Influence of Land Use and Water Management On Dissolved Organic Carbon Fluxes From Rice-Dominated Watersheds.

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 mutagenic trihalomethanes (THMs). Rice fields are a potentially significant source of DOC to the Sacramento River because approximately 95% of the 213,000 ha of land that is used to grow rice in California are located within the Sacramento Valley.  This study investigated the impact of land use, rice water management practices, and soil properties within rice-dominated watersheds on DOC flux, chemical characteristics in outflow water, and potential to form trihalomethanes (THMFP).  During the course of this study, 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 concentrations and for UV absorbance, which was used to calculate specific ultraviolet absorbance (SUVA), a surrogate measure of DOC reactivity, and to calculate spectral slopes, S, which are surrogate measures of DOC chemical characteristics.  Flow and land use data were obtained from the local water irrigation district and soil property data from the SSURGO database. Initial results indicate that both DOC 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, DOC becomes more reactive.  Rice land use ranged from 42 to 95% within the watersheds. The dominated soil orders within the watersheds were alfisols, mollisols, and vertisols, with percent aquic soils ranging from 21 to 86%.  However, no relationship was found between DOC and watershed land use cover and soil properties.  This implies that rice water management activities are the dominant factor controlling the quantity and quality of DOC leaving rice-dominated systems within the Sacramento Valley.