Preliminary Assessment of DOC and THM Precursor Loads from a Freshwater Restored Wetland, an Agricultural Field, and a Tidal Wetland in the Sacramento-San Joaquin River Delta

Roger Fujii¹, B.A. Bergamaschi¹, N.K. Ganju¹, J.A. Fleck², K.R.Burow- Fogg¹, David Schoellhamer¹, and S.J. Deverel^3
1U.S. Geological Survey (USGS), 6000 J St., Sacramento, CA 95819
²California State University Foundation, 6000 J St., Sacramento, CA 95819
³Hydrofocus, Inc., 2727 Del Rio Pl., Suite B, Davis, CA 95616

Water exported from the Sacramento-San Joaquin River Delta supplies drinking water to more than 22 million people in California. At certain times of the year, Delta waters contain relatively high concentrations of dissolved organic carbon (DOC) and bromide. During these times, chlorination of Delta water for drinking water disinfection will form disinfection byproducts, such as trihalomethanes (THMs), that can exceed the U.S. Environmental Protection Agency's maximum contaminant level for THMs of 80 mg/L. Important sources of DOC and THM precursors (types of DOC that form THMs when chlorinated) to the Delta include rivers, drainage water from peat islands, water from wetlands and areas with extensive riparian vegetation, and in-channel growth of algae and macrophytes. Due to proposed ecosystem restoration and creation of wetlands in the Delta, there is an urgent need for information on the relative loads of DOC and THM precursors produced from three different land uses: restored wetlands constructed for subsidence mitigation, tidal wetlands, and agricultural operations.
We have been conducting research in the Delta to provide this information. A restored wetland and agricultural field located on Twitchell Island, and a tidal wetland on Browns Island have been monitored for flow, DOC, and THM precursors. Initial results indicate that the loads of DOC and THM precursors are similar for the restored wetland (surface water only) and the agricultural field. These land uses produce DOC loads of about 14 and 11 g C/m2/yr, respectively, and THM precursor loads of about 1.7 and 1.0 g THM/m2/yr, respectively. Estimates of DOC and THM precursor loads for the tidal wetland site on Browns Island and seepage associated with the restored wetland are being developed.