Enhancing hydrological analysis by incorporating environmental and artificial tracers of an altered vadose zone : a systematic review

Abstract

This review investigates the integration of environmental and artificial tracers for evaluating water flow within modified vadose zone environments and assesses how heterogeneous structures and preferential flow influence transport mechanisms, residence times, and flow pathways. Utilising both single and multiple tracers the precision of flux estimations between the vadose zone and water table can be enhanced. Altered vadose zones substantially influence tracer storage and release based on shifting moisture levels, highlighting the hydrological importance of these altered zones. The review underscores the importance of high-frequency measurements for diverse hydrological systems and understanding contaminant transport processes, with vadose zone thickness influencing residence times and hydrological behaviour and enhancing groundwater protection. Insights into the study of karst aquifers using fluorescent dyes and natural tracers shed light on rapid flow dynamics, and how to improve modelling techniques to capture these complexities. This further highlights the need to address safety and regulatory considerations related to tracer use, particularly toxicity and ecotoxicity effects, which are critical when borehole water is utilised for domestic purposes. In this regard, the development of a standardised regulatory framework in South Africa, given the absence of specific tracer test guidelines, drawing from international examples should be established. Numerical models for vadose zone flow and contaminant transport tackle challenges like non-equilibrium processes, complex geometries, and heterogeneity. Methods like Picard and Newton iterations can therefore enhance model accuracy, vital for sustainable water resource management and understanding modified vadose zone processes.