RESOLVING AQUATIC AND TERRESTRIAL SOURCES OF DISSOLVED ORGANIC MATTER IN THE SACRAMENTO-SAN JOAQUIN DELTA BY RADIOCARBON DATING

Radiocarbon ages were used to estimate the relative contribution of dissolved organic carbon (DOC) from agricultural peat-lands and riverine sources to DOC loads in the California State Water Project (SWP). Organic carbon loading to the SWP is a growing concern for water utilities serving 23 million Californians owing to the potential for DOC to form harmful disinfection by-products. An accurate conceptual understanding of carbon dynamics within the Sacramento-San Joaquin Delta and SWP is critically needed to assess the impact of proposed CALFED ecosystem restoration activities (e.g., construction of in-Delta reservoirs and tidal wetlands). The 14C content of DOC was measured in riverine input to the Delta and in three agricultural drains during the spring runoff period (April 2003) and at the start of the irrigation season (June 2003). DOC fractions measured included bulk DOC and dissolved hydrophobic acids (HPOA). In the agricultural drains the 14C content of HPOA compounds and bulk DOC overlapped and ranged from 0.73 to 0.86 pmc. The 14C content of bulk DOC and HPOA fractions in riverine input to the Delta was higher, ranging from 0.96 to 1.07 pmc. The 14C content of SWP waters, ca. 1.00 pmc, was bracketed by the riverine and agricultural values and showed little variation between the two sampling dates. In contrast, mean 14C content of HPOA in the rivers increased from 0.96 to 1.08 pmc between April and June, while in the agricultural drains mean values declined slightly from 0.81 to 0.79 pmc. Using HPOA 14C content and a simple two-component mixing model we estimated that, during April 2003, 93% of the carbon in the SWP was derived from riverine inputs. During June 2003 DOC loading to the SWP was split about equally between riverine inputs and agricultural drainage.