Understanding Effects of Recharge and Dissolved Nitrate on Selenium and Salinity Mobilization Using Geochemical Modeling and Laboratory Testing
Selenium (Se) and salinity (total dissolved solids) are waterquality concerns in much of the arid western U.S. where agricultural and urban irrigation overlie Cretaceous-age shale. Agricultural and urban land uses increase recharge to near-surface ground water and contribute to perennial streamflow in historically arid areas. Leaching of Se and salinity from shale can degrade water quality and negatively affect aquatic biota in receiving water bodies. Multiple local, State, and Federal agencies, including the Bureau of Reclamation (Reclamation) and the U.S. Geological Survey (USGS), have worked to quantify the effects of Se on water quality, fish, and birds in the western U.S.. Increased salinity can affect downstream water use for irrigation or water supply. In the upper Colorado River Basin, programs since 1985 have worked to reduce Se and salinity loads to the Colorado River through implementation of best management practices; however, there is still a lack of understanding of how periodic recharge (wetting and drying) control mobilization of Se and salinity from shale. Monitoring results also indicate statistical relations between dissolved nitrate and Se concentrations, suggesting that nitrate may play an important role in Se mobilization. This study proposes to use geochemical modeling in conjunction with laboratory testing of shale cores from the upper Colorado River Basin to evaluate physical and chemical processes controlling Se and salinity mobilization. Specific research questions to be addressed by this study include: (1) How do recharge and water table fluctuations (wetting and drying) affect the mobilization of Se and salinity from shales? (2) How does the presence of dissolved nitrate in ground water affect Se mobilization from shales?
Need and Benefit
Selenium is an essential trace element and dietary micronutrient for humans and animals. However, elevated concentrations of Se can be toxic to humans, fish, and wildlife. In the aquatic environment, Se accumulates through the food chain, leading to elevated Se concentrations in tissues of fish and birds, the toxic effects of which can include mortality, decreased reproduction, and deformities in offspring. Se is associated with Cretaceous shales throughout the western U.S., and in Colorado, most Se-impaired streams are associated with nonpoint source ground water discharge from irrigated areas underlain by Se-bearing Cretaceous shale such as the Mancos Shale in western Colorado or its equivalent, the Pierre Shale, in central and eastern Colorado. Multiple weathering processes can mobilize Se from the Se-bearing rocks into the shallow ground water system where it is transported to downgradient receiving streams. During weathering, oxidation and hydrolysis release anions and cations (salinity) including calcium (Ca), magnesium (Mg), sodium (Na), sulfate (SO4), bicarbonate (HCO3) and Se from shale. Selenium is converted from a reduced form such as elemental Se, selenide in pyrite, or organic Se to an oxidized form such as selenite or selenate. The more mobile oxidized forms of Se are then transported to streams via ground water movement. In arid climates, potential evaporation exceeds precipitation such that salts of Ca, Mg, Na, and SO(v4) accumulate by evaporative concentration in the shallow subsurface. Oxides and hydroxides of iron also are common in the oxidized weathering zone. Trace elements such as Se co-precipitate with the salts and are concentrated by evaporation. Selenium also can weakly adsorb to Fe-oxide and clay surfaces under alkaline conditions. As stream and ground water levels fluctuate throughout the year, Se cycling between dissolved and solid phases can occur. Evaporative Se-bearing salts can be dissolved, mobilized, and flushed to streams when water levels rise. When water levels fall, evaporative deposits can precipitate near the capillary fringe. In irrigated agricultural areas, the presence of dissolved nitrate in recharge and ground water has been statistically related to elevated ground water Se concentrations. A few previous studies have hypothesized that nitrate serves as an oxidizing agent for Se dissolution; however, no studies have undertaken systematic geochemical modeling and laboratory testing of Se-bearing shales to quantify the effects of dissolved nitrate on Se mobilization processes.
The research will benefit and provide information for current Reclamation programs, by furthering the understanding of Se and salinity mobilization beneath irrigated areas that overlie Se-bearing shale. Reclamation is actively working in collaboration with the Colorado River Basin Salinity Control Program (CRBSCP) and the Gunnison Basin Selenium Task Force (GBSTF) to reduce Se and salinity loading to the Colorado River. Results from this proposed work will identify recharge conditions and nitrate concentrations that promote and/or deter Se and salinity mobilization, which will assist Reclamation in implementing best management practices for irrigation in the western U.S.. Because the study will evaluate generic geochemical processes rather than site-specific processes, study results will be directly transferrable to other locations in the western U.S. underlain by Se-bearing shales. Results from the study will thus benefit local, State, and Federal agencies as they develop and implement best management practices as part of Se and salinity control programs in Colorado and throughout the western U.S..
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Study results will identify processes and conditions controlling Se mobilization and will be published in a peer-reviewed journal article. Associated data will be served through the USGS Colorado Water Science Center website.