2009 Darcy lecturer
Peter Cook
The 2009 Henry Darcy Distinguished Lecturer Peter Cook, Ph.D., is a senior principal research scientist with CSIRO Land and Water. He received a B.A. in geography from Australian National University in 1986 and a Ph.D. in Earth sciences from Flinders University of South Australia in 1992. Between 1992 and 1994, Cook carried out postdoctoral research at the U.S. Department of Energy and University of Waterloo, Canada, before returning to Australia. Cook’s research interests span the fields of ground water hydrology, ecohydrology, isotope hydrology, and unsaturated zone flow, but have mostly focused on the use of environmental tracers, including the integration of tracer and hydraulic methods. Specific research projects have involved estimation of aquifer recharge, quantification of ground water discharge to streams and wetlands, prediction of stream and ground water salinisation rates, and assessment of ground water-dependent ecosystems. He has cowritten books on environmental tracers and ecohydrology.
"Environmental Tracers in Modern Hydrogeology: Reducing Uncertainty in Ground Water Flow Estimation" is the title of Cook's Darcy Lecture. 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 Australia, 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.
There are a large number of environmental tracers, all with different properties and hence different potential uses. While environmental tracers that readily undergo chemical reactions can sometimes be used to determine reaction pathways, tracers that behave more conservatively may yield information on transport processes. Calculation of ground water residence times is one of the more common applications. Tracers that can be used for this purpose include radioactive isotopes, which decay at a known rate (e.g., 14C, 3H), tracers that are produced and accumulate in the subsurface (e.g., He), and tracers that are neither produced nor consumed in the subsurface, but have a variable and well-known input history (e.g., CFCs, SF6). Ground water residence times in unconfined aquifers can be used to infer aquifer recharge rates, whereas in confined aquifers they allow quantification of horizontal flow velocities. Tracers present in much higher concentrations in ground water than in surface water have great potential for quantifying ground water discharge to surface water. In particular, dissolved gas tracers such as radon and helium will rapidly volatilise from surface water and so provide important tracers of recent ground water inflow. Radon (with a half-life of 3.8 days), in particular, can be used in quantifying rates of ground water discharge to streams, wetlands, and to the ocean, and also to determine the rate of water exchange between a river and its underlying hyporheic zone.\
Submit your request to have the 2009 Darcy Lecture presented at your event. Requests must be received by October 15, 2008.
Please note that Cook will be available to present his lecture in the United States, Canada, and Mexico during the months of March, April, May, June, September, and October 2009; for locations outside of those three countries, he will be available during the months of January, February, July, August, November, and December 2009.