Combined use of the EPA-QUAL2E simulation model to assess the source apportionment of point and non point loads of nutrients to surface waters

Diffuse pollution is generally indirectly estimated by area and specific emission factors as function of land use. However in many cases these estimates were proven to be remarkably inaccurate. Aim of this study was to combine a water quality simulation model, (USEPA-QUAL2E) and Factor Analysis to increase the understanding of the water pollutants source apportionment. The study concerned two different watersheds, an upland area characterised by a very scarce agricultural use, and another area covering both the upland and the lowland physiographic regions. Particularly the lowland region is included in one of the most productive agricultural areas in Italy. By comparing instream measurements with QUAL2E simulations during dry and wet weather conditions, a good fit (errors ± 20%) was found for the dry weather scenario, whereas very poor was the model performance on the wet weather scenario. This was in the same way expected since the rainfall-driven pollutants scenario deviates significantly from QUAL2E general assumptions of constant emissions in steady state streamflow conditions. However the poor fit was also due to the scarcer reliability of the adopted non point emission estimates. Despite of approximations the model wet weather simulations enabled to estimate the non point contribution to the instream load at the rainfall event scale resolution. Such diffuse sources contribution was found around 80% in the area of extensive agricultural land use, and around 40% in the upland region. Factor Analysis applied to the instream measurements data shed light on the exchange from the groundwater to the surface water system that occurred in the upland region. The hypothesis of a groundwater contribution to the instream total load of nitrates was also supported by QUAL2E simulations that, when considering only the point loads, systematically underestimate the dry weather nitrate concentrations. The same pattern was not observed for the lowland region.