The aim of the investigations in Muttenz, Switzerland, was to explore the hydraulic influence of tectonic structures of the Tafeljura on the propagation of groundwater solutes, e.g. from contaminated sites or the dissolution of evaporites. For this purpose, oriented structural images from 12 boreholes were compared to hydraulically relevant measurements (flowmeter, packer and pump tests, temperature and electrical conductivity of the pore water) and estimations of the hydraulic conductivities.

Results show that bedding planes generally remain close to the horizontal (Figure). In contrast, partly of more largely open fractures frequently dip at an angle of almost 60° to the ESE and conjugate to the WNW and thus parallel to the tectonic horst and graben structures of the Tabular Jura which have been mostly formed in the Oligocene. Both, open bedding planes and open fractures, have an important influence for the groundwater flow field. The closed foliation surfaces observed in evaporites show a rather uniform dip to the SW or NE with an angle of mostly less than 45°. This suggests that this overprint caused by ductile deformation is related to the formation of the NNW-ESE to NW-SE striking anticline structure, which could be of Miocene age.

The mean values of hydraulic conductivity (K) determined by flowmeter and packer tests for the open fractures are consistently higher than for the open bedding planes. The highest K values in the 10^-3m/s range were measured on average in the fractures of the dolomitic, as well as in bedding planes and fractures of the calcitic part of the Schinznach Formation. In contrast, the values in the dolomite zone are half an order of magnitude lower in the flowmeter and packer tests. The mean values of K determined next to the open structures in the Upper Sulphate Zone are consistently approx. 2 orders of magnitude lower.

The analyzed data set forms a basis for the further development of groundwater models. Three approaches are proposed: (1) model approach with single continuum and homogeneous model anisotropy based on the dominant orientation of open layer joints and fractures; (2) model approach with dual continuum and an isotropic/anisotropic matrix and an additional model anisotropy based on discrete 2D elements for open layer joints and fractures; and (3) model approach with equivalent simple continuum and a model anisotropy based on a discrete network based on principal directions and density of beds and fractures. The chosen modeling approach could then be compared with existing field data (e.g. groundwater solutes, isotopic compositions or groundwater temperatures), which are sensitive to a distribution of the groundwater flow field.

Commissioned by: AUE BL

Project duration: 2022-2023

Eric Zechner

Figure: Top left: Poles of all structures, mean angle of dip and mean angle of azimuth. Top right: all partially open and open beds. Bottom from left to right: Closed fractures, partially open and open fractures and foliations in resulting Schmidt polar projections of the upper hemisphere with contouring of the polar density from white to blue to red.