The Use of a Novel Aerobic Wetland to Reduce Agricultural Pollutants Associated with Subsurface Tile Drainage

Pollution from agricultural run-off is a significant contributor to the eutrophication and declining health of surface waters. Often agricultural chemicals are transported off site in subsurface tile drain lines, which act as conduits between fields and surface waters. The use of constructed wetlands has increased as a way to manage, treat, collect and reuse this wastewater stream Our research examined a novel recirculating vertical flow constructed wetland (RVFCW) as a means to collect, treat, and reuse tile drainage waters. The RVFCW is a modular system designed to reduce agricultural pollutants. The RVFCW is comprised of three modules: 1. a vegetated layer providing nutrient uptake through biomass assimilation. 2. a filtration module incorporating porous media; and 3. a recircultion module designed to enhance oxygenation of the wastewater during treatment. RVFCW's combine numerous biotic and abiotic treatment properties to provide a low cost, decentralized treatment system. Synthetic tile water, [nitrate (25 mg/L NO₃–N), reactive phosphorus (0.19 mg/L) TOC (3 mg/L)] was used as a wastewater source. Four vegetated microcosms (Carex stipata) were compared to 3 non-vegetated units and to units where weeds were allowed to grow for the treatment of synthetic drainage water. Systems were recharged twice a week with 20L of synthetic tile water. Influent nutrient loads were recorded and compared to effluent loads. One vegetated unit was routinely harvested providing insight into plant nutrient uptake. After four months both types of vegetative units had reduced nitrate loads by an avg. of 62%. Effluent NO₃ (N) concentrations averaged 3.5 mg/L and were below the MCL of 1Omg/L in 70% of the samples. Phosphorus removal was constant in all units. Systems with native weeds had the greatest water loss due to evapotranspiration and the highest conductivity values of all effluent samples. These results indicate the potential for significant nitrate reduction in aerobic, vegetated treatment systems. Prolific growth occurring in both sedge and weed units showed the ability of these systems to sequester both N and P throughout the growing season. The largest nutrient load reductions and evapotranspiration rates were observed in systems with native S.E. Pennsylvania weeds, demonstrating possible phytoremediative potentials for these species.