Transition probability analysis of lithology descriptions for probabilistic delineation of well capture zones.

Source protection area delineation has evolved over recent decades from fixed radius, analytical and numerical methods, which do not consider uncertainty, to more complex stochastic numerical approaches where uncertainties are often considered in a Monte Carlo framework. Representation of aquifer heterogeneity in these studies is typically based on geostatistical representation of hydraulic properties. In this paper we present a transition probability analysis of lithol ogical descriptions, sourced from regional council records of drillers' bore logs. Lithologies are categorised according to their inferred hydraulic flow and transport properties, using two to four hydrofacies groupings. The spatial connectivity of these hydrofacies is described by the probabilities of transitioning from one hydrofacies to another at discrete lags. This geostatistical representation of heterogeneity can often provide more realistic depictions of preferential flow paths, which are of great importance for the simulation of transport processes in alluvial gravel aquifers. We present the steps that are required for the geostatistical analysis, the assumptions that practitioners will need to make in similar applications, particularly around the categorisation of lithologies into hydrofacies, and the implications of these assumptions for practical applications. The outputs of this paper illustrate challenges and uncertainties associated with the characterisation of aquifer heterogeneity for contaminant transport simulations. [ABSTRACT FROM AUTHOR]