Chemical Hydrogeology: Importance for Aquitard Science and Implications
Dr. John Cherry, University of Waterloo Distinguished Professor Emeritus, Adjunct Professor, University of Guelph and founder of the Adrian Lecture Series.
The lecture will be followed by a reception in the EIT foyer. Please join us in congratulating Dr. Cherry on winning the 2016 Lee Kuan Yew Water Prize.
As distinct from ‘physical hydrogeology’ and ‘contaminant hydrogeology’, the science of chemical hydrogeology also has puzzles to solve with practical relevance. Chemical hydrogeology concerns the use of natural dissolved constituents, including isotopes, in groundwater to better understand flow systems and vice versa. This talk describes a few such puzzles that I have worked on with students and colleagues beginning in 1967. The first one concerns salt in the glaciated Prairie Region of Canada where natural salt occurs nearly everywhere resulting in too little fresh groundwater and extensive and expanding saline soil damaging agriculture. The origin of the salt, which has sulfate as the major anion (minimal chloride) was the subject of speculation beginning in the 1950’s with a two hypotheses in play. One attributed the sulfate ions in the Quaternary deposits to glacial squeezing and the other to sulfate minerals distributed in the deposits by glacial erosion of bedrock. I championed one of the glacial squeezing hypothesis in an early paper (Cherry 1972) but this turned out to be wrong as shown by a third hypothesis established by a 1984 UW PhD thesis by M.J. Hendry (WRR 1985) showing that ‘trace amounts’ (below detection by conventional analyses) of solid phase organic carbon and pyrite common in the glacial deposits when exposed to oxygen in the partially saturated vadose zone produce sulphate and carbonate, and then precipitation of gypsum from carbonate mineral dissolution. Since deglaciation, the sulphate leached from the vadose zone in recharge areas and then transported by groundwater has been accumulating and evapo-concentrating in groundwater discharge areas. However the degree to which salinized soil expands or shrinks depends on how we alter the land drainage by agricultural practices guided by government policies. The policies often turn out to be wrong because they are counter to the way the chemical hydrological system works. It is instructive to examine how the third hypothesis originated and why its discovery took so long. We noticed in 1976, concerning another puzzle, that the shapes of sulphate profiles with depth in low permeability near-surface, clayey glacial lacustrine deposits (15 m thick) in Winnipeg were unexpected, so we added profiles of oxygen-18, deuterium and tritium determined in the environmental isotope lab newly established by Professor Peter Fritz (UW). This resulted in discovery of relic glacial water in these deposits and recognition, based on a paper by Foster (1975) about tritium diffusion in chalk in the UK, that molecular diffusion can govern solute distributions in aquitards where fractures are small or absent. This was followed up by exploration in several thick (25-35 m) clayey Quaternary deposits in Ontario, Quebec, Manitoba, Saskatchewan, Wisconsin and North Dakota (UW PhD theses (eg Desaulniers 1984, Remenda 1989, Hussain 2002)) that the presence of paleowater with diffusion control on solutes/isotopes is the rule rather than the exception, and that chemical hydrogeology is the key to understanding aquitard hydrology/integrity. It is increasingly recognized that the natural water quality of many aquifers is governed by the hydrochemistry of their associated aquitards. Recent examples are provided by the aquifer/aquitard systems in the Pearl River Delta and the Shanghai area, China. By the early 1980’s it was recognized that thick diffusion-controlled Quaternary aquitards are most suitable for entombment below the water table of low-level radioactive waste and some types of solid hazardous industrial wastes. However, it took another two decades for recognition that deep Paleozoic rock strata such as shale that show diffusion control and essentially no water flow offer best prospects for deep geologic repositories for high-level, radioactive waste. These strata are now what nearly all countries with reliance on nuclear power are seeking for their repositories after finding that their initial selection of granite decades ago was flawed due to presence of fractures not anticipated. This delay, caused by gaps between science and policy, has contributed substantially to the strong public resistance to nuclear power though this power source does not contribute greenhouse gases.
John A. Cherry holds geological engineering degrees from the University of Saskatchewan and University of California Berkley, and earned a PhD in hydrogeology from the University of Illinois. He joined the faculty at the University of Waterloo in 1971 for field research on the migration and fate of contaminants in groundwater and their remediation. He retired from Waterloo in 2006, but he continues research as a Distinguished Professor Emeritus. He co-authored the textbook “Groundwater” with R.A. Freeze (1979) and co-edited and co-authored several chapters in the book “Dense Chlorinated Solvents and Other DNAPLs in Groundwater” (1996). He has participated in the development of technologies for groundwater monitoring and remediation, co-holds several patents, is a Fellow of the Royal Society of Canada, and has received awards from scientific and engineering societies in Canada, the United States, and the United Kingdom. He held the Research Chair in Contaminant Hydrogeology at the University of Waterloo from 1996 to 2006 and is currently the Director of the University Consortium for Field-Focused Groundwater Contamination Research, Associate Director of G360 Centre for Applied Groundwater G360 – The Centre for Groundwater Research and Adjunct Professor in the School of Engineering at the University of Guelph.
The Adrian Smith Lecture Series
The Adrian Smith Lecture Series on Environmental Geochemistry, an annual lecture in environmental geochemistry, was established in the Department of Earth and Environmental Sciences at the University of Waterloo by colleagues and friends in memory of Dr. Adrian Smith.
For more than a decade Adrian Smith was a regular visitor to our department, initially as guest speaker and later as an adjunct faculty member, a colleague and a supervisor to our students. Adrian generously gave his time, participating in technical discussions of thesis projects, providing comments and advice for improvements and fostering philosophical debates. These efforts are reflected in new approaches and attitudes that our students take toward research and science.
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