Geochemistry Of Naturally Occurring Arsenic In The Humboldt River Basin, Nevada
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The geochemistry of naturally occurring arsenic (As) has been investigated in the surface waters and ground waters of the shallow alluvial aquifers of the Humboldt River Basin (HRB) in northern Nevada using laboratory experiments, geochemical reaction path modeling, and statistical analysis methods. A total of 15 surface water samples and 19 sediment samples were obtained from the Humboldt River and river-bottom sediments in the field in September, 2007. The ground water data from 72 wells, selected from a total of 47,500 samples from 18,800 springs and wells, in the HRB and northern Nevada were obtained from a public domain database provided by the Nevada Bureau of Mines and Geology. Concentrations of dissolved As in the waters of the Humboldt River (HR) range from 0.012 to 0.066 mg/L, with an average of 0.032 mg/L. The concentrations of As in shallow alluvial ground waters range up to 0.55 mg/L with an average of 0.06 mg/L. The current Environmental Protection Agency (EPA) maximum contaminant level (MCL) of As is 0.01 mg/L. The study of the distribution, sources, and processes controlling As in the Quaternary alluvial aquifers of the HRB and northern Nevada has found that the distribution of high As concentrations can be correlated with local and regional geology and geomorphology. The highest concentrations of As in the ground waters occur in the mineralized zones of metallic-sulfides around Boulder Valley, followed by the Quaternary playa deposits around Lovelock Valley and the Humboldt Sink, where evaporation predominates. Bi-variate correlations and factor analyses of the dissolved components in the ground water suggest that the sources of dissolved As are likely from dissolution of As-bearing sulfides, iron-oxyhydroxides, weathering of ferromagnesian silicates, and mixing with geothermal waters, where oxidation of As-bearing sulfides is alocal source of As in the Boulder valley area. The statistical and geochemical analyses of the HR bed sediments suggest that oxidation of As-bearing sulfide minerals is the source for high dissolved As in the upstream area. Concentrations of dissolved As and other trace elements are affected by mixing with ground water inflows occurring as river base flows in the upper HR and middle HR areas. Mixing with high-As geothermal waters locally enriches As near known areas of geothermal hot springs. Evaporation further enriches As in lower reaches of the river. Sequential extraction analyses of the river sediments demonstrate that As is mostly (83%) bound to the residual fraction which contains silicate minerals, and a lesser amount of As (13%) is held by iron-oxyhydroxides.The results of geochemical reaction path modeling indicate that oxidation of As-bearing sulfide minerals plays the most important role for source of As and sulfate in the upstream region with approximately 3.75 mmoles (449 ppm) of pyrite and 6.88 x10-4 mmoles (0.11 ppm) of arsenopyrite oxidized per liter of river water. The acidity produced by the oxidation of sulfide minerals is buffered by carbonate equilibria as a result of dissolution of Pre-Cenozoic carbonate rocks in the country rocks of the upstream region. Modeling results demonstrate that the source of As from oxidation of sulfide minerals is less significant in the downstream, where mixing with shallow ground water inflows and local geothermal spring water control the enrichment of dissolved As. The effects of evapo-transpiration further control the dissolved As concentrations in the lower HR waters. The process of sorption-desorption, which is pH-dependent, is less significant in the HR unlike many other similar semi-arid environments, and needs further evaluation.