Hydrochemical variations during flood pulses in the south‐west China peak cluster karst: impacts of CaCO₃–H₂O–CO₂interactions

Abstract

Abstract:High-resolution measurements of rainfall, water level, pH, conductivity, temperature and carbonate chemistry parameters of groundwater at two adjacent locations within the peak cluster karst of the Guilin Karst Experimental Site in Guangxi Province, China, were made with different types of multiparameter sonde. The data were stored using data loggers recording with 2 min or 15 min resolution. Waters from a large, perennial spring represent the exit for the aquifer's conduit flow, and a nearby well measures water in the conduit-adjacent, fractured media. During flood pulses, the pH of the conduit flow water rises as the conductivity falls. In contrast, and at the same time, the pH of groundwater in the fractures drops, as conductivity rises. As Ca 2C and HCO 3 were the dominant (>90%) ions, we developed linear relationships (both r 2 > 0Ð91) between conductivity and those ions, respectively, and in turn calculated variations in the calcite saturation index SI C and CO 2 partial pressure P CO 2 of water during flood pulses. Results indicate that the P CO 2 of fracture water during flood periods is higher than that at lower flows, and its SI C is lower. Simultaneously, P CO 2 of conduit water during the flood period is lower than that at lower flows, and its SI C also is lower. From these results we conclude that at least two key processes are controlling hydrochemical variations during flood periods: (i) dilution by precipitation and (ii) water-rock-gas interactions. To explain hydrochemical variations in the fracture water, the water-rock-gas interactions may be more important. For example, during flood periods, soil gas with high CO 2 concentrations dissolves in water and enters the fracture system, the water, which in turn has become more highly undersaturated, dissolves more limestone, and the conductivity increases. Dilution of rainfall is more important in controlling hydrochemical variations of conduit water, because rainfall with higher pH (in this area apparently owing to interaction with limestone dust in the lower atmosphere) and low conductivity travels through the conduit system rapidly. These results illustrate that to understand the hydrochemical variations in karst systems, considering only water-rock interactions is not sufficient, and the variable effects of CO 2 on the system should be evaluated. Consideration of water-rock-gas interactions is thus a must in understanding variations in karst hydrochemistry.

Response of dissolved inorganic carbon (DIC) and δ13CDIC to changes in climate and land cover in SW China karst catchments

“…In general, lower pH, EC, HCO 3 À concentrations and d 13 C DIC result from the high rainfall and water temperatures of the rainy season, and vice versa during the dry season. During the rainy season, fresh inflows of precipitation at low pH dilutes the karst water and dissolves further soil CO 2 to it (Liu et al, 2004(Liu et al, , 2007. The higher temperatures in the rainy season lead to increased biomass productivity, strong root respiration and organic decomposition, providing more soil CO 2 with low d 13 C (Atkin et al, 2000).…”

“…CO 2 partial pressure (pCO 2 ) and the calcite saturation index (SI c ) are calculated from a geochemical model with pH, temperature and concentrations of seven major ions (Liu et al, 2004(Liu et al, , 2007Zhao et al, 2010). Since the host rocks in the regions are limestone and dolomite, intercalated with gypsum strata, Ca 2+ , Mg 2+ , HCO 3 À and SO 4 2À are the major ions.…”

“…The processes not only vary seasonally but also control carbon input and output in the aquatic system, creating variations in DIC and d 13 C DIC . Although investigations of DIC concentration and d 13 C DIC in freshwater environments have been carried out recently by many researchers (Han et al, 2010;Zhao et al, 2010;Kanduč et al, 2012;Tsypin and Macpherson, 2012), studies on variations of inorganic carbon systems in the karst catchments on multiple time scales (e.g., the annual, seasonal and storm-event scale) are still very limited (Hélie et al, 2002;Liu et al, 2004Liu et al, , 2007Guo et al, 2008;Li et al, 2010;Zhao et al, 2010). Thus, our understanding of the DIC and d 13 C DIC variations and their controlling factors at seasonal to long term scales in karst aquatic systems needs further investigation, especially under different land covers in different karst areas due to changes in biological activities, nutrient loads and the processes of mutual interaction that may occur (Harris, 1999).…”

Seasonal, diurnal and storm-scale hydrochemical variations of typical epikarst springs in subtropical karst areas of SW China: Soil CO2 and dilution effects

“…(1) Determining partial pressure of CO 2 and calcite saturation within these waters (Neal et al, 2002;Liu et al, 2004); (2) Assessing the balance between photosynthesis and respiration by biota and CO 2 transfers at the airwater interface (Maberly, 1996); (3) Assessing the biological influences on CO 2 fluctuations in the water with respect to the precipitation/dissolution of calcite (Guasch et al, 1998;Liu et al, 2006); (4) Understanding geochemical processes in karst aquifers and characterizing carbonate aquifers (Shuster and White, 1971;Andreo et al, 2002); (5) Assessing karst-related carbon sink from atmosphere (Liu and Zhao, 2000).…”

Natural variations in calcite dissolution rates in streams: Controls, implications, and open questions

“…However, the observed low variability in time also suggests that the typical approach of constant input chemistry is a reasonable approximation in many cases. Though a few site-specific studies have examined controls on carbonate system dynamics using time series data [e.g., Liu et al, 2004;Groves and Meiman, 2005;Pu et al, 2014], more work is needed to fully understand what determines whether dilution or CO 2 is important in a given setting or under given conditions. Prior work has shown that dilution is an important control on dissolution where runoff from noncarbonate rocks, which is undersaturated, mixes with or displaces groundwater from carbonate aquifers, which is saturated [Gulley et al, 2013[Gulley et al, , 2014.…”