| Karst region in southwest of China is located in the sub tropic with good combination of water and temperature. Water resource is relatively abundant. Subterranean is the major resource of karst water and under control by the natural conditions as geology, structure, hydrogeology and so on. The unique structure of dual layers, indeed absent from the soil cover, is vulnerable to be contaminated, especially in agricultural zone, as the main living and producing water resource, the quality of the karst subterranean water is much more important.Qingmu guan Underground River (QUR) is the typical karst underground river in southwest karst valley area, agricultural production is lagging behind the level of average, land-use patterns are based primarily on cultivation land. Some abandoned coal mines and quarries are spotted in the area. The downstream export of underground river, Jiangjia Spring, is the vital water resource for nearby residents, the water quality has a direct effect on the local people's health. In this study, QUR dynamic characteristics of subterranean basins, affecting factors and sources of nitrate pollution will be in analysis and discussion, which will help understand the formation of underground water, distribution, spatial and temporal variation, to provide a basic consideration for groundwater development and utilization.1.The weather stations, monitoring stations is established to monitor the amount of underground rivers, and physical characteristics of the flow control output system and the main physical indicators include water temperature (T), electrical conductivity (EC), pH value variations. And moreover, field observations are coincided to describe the physical changes of the year in QUR.2.The combination method of field sampling, field observations and laboratory analysis is adopted in QUR Basin to analyze different water types with continuous sampling, from April 2008 to June 2009 (once per month). 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. 3.Study area is in the agricultural region, according to this article, due to the frequently agricultural activities, the "fertilization activities" period is monitoring intensively, carried on observation and sampling from May 6,2008 to June 2,2008 (once per two days).Observing content and indicators do as the same as 2nd mentioned. The analysis revealed a strong disturbance of agricultural activities in the short time scale, to the variation of karst groundwater quality.4. Analyzed the nitrogen and strontium isotopic characteristics of QUR water by means of isotope methods, and which is combined with water chemistry analysis methods to identify and distinguish natural factors from human ones on the impact of groundwater quality in agricultural areas.Finally, the consequences are concluded in this paper:the water quality of QUR changes seasonally for sure, which shows up normal all the year except the fertilizing period in Jiangjia Spring. The water in Jiangjia Spring is vulnerably contaminated by human farming activities. By means of Sr isotope and N isotope with combination of hydro-chemical analyzing, it shows paddy field and Yankou Sinkhole in the upriver as the input, are affected by the fertilizer. Jiangjia Spring derived from limestone shows high nitrate concentration and 815N, simultaneously,87Sr/86Sr is over 0.708, which indicates the fertilizer contamination throng water-rock interaction, moreover, the fertilizer and nitrogen in soil in dry land at the middle and downriver is the other main nitrate contribution to the Spiring. The coal mine water is main from the rainwater deduced by 87Sr/86Sr while absent from theδ15N. By and large, the main nitrate sources of QUR are the soil organic nitrogen, fertilizer and those comprehensive effects.
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