WET SEASON HYDROCHEMISTRY OF THE SOUTHWEST CHINA PEAK CLUSTER KARST

High resolution measurements of stage, pH, conductivity, temperature, and carbonate chemistry parameters of groundwater were made at two adjacent locations within the 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 aquifer's conduit flow, a nearby well measures water in the conduit-adjacent, fissured media. During flood pulses, the pH of the conduit flow water rises while conductivity falls. Inversely, and at the same time, the pH of groundwater in the fissures drops, while conductivity rises. As Ca²⁺ and HCO₃^- were the dominant (>90%) ions, we developed relationships (both r²>.91) between conductivity and those ions, respectively, and in turn calculated variations in the calcite saturation index (SI_c) and CO₂ partial pressure (Pco₂) of water during flood pulses. It was found that the Pco₂ of fissure water during flood periods is higher than that at lower flows, while its SI_c is lower. Simultaneously, Pco₂ of conduit water during the flood period is lower than that at lower flows, and its SI is also lower. From these results we conclude that at least two key processes are controlling hydrochemical variations during flood periods: 1) dilution by precipitation, and 2) water/rock/gas interactions. To explain hydrochemical variations in the fissure water, the water/rock/gas interactions may be more important. For example, during flood periods, soil gas with high CO₂ concentrations dissolves in water and enters the fissure system, the water which has in turn become more highly undersaturated dissolves more limestone, and the conductivity increases. Dilution of rainfall is more important in controlling hydrochemical variations of conduit water, because rainfall with higher pH (in this area apparently due to interaction with limestone dust in the lower atmoshphere) and low conductivity travels through conduit system rapidly. These results illustrate that to understand the hydrochemical variations in karst systems, considering only water/rock interactions is not sufficient, and the variable effects of CO₂ on the system should be evaluated. Consideration of water/rock/gas interactions is thus a must in understanding variations in karst hydrochemistry.