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Effects of Urbanization on Organic Carbon Loads in the Sacramento River, California

To gain better understanding of the effects of urbanization on organic matter transport in rivers, we quantified total organic carbon loading from point and non‐point urban sources within the metropolitan area of Sacramento and compared these loads to the amount of organic carbon carried in the downstream Sacramento River. Median total organic carbon (TOC) concentrations in the Sacramento River, non‐point urban runoff and wastewater treatment plant effluent were 2.1, 8.9, and 23 mg L−1, respectively. Dissolved organic carbon (DOC) in non‐point runoff and the river had similar specific UVA absorbance and disinfection by‐product formation potential, but based on radiocarbon measurements, non‐point DOC was substantially older (age > 2000 a) than DOC in the Sacramento River. This finding suggests that DOC in non‐point runoff is derived primarily from leaching of older soil organic matter. The 10th, 50th, 90th and 99th percentile contributions of urban sources to daily TOC load in the Sacramento River were 10%, 20%, 38% and 80%, respectively. Total urban TOC yield was 150 kg ha−1yr−1 and urban sources contributed ∼17% of the annual load of TOC in the Sacramento River below Sacramento.