On Soil Control Of Hydro Chemical Kinetics Of Cave Drip Water A Case Study At Shihua Cave, Beijing, China

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, a study was conducted of drip water geochemistry and its response to rainfall and soil water regime by field monitoring at Shihua Cave, Beijing, China from December 2003 to December 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 the drip site. In spring, the small amount of precipitation filled only the fissures and did not reach the karst water table and without resulting in increase of the drip rite. However, in summer, under higher rainfall, water filled the soil cracks developed after a long period of drought in the precedent autumn, winter and spring, 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, Mg2+and SO42-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 the soil above the cave has also been determined to infer its control on karst water hydrogeochemistry. There is a considerable amount of soluble Ca2+ and Mg2+ in soil solution above the cave, which may be leached down to the underlying aquifer by rainwater. On the other hand, rainwater in summer carries large amount of CO2 derived from soil produced by soil respiration, which dissolves the soilcarbonates and the underlying carbonate rock and results in an increase of Ca2+ concentration in drip waters and contributing to an active deposition of secondary calcite during summer.Previous studies showed that the variability in dripwater Mg/Ca ratio could indicate the water-rock interaction processes. 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 of dolomite. Furthermore, calcite precipitation prior to dolomite occurred along the flow path due to degassing of CO2 from fissure solution, resulting in depletion of Ca2+ and relative enrichment of Mg2+ in drip water. However, the dripwater Mg/Ca ratio in Beijing Shihua Cave showed higher value in summer/wet period than in the autumn, winter and spring seasons/dry period of the investigation period, which was attributed to by the more favorable leaching of Mg2+ than of Ca2+ from the soil. Variation of speleothem Mg/Ca ratio with cave temperature is insignificant for the temperature in the Shihua Cave is relatively constant within a year, so variation of speleothem Mg/Ca ratio could be indirect indicator of drip water Mg/Ca ratio and, in turn, an indicator of rainfall in warm seasons of the epikarst environmental conditions. However, precise and accurate interpretation requires more detailed and long term monitoring and calibration of Mg/Ca ratio variation to climate conditions.The data of dissolved organic carbon (DOC) showed there were seasonal variations in the DOC concentration in all drip waters. High level DOC concentration was introduced during rain period in July, which resulted in the formation of organic substances laminae of stalagmite.