HR: 15:10h
AN: H32D-06    [PDF]
TI: Selenium removal during a flood experiment: Best management practice for a contaminated wetland?
AU: * Naftz, D L
EM: dlnaftz@usgs.gov
AF: USGS, 2329 Orton Circle, Salt Lake City, UT 84119 United States
AU: Yahnke, J
EM: jyahnke@do.usbr.gov
AF: Bureau of Reclamation, Denver Federal Center, D-8570 P.O. Box 25007, Denver, CO 80225 United States
AU: Miller, J
EM: JMILLER.4ucro@uc.usbr.gov
AF: Bureau of Reclamation, Mail Room 6107, UC-242 125 South State Street, Salt Lake City, UT 84138 United States
AU: Noyes, S
EM: snoyes@uc.usbr.gov
AF: Bureau of Reclamation, Provo Area Office, PRO-470 302 East 1860 South, Provo, UT 84606 United States
AB: Constructed and natural wetlands can accumulate elevated levels of selenium (Se); however, few data are available on cost-effective methods for remobilization and removal of Se from these areas. The experiment was conducted at Stewart Lake Waterfowl Management Area (SLWMA), a Se-contaminated wetland in northeastern Utah. The purpose of the experiment was to assess the effectiveness of flooding on the removal of Se from surface sediments and transport to the chemically reducing ground water 1.8 meters (m) below land surface. The 84-m$^{2}$ flood-experiment plot contained 10 monitoring wells, a water-quality minimonitor (continuous measurement of pH, specific conductance, water temperature, and dissolved oxygen), a down-hole bromide (Br) 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. Standing water depth in the flood plot was maintained at 0.3 m through September 1, 2002. Mean vertical water velocities were estimated to range from 0.3 to 1.3 centimeters per hour. Dissolved (less than 0.45 micron) 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 (1.8 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 54.9 to 172 milligrams per square meter (mg/m$^{2}$). Pre- and post-flood surface soil sampling indicated a mean Se flux of 750 mg/m$^{2}$ through the top 15 centimeters of soil. Measurable Se flux to the deep ground water would have increased if the flood experiment had continued beyond September 1, 2002. Water samples from the deep ground water collected in April 2003 (8 months after the flood experiment) contained Se concentrations of less than 20 ug/L and dissolved oxygen concentrations of less than 0.1 milligrams per liter. 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 observed during the experiment of 0.65 mg/hr/m$^{2}$ (n = 52), seven flooding cycles would be required to meet the 4 milligrams per kilogram remediation goal in surface soils from the SLWMA wetland.
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
DE: 1842 Irrigation
DE: 1890 Wetlands
SC: Hydrology [H]
MN: 2003 Fall Meeting