- The Clean Water Act imposes Total Maximum Daily Load (TMDL) limits on pollutant concentrations within wastewater effluent; in Oregon, thermal discharge is one of the pollutants subject to regulation. The City of Woodburn, Oregon, funded a series of pilot scale studies to investigate the utility of natural systems to reduce wastewater effluent temperature. The research discussed here examined the groundwater response to an unlined, 0.15 hectare wetland used to infiltrate treated wastewater into the subsurface and the potential efficacy of an upscaled discharge system.
An array of geotechnical holes and monitoring wells were placed around the wetland and were equipped with pressure transducers. Groundwater level, water temperature, and water chemistry were then monitored for 1.5 years. These data were used to calibrate a series of numerical models, which were in turn utilized to assess the rate, flow direction, and spatial extent of infiltration from the wetland. Results suggest that the water table rises to the level of the wetland, and that infiltration occurs primarily through lateral flow from the wetland. Numerically-simulated and observational data found maximum water displacement velocities of 1.5 m/day in the horizontal direction, and 0.1 m/day in the vertical direction.
Next, the numerical groundwater simulation model was used to predict how infiltration rate would be affected by moving from a pilot-scale to a large-scale system. It was determined that because a large percentage of flow occurs laterally through the perimeter, the specific infiltration rate will decrease as wetland area increases, due to a smaller perimeter-to-area ratio. Simulations estimated that the water displacement velocity for the large-scale (5.5 hectare) system was 0.06 m/day in the vertical direction and 0.4 m/day along the perimeter. These results were deemed consistent with a water budget performed on a separate inundated portion of the floodplain.
Finally, the observed temperature and numerical simulations were used to simulate the subsurface temperature profile. It is predicted that the heat which enters the subsurface spreads out beyond a radius of 20 - 50m, and the wastewater will have its temperature transition from having daily fluctuations to seasonal fluctuations, before eventually reaching steady state. Further, it was found that mean annual temperature is a function of the distance from the wetland, thus effectively dispersing the thermal load to the river across the entire year.
In an effort to translate these results to other locations, two sets of non-dimensional numbers were formulated. The first set of parameters examines the potential for cooling due to conductive losses to the atmosphere, and may indicate systems where the hydraulic retention time will be sufficient to partially or fully cool the wastewater. The second set of non-dimensional numbers provides a means by which a site's potential for hyporheic discharge may be assessed, by comparing hydraulic retention time against infiltration capacity.
Altogether, the results suggest that infiltration wetland systems will reduce the peak wastewater discharge temperatures, and thus enable the City of Woodburn and other wastewater agencies to comply with their permit requirements through use of wetland recharge systems.