by Cassandra Willyard Thursday, January 5, 2012
Over the past century and a half, the river delta that provides Central and Southern Californians with drinking water and farmers with productive cropland has sunk about two and a half centimeters each year. Some of the delta’s islands now lie more than six meters below sea level. Two thousand kilometers of aging levees keep them dry for now, but as the land sinks, the pressure on the levees grows. Researchers at the U.S. Geological Survey in California have uncovered a way to rebuild the delta and sequester carbon in the process. Now, with a $12.3 million grant from the California Department of Water Resources, they will test their strategy on a much larger scale.
The Sacramento-San Joaquin River Delta, a 1,200-square-kilometer web of islands and channels located about 50 kilometers northwest of San Francisco, was once a vast tidal marsh. “Historically the delta was a very sustainable, resilient system,” says Steve Crooks, a wetland scientist at Philips Williams & Associates, Ltd., who is not involved in the research. Then in the late 1800s, Californians drained the marshes and built levees. Wetlands became productive farmlands, and the land began to subside.
Delta soil is made up of abundant organic matter — dead marsh plants that accumulated over the course of 6,000 years or so. When covered by water, these plants decompose slowly. But once exposed to air, they decay at an alarming rate. Since 1900, the delta has lost about 2.5 billion cubic meters of organic-rich soils.
A decade ago, USGS began trying to figure out how to reverse the trend. Scientists at the USGS California Water Science Center in Sacramento flooded some farmland on Twitchell Island and constructed two small wetlands that they populated with native plants such as cattails and tules. In some parts of the delta, the land sinks up to a few centimeters a year. But in the test plots, it rose 3.3 to 5.6 centimeters. At those rates, rebuilding the delta could take more than a century. But given that the soils accumulated naturally over millennia, says Robin Miller, a USGS biogeochemist and one of the lead researchers on the project, “it’s happening amazingly fast.”
Rebuilding the delta would take some of the pressure off the region’s aging levees, lessening the likelihood of levee failures and flooding. That’s important, Miller says, because breaches could bring an influx of brackish water into the delta, contaminating the drinking water supply. “Once the saltwater gets into that system,” she says, “it can take up to two years to flush it out, depending on the extent of levee failure.”
Managed marshes could provide another benefit as well: carbon sequestration. Miller and her colleagues found that the wetlands sequestered about 6,000 metric tons per square kilometer per year. The state hopes that once a state-run carbon market is in place, farmers will have an economic incentive to convert their fields into managed wetlands. The results are promising, but several possible pitfalls still need to be addressed.
One concern is mercury: “We have a lot of mercury in the system because of the Gold Rush,” Miller says. Most is in a form not readily absorbed by living organisms. Wetlands, however, can mobilize mercury, turning it into a form that “critters can ingest,” she adds. Because mercury accumulates up the food chain, that could mean trouble for the delta’s top predators, including humans.
Another concern is the dissolved organic carbon that is released when plants decompose. Normally, dissolved organic carbon is not a problem, Miller says. But when it reaches water treatment plants and mixes with chlorine it can form “disinfection byproducts,” some of which are carcinogenic, she says. Given that much of California’s drinking water, which must be chlorinated, passes through the delta, dissolved organic carbon could be an issue.
Lastly, although freshwater wetlands can sequester carbon, they also tend to release more potent greenhouse gases, such as methane. “The measurements that we have indicate that the methane emissions are much lower than the carbon being stored,” Miller says. But “it’s one of the things we need to measure more accurately.” She and her colleagues are trying to manage the experimental wetland to minimize emissions.
Miller and her colleagues will examine these and other issues, such as soil accumulation and the economic viability of converting farms to wetlands in the new, larger wetland. The California Department of Water Resources has yet to determine a location for the experimental marsh, which will encompass about one or two square kilometers, but construction is set to begin in the spring of 2009. The grant will supply funding for three years, according to Ralph Svetich of the California Department of Water Resources, but the project could continue through 2016. One of the main goals, Svetich says, is to establish protocols so that a third party can independently verify the wetlands' potential for carbon storage. Wetland carbon sequestration is not yet an approved carbon offset project in any climate exchange.
USGS estimates that converting all of the delta’s lands to managed wetlands could have as much of an impact on carbon dioxide emissions as doing away with all of California’s residential air conditioners. But it’s unlikely the entire delta would be converted. Miller says it makes sense to start with the lands that have the deepest peat remaining.
Crooks agrees and notes that it might make sense to start with the fringes of the delta, where subsidence has had less of an impact. “There is broad acceptance across the community that what we have now is unsustainable,” he says. “What is not yet determined is how to get to a sustainable future.”
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