Abstract
Karst water is vital for local drinking and irrigation but is susceptible to contamination. Hydrochemistry, which is highly related to carbonate weathering in karst catchments, can affect water quality and respond rapidly to climate change. In order to explore hydrogeochemical sources, dynamics, and their responses to rainstorms, rainwater, throughfall, hillslope runoff, surface water, and groundwater were sampled synchronously during rainstorms at a karst Critical Zone Observatory (CZO), Southwest China. Results showed that the total dissolved solids (TDS) concentration in throughfall increased by 30.1 ± 8.0% relative to rainwater, but both throughfall and rainwater contributed little to TDS in surface water and groundwater compared with terrestrial sources. Hydrochemistry in surface water and groundwater was diluted by rainstorms but displayed chemostatic responses with different intensities to increasing discharge. This is possibly regulated by hydrogeological conditions, available sources of various solutes, and the difference between solute concentrations before and after rainstorms. Ca2+ and Mg2+ dynamics were mainly regulated by carbonate weathering, gypsum dissolution, and gypsum-induced dedolomitization (geological sources), which also affect Ca2+, Mg2+, and SO42- in deep confined groundwater draining a gypsum stratum. For HCO3-, CO2 from respiration and microbiologic activities is one dominant contributor, especially for spring. The chemostatic behaviors of NO3-, Cl−, and K+ were related to agricultural activities, especially in surface water. These controls on hydrochemistry may already exist as hillslope runoff occurs, which has be further demonstrated by principle component analysis (PCA). The heterogeneous permeability of epikarst can affect the mixture of groundwater from different sources and flowing pathways, enabling hydrochemistry at different hydrogeological conditions to display discrepant responses to rainstorms. The epikarst aquifer with high permeability is susceptible to changes in external environment, such as rainstorms and agricultural activities, increasing the potential risk of water environment problems (chronic pollution of nitrogen and high hardness of water) during a certain period. Drinking water safety thus deserves consideration in the agricultural karst catchment.
Highlights
Karst landforms cover 2.2 × 107 km2 and are scattered around the world, especially in Southeast Asia, South America, and the Mediterranean coasts (Yuan and Cai, 1988; Ford and Williams, 2007)
Throughfall, hillslope runoff, surface water, spring, and well water at different hydrogeological conditions were collected synchronously during rainstorms to analyze the dynamic responses of hydrochemistry and its variation at different spatial levels at a karst Critical Zone Observatory (CZO), Southwest China
Chemical compositions in rainwater and throughfall could be affected by both anthropogenic emission (NOx, NH3, and SOx) and some weathering products (Ca2+ and Mg2+) released to ambient atmosphere with dust
Summary
Karst landforms cover 2.2 × 107 km and are scattered around the world, especially in Southeast Asia, South America, and the Mediterranean coasts (Yuan and Cai, 1988; Ford and Williams, 2007). Comprised of chemically soluble rocks with large passages or network of conduits and caves inside, karst aquifers are very permeable and can store and transport large amounts of water, supplying drinking and irrigation water to ∼25% of the world’s population (Ford and Williams, 2007). The special hydrogeological conditions (high permeable soil/rock systems with caves and fractures inside) of karst systems benefit water and solutes migration, making the CO2-H2OCaCO3 system sensitive to hydrological changes (Yuan and Zhang, 2008; Beaulieu et al, 2012), and making karst aquifers vulnerable to contamination (Kacaroglu, 1999). Researching karstic hydrochemical dynamics during rainfall storms is conducive to understanding the transfer of contaminants and solutes and their responses to climate change
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