Existing studies showed that conductive fault zones within carbonates and evaporites (gypsum/anhydrite, rock salt) may act as a bypass for both the access of subsaturated groundwater to vulnerable rock salt formations and the transport of the resulting brine into near-surface aquifers (Zidane et al., 2014). As a consequence, rock salt karst develops more likely near the conductive fault zones, which may lead to land subsidence above the fault zones. However, mapping and hydraulic characterization of subsurface fault zones is often not possible due to the lack of relevant data.
The presented multi-tracer study in the regional Middle Triassic carbonate aquifer of Northwestern Switzerland aims at delineating areas, where upwelling of saline waters suggests the presence of conductive subsurface fault zones. A total of 60 groundwater wells reaching depths between 10m and 270m were sampled for physico–chemical parameters, hydrochemistry (major ions and trace elements), and water isotopes (18-O, 2-H, 3-H, 13-C, 14-C, 34-S, 87-Sr/86-Sr). The multi-tracer data was structured with multivariate data analysis tools such as principal components analysis, and compared to simulated groundwater flow velocities from a 3D groundwater flow model from the aquifer system.
Data of 34-S composition together with sulphate concentration were used to distinguish between dissolution of gypsum/anhydrites aquitards lying either above, or below the Middle Triassic aquifer. Isotopic tracer information used for groundwater age determination found that large parts of the aquifer have been subjected to mixing between very recently infiltrated rainwater, infiltration from the nearby river Rhine, and existing older Holocene, or Pleistocene groundwater (>10’000 years). The mixing process is accelerated due to both large-scale groundwater pumping and artificial groundwater recharge, which significantly increase flow velocities in affected parts of the aquifer as confirmed by groundwater flow simulations. Also the observed distribution of sampled chloride concentrations ranging from a few 10s of mg/l to several g/l is influenced by the accelerated mixing process driven by artificial groundwater recharge and groundwater withdrawal. Mapping of conductive fault zones with groundwater sampling appears therefore more appropriate in areas with less anthropogenic influence to the groundwater flow field, which corresponds to sampled areas with low groundwater flow velocities.
Zidane, A., Zechner E., Huggenberger P., Younes, A. (2014): Simulation of rock salt dissolution and its impact on land subsidence, Hydrol. Earth Syst. Sci., 18, 2177–2189, doi:10.5194/hess-18-2177-2014.