Environmental Tracers in Modern Hydrogeology: Reducing Uncertainty in Ground Water Flow Estimation

Environmental tracers can reduce uncertainty of hydrogeological predictions in all environments, but are particularly valuable in highly heterogeneous systems, where spatial variations in aquifer hydraulic conductivity may range over several orders of magnitude, and so hydraulic approaches are inherently uncertain. Despite the rapid growth of environmental tracers during the past few decades and their adoption by the research community, they are not widely used in routine hydrogeological assessments. This lecture illustrates the potential of environmental tracers through illustration using field sites in North America and, and discusses methods for bridging the gap between research and practice.

Quantitative hydrogeology is often traced back to Darcy who, in the mid-19th century, observed a linear relationship between flow rate and hydraulic gradient, the proportionality constant later becoming known as hydraulic conductivity. Even today, ground water flow rates are most frequently determined as the product of measured hydraulic gradients and hydraulic conductivities, the latter determined using pumping tests. Although the last 150 years have seen considerable improvement in interpretation of pumping tests, and understanding of isotropy and heterogeneity, estimation of aquifer hydraulic conductivity values at appropriate scales remains a significant source of uncertainty. Within the past few decades, however, environmental tracer methods have been developed that can provide independent estimates of ground water flow rates, which have helped to overcome some of the problems associated with hydraulic approaches, particularly in heterogeneous systems. However, despite the ability of environmental tracers to constrain conceptual models of ground water systems and significantly reduce uncertainties in prediction, the methods are underrepresented in hydrogeological textbooks and are still not widely used for hydrogeological assessment.