TEMPORAL AND SPATIAL VARIATION OF CAVE DRIPWATER GEOCHEMISTRY IN SHIHUA CAVE, BEIJING,CHINA
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摘要: 通过对北京石花洞滴水地球化学一个水文年的观测,揭示了洞穴滴水水文地球化学季节变化与外界气候变化的关系,3个滴水点的滴率随降雨量的增加都有明显的变化,但不同滴水点滞后时间不同。滴水滴率、Mg2+和SO24含量的季节变化数据显示,雨季洞穴滴水主要来源于当季降水,但也存在岩层滞留水的混入。滴水中Mg/Ca比值存在明显季节变化,旱季较低而雨季较高,但在雨季初期出现较大的波动。分析洞穴上覆土壤和洞内裂隙土壤数据,认为雨季初期滴水中Mg/Ca比值的波动是由土壤中Mg2+的快速淋溶造成的,上覆土壤结构性质和组分变化均影响滴水地球化学特征。Abstract: In recent years there has been a growing interest in cave speleothems as archives of paleoclimate change. An understanding of temporal and spatial variability of cave dripwater geochemistry is thus important in studying the stalagmite records for interpreting the paleoclimatic change. In this paper, the authors reported a monitoring of drip water geochemistry and its response to rainfall and soil water regime in Shihua Cave, Beijing, China from December 2003 to November 2004. As monitored in three drip sites, there was marked temporal and spatial changes in dripping rates, with the lag times varying with the different features of micro-fissure/porosity in the soil and limestone rock above cave the drip site. Spring rainfall water filled the soil cracks developed after a long period of drought in the precedent autumn and winter, and infiltrated into the epikarst zone, passing the karst conduits that fed the cave drip water. Rainfall water migration was predominantly along low-permeability, diffuse flow-routes during periods of low rainfall and correspondingly, low recharge. As rainfall and recharge increased, the capacity of low-permeability diffuse flow-routes was exceeded and groundwater migration along pathways of high-permeability conduit flow-routes increased. The analysis of the seasonal variation of drip rate, Mg 2+ and SO 2- 4 demonstrates that most of the drip water came from the rainfall mixed with older water stored in the pores and micro-fissure network of epikarst zone. The chemical composition of soil above the cave has also been determined to address the role of soil in karst water hydrogeochemistry. There are a considerable amount of soluble Ca 2+ and Mg 2+ in the soil above the cave, which may be leached to the underlying aquifer by rainwater. On the other hand, rainwater contains large amount of CO 2 derived from soil with higher concentration in summer dissolving the underlying carbonate, resulting in the increase of Ca 2+ concentration in drip waters, indicating active deposition of secondary calcite during summer. The observed variability in dripwater Mg/Ca ratio indicates that water-rock interaction processes are neither spatial nor temporal constant. The variation of karst water residence time in the epikarst zone may be interpreted by the drip water Mg/Ca. In the dry season, there could exist a durable contact of rock with moisture, probably resulting in an increased Mg/Ca ratio of the solution due to quicker saturation of calcite than dolomite. Furthermore, calcite precipitation prior to dolomite occurred along the flow path due to degassing of CO 2 from fissure solution, resulting in depletion of Ca 2+ and relative enrichment of Mg 2+ in drip water. This process is highly sensitive to temperature both of water and the soil above cave. While variation of speleothem Mg/Ca ratio with temperature is insignificant for the temperature in the Shihua Cave is relatively constant within a year, variation of speleothem Mg/Ca ratio could be indirect indicator of drip water Mg/Ca ratio and, in turn, an indicator of the epikarst environmental conditions affecting the hydro-geochemical flow in the cave. However, precise and accurate interpretation requires more detailed and long term monitoring and calibration of Mg/Ca ratio variation to climate conditions.
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Key words:
- cave drip water /
- geochemistry /
- Mg/Ca ratio /
- soil /
- epikarst zone
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