The Challenges

Historically, the Sacramento—San Joaquin Delta (Delta) in California was a vast inland freshwater wetland where organic soils over 50 feet deep formed over several millennia. Beginning in the late 1800’s, levees were constructed and pumps were used to drain the area for agricultural use. Draining these lands has led to land-surface subsidence A dropping of the land surface.
While land-surface loss can be due to dewatering, groundwater pumping, or soil compaction, in the Delta the dominant cause is loss of the organic-rich peat soils due to microbial degradation. Put simply, microbial activity — which is enhanced under drained (oxygen rich) conditions compared to flooded (oxygen poor) conditions — converts the organic rich soil to carbon dioxide gas (CO₂). to more than 20 feet below sea level in some areas, primarily due to oxidative loss of the organic rich soils. Because the Delta supplies drinking water for over 23 million Californians, protecting both water supply and water quality in this region is of great importance.

Map of the Sacramento-San Joaquin Delta. Click to Enlarge.

Dissolved Organic Matter and Water Quality

Rivers, wetlands, and agricultural operations supply natural organic material to the Sacramento-San Joaquin Delta (Delta) and the San Francisco Estuary. This natural organic matter provides many ecosystem benefits, but it also adversely affects drinking water. This occurs because during drinking water treatment, chlorine added for purposes of pathogen control dissolved organic carbon (DOC) in the water to form carcinogenic and mutagenic disinfection by-products (DBPs). Concentrations of these compounds in tap water are regulated by the Environmental Protection Agency.

Because much of the land in the Delta is below sea level, drainage water must be continuously pumped into adjacent Delta channels to prevent these areas from flooding. This drainage water typically contains high concentrations of DOC and DBP precursors because they have passed over and through high organic matter peat soils (Fujii et al. 1998, Fleck et al. 2007). Drainage water from Delta peat islands has been shown to represent a significant source of these constituents of concern to drinking water diverted from the Delta (Kraus et al. 2009). Management actions that reduce the export of these constituents from subsided islands will improve water quality in the Delta.

Land Subsidence and Levee Stability

As land surface elevations decrease, costs for levee maintenance and repair increase, as do the risks of catastrophic levee failure. Currently 98 percent of the Delta is below sea-level (Knowles 2010). In addition to immediate loss of life and property associated with levee failure, saltwater intrusion into this freshwater system could result and halt water exports for an extended period of time (Mount and Twiss 2005). Management actions that reduce, or better yet reverse, subsidence in the Delta would reduce these costs and risks.