Migration Mechanism Of Nitrogen In Peak Forest Plain Karst Water System In Eastern Guilin

In southwest China, centered upon Guizhou province, there is an area of about600,000km2of carbonate rock mountains, it is the most developed karst area in China. Karst water resources is one of the most important support for social and economic development in southwest karst area. With a lot of rainfall, there is a large plenty of karst water resources in southwest karst region, but due to the strong karstification and the destruction of vegetation, southwest karst area become one of the serious water shortage areas in China, the issue of water resources has become a key factor in restricting the economic development of the region. Groundwater, as a crucial component of water resources, has tremendous influences on the sustainable development of social economy.In recent years, karst groundwater pollution in China has been so serious that the usable quantities of groundwater resources are very little and the protection of karst groundwater couldn’t be delayed.In southwest karst area, the groundwater is an important source of water for life and production, but did not get the appropriate protection. Use large amounts of fertilizer in agricultural production, garbage dumping,livestock manure random discharged, there phenomenons can be seen everywhere, the3066underground rivers of the southwest karst area facing the challenges of multiple pollution, and are being changed to the threat of sewage sewers, also caused nitrogen pollution, therefore, reduce nitrogen pollution in karst groundwater is very important for ensuring the people’s livehood and social harmony and stability. Karst groundwater nitrogen pollution as well as the migration of nitrogen transformation mechanism has been widely reported, but most of these studies focused on non-karst areas, karst area, especially the peaks plains nitrogen pollution of groundwater system involves less."3-N"pollution situation as well as the migration and transformation mechanism has been widely reported, but most of these studies focused on non-karst areas, karst area, especially in the peak forest plains,nitrogen pollution of groundwater system involves less.In this paper, we choose a typical peak forest plain in eartern Guilin as our sample. use groundwater tracer tests to identify the connectivity of the karst aquifer in this area,to reveal the structural characteristics of peak forest plain karst aquifer, and use nitrate nitrogen and oxygen isotopes to analyse nitrogen distribution, change and sources in the karst water system, probe into the natural and factitious factors.It can be drawn from the results of the tracer tests that there is great difference in the direction and rate of the groundwater flow in the study area, northwest and southwest are both of the main flow directions, while the flow rate mainly reflects the different types of aquifer medium. Karst aquifer medium of the peak forest plain in eastern Guilin is relatively uniform, the channel is complex and changeable. Seen from the groundwater migration characteristics, the groundwater flow rate is slower, the ratio of the maximum visual flow rate and the average visual flow is3.98, shows that the underground karst was very development, mainly was karst fissure, groundwater migration pathways are very complex, form a intricate karst fissure net, the groundwater flow present planar migration. It also shows that the update rate of the groundwater is slower in peak forest plain area than in the karst mountain peak cluster depression area, there is a greater risk of groundwater pollution.During the rainy season, nitrate content of groundwater in eastern Guilin was0.088-46.69mg/L, the average was12.52mg/L,higher than10mg/L which ruled by World Health Organization; ammonia and nitrite nitrogen content were lower, only three sampling points detected the presence of ammonia.In the dry season, the nitrate content in the groundwater decreased to an average of8.8mg/L, exceeding rates dropped significantly; ammonia and nitrite nitrogen content increased, may be related to low temperatures, microbial nitrification rate slowed down. Thus, the presence of nitrogen in the groundwater in the form of nitrate, constitutes a serious threat to the health of local residents. Though ammonia and nitrite content were not high, but view from the special hydrogeological conditions as well as the nitrogen cycle, there is still a greater risk of contamination. Through the analysis of the relationship between the different land use patterns and the nitrate content, we found that the nitrate pollution of the living area was the most serious, the highest average nitrate content was paddy growing areas,followed by vegetable growing areas and other areas of unused, nitrate content of groundwater was not exceeded in vegetables and paddy hybrids areas as well as other unused areas.Analysed nitrate nitrogen and oxygen isotopes characteristics,δ15N values was5.54‰～25.43‰,δ18O values was3.18‰～13.21‰, weres basically within the range of feces or sewage(10‰-20‰)and soil organic(4‰～9‰), showed that the nitrate sources of groundwater in this area were livestock manure and sewage, and also may had a mixture of soil organic nitrogen and fertilizer. Sampling points which distributed in the runoff discharge zone, N and O isotope ratios in the range of1.3-2, indicating that the occurrence of denitrification, the sampling points which distributed in the recharge area, N and O isotope ratios were not within this range, indicating that denitrification were not obvious, probably not the main factor of the isotopic variation.