Wetlands clean selenium from agricultural runoff
Researchers say bulrushes and grasses work better than high-tech approaches to remediating polluted waters

By Sarah Yang, Public Affairs

22 January 2003 | Researchers at Berkeley have found a natural “detox program” for selenium-contaminated farm runoff, in the form of wetland vegetation and microbes.

Results from a two-year study by Berkeley researchers show that man-made wetlands in the San Joaquin Valley were able to remove an average of 69.2 percent of the selenium in agricultural drainage water. More significantly, some plant populations showed remarkable promise at converting selenium into a harmless gas consisting primarily of dimethyl selenide. That means less of the selenium from agricultural wastewater would end up in sediment or plant tissue, from where it can be absorbed by aquatic wildlife, leading to deformities and affecting the species’ reproduction.

The new study, published early this month in the journal Environmental Science and Technology, follows previous re-search at the Chevron oil refinery in Richmond. There researchers found that manmade wetland ponds could take out as much as 89 percent of the selenium from millions of gallons a day of refinery discharge, preventing it from reaching San Francisco Bay. Sele-nium is stripped from crude oil during the refining process.

“We thought that if wetlands could filter selenium from oil-refinery wastewater, then they could probably be used for agricultural runoff,” says Norman Terry, professor of plant biology at Berkeley’s College of Natural Resources and principal investigator of the study. “We’re basically learning that some of the best, most efficient filters for pollutants can be found in nature.”

‘An amazing process’
According to Terry, the entire wetland ecosystem is acting as a bio-geo-chemical filter. “Everything is working in concert to take the selenium out of the drainage water,” says Terry. “The extensive root system of the plants slows down the water flow so the selenium gets trapped in the sediment. 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.”

The toxic effects of selenium made headlines in 1983 when high levels from polluted farm water were found at the Kesterson National Wildlife Refuge in the San Joaquin Valley. The soil on the west side of the valley is naturally rich in selenium, which leaches into the shallow groundwater of the region. Excessive agricultural irrigation accelerates this leaching process.

A large quantity of selenium-polluted agricultural drainage water was being discharged into a reservoir at Kesterson in the early 1980s. The selenium was linked to severe deformities suffered by birds and other wildlife at the Kesterson refuge. Its discovery put the brakes on the construction of a drain that would have carried irrigation water from the Central Valley to the Delta. (Farmers say the disruption of the irrigation drain, however, allowed salt to build up in the soil, leaving their land fallow.)

To test the effectiveness of wetlands in cleaning selenium out of agricultural drainage water, researchers from the UC Salinity/ Drainage Program built 10 separate wetland ponds in the Central Valley at a site in Corcoran. The ponds, or “cells,” contain a single plant species — such as cordgrass, saltmarsh bulrush, and rabbitfoot grass — or a combination of plants. One pond was left unplanted as a control.
Separate pipes brought water in and out of the ponds, which are roughly the size of two basketball courts.

In measurements taken from 1997 to 1999, researchers found that most of the selenium was retained in the sediment, with less than 5 percent accumulating in plant tissue.

“Selenium is not considered an essential nutrient in plants,” says Zhi-Quin Lin, lead author of the study and a former post-graduate researcher with Terry at Berkeley. He is now assistant professor of environmental ecology at Southern Illinois University at Edwardsville. “However, selenium is a chemical analogue to sulfur, which is essential to plants. One theory holds that plants metabolize selenium through similar bio-chemical pathways as sulfur.”

Wetlands take over
The researchers say constructed wetlands can be retired and drained when the concentration of selenium in the sediment and plant tissue gets too high. This would allow another process of selenium removal to kick into gear.

“Once the water and wetland plants are removed, we can plant pickleweed or other vegetation into the soil,” said Lin. “In lab tests, these plants and various strains of bacteria associated with them take over the remediation process and volatilize the selenium in the soil.”

The researchers were particularly excited by the amount of selenium volatilized by the wetland ponds. In one summer month, nearly half of the selenium entering the pond containing rabbitfoot grass was volatilized into a gas consisting mostly of dimethyl selenide.

“Grasses that have extensive root systems, such as rabbitfoot grass and cordgrass, do a better job of providing surface area for microbes that help volatilize selenium into dimethyl selenide,” says Terry.

“Converting the selenium into gas helps get the chemical out of the area entirely,” he continues. “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.”

Terry notes that the air in the northern hemisphere already contains about 10,000 metric tons of volatile selenium from volcanoes, soil and plants. “The amount of dimethyl selenide released by wetlands would be negligible in comparison,” he says.

The researchers are studying ways — including using genetically engineered plants — to improve volatilization rates throughout the year. Currently, volatilization is greatest during warmer months. When winter and fall periods were taken into account, an average of 9.4 percent of the total selenium entering the rabbitfoot grass pond was volatilized.

Terry says wetland plants could become a major wastewater remediation tool for both agriculture and industry. “Wetlands are a very efficient and affordable solution to ridding polluted water of a toxic chemical,” he says.