Hydraulic heterogeneity and its impact on kinematic porosity in Swedish coastal terrains

Abstract

Hydrogeology in crystalline rock aquifers is often problematic due to the heterogeneity and anisotropy in the fracture network. Kinematic porosity of the host rock is exceedingly important for municipal decision makers in assessing sustainably extractable water supply volumes and assessing contaminant transport behavior within the matrix. This study explores heteroscedasticity in the hydrogeological characteristics of the fracture network and estimation of kinematic porosity from superficial fracture measurements. Estimates were based on the geometrical properties of the fractures including: fracture frequency, aperture and orientation. The estimates were adjusted for aperture changes with depth, connectivity of the fracture network, fracture continuity and measurement orientation bias. The results were compared with well archive data and correlations were found to be significant with more than 95% confidence. Erratic behaviour of well data relative to fracture measurements indicates that well orientation with respect to the fracture network gives incomplete hydrogeological data. Spatial heterogeneity of the bedrock was examined using spatial statistics and geographic information systems. The results from the spatial statistical analyses of well data showed that the heterogeneity within the bedrock is sufficiently high that spatial correlations cease to exist in nearly all investigated rock types at distances greater than 500 m, and in some rocks, particularly sedimentary gneisses, no spatial correlations were observed. Arbitrarily grouped samples with similar geology and topography showed evidence of non-stationary variance. Results indicate that regional generalizations based on sparse point measurements are highly error prone and potential exists in complementary field-based estimates.