Nitrogen and phosphorus leaching from growing season versus year-round application of wastewater on seasonally frozen lands

Land application of wastewater has become an important disposal option for food-processing plants operating year-round. However, there are concerns about nutrient leaching from winter wastewater application on frozen soils. In this study, P and N leaching were compared between nongrowing season application of tertiary-treated wastewater plus growing season application of partially treated wastewater (NGS) vs. growing season application of partially treated wastewater (GS) containing high levels of soil P. As required by the Minnesota Pollution Control Agency (MPCA), the wastewater applied to the NGS fields during October through March was treated such that it contained ≤6 mg L^-1 total phosphorus (TP), ≤10 mg L^-1 NO₃-N, and ≤20 mg L ^-1 total Kjeldahl nitrogen (TKN). The only regulation for wastewater application during the growing season (April through September) was that cumulatively it did not exceed the agronomic N requirements of the crop in any sprayfield. Application of tertiary-treated wastewater during the nongrowing season plus partially treated wastewater during the growing season did not significantly increase NO₃-N leaching compared with growing season application of nonregulated wastewater. However, median TP concentration in leachate was significantly higher from the NGS (3.56 mg L^-1) than from the GS sprayfields (0.52 mg L^-1) or nonirrigated sites (0.52 mg L^-1). Median TP leaching loss was also significantly higher from the NGS sprayfields (57 kg ha^-1) than from the GS (7.4 kg ha ^-1) or control sites (6.9 kg ha^-1). Tins was mainly due to higher hydraulic loading from winter wastewater application and limited or no crop P uptake during winter. Results from this study indicate that winter application of even low P potato-processing wastewater to high P soils can accelerate P leaching. We conclude that the regulation of winter wastewater application on frozen soils should be based on wastewater P concentration and permissible loading. We also recommend that winter irrigation should take soil P saturation into consideration.