The efficacy of using constructed wetlands (CWs) to sequester organic carbon and nutrients from irrigation tailwaters was studied in the San Joaquin Valley, California. Two CWs were monitored during the 2004 irrigation season, a new CW (W-1) and 10-year-old CW (W-2). Input/output waters from CW were collected weekly and analyzed for a variety of water quality contaminants. Organic carbon, nutrient and sediment retention efficiencies were evaluated from input/output concentrations. Characteristics of sediment were examined spatially at W-2. Results indicate that W-2 was more efficient at contaminant removal. Average particulate organic carbon retention, was 70 ± 13\% (mean ± standard deviation) in W-2 and 48 ± 32\% in W-1. Chlorophyll-a, a measure of algal biomass, was higher at W-1, especially in input waters. Initially, output concentration of chlorophyll-a increased 15-fold in W-2, however over time, as emergent vegetation established, chlorophyll-a decreased to 35\% of input levels. Average total N removal efficiency was 45 ± 18\% for W-2 compared to 22 ± 32\% in W-1. Total P removal efficiency was 72 ± 14\% at W-2 compared to 18 ± 26\% at W-1. CWs were most effective at removing total suspended solids, 84 ± 15\% and 97 ± 2\% for W-1 and W-2, respectively. Results demonstrate that CWs are effective at capturing POC, sediment and nutrients from irrigation tailwaters. \textcopyright IWA Publishing 2007.