Nitrate leaching in an on-site wastewater system: Monitoring and modeling with HYDRUS 1D

Nitrate leaching from traditional on-site wastewater treatment systems (OWTS) is a global challenge affecting groundwater quality. This study investigates the optimal conditions for wastewater treatment, focusing on the influence of soil characteristics and hydraulic parameters. Three percolation trenches with different backfill materials—two types of gravel and fine sand—were employed in a clayed subsurface with a bimodal pore size distribution. The trenches received septic tank effluent from a local facility. Using the inverse function of HYDRUS 1D, we modeled daily water content and deduced water flux over 15 weeks of sampling. Soil volumetric water content and electrical conductivity were continuously monitored using reflectometers. Porewater samples were collected for chemical analysis, including dissolved inorganic nitrogen ions. The study concluded that while the choice of backfill material is important, it does not have a real impact. Still, the overall soil structure and properties substantially affect wastewater treatment efficiency. Results indicate that soil characteristics and hydraulic conditions are fundamental in optimizing nutrient transformation and minimizing nitrate leaching.