Nutrient Removal in Constructed Wetlands Treating Agricultural Tile Drainage

Abstract

Agricultural runoff can carry substantial loadings of nitrogen and phosphorus that can impact local surface water quality and contribute to impairment of water bodies further downstream. Subsurface tile drainage, a drainage water management practice commonly used in the Midwest, is known to contribute to elevated levels of these contaminants. Strategies to improve drainage water quality must be implemented in a way that minimally impacts land utilization and crop yield. In this study, three constructed wetlands were utilized to treat runoff from tile outlet terrace (TOT) agricultural fields managed under either a no-till corn-soybean rotation with wheat prior to soybean, or a no-till soybean crop. Nutrient and sediment removal efficiencies and runoff impact on receiving streams were determined during two growing seasons in 2014 and 2015. Water samples were collected with an auto-sampler at the wetland influent and effluent locations at Harvest Hills North (HHN/site 1), Harvest Hills Middle (HHM/site2), and Dan Cain site (Cain/site3). Using stream bottles, samples were also collected from two local streams that receive TOT runoff during and after storm events. Over the two years, changes in nutrient and sediment loads to the wetlands were observed. Runoff quality was affected by changes in crop type, fertilizer application rate, and precipitation pattern, frequency and intensity. During the two growing seasons, TOT runoff was responsible for 99.5, 71.2 and 197.7 kg of TN entering the wetlands at sites 1, 2 and 3, respectively, of which 67.7, 59.3 and 93.8 kg exited the system in the wetland effluent (32, 17 and 53% load removal). For TP, approximately 16.54, 8.75 and 45.18 kg entered the wetlands, of which 10.24, 5.25 and 19.67 kg exited (38, 40 and 56% removal). For total suspended solids (TSS), roughly 14793, 4023 and 64624 kg entered, of which 4824, 1748 and 10876 kg exited (67, 57 and 83% removal). Compared to the year with soybean crop coverage (2014) at the sites with a no-till corn-soybean rotation (sites 1 and 2), higher sediment concentration in TOT runoff was observed at the site with a no-till soybean crop both years (site3). The wetlands’ performance was typically better with higher influent concentrations, although the wetland design and inflow volume also seemed to contribute as well. Variations in behavior between two similar wetlands (sites 1 and 2) were likely due to differences in seepage rates and flow distribution through the wetlands, which is believed to have changed as sediments built up near the influent discharge pipe at site 1. Stream monitoring results showed that median concentrations of TN and TP were higher than the benchmark values for streams in U.S. EPA Region 7, with no measureable impact from either the treated (wetland effluent) or the untreated runoff. Potential reasons for why no significant impact to stream quality was observed are the relatively low volume of discharge relative to stream flow, and the relatively high stream levels of nutrients and sediments even upstream of the discharge location.