Wastewater treatment at the Houghton Lake wetland: Temperatures and the energy balance

This paper describes the temperatures in surface water and soils in a very long-running study of the capacity of a natural peatland to remove nutrients from treated wastewater. Two zones were found, an adaptation zone near the discharge to the wetland, and a background zone comprised of areas more than about 100 m from the discharge. The discharge zone was transformed to a floating mat during the 30-year course of the project. Strong diurnal cycles in surface water temperatures were measured, with a median daily swing of about 6-10 °C. Pumped water was a few degrees warmer than the wetland background, and was reduced in temperature by passage through the adaptation zone. The time constants for adaptation (63% of change) were approximately one-half to 1 day. Soil temperatures followed a cyclic pattern, with decreasing amplitude with depth, and a time delay increasing with depth. The seasonal surface maximum was about 18 °C. The irrigation season started on May 1, with water at 10 °C, and ended in early October, with water at 10 °C. The soil conduction model was used to infer cyclic surface temperatures, with a smoothed result compared to synoptic temperature measurements in surface water. Background zone fitting parameters were the Julian day of surface maximum temperature (196), mean temperature (7.9 °C), surface amplitude (10.3 °C), and penetration depth (1.0 m). Soil heat fluxes were vertically downward during the warm season, and back up toward the surface with maxima of 1.4 MJ/m2 d in the discharge zone. This vertical soil heat flux was of small importance to the summer energy budget, which was dominated by solar radiation and evaporative cooling.