| Karst region in southwest of China is located in the subtropic with good combination of water and temperature and water resource. Groundwater in karst is the major resource of karst water and is significant to local residents living and production water. The unique structure of dual layers under control by the natural conditions as geological structure and geographical position.Frequent human activity make it very easy to be polluted, especially in agricultural areas, the water quality of the karst underground river is more important.Qingmu guan Underground River (QUR) as the typical karst underground river in southwest karst valley area, land-use patterns are based primarily on cultivation land agricultural production is lagging behind the level of average and few industry are there. Some abandoned coal mines and quarries are spotted in the area.Underground river from the upstream Tianci to dingjialong cave as the vital water resource for nearby residents, the water quality has a direct effect on the local people's health.This article begin with the protection of underground river resource and environment, related to QUR Underground river system in Chongqing as an example,underground water physical-chemistry characteristics, affecting factors and sources of nitrate pollution are in analysis and discussion, which will help and understand the formation of underground water, distribution, spatial and temporal variation, and provide a basic consideration for groundwater development and utilization.1.The combination method of field sampling, field observations and laboratory analysis is adopted in QUR Basin to analyze different water types with continuous sampling. And moreover, field observations are coincided to describe the physical changes of rainy season in QUR.2.. observations and sampling are from May 2010 to Nov 2010. Field observations include temperature (T), electrical conductivity (EC), pH value and Ca2+, HCO3- and so on. Laboratory analysis, including major cations such as, K+,Na+,Ca2+,Sr2+,Mg2+; and the main anion, HCO3-,NO3-,SO42-,Cl-. Afterwards, chemical characteristics of water changes are acquired in QUR throughout the year, chemical characteristics of water in this district are CaHCO3 and Ca,Mg-HC03,and major cations are Ca2+.3. As in the agricultural area, agricultural activities is frequent,the main man-made fertilizer of agricultural production activities are nitrogenous fertilizer and potash fertilizer and a great deal of manure. Analyzed the nitrate nitrogen and nitrogen isotopic characteristics of QUR, and combined with water chemistry analysis methods to identify and distinguish natural factors from humans'on the impact of groundwater quality in agricultural areas.Finally, the consequences are concluded in this paper:the water quality of QUR changes seasonally:The south of Underground river,Tianchi,is the input end and Laijiacao and Dingjialong cave are the output end. Dingjialong cave derived from limestone shows high nitrate concentration and 815N which indicates Underground river are polluted by fertilizer contamination and animal wastes throng water-rock interaction.And paddy field in the north of Underground river and Yankou Sinkhole in the upriver as the input, are affected by the fertilizer. In a word, the main nitrate sources of QUR are different from the south and north,in the south the main are fertilizer and wastes,but the north are the soil organic nitrogen and fertilizer.
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