The role of tributary mixing in chemical variations at a karst spring, Milandre, Switzerland

Abstract

Summary Solute concentration variations during flood events were investigated in a karst aquifer of the Swiss Jura. Observations were made at the spring, and at the three main subterraneous tributaries feeding the spring. A simple transient flow and transport numerical model was able to reproduce chemographs and hydrographs observed at the spring, as a result of a mixing of the concentration and discharge of the respective tributaries. Sensitivity analysis carried out with the model showed that it is possible to produce chemical variations at the spring even if all tributaries have constant (but different for each of them) solute concentrations. This process is called tributary mixing. The good match between observed and modelled curves indicate that, in the phreatic zone, tributary mixing is probably an important process that shapes spring chemographs. Chemical reactions and other mixing components (e.g. from low permeability volumes) have a limited influence.Dissolution-related (calcium, bicarbonate, specific conductance) and pollution-related parameters (nitrate, chloride, potassium) displayed slightly different behaviours: during moderate flood events, the former showed limited variations compared to the latter. During large flood events, both presented chemographs with significant changes. No significant event water participates in moderate flood events and tributary mixing will be the major process shaping chemographs. Variations are greater for parameters with higher spatial variability (e.g. pollution-related). Whereas for large flood events, the contribution of event water becomes significant and influences the chemographs of all the parameters. As a result, spring water vulnerability to an accidental pollution is low during moderate flood events and under base flow conditions. It strongly increases during large flood events, because event water contributes to the spring discharge.

Characterization of recharge and flow behaviour of different water sources in Gunung Kidul and its impact on water quality based on hydrochemical and physico-chemical monitoring

“…This so called piston flow (Ford and Williams 1989) is a typical feature of karst aquifers and was reported from several other studies (e.g. Hess and White 1988;Birk et al 2004;Genthon et al 2005;Massei et al 2007;Perrin et al 2007;Baena et al 2009). The physico-chemical properties change only after the actual recharge water consisting of ''new'' surface water arrives at the observation site in Gua Seropan during Period B.…”

“…Massei et al 2003;Fournier et al 2007;Barberá and Andreo 2012). Due to the rapid flow, the thermodynamic equilibrium with respect to calcite is not reached which explains the low EC during floods (Perrin et al 2007). The relatively small and distinct peaks are indicative of concentrated infiltration close to Seropan.…”

“…The base flow in Seropan in February 2012 is characterized by an EC of 506 lS/cm which is about equal to the EC during dry season (data not shown). Because EC in karst systems is mainly controlled by the calcium-carbonate equilibrium Perrin et al 2007), the constant and about 20 times higher EC compared to precipitation (20-30 lS/cm) indicates that base flow is continuously recharged by a water mass with sufficient long residence time in the limestone that allows equilibration with calcite (Hess and White 1988). This assumption is supported by a SI Cc of around 0 in Seropan River water during dry season 2009 and the weak rainy season in 2010 (Table 1).…”

Groundwater management of a highly dynamic karst by assessing baseflow and quickflow with a rainfall-discharge model (Dardennes springs, SE France)

“…For instance Padilla et al (1994) and Fu et al (2016) quantified the quickflow and baseflow of karst springs. Discharge variation versus chemical, electric conductivity, temperature or turbidity variation is also a classical tool to investigate aquifer behavior at the flood scale, using graphical analysis of chemographs or a hysteresis loop in XY plots (Bakalowicz, 1979;Chanat et al, 2002;Perrin et al, 2007;Valdes et al, 2006). Time series analyses have been successfully applied to karst aquifers to investigate the rainfall-discharge relationship at the global scale (e.g., Delbart et al, 2016;Denic-Jukic and Jukic, 2003;Mathevet et al, 2004;Padilla and Pulido-Bosch, 1995).…”

Radon and CO2 as natural tracers to investigate the recharge dynamics of karst aquifers

“…This approach made it possible to study a number of flood events of variable intensity. Rather than measuring at the spring where the hydrochemical response can be strongly affected by mixing of water from different subcatchments (Perrin et al, 2007), the measurements were carried out directly in an underground stream draining a smaller subsection of the karst system. In addition to 222 Rn and CO 2 , the discharge of the underground river, electrical conductivity and temperature of water were recorded.…”