Performance Analysis of a Wet-Retention Pond in a Small Agricultural Catchment

Urban wet-retention pond performance has been extensively studied for many water contaminants. However, very few studies have focused on wet-retention pond efficiency under agricultural production systems. This study presents the results drawn from a 5-year data set collected on a retention pond constructed in Saint-Samuel (Canada) at the outlet of a 23.1-hectare watershed under cash crop production. A total of 62 runoff events were analyzed for hydrological performance, 20 events for total suspended solids (TSS), 11 events for total nitrogen (TN), and 14 events for total phosphorus (TP). Peak flows were reduced by 38% on average and the retention pond reduced the frequency and duration of exceedance of the erosion threshold. Drawdown times for 50, 75, 90, and 100% of the runoff volume were 7, 18, 28, and 42h, respectively. The mean detention time was 3.2h compared to the watershed lag time of 3.4h. Mean removal efficiency ratios based on event-mean concentrations (EMC) and event-total loads (ETL) were similar to those reported for urban areas, with values of 50-56%, 42-52%, and 48-59% for TSS, TN, and TP, respectively. Effluent water quality analyses have shown that environmental criteria were often exceeded and threshold concentrations below which treatment is unlikely to occur were similar to irreducible concentrations reported for urban areas with estimated values of 30, 1, and 0.2mg/L for TSS, TN, and TP, respectively. The primary treatment mechanism of the retention pond is most likely the sedimentation of sand and silt particles and particulate nutrients.