The team from the US Geological Survey (USGS) California Water Science Center reckons that the method could sequester around 3 tons (US) of carbon per acre each year. If implemented over the whole 1,150 square miles of the Delta, that would be equivalent to storing the emissions of all the electricity used for residential air conditioning in California.
The technique works by flooding the Delta’s peat soil to a depth of roughly half a metre. This water prevents oxygen from reaching the ground so that when the bulrush and reed species growing there die, they decompose very slowly. The resulting mat of degradation-resistant organic material gradually increases in size, raising the ground level by 1.5 to 2 inches a year and storing carbon. Without the technique, land in the region typically sinks by one to three inches a year as the organic carbon in the peat soils decomposes through microbial oxidation.
The reversal in land subsidence reduces the hydraulic pressure on levees around the wetland and lessens the need to dig deeper drainage ditches. As well as being used for agriculture, the Delta provides drinking water to more than 23 million Californians.
In order to optimize and find out more about the subsidence reversal process, Robin Miller of the USGS and colleagues are currently running the Twitchell Island Demonstration Project. Set up in 1997, this features two 7.5 acre wetland regions that are continuously flooded; one to a depth of 10 inches and one to a depth of 22 inches. The land surface has gained around 2 inches of height each year.
If done on a large scale, the technique could reduce the amount of the Delta that’s below sea level by 70% in 50 years and by more than 99% in 100 years, according to the researchers. It would also provide additional wildlife habitat and could, if some form of carbon credit scheme were in place, provide income to farmers.
Now the researchers would like to set up a farm-scale wetland of about 150 acres. They are aiming to assess issues potentially caused by introducing the wetland, such as mercury methylation, the emission of the greenhouse gases methane and nitrous oxide, and the presence of dissolved organic carbon in the water that could form harmful disinfection byproducts when the water is treated for drinking by humans.