Modeling fate and transport of ammonium, nitrite, and nitrate in a soil contaminated with large dose of urea

Soil or groundwater potential contamination generated by urea spills during its manufacture process is unknown. The aim of this work is determining the fate and transport of Nitrogen (N) species (ammonia, nitrites, and nitrates) when high doses of urea are spilled on to the soil surface. Fertilizer concentrations applied to soil surface were in the range from 167 to 301 g N m−2. HYDRUS 2D was applied to simulate N-species transport in the unsaturated zone. Simulated soil water contents correctly represent field measurements during the period March 2018–December 2019 (rRMSE = 0.099). Measured and simulated concentrations of N–NH4+, N–NO2−, and N–NO3− presented a general r2 of 0.76 and RMSE of 0.0435 mg g−1 (rRMSE = 0.096). A considerable decrease in soil pH was observed after urea application. Simulated N losses by volatilization were found in the range of 7.9–13.3% of total urea-N input, in agreement with field measured quantities. According to N mass balance the predominant species are ammonium and nitrate. After hydrolysis of urea, nitrification is the dominant process in soil. At the end of the experiment, ammonium corresponded to 22% of total urea-N input, majorly concentrated in the first forty centimeters of soil profile. Otherwise, nitrate represented 52.7% of total urea-N input. Although simulated quantities of N–NO3− leached into the aquifer are negligible (0.23 g m−2), a significant solute load remains in soil, which could be mobilize in the future. Numerical modeling of N transport is a fundamental tool to predict and evaluate the characteristics of a pollutant event of this nature.