Do constructed flow through wetlands improve water quality in the San Joaquin River?
The efficacy of using constructed wetlands (CW) to improve water quality of irrigation tailwaters was studied in the San Joaquin Valley, California. Two CWs were monitored during the 2004 and 2005 irrigation season, a new CW (W-1) and 12-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. Results indicate that CW-2 was more a more efficient contaminant removal system for most water quality constituents. CWs were most effective at removing total suspended solids (TSS). Average TSS removal at CW-2 was 98% in 2004 and 83% in 2005. At CW-1, mean TSS removal was 90% in 2004 and 87% in 2005. Average total N removal efficiency was 41% in 2004 and 29% in 2005 for W-2, compared to 31% in 2004 and 21% in 2005 at W-1. Total P removal efficiency was 63% in 2004 and 24% in 2005 at W-2, compared to 27.5% in 2004 and 11% in 2005 at W-1. Chlorophyll-a, a measure of algal biomass, was higher at W-1, especially in input waters. Initially, in 2004, output concentration of chlorophyll- a increased, however over time, as emergent vegetation established, chlorophyll-a decreased to 35% of input levels. In 2005, CW-2 was a large source of algal biomass because vegetation was not present. Results demonstrate that CWs are effective at capturing sediment and nutrients from irrigation tailwaters, but may be a source of algae if not managed carefully.