Selenium in the prairie waters of southern Manitoba : concentration, speciation, and temporal variation

The prairie waters of southern Manitoba share many similarities in geology, climate, water chemistry and hydrology with those in the western United States, where elevated concentrations of selenium from irrigation practices have resulted in adverse impacts on fish ald waterfowl.To assess the current range of selenium exposure in southem Manitoba, the concentration, speciation and temporal variation of selenium were studied at Delta Marsh, Stephenfield Reservoir and the South Tobacco Creek Watershed between July 2005 and July 2007 .The sites were selected based on their perceived level of impact from agricultural practices.Delta Marsh was chosen as a reference site and was to provide a background level of selenium in southern Manitoba.Stephenfreld Reservoir and the South Tobacco Creek Vy'atershed were chosen as potentially selenium contaminated areas by irrigation and other agricultural activities, respectively.A new analytical technique was developed to determine the selenium speciation in natural waters, which was applied to examine the fate of selenium under elevated conditions by a mesocosm study.At Delta Marsh and Stephenfield Reservoir, the total selenium concentrations were low (<1 pg/L) in the surface water, less than 1.3 pglg (dry weight) in the sediment cores, and less than 0.12 pglg @ry weight) in the macrophytes tested.At the South Tobacco Creek Watershed, elevated selenium concentrations (>11 pgll,) in the surface water were found during the spring freshet in 2007 with lower levels observed for the rest of the year, indicating a potential selenium source from airbome particles accumulated during the winter and/or increased soil erosion during snowmelt.A high performance liquid chromatography-inductively coupled plasma-dynamic reaction cell-mass spectrometry (HPLC-ICP-DRC-MS) method was developed for determination of selenium speciation in natural waters.It allows for the isocratic separation of selenite, selenate, and several organoselenium species (e.g., selenomethionine, Se-metþlselenocysteine, selenocystine) in a rapid fashion.No pre-concentration or pre-reduction step is required, which minimizes the risk of cross-contamination and alterations in speciation, and increases the sample throughput.In the surface water samples from the South Tobacco Creek Watershed, selenium speciation is dominated by selenate, with a small fraction being in the form of selenite.No detectable organoselenium species were found.A mesocosm study on the transport and transformation of selenium was carried out from May to August 2007, in Crescent Pond of Delta Marsh.Three mesocosms were deployed in the pond, two of which were spiked with 6.3 and,15.7 ¡rgll, 82Se1IV), respectively, in the overlying water column.The concentration of spiked selenium decreased to 2 ¡tglL in 20 days after addition, and was almost completely removed from the surface water and below the detectable concentration in about 50 days.The overall results of this thesis indicate that selenium concentrations in the prairie waters of southem Manitoba are low at present, and if selenium is discharged as selenite, it will be quickly removed from the surface water.However, selenium can be elevated during the snowmelt season and in areas of intensive agricultural activities such as at the South Tobacco Creek Watershed.A direct link between surface water selenium concentration and irrigation cannot be established at this time, but continuing of the monitoring of selenium in the prairie waters in southern Manitoba is warranted given the projected increase in irrigation and other agricultural activities in this area.

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