Agricultural pollutant removal by constructed wetlands: Implications for water management and design

Several agricultural non-point source pollutants impair water quality in the Sacramento–San Joaquin River system of California's Central Valley. Constructed wetlands are a water management option available to growers in this region to economically reduce pollutants in agricultural runoff. This study assessed the efficacy of using constructed wetlands to improve water quality of irrigation return flows prior to discharge into surface waters. Seven constructed surface flow-through wetlands having contrasting design and water management were evaluated for their effects on the fate of chemical (salts, nutrients, and dissolved organic carbon), physical (suspended solids), and biological contaminants (algal pigments and bacteria indicator). Based on wetland input–output measurements, load removal efficiencies (LRE) were used to determine wetland performance. All wetlands efficiently removed nitrate and total suspended solids showing LRE ranging from 22 to 99% and from 31 to 96%, respectively. However, for the rest of studied parameters, removal dynamics were mixed, varying from source (negative LRE) to sink (positive LRE). In general continuous flow-through wetlands were more effective in removing most pollutants in comparison to flood-pulse wetlands. Among continuous flow-through wetlands those with lower vegetation cover (<5%) acted as algae incubators contributing to biological oxygen demand, while those with long hydrologic residence time (>10 days) promoted increased concentrations of dissolved organic carbon. Water evapotranspiration and seepage processes, vegetation characteristics and hydrologic residence time were key factors affecting concentration and load reductions. This study demonstrates the potential benefits and limitations of constructed wetlands for treating water from agricultural non-point sources, providing useful information for optimizing constructed wetland design and management.

► Performance of constructed wetlands for removing agricultural non-point source water pollutants is very heterogeneous.
► Continuous flow-through wetlands are more effective in removal of water pollutants compared to flood-pulse wetlands.
► Flow-through wetlands with a low percentage of vegetation cover act as algae incubators.
► Flow-through wetlands with long hydrologic residence time promote dissolved organic carbon production.