Conceptual geological models, its importance in interpreting Vadose zone hydrology and the implications of being excluded

Abstract

Vadose zone conditions are becoming increasingly important in site investigation, including, for instance, (1) the protection of the phreatic zone from surficial contaminants (aquifer vulnerability), (2) surface water-groundwater interaction and biodiversity, (3) water influencing infrastructure development and (4) problem soil behaviour resulting in surface expressions of subsurface volume change. This multidisciplinary paradigm involves a wide range of specialists, but at the root of issues pertaining to the vadose zone is the geological regime in which it occurs. A conceptual model should include all fundamental branches of geology, viz. stratigraphy, mineralogy and petrology, structural geology and physical geology. The importance of a proper geological model is addressed at the hand of selected case studies in urban and peri-urban South Africa. In all instances, the sites were developed with subsequent issues arising due to inadequate vadose zone investigation. To evaluate, geological models were compiled based on available geological and geomorphological information. Physical properties such as detailed soil profiles and grading analyses were inferred to address vertical and spatial material variability. Mineralogy of the various soil horizons was used in conjunction with bulk dry densities to determine porosity and to address pedogenetic and eluviation processes. Hydrological data include percolation tests and Atterberg limits and were inferred onto the model to clarify anticipated hydrological behaviour. The additional geological data improve the understanding of the case studies incorporating ephemeral hillslope wetlands, constructed fill and water addition through leaking pipelines and irrigation, through the addition of knowledge overlooked by hydraulic testing exclusively and accentuate the importance of proper geological understanding prior to hydrological interpretation. The major issues arising, apart from damage to infrastructure and contamination of water resources, are excessive rehabilitation costs, decrease in aesthetic value and general discontent of land owners and proximate residents. The geological model is imperative and should not be excluded or overlooked due to increasingly popular alternative methods of investigation.