Evaluation of Passive Bioreactors to Reduce Selenium Impacts from Reclamation Projects
The biologically-assisted removal of selenium from water has been demonstrated using active but expensive methods that involve nutrient addition, precipitation, and filtration. The bench-scale tests proposed herein would be conducted for about six months and will develop data necessary to evaluate the kinetics and feasibility of using *passive* selenium reducing bioreactors to replicate the success of active treatment methods, but at a significantly lower cost per acre-foot treated. The tests will evaluate different construction materials for the proposed bioreactors, including inexpensive agricultural wastes (wood chips, moldy hay, sawdust, and animal manure). Selenium reduction has been observed as a secondary benefit in the treatment of mining-impacted water using similar materials.
Successful testing of this concept will provide Reclamation and others with a valuable tool to address selenium-related problems caused by irrigation drainage from Federal projects in western Colorado and other western states.
Need and Benefit
The lower Colorado and Gunnison Rivers in western Colorado and many of their tributaries are on the State of Colorado's 303(d) list of impaired waters for selenium, primarily due to irrigation drainage that mobilizes selenium from the local Mancos shale. These river segments also are critical habitat for four endangered fish species, and Reclamation is working with its partners to recover those fish. However, Endangered Species Act (ESA) Section 7 consultations could eventually place restrictions on water use by Reclamation projects and affect Reclamation's ability to deliver water.
Much of the source of this drainage is water delivered through Reclamation project facilities (Grand Valley Project & Uncompahgre Project). This same problem is occurring in other Reclamation project areas, including the lower Arkansas basin in eastern Colorado and many areas in California (San Joaquin Valley and Salton Sea).
Because of the diffuse and widespread nature of the source, a combination of selenium reduction efforts is needed locally, ranging from best management practices (BMP) for irrigation (e.g., lining/piping canals and laterals) to treatment of selenium-contaminated surface water at strategic locations.
This Science and Technology (S&T) Program research project aims to determine the optimum substrate and loading rates for a passive selenium-reducing bioreactor, which could be a practical, low-cost treatment method with the potential to be used throughout western Colorado and other Reclamation project areas. It would likely be transferable to other locations where the presence of marine shales has led to selenium-impaired surface waters. Locally, it could be applied to drainages where State and Federal regulatory agencies are pushing for the reduction of selenium loading. Use of this technology could avoid future water conflicts with potentially substantial cost savings.
Although regulatory restrictions on water use are not in place at this time, the "writing is on the wall, " and this technology could lead to local entities undertaking remediation, thus benefiting Reclamation and their project water users.
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