Study Of Accumulation And Transformation Of Nitrogen In The Heterogeneous Unsaturated Zone With Stimulated Rainfall

Nitrogen pollution of groundwater has become a popular problem in the world, which has been posed a great risk to the drinking water supply. During the process of the contaminants infiltrate from land surface to the groundwater, the unsaturated zone soil texture, structure and thickness have large effects on the nitrogen transport and transformation, and then decide the possibility and degree of groundwater contamination. An area along Wenyu River was selected as the study area which is polluted to various extents of ammonium nitrogen in shallow groundwater. Soil samples were collected from a series of boreholes in unsaturated zone and the vertical distribution characters of nitrogen in the profiles were analyzed. Laboratory experiments with simulated rainfall were conducted using a sandbox to investigate the influence of unsaturated zone heterogeneous structure on the process of moisture transfer, nitrogen transport and transformation. The results were summarized as follows:(1) Vertical distribution characters of nitrogen in unsaturated zone profiles indicate that: An accumulation of ammonium nitrogen is found in the lower parts of 9 soil profiles in the study area; There are two main types of the residual nitrate nitrogen distribution: one was the accumulation of nitrate appeared in the surface soil, and the other was the peak value of nitrate concentration appeared in the middle of profile. The results have shown that no obvious relations between the distribution of nitrogen and the soil texture, which suggests that the soil strcture and layered-structure may have an effect on the nitrogen transport and transformation in the unsaturated zone and finally result in the complex of its distribution.(2) Experimental study on moisture transfer shows the preferential flow occurred in the heterogeneous unsaturated zone, and the difference between soil textures has great effect on preferential flow formation. Water content is 29% in the two clay lenses which suggests that the clay lenses keep more water than fine sand layer. Even in the breakthrough experiment, the distribution of ammonium nitrogen in the soil keeps the same characteristic with the soil water. Both experiments demonstrate that soil water and nitrogen are maintained at a higher level in the clay lenses.(3) Study on intermittent contamination of ammonium nitrogen reveals that the dynamic equilibrium of absorption and biodegradation is reached after 89 days. The unsaturated zone soil texture and layered-structure are very important to the nitrogen transport and transformation which lead to an accumulation of ammonium nitrogen and nitrite nitrogen in the upper surface of two sandy-clay interface. Nitrate nitrogen is accumulated in the topsoil while an accumulation of ammonium nitrogen and nitrite nitrogen is found in the sublayer soil. The leaching test suggests that nitrogen concentration in outflow were reduced more than 90 percent after 9 periods. Stimulated rainfall has a significantly effect on the leaching of nitrogen while the clay lenses can retard the transport of nitrogen.(4) In a study of nitrogen balance, it is found that total leaching losses of nitrogen and denitrification loss account for 70% and 17% respectively; It is also suggested that nitrification is intensity high and denitrification is comparatively weak in the simulated sandbox; Most of nitrogen is lost in outflow water in the forms of ammonium, nitrate and nitrite nitrogen.