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Title: The influence of nitrate on selenium in irrigated agricultural groundwater systems

Author
item BAILEY, RYAN - Colorado State University
item Hunter, William
item GATES, TIMOTHY - Colorado State University

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/12/2012
Publication Date: 5/1/2012
Citation: Bailey, R.T., Hunter, W.J., Gates, T.K. 2012. The influence of nitrate on selenium in irrigated agricultural groundwater systems. Journal of Environmental Quality. 41:783-792.

Interpretive Summary: The contamination of groundwater by toxic selenium oxyanions is an environmental concern in many regions of the world. This is particularly so in areas where aquifer systems are underlain by marine shale, an important selenium containing geologic formation. The leaching of selenite and selenate from these deposits is impacted by the influence of other ions present in the groundwater. This study examined the influence of nitrate on the release of selenate and sulfate into groundwater. Sulfate is an anion often associated with selenate. Laboratory tests included batch and flow-through column studies to assess the release of sulfate and selenate and the inhibition of their reduction by nitrate. Results showed a strong correlation in groundwater between the amount of nitrate and of selenate (R = 0.82) as well as between nitrate and sulfate (R = 0.73). Both dissolution and oxidation were responsible for the release of selenate and sulfate from shale. Nitrate concentrations > 5 mg L-1 was found to greatly diminished the reduction of selenate in groundwater. The results demonstrate the importance of considering the presence of nitrate in groundwater when investigating the fate and transport of selenium oxyanions in soil and groundwater systems.

Technical Abstract: Selenium (Se) contamination of groundwater is an environmental concern in several regions 25 around the world, particularly in areas where aquifer systems are underlain by Se-bearing 26 geologic formations such as marine shale. The fate and transport of Se is complicated by the 27 influence of other chemically-active species such as nitrate (NO3), which inhibits the chemical 28 reduction of toxic selenate (SeO4) as well as releases SeO4 from shale through oxidation of 29 reduced Se. This study provides a review of the influence of NO3 on Se species in soil and 30 groundwater systems and presents results of field sampling and laboratory tests that provide 31 further clarification on this influence. Field tests include groundwater sampling near the 32 alluvium-bedrock shale interface and an analysis of bedrock shale samples. Laboratory tests 33 included batch and flow-through column studies to assess the release of sulfate (SO4) and SeO4 34 in the presence of NO3 as well as the inhibition of SeO4 reduction in the presence of NO3. 35 Results showed that a strong relationship in the groundwater exists between NO3 and SeO4 (r2 = 36 0.67, a = 0.05) as well as NO3 and SO4 (r2 = 0.54, a = 0.05), that both dissolution and oxidation 37 are responsible for the release of SO4 and SeO4 from shale, and that a concentration of NO3 of 38 approximately 5 mg L-1 or greater will diminish the reduction of SeO4. Although not trivial, the 39 inclusion of the fate and transport of NO3 is imperative when studying or simulating the fate and 40 transport of Se species in soil and groundwater systems.