Nutrient Fate and Transport in the Vadose Zone below an At-Grade Wastewater Distribution System in a Cold Climate

Abstract

At-grade distribution systems for wastewater treatment represent potential sources of nutrient input to the environment. The efficacy of nutrient treatment in the vadose zone below an at-grade distribution system receiving secondary treated municipal wastewater effluent was studied on silt loam soil at a site in Calgary, Alberta, Canada, where there are large seasonal temperature fluctuations and cold winter conditions. In addition to wastewater effluent characterization, field monitoring of pore water chemistry, soil temperature, and interstitial soil gas concentrations was conducted at five different depths (up to 150 cm below surface) over the 47-week effluent dosing period. Results showed that ammonium-N from the applied effluent was effectively nitrified in the top 30 cm of the soil profile. While there was evidence for subsequent nitrate removal by denitrification in this same zone, it was limited to the warm summer months and did not appear to result in significant nitrogen mass removal (5–15%) at greater depths in the soil profile. Phosphate was rapidly attenuated in the top 60 cm of the soil profile. This study demonstrated the importance of climatic conditions on biochemical nitrogen transformations in the shallow subsurface and suggested a greater potential for nitrate leaching to groundwater in the winter.