Plants transfer selenium from polluted to barren soils

A two-year study by researchers from the University of California found that man-made wetlands in the state’s San Joaquin Valley removed 69% of the selenium in agricultural drainage water. The scientists also discovered species of plants able to convert selenium into non-toxic dimethyl selenide gas, thus reducing the amount of selenium built up in soil.

The study, published in the journal Environmental Science and Technology, builds on previous research where wetland ponds were found to remove 89% of selenium from daily discharges from an oil refinery.

“We thought that if wetlands could filter selenium from oil refinery wastewater, then they could probably be used for agricultural runoff,” says Professor Norman Terry, principal investigator.

Soil on the west side of the San Joaquin Valley is naturally rich in selenium, which leaches into the region’s groundwater. Excessive agricultural irrigation accelerates this leaching process. In the early 1980s, groundwater polluted with inorganic selenium was linked to severe deformities suffered by birds and other wildlife in the area.

The discovery of selenium contamination halted the construction of a drain that would have carried irrigation water from the valley to nearby farms, which then suffered from salt build-up in the soil, leaving the land fallow. The government was obliged to compensate farmers to the tune of US$107 million for 34,000 acres of salt-poisoned farmland.

In an attempt to overcome the leaching process, scientists designed a series of wetland ponds planted with cordgrass, saltmarsh bulrush, rabbitfoot grass and other plants capable of volatilizing the selenium in the soil. The researchers were delighted to find that over one summer month, nearly half of the selenium entering the pond containing rabbitfoot grass was volatilized.

The extensive root system of the plants slows down the water flow so the selenium gets trapped in the sediment, explains Terry. “The plants also provide a source of fixed carbon to fuel microbes, which metabolize the selenium into non-toxic gas. It is truly an amazing process,” he adds. When concentrations become too high in the soil and plant tissues, the wetlands can be retired, drained and replaced by plants like pickleweed that continue the remediation.

The process of volatilizing selenium also benefits others. “Air currents carry away the dimethyl selenide to the eastern part of the state where the soil is so deficient in selenium that farmers there actually feed their livestock selenium supplements to keep them healthy,” says Terry.

The study will now focus on improving volatilization rates throughout the year using genetically engineered plants. Currently, volatilization is greatest during warmer months. During autumn and winter less than 10% of selenium entering the rabbitfoot grass pond is volatilized.

“The upshot is that wetlands are a very efficient and affordable solution to ridding polluted water of a toxic chemical,” concludes Terry. “Plants grow year after year, and while a constructed wetland system would need to be monitored, it would be relatively easy to maintain.”