South China karst aquifer storm-scale hydrochemistry

The peak cluster and peak forest karst regions of Southeast Asia form one of the earth's most extensive karst regions. Although there exists a rich, descriptive tradition of geomorphic work performed there, little quantitative study has been made of carbonate hydrochemistry and related aquifer/landscape behavior and evolution. In this paper, high-resolution measurements of ground water carbonate chemistry and flow were made and analyzed at two adjacent locations within the subtropical peak cluster karst of the Guilin Karst Experimental Site in Guangxi Province, China. While waters from a large, perennial spring represent the exit for the 2 km2 catchment's conduit flow, a nearby well (within 5 m) measures water in the conduit-adjacent, fractured media. Results indicate that within peak cluster karst aquifer flow systems, spatially heterogeneous flow conditions can exist with respect to timing, magnitude, and, in some cases, direction of responses, as different controls can operate in the different flow system components. Storm-scale chemical responses are controlled by dilution from rapid infiltration of rain water, CO2 gas sources and sinks, and water-carbonate rock interactions. At this particular location, there is also an influence from high pH recharge, apparently buffered by atmospheric limestone dust. An example of the varying controls on storm-scale responses within the flow system is that within the fractured medium, variations in the ground water calcite saturation index, a key parameter influencing rates of aquifer/landscape evolution, are small and controlled by CO2 gas, while in the conduit they are more significant and dominated instead by dilution with rain water.