Geochemical evidence of water source characterization and hydrodynamic responses in a karst aquifer

Abstract

► Water types identification and their relation with hydrodynamics and chemistry. ► Groundwater mineralization: water sources, flow-paths and water–rock interactions. ► Study of anthropogenic sources of contamination by the use of bacteria monitoring. ► Proposition of a conceptual circulation model for the aquifer functioning. The Lez karst spring, the main perennial outlet of the Lez karst system in southern France, plays an important role in supplying drinking water to the Montpellier metropolitan region. In order to investigate the origin of groundwater, its circulation patterns, and to understand the connectivity and compartmentalization of a karst system, a multi-tracer approach was used to describe the hydrogeology of the Lez karst system. Groundwater samples were collected from Lez karst during a range of hydrologic conditions (between March 2006 and August 2009) and analyzed for major and trace elements, total organic carbon, fecal, and total coliform. During the first recharge event of autumn, highly-mineralized water was observed at Lez Spring during the studied years. Multiple parameters of water during this rise were monitored with a fine time-step in 2008. Discriminate Factorial Analyses revealed the existence of different water-types discharging at Lez Spring. During high stage periods, highly mineralized water initially discharges from the spring, followed by rapid infiltration water. This behavior suggests that hydrodynamics affect groundwater circulation by soliciting different endmembers. These characteristics were observed on a larger scale when monitoring three intermittent springs connected to Lez Spring. A detailed analysis using bivariate diagrams of major, trace elements and elemental ratios provided insight into different water origins, associated lithologies, and mineral-solution reactions related to hydrodynamic responses. From the five identified water-types, the two more contrasting ones are emphasized: the first one corresponds to the most geochemically evolved waters, issued from deep layers where evaporite chemical fingerprinting has been identified. They are characterized by high mineralization and high concentrations in Cl, Na, Mg, Li, B and Br elements, and high Sr/Ca, Mg/Ca and Cl/Br ratios. The second water-type corresponds to the most diluted (low mineralized) waters with high concentrations of NO 3 , bacteria, Total Organic Carbon (TOC), and represents the flux of rapid infiltration waters. This endmember underlines the vulnerability of the system to surface infiltration and anthropogenic contamination through the infiltration of water via sinkholes and well-developed fracture networks.