SELENIUM MOBILIZATION DURING A FLOOD EXPERIMENT IN A CONTAMINATED WETLAND, UPPER COLORADO RIVER BASIN, UTAH

A field experiment was conducted to assess the effectiveness of flooding on the removal of Se from dry surface sediments in a contaminated wetland in eastern Utah. The 83-square-meter plot contained 10 monitoring wells, a water-quality minimonitor (continuous measurement of pH, specific conductance, water temperature, and dissolved oxygen), a down-hole bromide electrode, and 2 pressure transducers. Flooding was initiated on August 27, 2002, and a Br tracer was added to water delivered through a pipeline to the flood plot during the first 1.2 hours (hr). Standing water depth in the flood plot was maintained at 0.3 meter (m) through September 1, 2002. The Br tracer data indicate a dual porosity system that includes fracture and matrix flow components. Mean vertical water velocities for the matrix flow component were estimated to range from 0.002 to 0.012 m/hr. Dissolved Se increased from pre-flood concentrations of less than 10 micrograms per liter (ug/L) to greater than 800 ug/L during flooding in samples from deep (2.0 m below land surface) ground water. Se concentrations exceeded 5,500 ug/L in samples from shallow (0.8 m below land surface) ground water. Ratios of Se to Br in water samples indicate that Se moved conservatively during the experiment and was derived from leaching of near-surface sediments. Cumulative Se flux to the deep ground water during the experiment ranged from 9.0 to 170 milligrams per square meter (mg/m2). Pre- and post-flood surface soil sampling indicated a mean Se flux of 720 mg/m2 through the top 15 centimeters of soil. Ground-water samples collected 8 months after termination of the flood experiment contained Se concentrations of less than 20 ug/L. The minimonitor data indicate a rapid return to chemically reducing conditions in the deep ground water, limiting the mobility of the selenium dissolved in the water pulse introduced during the flood experiment. Ratios of Se to Br in deep ground-water samples collected 8 months after the experiment confirmed the removal of Se from the aqueous phase. Based on the median Se flux rate estimated during the experiment of 0.65 mg/hr/m2 (n = 52), seven flooding cycles would be required to meet the 4 micrograms per gram remediation goal in surface soils for this wetland.