Exploration of the nitrogen contamination from sewers exfiltration to the unsaturated zone by modeling

Numerous elements, such as the degree of sewer degeneration, hydraulics, and geological conditions, influence the extent to which sewage pollutes the unsaturated zones of urban. The present study discussed the influence of sewer exfiltration on the urban unsaturated zone, using nitrogen from domestic sewage as a representative contaminant in combination with experiments, literature studies, modeling and sensitivity analysis. The study shows that soils with high sand content exhibit high permeability and strong nitrification capacity, and groundwater is more susceptible to contamination with nitrate. In contrast, the nitrogen in the clay texture or wet soils has short migration distances and a weak nitrification capacity. However, under such conditions, the accumulation of nitrogen can last for more than 10 years, and there is a possible threat of groundwater contamination due to the detection difficulty. The presence of sewer exfiltration and the damage degree of a sewer can be determined by the ammonium concentration at 1-2 m near the pipe or nitrate above the water table. The sensitivity analysis revealed that all parameters impact the nitrogen concentration in the unsaturated zone to varying degrees, four of which are the primary parameters: defect area, exfiltration flux, saturated water content and first-order response constant. In addition, changes in environmental conditions significantly influence the boundaries of the pollution plume, especially the horizontal. The research data collected in this paper will not only allow for a rigorous assessment of the study scenarios but will also provide data support for other researchers.