Modeling the migration of excess nitrate from agricultural activities in Amasya Gümüşhaciköy

Excessive use of nitrogenous and phosphorus fertilizers in agricultural practices disrupts the soil ecosystem, and the excess fertilizer used is washed away by rain and agricultural irrigation, causing nitrate (NO₃⁻) pollution in surface and groundwater. Nitrate (NO3-) leaching, one of the negative effects of uncontrolled agricultural activities on the environment, is affected by many complex structures and sections along the soil profile from the plant root zone to the groundwater, and determining this situation requires a fairly comprehensive study. In this study, nitrate (NO₃⁻) leaching was modeled using the HYDRUS-1D program in the unsaturated zone above the groundwater table at depths of 0-120 cm and 0-90 cm at 12 selected profile points within a 25 km² agricultural area in the Gümüşhacıköy district of Amasya Province. Within the scope of the thesis, 5 different scenarios were created, and predictions were made by comparing the simulation results with existing measurements for each scenario. Using model results, the compliance of nitrate leaching amounts determined from different soil types in the region's groundwater was evaluated with legislation. Furthermore, scenario analyses were conducted to determine the most effective method, taking into account 15-year projection data. The region's unique geographic, climatic, and agricultural characteristics were systematically evaluated, not only for physical processes but also for the impacts of anthropogenic activities on the vadose zone. The results of this study indicate that the Gümüşhacıköy region is at medium-to-high risk for nitrate pollution. While current nitrate levels in the soil range between 15 and 45 mg/kg on average, and these values are within safe limits by national and international standards, the consistent exceedance of critical levels and the continuous increase observed in most profiles emphasize the need for proactive measures. If current trends continue, average concentrations could exceed the 50 mg/l limit in the future. Model analysis results indicate that nitrate leaching rates range from 13.3% to 34.4%, with the risk of leaching being two to three times higher in sandy soils. The leaching rates detected at profile points P3, P7, and P9 reached a range of 25-34%, indicating that the sensitive area criteria stipulated in the regulation are being met. In the five different scenarios developed as part of this thesis, the most successful results in terms of nitrate concentration reduction effectiveness were found to be those designed for a 35% and 25% reduction in fertilizer use. Based on model and scenario results, it appears that the region could experience significant nitrate pollution, particularly in the event of extreme weather events or inappropriate agricultural practices. Therefore, sustainable agricultural practices, the use of precision agriculture technologies, and the urgent implementation of environmental monitoring and assessment systems are critical to reducing the risk of nitrate pollution.