QUANTIFYING NUTRIENT REMOVAL FROM GROUNDWATER SEEPAGE OUT OF A CONSTRUCTED WETLANDS RECEIVING WASTEWATER EFFLUENT

The goal of this study is to quantify the removal of nutrients from water that seeps out of a constructed wetland receiving treated wastewater effluent. The Bloomington-Normal Wastewater Reclamation District’s treatment facility south of Bloomington, Illinois is currently using constructed wetlands as a final treatment for nutrients by putting a fraction of their treated wastewater effluent in the wetlands before being released into a stream. Shallow and deep wells have been installed around and beneath the wetland. Samples from these wells, the effluent channel and outlet are being analyzed for nitrate, phosphate and chloride. These data will be used to determine the quantity of nutrients being removed from the seepage compared to the total amount of water flowing through the wetland, through groundwater modeling. After sampling it appears that a majority of the nitrate and phosphate seeping out of the wetlands is being removed by groundwater processes. Effluent-groundwater mixing calculations were completed using an effluent chloride concentration end member from the day samples were collected and a groundwater chloride concentration end member from wells located up-gradient of the wetland. The calculations show that a majority of the water sampled from the wells is effluent rich (50-100%), although 3 wells on the berm closest to the wetland are showing low effluent concentrations (<1-40%) possibly due to surface recharge. Data collected when the facility first started shows that there is a lag time of 26 days between when the effluent is released from the plant and when it reaches the groundwater wells between the wetland and stream. From cross-sections constructed using boring logs, seismic data, a pumping test and geochemical data; a numerical model will be constructed using MODFLOW to quantify the amount of water flowing out of the wetland as seepage. The amount of nutrients being removed from the groundwater will be calculated using the modeled seepage and the reduction in nutrient concentrations between effluent and groundwater.