Abstract
Characterizing the subsurface flow in karstic areas is challenging due to distinct flow paths coexisting, and lithologic heterogeneity makes it more difficult. A combined use of hydrochemical, environmental isotopic, and hydrograph separation study was performed to understand the subsurface flow in a karst terrain where Ordovician carbonate rocks overlie Jurassic sandstone and shale along thrusts. Spring water collected was divided into Type Ⅰ (n= 11) and Ⅱ (n= 30) based on flow patterns (i.e., low and high discharge, respectively). In addition, groundwater (n= 20) was examined for comparison. Three Type Ⅱ springs were additionally collected during a storm event to construct hydrographs using δ18O and δD. As a result, Type Ⅱ had higher electrical conductivity, Mg2+, HCO3−, and Ca2+/(Na++ K+) than Type Ⅰ and was mostly saturated with calcite, similar to deep groundwater. The hydrochemical difference between Types Ⅰ and Ⅱ was opposite to the expectation that Type Ⅱ would be undersaturated given fast flow and small storage, which could be explained by the distinct geology and water sources. Most Type Ⅱ springs and deep groundwater occurred in carbonate rocks, whereas Type Ⅰ and shallow groundwater occurred in various geological settings. The carbonate rocks seemed to provide conduit flow paths for Type Ⅱ given high solubility and faults, resulting in 1) relatively high tritium and NO3−and Cl−viashort-circuiting flow paths and 2) the similar hydrochemistry and δ18O and δD to deep groundwaterviaupwelling from deep flow paths. The deep groundwater contributed to 83–87% of the discharge at three Type Ⅱ springs in the dry season. In contrast, Type Ⅰ showed low Ca2++ Mg2+and Ca2+/(Na++ K+) discharging diffuse sources passing through shallow depths in a matrix in mountain areas. Delayed responses to rainfall and the increased concentrations of contaminants (e.g., NO3−) during a typhoon at Type Ⅱ implied storage in the vadose zone. This study shows that hydrochemical and isotopic investigations are effective to characterize flow paths, when combined with hydrograph separation because the heterogenous geology affects both flow paths and the hydrochemistry of spring water passing through each pathway.
Highlights
Karst aquifers differ from unconsolidated granular aquifers regarding subsurface flow behavior because of various types of porosity, e.g., matrix, fractures, and conduits (Ghasemizadeh et al, 2012; Hartmann et al, 2014; Demiroglu, 2016)
Spring water samples were divided into Type I (n 11; much lower than the rates in Figure 2A) and Type II (n 30; discharge rates similar to those in Figure 2A (> 480 L/min)) based on the flow type observed during sampling to assess the hydrochemical and isotopic differences of spring water depending on flow patterns and to determine the factors contributing to the differences in the study area with stratigraphic overturning along thrusts
Spring water quality was mainly controlled by three hydrogeological processes in the studied karst area: 1)
Summary
Karst aquifers differ from unconsolidated granular aquifers regarding subsurface flow behavior because of various types of porosity, e.g., matrix, fractures, and conduits (Ghasemizadeh et al, 2012; Hartmann et al, 2014; Demiroglu, 2016). Groundwater recharge occurs by either diffuse infiltration through the fissured matrix, called diffuse-dominated, or direct point infiltration into the conduit network, called conduitdominant. The diffuse-dominated flow is associated with slow and delayed responses of water seepage from the aquifer matrix, less connected fractures, or overlying soils, whereas the conduitdominated flow is fast through the interconnected voids and passages of the aquifer (Schilling and Helmers, 2008). Conduit flow makes the karst aquifer vulnerable to contamination through direct and fast infiltration (Stueber and Criss, 2005; Ghasemizadeh et al, 2012; Parise et al, 2015) or to rock collapse due to open caves (Waltham and Lu, 2007; Pogačnik et al, 2017). An understanding of conduit flow paths is essential when managing water resources or building facilities (e.g., dams) in the karst area (Milanović, 2021)
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