Groundwater systems in bare and covered karst aquifers: evidence from tracer tests, hydrochemistry, and groundwater ages

摘要

The east Guilin region contains a karst dominated hydrological system along the Lijiang River. The two main topographic characteristics of the basin are peak cluster depression and peak forest. Despite the fact that these areas represent adjacent units, they have different groundwater movement patterns. This study describes the groundwater and solute movement in the different hydrogeological sub-regions via several techniques. Our results indicated that the topographic boundary between the peak cluster depression and the peak forest is clear. However, a transition zone exists between these topographic zones, and it can be determined in terms of groundwater movement. We employed several methods that are widely considered to be effective. Tracer tests were conducted in the transition zone, hydrochemistry techniques were used in the peak forest, and groundwater age dating based on CFCs was employed throughout the study area. The main conduits could be found in the transition zone, but the groundwater flow was much slower in the transition zone than in the mountain peak cluster area. Minor conduits also accounted for a high proportion of the total flow in the transition zone. The solute migration within the plain, which was determined by analyzing the nitrates, was controlled by mixing and distance effects. The maximum nitrate concentration was limited at the local scale. The nitrate concentration gradient at the regional scale was not related to the groundwater movement, indicating that the groundwater recharged in a dispersed manner and discharged at discrete locations along the river. The age dating revealed that the groundwater was older in the plain than in the bare mountain zone. This was due to the strong mixing of young and old water, which was the result of the characteristics of the karst media in the aquifer. Our investigation of the groundwater system in a bare/covered karst aquifer provides data for decision-making in effective groundwater management.