Hydrogeochemical characteristics of surface water and groundwater in the karst basin, southwest China

摘要

Groundwater is a very significant water source used for irrigation and drinking purposes in the karst region, and therefore understanding the hydrogeochemistry of karst water is extremely important. Surface water and groundwater were collected, and major chemical compositions and environmental isotopes in the water were measured in order to reveal the geochemical processes affecting water quality in the Gaoping karst basin, southwest China. Dominated by Ca~(2+), Mg~(2+), HCO_3~- and SO_4~(2-), the groundwater is typically characterized by Ca-Mg-HCO_3 type in a shallow aquifer, and Ca-Mg-SO_4 type in a deeper aquifer. Dissolution of dolomite aquifer with gypsiferous rocks and dedolomitization in karst aquifers are important processes for chemical compositions of water in the study basin, and produce water with increased Mg~(2+), Ca~(2+) and SO_4~(2-) concentrations, and also increased TDS in surface water and groundwater. Mg~(2+)/Ca~(2+) molar ratios in groundwater decrease slightly due to dedolomitization, while the mixing of discharge of groundwater with high Mg~(2+)/Ca~(2+) ratios may be responsible for Mg~(2+)/Ca~(2+) ratios obviously increasing in surface water, and Mg~(2+)/Ca~(2+) ratios in both surface water and groundwater finally tending to a constant. In combination with environmental isotopic analyses, the major mechanism responsible for the water chemistry and its geochemical evolution in the study basin can be revealed as being mainly from the water-rock interaction in karst aquifers, the agricultural irrigation and its infiltration, the mixing of surface water and groundwater and the water movement along faults and joints in the karst basin.