Contamination Mechanism Of Nitrides On Subsurface Environment And Safety Assessment On Groundwater Quality In Guanzhong Basin

The nitride pollution has posed a potential danger on the ecological environment and human health, which is always a hot and difficult science problem in the study region of environmental science and hydrogeology. The nitride pollution mechanism and the safety of water quality in the vadose zone and shallow groundwater of the typical study area in Guanzhong Basin were studied. The data collection, in situ test, lab experimentations, numerical simulation and theoretical analysis were used to research the migration and transformation and fate of "three nitrogen". Through these works, some main research results were obtained, as follows:(1) The "Three Nitrogen" exists generally in vadose zone and shallow groundwater in study region. The nitrate content was significantly higher than the content of ammonium nitrogen and nitrite. The nitrate presents the face-shaped pollution form in shallow groundwater, and the ammonium nitrogen and nitrite present point pollution form. The human activity is the excitation factor of nitrogen pollution in ground environment. The vadose zone lithology and thickness, stratum construction, the hydraulic condition of aquifer and environment factor are the critical factors of "Three Nitrogen" existence form and migration and transformation in ground environment.(2) According to the systematic analysis of the nitride geochemical behavior, the ammonium nitrogen adsorption/desorption process was studied. The ammonium nitrogen adsorption/desorption experiments results show:The loess isothermal adsorption/desorption equations are consistent with Langmuir adsorption kinetics models, and the loess desorption hysteresis is the strongest. The silty clay and silt isothermal adsorption/desorption equations accords with the Henry model, and the silty clay and silt desorption hysteresis are stronger than fine sand. The fine sand isothermal adsorption equations is consistent with the Langmuir adsorption kinetics models and the isothermal desorption equations is consistent with Henry model, and the fine sand desorption hysteresis are the weakest. So, the loess vadose zone has the strongest hysteresis for the ammonium nitrogen, the silty clay and silt vadose zone has stronger hysteresis, but the fine sand vadose zone has the weakest hysteresis.(3) The nitrification and denitrification experiments show:the ammonium nitrogen nitrification kinetics is divided into the slow phase, fast phase and decay phase (denitrification phase), and the nitrate denitrification process consist of the denitrification phase and the weak nitrification phase. The Nitrate denitrification rate is 22.13%-70.23%, which means the nitrate can not be completely biodegradable in the native soil environment (lack of man-added nutrient solution). So, the more nitrate go into the groundwater, which supply abundant substance for the nitride pollution of the groundwater.(4) The mixture and sealed soak experiment approved that the mixture dilution cannot decrease the content of nitrate, and the chemical reduction and biological denitrification are the important paths to decrease the content of nitrate, that the nitrate natural degradation is dominated by chemical reduction, and supplemented by biological denitrification. The nitrate natural degradation rate is 25.7-39.9% in aquifer. The self-purification capacity of aquifer is bad and the nitrate is easy accumulating in the aquifer. The research proves the artificial strengthen chemical reduction (add the ferrous iron salt) is more than two times higher than the original ecological degradation. This study result can provide a theoretical basis and technical support for groundwater nitrogen pollution prevention and control.(5) Through the in situ testing, the migration and transformation rule of "Three Nitrogen" in loess and sand vadose zone was studied. According to the vadose zone soil moisture and solute transport model, the plant root water uptake and nitrogen absorption reduces the residual value of nitride and maximum migration distance was studied. The corn root water uptake and nitrogen absorption was stronger than that of wheat roots, nitrogen absorption of wheat and corn root accounted for 41.3% of the total nitrogen. Nitrate flux, correspondingly through the loess, silt and fine sand vadose zone to the shallow groundwater, the nitrate is a difference of 104 times from the input flux of ammonium nitrogen and nitrite. A mathematical model based on conservation of mass is set up to estimate nitrate accumulation in the aquifer, and compute the threshold influencing the annual nitrate accumulation; The research complement the nitride environmental behavior and fate theory in aquifer.(6) At last, the groundwater health risk assessment in study region was evaluated, the assessment results shows that the nitrate non-carcinogenic chronic toxic index for people health is higher in study region shallow groundwater. The high risk region is occupied 78.2% in study area, which distribute chiefly in the irrigation area and town with dense-residence, advanced-industry. The groundwater containing 12.6-20mg/L nitrate has high risk to people health. Through groundwater health risk assessment in Xi'an region, the author analysis put forward that the nitrate is the priority remediation matter. For, the safety management program and safeguard measures of underground water quality based on health risk, is proposed initially to put forward a new model for managing and protecting the groundwater resource.