Abstract:

Some kinds of runoff were identified in a karst hydrological system by observation and monitoring in field during rain storm events at YAJI hydro-geological site, Guilin, China. Runoff includes many forms such as epikarst springs, slope return flow, overland flow and soil interflow in a depression. The runoff aroused by rainfall has different residence time and hydrochemistry. A multi sensor for pH and specific electrical conductivity together with calcium and bicarbonate concentration were used to determine the chemical composition of stream water, spring, drop water and stored water in rock holes. The results are quite different among these kinds of karst water, indicating that the chemical items of water can be used for tracing the sources of spring. Calcite dissolution in pure water with different p_CO2 was calculated by PHREEQC. We get a line of Ca²⁺ concentrations at different p_CO2. The dots representing different water in the system were near the theoretical line of Ca²⁺ concentration VS p_CO2 in CaCO₃-H₂O-CO₂ (g) system. It showed that the chemical character of runoff was controlled by p_CO2. Runoff was classified into two groups according to its hydrochemistry based on difference of p_CO2 in atmosphere and soil. The two groups were end numbers of spring water in flood. It showed that event water controlled by atmosphere but not soil air occupied about 70% when storm by end-number method.

Key words: karst water, calcite dissolution, partial pressure of CO₂, runoff