Hydrogeochemical indicators of hierarchically nested structure of karst groundwater flow systems in the ejective folds area

Abstract

The karst groundwater flow system in the ejective folds area in the eastern Sichuan Basin shows the characteristics of a hierarchically nested structure. However, previous studies were mainly focused on hydrogeological conditions analysis, lacking qualitative evidence directly from hydrochemistry. In this study, the authors collected and sorted the chemistry data of representative karst springs and boreholes in the study area, namely, 41 shallow karst waters and 19 deep ones. The Pipper chart shows that the shallow karst water is HCO3-Ca·Mg and low Mg2+ concentration, while the deep karst water is SO4-Ca·Mg and high Mg2+ concentration. For both shallow and deep karst groundwater, the analysis of the groundwater flow system of a typical profile shows that the Mg2+ concentration value can be used to evaluate the groundwater residence time. The shallow karst groundwater system is highly controlled by geomorphology, among which the shallow incised valley springs are the discharge points of the local groundwater flow system. However, springs in deep gullies correspond to intermediate or even regional flow systems. This pattern is verified by the groundwater residence time evaluation from the Mg2+ concentration, which also reflects the movement characteristics of deep-circulation groundwater flow revealed by hot springs and boreholes.The hydrochemical characteristics can effectively indicate the pattern of groundwater flow systems. This paper provides a reference for identifying the hierarchically nested structure of groundwater flow systems in similar karst areas.

Keywords

• ejective folds
• groundwater age
• hydrochemical identification
• groundwater flow system
• hierarchically nested structure