Correlation between English Permo-Triassic sandstone lithofacies and permeability and its importance for groundwater

Predicting contaminant movement in aquifers is essential for the protection of groundwater resources. Contaminant movement is strongly influenced by hydraulic property heterogeneity. The aim of this project has been to determine if hydraulic property values are correlated with geological structures, even if the aquifer is a cemented sandstone: if they are, the correlations could be used in gaining insight into, and quantitatively estimating, solute transport. Example Permo-Triassic sandstone lithofacies associations across England were examined in order to determine common lithofacies geometries. Several thousand measurements of permeability and several hundred measurements of porosity were determined on a core from northern England and two outcrops from the Midlands in order to examine correlations with lithofacies. Two types of synthetic fluvial multiple channel systems with flow a long and across channels were then numerically simulated to determine the significance of the correlations found. Results indicate that the Permo-Triassic sandstones have a wide range of lithologies and permeabilities, varying between and within sequences and regions. Permeability is generally greater for aeolian sandstone than for fluvial sandstone. The borehole core stud y shows that lithofacies correlate with porosity, permeability, and permeability to porosity ratio, though some significant overlaps occur for permeabilities of the three coarser lithofacies. Mudclasts can reduce sandstone permeability of the two coarser lithofacies by up to 6 % and 8% respectively. Markov chain analysis indicates that the sequences of lithofacies are structured, and hence the permeability is also similarly structured. The two outcrop studies confirm that permeability structures vary more between than within beds. Solute transport modelling indicates that differing solute breakthrough patterns will occur with different types of hydraulic property distributions based on lithofacies geometry. Thus lithofacies correlations can be used to help predict solute movement, but transverse dispersion, not explicitly studied here, also appears particularly important. Generally, the heterogeneous porous medium can be reasonably represented by an equivalent homogeneous medium for prediction purposes, however determining appropriate properties for the equivalent homogeneous medium is challenging