With the peat exposed to air, it oxidized rapidly. The soil surface descended below sea level, sometimes by inches per year. The levees were reinforced, and the water table lowered some more by pumping, so that farming could continue. By the 1990s the soil surface on some of these peat lands was down to 15, 20, and even 25 feet below sea level. The biological crater left by the oxidation of peat soils in the Delta had grown huge, about the size of the debris flow from the eruption of Mount St. Helens in 1980.
If you double the height of a levee or dike, you quadruple the water pressure on it. If or when these levees fail, salt water from San Francisco Bay will fill the crater, compromising the water supplies for about two-thirds of California, including much of the irrigated agriculture of the central valley.
About ten years ago the US Geological Survey began a small experiment to see if this loss due to oxidation could be reversed. On Twitchell Island, about 15 feet below sea level, they flooded the land shallowly, and put in some clumps of tules and cattails. As the plants grew, they raised the water level. After ten years, this experiment has built two feet of peat soils that you can stand on, and recording a carbon accrual of 10 metric tons C per ha per year.
The plants drew carbon dioxide from the air in photosynthesis, and built complex carbon compounds. The partially decayed dead stalks and roots, protected from further oxidation by careful management of the near-stagnant water, became organic matter or peat. More carbon went in than out. This biological system converted atmospheric carbon into organic matter at the rate of several inches per year, surpassing expectations, and the experiment is being scaled up.
Links for further info:
http://ca.water.usgs.gov/news/carbon_briefing.pdf
Robin Miller's paper: http://repositories.cdlib.org/jmie/sfews/vol6/iss3/art1/
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