A serious threat to human health is posed by the reaction of dissolved organic carbon (DOC) with chlorine during drinking water treatment because of the formation of carcinogenic and mutanogenic trihalomethanes (THMs). Rice production may be a significant source of DOC to the Sacramento River. Approximately 95% of the 213,000 hectares of rice grown in California are located within the Sacramento Valley. We investigated the impact of land use, water management practices, and soil properties within rice-dominated watersheds on DOC flux, chemical characteristics in outflow water, and the trihalomethane formation potential (THMFP) of DOC. 

The input and outflow waters from eleven rice-dominated watersheds were sampled on a weekly to bimonthly basis during the 2008 rice growing season (May thru September). Samples were analyzed for DOC concentration and for UV absorbance, which were used to calculate specific ultraviolet absorbance (SUVA), a surrogate measure of DOC reactivity, and to estimate THMFP. Flow and land use data were obtained from the local water irrigation district and soil property data from the SSURGO database.

Results/Conclusions

Initial results indicate that both DOC and THM concentration and flux are highest during the onset of rice field flooding and plant establishment (May to late June), decrease during the maintenance flow of the fields (late June to mid-August), and are lowest during the field draining period (mid-August through September). However, during these same time periods, SUVA was found to increase, which implies that although the quantity of DOC leaving these systems decreases over time, the DOC becomes more reactive.

The watersheds ranged from 700 to 5100 hectares in size, and rice land use ranged from 42 to 95%.  The dominant soil orders within the watersheds are alfisols, mollisols, and vertisols. Percent rice land use, watershed size, and outflow were found to be positively correlated with DOC and THM flux.  SUVA was positively correlated with the amount water reuse within a system.  No relationships were found with soil properties. These results suggest that land use and water management are the dominant factors controlling the quantity and quality of DOC leaving rice-dominated systems within the Sacramento Valley.