HR: 0800h
AN: H41C-0665    [Abstracts]
TI: Dissolved Organic Carbon as a Drinking Water Constituent of Concern in California Agricultural Watersheds
AU: * Pellerin, B A
EM: bpellerin@usgs.gov
AF: US Geological Survey, 6000 J Street, Sacramento, CA 95819, United States
AU: Bergamaschi, B A
EM: bbergama@usgs.gov
AF: US Geological Survey, 6000 J Street, Sacramento, CA 95819, United States
AU: Downing, B D
EM: bdowning@usgs.gov
AF: US Geological Survey, 6000 J Street, Sacramento, CA 95819, United States
AU: Bachand, P A
EM: phil@swampthing.org
AF: Bachand and Associates, 2023 Regis Drive, Davis, CA 95618, United States
AU: Deverel, S
EM: sdeverel@hydrofocus.com
AF: Hydrofocus Inc., 2727 Del Rio Place, Suite B, Davis, CA 95618, United States
AU: Kendall, C
EM: ckendall@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, MS 434, Menlo Park, CA 94025, United States
AB: Dissolved organic carbon (DOC) from the breakdown of plant and animal material is a concern for drinking water quality in California due to the potential formation of carcinogenic byproducts during disinfection. Agricultural DOC loading to surface water is a significant concern, but the sources and reactivity in agricultural runoff remains poorly understood. Here we present data on DOC dynamics in surface water from the Willow Slough watershed, a 425\- km² agricultural catchment in the Sacramento Valley, California. Samples collected weekly during 2006 and 2007 were analyzed for DOC concentration, optical properties (UV absorbance and fluorescence), ¹³C\- DOC isotopes, and trihalomethane formation potential (a regulated disinfection byproduct formed during chlorination). DOC concentrations at the watershed mouth ranged from 2 to 4 mg/L during winter and spring, with a clear increase in DOC concentrations to more than 7 mg\/ L following the onset of summer irrigation. The ¹³C\- DOC values revealed a large range (-19 to -27 ‰), with lowest values during winter baseflow and higher values during summer and winter storms. Spectral slopes also varied seasonally (0.012 to 0.020), with steeper slopes during winter baseflow. Both isotopic and optical data provide evidence for algal\- derived DOC during the winter baseflow and terrestrial sources during winter storms and summer irrigation. Total THM formation potential was higher in winter than summer, and is strongly correlated to DOC concentrations in surface waters (r² = 0.87). In contrast to the total THM formation potential, the specific THM formation potential (e.g., total THM normalized to DOC) decreased during the summer irrigation season, suggesting a change in reactivity related to DOC source or degradation. Additional data from plant leachates and ground water will be discussed, as well as the implications of watershed management on DOC dynamics and reactivity in agriculturally-dominated landscapes.
DE: 0402 Agricultural systems
DE: 1834 Human impacts
DE: 1871 Surface water quality
DE: 1879 Watershed
SC: Hydrology [H]
MN: 2007 Fall Meeting