Quantifying nutrient removal from groundwater seepage out of constructed wetlands receiving treated wastewater effluent

Wetlands play an important role in the reduction of nutrients in agricultural runoff and treated wastewater effluent. A component of wetlands systems, the seepage out of the wetland and subsequent groundwater flow, has been virtually ignored in understanding the transport and fate of nutrients. This study examined the flow of groundwater infiltrating from a constructed wetland receiving municipal wastewater effluent, and the fate of dissolved nutrients within the groundwater. Concentrations of nitrate, ammonium, phosphate and chloride were used to assess the quantity of nutrients being removed/added from the seepage compared to the total amount removed/added by surface water processes. Nitrate–Nitrogen concentrations in the surface water ranged 10.0–26.8 and ND-1.5 mg/L in groundwater. Ammonium–Nitrogen concentrations in the surface water ranged 0.05–2.3 and 0.004–4.5 mg/L in groundwater. Phosphate concentrations in the surface water ranged 0.1–3.2 and ND-0.4 mg/L in groundwater. Effluent-groundwater mixing calculations showed that a majority of the water sampled from the wells is effluent rich (50–100 %). MODFLOW simulations suggest that groundwater seepage flux from the wetland was between 3 and 11 % of the surface water flux through the wetland. Nitrate and phosphate removal was determined to be significant in the groundwater—1930 and 175 g/day, respectively, but ammonium concentrations increased in the groundwater by 29 g/day. Dependent on surface water retention times in the wetland, nitrate removal from the surface water ranged from 4100 to 14,450 g/day. Ammonium and phosphate were added at rates between 21–74 and 221–780 g/day, respectively.