News Stories

The daunting drainage water problems of the western San Joaquin Valley immediately came to mind when scientists learned by chance that wetlands naturally clean selenium from wastewater.

In a vast study near Corcoran, UC scientists are now putting this revolutionary new idea to the test.

The clean up capability of wetlands was discovered in the early 1990s at the Chevron Oil Refinery in Richmond, Calif. The company wanted to beautify a 90-acre industrially spoiled area by turning it into a brackish marsh habitat for wildlife. Complying with government regulations, Chevron monitored the water as it entered and exited the wetland. To their surprise, they found the wetland improved water quality far better than other filtration or remediation systems.

Working with Chevron, UC Berkeley plant biologist Norman Terry concluded, among other findings, that selenium levels dropped 89% as the water flowed through the facility. He realized Chevron’s serendipitous discovery might be the solution sought by west side farmers since the Kesterson Wildlife Refuge was closed in 1983. Selenium that had flowed into the refuge in agricultural drainage water resulted in waterfowl deformities and deaths.

Since that time, farmers have used a variety of costly methods for drainage water disposal, including carefully monitored evaporation ponds and the construction of compensation wetland habitat. Flow-through wetlands may be a more economical and ecological solution.

In what Terry called a "wonderful example of American can-do spirit," the Corcoran wetland study was initiated in 1996. On land donated by J. G. Boswell and Company, the Tulare Lake Drainage District dug 10 quarter-acre wetland cells, developed pipes and pumps and maintain the system.

With funding from the UC Salinity/Drainage Taskforce, the California Department of Water Resources and the Electric Power Research Institute, Terry’s staff planted the cells with different combinations of wetland species to compare their effectiveness in remediating selenium.

In each of seven cells, scientists are growing a single species of wetland plant, including saltmarsh bulrush, baltic rush, cordgrass, rabbitfoot grass, salt grass and cattail. Two cells are planted with combinations of plant species, one cattail and widgeon grass, the other bulrush and tule. One cell was left unvegetated as a reference and contains algae and microbes that naturally occur in drainage water and sediment at the Corcoran site.

The first data were recently submitted to members of the UC Salinity/Drainage Taskforce.

"I think we have the proof of concept," Terry said. "However, we have not yet got it working optimally."

In the 1998 season, the average selenium concentration in the drainage water was 22 parts per billion. Terry found that, flowing drainage water through the cells for seven days, selenium remediation ranged from less than 30% in the unplanted cell and cells planted with cordgrass, salt grass, and a combination of bulrush and tule, to as much as 60% in the combination cell planted with cattail and widgeon grass. He said the efficiency of wetlands in removing selenium seems to be increasing from 1997 to 1998 and that the presence of certain key species of vascular plants is required to maximize selenium removal.

Doug Davis of the Tulare Lake Drainage District said scientists are still trying to understand exactly what happens to the selenium. "We believe that some precipitates into the sediment, some volatilizes into the atmosphere and some is absorbed into plants," he said.

The outflow, now with a reduced selenium level but still salty, could potentially be used to feed brackish wetland habitat in the San Joaquin Valley. However, Terry cautions that many questions remain.

"How important are the plant species? What exactly happens to the selenium? Will the flow-through wetlands themselves attract wildlife? And if they do, to what extend will selenium end up in wildlife?" Terry said. "We don’t want to create another Kesterson."

But first, Terry said, scientists must determine whether flow-through wetlands offer a practical and manageable solution for agricultural drainage water remediation.

"Once we’ve done that, we can look at how to make the system biologically safe," Terry said. "I’m confident we can."