QUANTIFYING THE EFFECTS OF GROUNDWATER ON NITROGEN CYCLING IN CONSTRUCTED WETLANDS RECEIVING AGRICULTURAL TILE DRAINAGE

This research is part of a larger project designed to gain a greater understanding of nitrogen removal processes for constructed wetlands receiving agricultural tile drainage. This part of the project was designed to identify and quantify the groundwater pathways that contribute to nitrogen cycling in the wetlands. Subsurface samplers and wells were installed beneath and around a wetlands consisting of 3 sequential cells. Data collection includes analyzing each sampler, well, and surface water for major anions and N species (nitrate and ammonium), measuring water levels in the wells and each wetland cell, and monitoring the flow of water at the inlet and outlet of each cell. A bromide tracer injection test is being conducted to detect groundwater flowing into (tracer dilution) or out of (tracer detection in samplers/wells) the wetlands. In the wetland surface water, ammonium concentrations increase from 0.01 mg/L to 0.21 mg/L from cell 1 to cell 3, and nitrate concentrations decreases from 13.9 mg/L to 0.9 mg/L from cell 1 to cell 3. Ammonium concentrations in the sediments beneath the wetlands range from 0.16 mg/L to 4.47 mg/L, and nitrate concentrations ranged from 0.4 mg/L to 3.0 mg/L. Groundwater ammonium concentrations ranged from 0.04 mg/L to 0.09 mg/L, and nitrate concentrations ranged from 0.1 mg/L to 5.7 mg/L. A water table contour map indicates lateral flow towards the wetlands resulting in possible input of N species. Data on flow into an out of each cell indicates that there is seepage out of cell 1, and seepage of groundwater into cells 2 and 3. Bromide was detected in the samplers beneath cell 1 and a well in the berm between cells 1 and 2, confirming seepage out of cell 1. Bromide has not yet been detected in subsurface samplers in cells 2 and 3, supporting the flow data that indicates seepage into the wetlands. To date, these data indicate that seepage out of cell 1 is a net loss of N due to near complete removal of nitrate. However seepage into cells 2 and 3 is a net source of N, mainly due to increased ammonium generated as the groundwater flows through the organic rich wetland sediments. Further analysis of the tracer test data will be used to quantify the groundwater N flux in/out of each cell.