Passage VII

In some locations, drainage water from agricultural areas contains selenium (Se), a substance that can be harmful to wildlife. Se can be removed from water by plant uptake or by adsorption onto soil and organic particles. Three studies were done to examine Se removal by human-made wetlands.

Study 1

Five open-water wetlands (Wetlands 1-5), each 15 m × 76 m in area, were constructed at a location in May 1997. At that time, 4 of the wetlands were planted with the same density of 1 or 2 types of wetland plants. From January 1998 through December 2000, drainage water containing an average Se concentration of 22 micrograms per liter (μg/L) was continuously fed into each wetland at a flow rate of 300 m³/day. The average residence time (RT, how long a volume of water remained in the wetland) was determined for each wetland (see Table 1).

Study 2

Every week from January 1998 through December 2000, the outflow water from each wetland was sampled and analyzed for Se. The average annual Se concentration of the outflow was determined for each year (see Figure 1).

Study 3

In December 2000, samples of each of the following materials were collected from several locations in each wetland: fallen plant litter (dead but not decayed plant matter), organic detritus (decayed organic matter on top of the soil), and the top 5 cm of the soil. The samples of each material were analyzed for Se, and their average Se concentration, in milligrams per kilogram dry weight (mg/kg dw), was determined (see Figure 2).