مدل‌سازی نیترات در آبخوان آبرفتی اسدآباد و ارزیابی آسیب‌پذیری

Recently, nitrate concentration in water resources has become an environmental problem of widespread concern. Mainly, high nitrate concentration is attributed to anthropogenic activities, such as the discharge of urban, rural and industrial sewage, and the excessive use of animal and chemical fertilizers in agriculture. In this work, a model was built to simulate large-scale migration of nitrate in water resources. Simulating and recognizing nitrate migration leads to control and reduce the negative environmental effects of nitrate leakage and its contamination. In this study, the amount of nitrate entering into the groundwater of Asadabad Plain was estimated. Asadabad is located in ۴۷°۴۷' to ۴۸°۱۶' E and ۳۴°۳۵' to ۳۴°۵۲' N, Hamedan province, Iran. The mean elevation from sea levels is ۱۶۵m. In this plain, groundwater reservoir is the main source of drinking water. Therefore, analyzing nitrate in groundwater is critical. Geological and hydrological data of the plain and especially the areas of absorption wells were gathered and used in MATLAB software for modeling. Then, in order to achieve the estimation, characteristics of the soil in the time zone were taken into account. Simplifying the model, it was assumed that there wasn’t any entrance of nitrate from surface runoff in to the wells. Due to informal questioning from city residents, the wells depth in the region was normally between ۵ to ۱۰ meters. Thus the wells depth was considered as ۶ meters for modeling. According to the basic data from region, ۵ people were defined as well users and ۰.۰۰۵ cubic meters per square meters as well inflow for each day. The soil texture was mostly silty and loam with the hydraulic conductivity of ۶.۹۳ meter per day. Also, infiltration rate was considered as ۰.۰۱ meter per day. The amount of and retardation coefficient determined as ۱, which indicated high mobility of nitrate in the soil. The discharge rate of groundwater was considered as ۰.۰۰۱۵ cubic meter per square meter per day. According to researches and data from the region, average nitrogen excretion per capita was ۷.۹ - ۱۲.۵ gr per day. A hybrid modeling approach was used to investigate the effect of nitrate produced from pit toilets and entered into the groundwater. This comprised an analytical solution for steady-state reactive transport through the unsaturated (or vadose) zone, which was then used as the input to a groundwater mixing model. To do this, the distance between the groundwater level and the bottom of the wells was calculated as an average of ۴۰ meters. According to lack of accurate data of nitrate decay rate in soil and in order to better cover the results, the half-life of nitrate was considered as ۵۰۰, ۱۰۰۰ and ۱۵۰۰ days. Hence, considering the distance from bottom of wells to the groundwater level and different time intervals (from ۱۲ months to more than ۵۰ years) and three half-lives, various amounts of nitrate concentration were calculated The results indicated the high probability of nitrate pollution at the groundwater level are higher than the global health standard for drinking around wells, over a period of ۵۰ years, with a medium to long half-life. Considering the groundwater feed rate as ۰.۰۰۱۵ cubic meters per square meter and the hydraulic gradient as ۰.۰۱۵ and the half-life of nitrate ۵۰۰, ۱۰۰۰ and ۱۵۰۰ days, passing less than ۵۰ years, the nitrate content in ۱۸ meters depth will be ۲۰۰ milligrams per liter,. Also, at a distance of about ۴۰ meters from the groundwater level, the concentration of nitrate will reach ۱۱۵ mg/L. At the same time, with a half-life of ۱,۵۰۰ days, this amount will reach about ۳۹۸ mg/L, and in just five years nitrate levels will pass through this global standard. The results showed that after a short period of ۵۰ years, for an aquifer located ۴۰ meters from the floor of an absorptive well, the probability of increasing the concentration of nitrate for drinking water is to a level above the WHO. An approach such as frequent drainage of wells, accurate location, and distance from the aquifer level as well as well-positioned changes can reduce the penetration of nitrate into groundwater. Of course, the implementation of these approaches will be limited due to technical and social considerations as well as economic ones.