Is in situ stress important to groundwater flow in shallow fractured rock aquifers?

SummaryIn situ stress affects the permeability tensor of fractured rock masses at depth but its effect on shallow to near-surface fractured rock aquifers has received little attention. This is partly because stress-related effects on groundwater flow at shallow depths are difficult to identify and characterise due to the complex interactions between all of the inherent properties of a fractured rock aquifer. These properties include the factors that dominantly control groundwater flow: fracture network density, geometry, connectivity and infill. Furthermore, surface processes such as weathering, erosion and unloading alter the original hydraulic nature (connectivity, transmissivity) of fractured rock masses resulting in higher degrees of spatial heterogeneity within shallow flow systems. These processes and interactions often mask the influence of in situ stress fields on fracture network permeability and groundwater flow. In this study, an integrated analysis of local area fracture networks, borehole geophysical logs, borehole groundwater yields and hydromechanical models demonstrate that in situ stress does affect groundwater flow in shallow (<200 m) fractured rock aquifers by altering fracture hydraulic aperture distributions, fracture network connectivity and groundwater flow rates via fracture deformation processes. In particular, a comparison between representative models of deformed (stressed state) and undeformed (zero stress state) fracture networks showed that below 100 m depth, groundwater flow rates could decrease several fold under the influence of the contemporary stress field. This prediction was highly consistent with the field observations. In contrast, groundwater flow modelling of shallow fractured rock aquifers is typically conducted under the assumption that permeability is independent of the state of stress. A key finding of this study is that in situ stress may be a more important control on both local and regional scale shallow groundwater flow systems than previously recognised. The methodology applied in this study also offers an alternative approach to investigating groundwater flow in fractured rock masses where field hydrogeological data are limited.

Research highlights
► Stochastic HM modelling is a useful alternative technique for areas with limited field data.
► Fracture deformation processes are depth dependent and result in anisotropic permeability.
► Surface measurements do not address sub-surface fracture density and deformation variations.
► Near surface in situ stress does have a discernible effect on groundwater flow patterns.
► In situ stress may play an important role with shallow groundwater flow systems.