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Water quality modeling is increasingly recognized as a useful tool for obtaining valuable information for optimal water quality management. In this study, the free software QUAL2Kw was used to evaluate the impacts of agricultural nitrogen (N) excess on river nitrate (NO3-N) concentrations. We explored the possibility to use QUAL2Kw in order to back calculate the exchange of water and N from the groundwater to the Oglio River, northern Italy, which drains a heavily irrigated and fertilized agricultural land. Along the river course water monitoring activities carried out in the dry, summer period revealed steep increases of NO3-N in different sectors, by up to 2 orders of magnitude, not explained by any significant point inputs. Such increases suggest the occurrence of large water exchange with nitrate-polluted groundwater and diffuse inputs. In turn, groundwater pollution is due to high N excess in the watershed (~200 kg N ha-1 yr-1), flood-based irrigation techniques and soil permeability. The QUAL2Kw model was calibrated using the average of 2 years' data collected in winter 2010 and 2011 and validated using the data of winter 2012. To minimize the error between simulation results and measured data, the constants of inorganic suspended solid (ISS), ammonium (NH4-N), nitrate and organic N were calibrated. The calibration and validation results showed a good correspondence between the calculated and measured values for most of water-quality variables. QUAL2Kw was then run separately with three years' summer data (2009, 2010 and 2011), and large gaps were found between the measured and predicted values of discharge, electrical conductivity, NO3-N and total N. Such gaps are discussed in terms of river-groundwater interactions, limited to the summer period and following irrigation by flooding, rise of the groundwater table and vertical transport of N. The gaps allowed to back calculate the volumes of water and the amount of N exchanged. The total load of NO3-N entering into the river from groundwater was estimated in 25.17, 25.63 and 29.89 ton per day for NO3-N in 2009, 2010 and 2011, respectively. Similar results were obtained in another study based on mass balance of N isotopes. The combination of experimental and QUAL2Kw modelled data proved to be a simple, low cost but effective tool in the estimation of NO3-N exchange between the surface and groundwater.