高分辨率监测岩溶地下水NO3-的动态变化及对外界环境的响应

High-Resolution Research on the NO_3^- Changes of Karst Groundwater and Its Responses to the Outside Environmental Variations

为深入揭示岩溶地下水文系统对外界环境的响应,利用多指标高分辨率在线监测技术对受农业活动影响的重庆青木关地下河水文地球化学变化进行研究.主要的高分辨率监测指标包括水位、电导率(EC)、pH值、降雨和NO 3-.在观测的6场降雨内,地下河水化学特征快速地反映着外界环境的变化.研究区地下水pH值的变化主要受到酸雨的影响,在降雨后都表现出明显的下降趋势,但在农业废水进入地下河系统后,它的变化受到两者的共同影响.EC受到雨水化学、稀释效应及农业活动废水的影响.NO 3-主要是农业活动的产物,受雨水水化学特征的影响较小,它的变化主要受农业废水及稀释效应的影响.在R1降雨影响下,水位上升,EC和NO 3-总体呈相反的快速变化过程而pH值的变化主要受酸雨影响而下降.在R2、R3、R4和R6降雨期,水位受降雨影响而快速的变化,pH值受酸雨影响而下降,EC和NO 3-受稀释效应影响也下降.但随着农业活动废水进入地下河系统,EC和NO 3-浓度在最快5 h最慢仅27 h的时段内同步地急剧升高,pH值也受影响而加速下降.在R5大暴雨期,水位急剧上升,EC和NO 3-受稀释效应影响而急剧下降,pH值先受到酸雨影响而下降,后由于稀释效应的影响而上升.在降雨期,农业活动废水的注入快速地改变着地下河的水质,使其呈现Ⅲ类水→Ⅳ类水→Ⅴ类水→Ⅳ类水→Ⅴ类水→Ⅳ类水→Ⅴ类水→Ⅳ类水→Ⅲ类水的变化过程,而采样分析结果所揭示的变化过程相对简单.通过自动监测结果和野外采样分析结果的对比发现两者之间存在较大差异,启示人们需要改进岩溶水文系统的监测研究手段.

In order to thoroughly reveal the karst groundwater system in response to the external environment,multi-index high-resolution auto-monitoring instruments were used to research the hydrogeochemistry variations of Qingmuguan subterranean stream. The monitoring indicators were pH value,electrical conductivity （ EC）,water level,rainfall and NO 3- . In the observed 6 times rainfall events,the pH value was mainly controlled by acid rain and EC was impacted by the rain chemistry,physical dilution effect of rainfall and agricultural wastewater. NO 3- derived from agricultural activities was less impacted by rain chemistry; and its variations were mainly affected by physical dilution effect of rainfall and agricultural wastewater. Under the influences of the R1 rainfall,the rapid changes of EC and NO 3- were contrary in generally,but pH value decreased due to the acid rain. In the R2,R3,R4 and R6 rain events,the water level was positively response to the rainfall and the pH value decreased due to the acid rain; however both of the EC and NO 3- decreased due to the dilution effect. With the afflux of agricultural wastewater,both of the EC and NO 3- were rapidly changed in synchronization within the shortest of 5 h of and the longest of only 27 h and the pH value showed accelerated decline. During the R5 torrential rain events,water level abruptly increased and all of the pH value,EC and NO 3- sharply decreased as a result of dilution effect. The groundwater quality was changed because the afflux of agricultural wastewaters in the entire rainfall periods. According to the National Groundwater Quality Standard,P. R. China （GB /T 14848-9）,the groundwater quality of the QSS showed the following order in the entire rainfall events：Grade Ⅲ→Grade Ⅳ→Grade Ⅴ→Grade Ⅳ→Grade Ⅴ→Grade Ⅳ→Grade Ⅴ→Grade Ⅳ→Grade Ⅲ. As a important drinking water source around local inhabitants,the groundwater quality of Qingmuguan subterranean stream was deteriorated to Grade Ⅴ in rain- fertilize periods. However,the sampling methods in field did show a simple variation curve. Consequently,the results from the research illustrate the need to redesign hydrochemical sampling methods for karst hydrology system,especially under the impact of human activities.