reactive transport was assessed by infiltrating columns of clean alluvial soil with tailings leachate that was dosed with a deuterated water tracer. During the reactive column experiment, probe-based measurements of pH, specific conductance, dissolved oxygen, and oxidation-reduction potential were continuously monitored in the column effluent, and samples were periodically collected and analyzed for Nevada Division of Environmental Protection Profile III analytes. Data from the batch reaction and reactive column experiments, including probe measurements, mass balance measurements, analytical chemistry analyses of column effluent for dissolved ions and deuterium, and geochemical analyses of residual solids were utilized for detailed geochemical interpretation and identification of potential attenuation mechanisms.Information from field characterization, laboratory experiments, and publicly available datasets were integrated to support the construction of hydrologic and conservative contaminant transport models of water movement and contaminant transport within the tailings impoundment and the underlying alluvium. As sulfate was identified to be the major contaminant of concern, and the reactive column experiment suggested that sulfate concentrations would remain elevated in the alluvium, a transport analysis of sulfate was undertaken. Pore water velocities from the hydrologic model were used as inputs for a conservative transport analysis of sulfate from the tailings impoundment through a section of unsaturated alluvium to a hypothetical water table at 150 m depth below the ground surface, and predicted a mean travel time of 10.1 years, with sulfate concentrations predicted to reach 95% of the source concentration after 19.0 years.From a hydrologic perspective, the most important result of this study was an increased understanding of the tailings impoundment hydrology. Specifically, the hydrologic sources and sinks of the tailings impoundment were identified, and it was clearly shown that run-on of water to the tailings impoundment from upslope areas comprised a significant portion of the water entering the tailings impoundment. The results of the hydrologic model also suggested that collection of leachate within excavated pits present on the surface of several tailings ponds could be increasing the amount of leachate that infiltrates the unsaturated zone compared to an undisturbed tailings surface.From a geochemical perspective, the most important results were the identification of several contaminants of concern, and the identification of their probable attenuation mechanisms in the unsaturated zone. From an analysis of the geochemical compositions of the tailings and tailings leachates, aluminum, antimony, arsenic, beryllium, cadmium, copper, iron, lead, manganese, selenium, zinc, fluoride, and sulfate were identified as contaminants of concern. Except for sulfate, all contaminant concentrations in the leachate were significantly reduced through interaction with the alluvial solids. Results from the batch reaction and reactive column experiments indicated that all contaminant concentrations except for antimony, arsenic, manganese, selenium, and sulfate were reduced to below drinking water standards in the column effluent. However, sulfate was the only contaminant that remained significantly elevated above drinking water standards �" the others were only slightly elevated above standards.Attenuation mechanisms related to the precipitation of, co-precipitation with, and sorption on to hydrous ferric oxide minerals were probably the most important attenuation mechanisms for the contaminants of concern identified at the Caselton tailings impoundment, and these mechanisms probably account for most of the attenuation capacity in the alluvial sediments underlying the tailings impoundment. Additional attenuation mechanisms that probably took place were the precipitation of some contaminant-bearing secondary minerals, and the precipitation of some carbonate solid solution minerals that contained contaminants of concern.