Investigation of Linkages Between Management Practices Used in Sacramento-San Joaquin Delta Rice Production and Loads of Dissolved Organic Matter and Disinfection Byproduct Precursors

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The primary objective of this study is to expand our current understanding of how to manage rice production in the Sacramento-San Joaquin Delta in a manner that minimizes loads of water quality contaminants to Delta waters.

Map of the Sacramento-San Joaquin Delta

Map of the Sacramento-San Joaquin Delta.

(Public domain.)

The Sacramento-San Joaquin Delta is the source of drinking water to over 22 million Californians and provides valuable habitat for a variety of biota including endangered fish and migratory birds. These interests are threatened because many of the Delta islands have subsided to as much as 20 ft below sea-level. The primary cause of subsidence today is oxidation of the organic soils when they are drained for agriculture. Under current agricultural management, Delta islands will continue to subside as their organic soils are subjected to oxidation. Further subsidence will require deeper drainage, increasing the hydraulic pressure on the levees further destabilizing the levees, and threatening drinking water supply, agricultural production, and crucial habitat.

In order to stop subsidence and halt further destabilization of the levees, managers and farmers are looking for alternatives to standard agricultural management. One solution that has been raised is through converting from their current agricultural crops to rice production in an attempt to stop oxidative subsidence of the organic soils by keeping them flooded for a longer portion of the year. However, it is still unknown whether rice management actually reduces oxidative subsidence of Delta island soils. Furthermore, conversion from current agricultural operations to rice production can be a concern may have deleterious effects on the Delta’s water quality.

The USGS, in collaboration with UC Davis, Bachand and Associates, Ducks Unlimited and Hydrofocus Inc, recently completed a pilot study investigating the effects of rice management practices on water quality. Results indicated that simple management practices are able to minimize the effect of rice production on organic carbon and disinfection byproduct precursor loadings relative to current operations. However, further study is required to improve the management practices for organic carbon as well as investigate the effects of these management practices on other contaminants of concern, such as mercury.

The primary objective of this study is to expand our current understanding of how to manage rice production in the Delta in a manner that minimizes loads of water quality contaminants to Delta waters by

  1. Evaluating techniques used to optimize the effectiveness of management practices identified in the pilot study.
  2. Evaluating whether the implementation of the recommended management practices will be cost-effective in minimizing loads of DOC and DBPPs at the large field- or whole island-scale.
  3. Evaluate the effect of rice production on mercury exports in addition to DOC and nutrients.
  4. Quantify the annual carbon budget of rice fields to determine whether rice production will help mitigate oxidative subsidence in the Delta.