The Fellows Speak
Presiding: G Hornberger, University of Virginia; R Bras, MIT
H32A-01 10:35h
How to be Unique in Hydrology, Just Like Everyone Else
The hydrological sciences present a wide range of unresolved issues that are of both intrinsic research interest and of key societal importance. And yet, as in many fields of scientific research, we see too much "more of the same science"; in many respects, we might conclude that there are "too many people working on too few problems". The key to being "unique", therefore, lies in identifying the truly important and challenging questions, in finding original means to attack these questions, and in following through to provide answers and solutions. My research philosophy is to question the "conventional wisdom", and to integrate such questioning with cross-disciplinary understanding. Following this philosophy, I have focused my research in several directions, two of which are surveyed briefly. (1) Since the early 1950's, dispersive transport in natural porous media has been quantified by the Fickian-based advection-dispersion equation, and variants therof (including most "stochastic hydrology" approaches), notwithstanding clear and repeated evidence that transport is non-Fickian. In a break with tradition, we have adapted a powerful theory for non-Fickian transport, developed originally in the physics literature to describe electron hopping in amorphous silicon. This theory has proven consistently effective in describing chemical transport in geological media. (2) By convention, there has been little interaction between geochemists and hydrologists. As a consequence, hydrologists usually treat effects of geochemical reactions by inclusion of a generic (and often inadequately) simplified retardation coefficient in a transport equation. On the other hand, geochemists usually treat these reactions by consideration of batch experiments and equilibrium theory, neglecting the dynamic nature of flowing systems. We have bridged these approaches by measuring the effects of geochemical reactions in flowing-column experiments. Such experiments have allowed us to gain fundamental understanding of a series of geochemical-hydrological phenomena, including precipitation and dissolution in rock fractures and in porous media.
H32A-02 10:50h
The World's Biggest Water Filter: The Vadose Zone
Soil is the fragile yet productive skin of our planet. The soil of the vadose zone occupies a critical position between the atmosphere and the subterranean realm. The vadose zone lies astride the main thoroughfare along which water and solutes enter our ground and surface waters. The soil of the vadose zone is the world's biggest filter. Water and chemical fluxes though the heterogeneous porous medium of the unsaturated soil of the vadose zone, in the active presence of plants, are necessary for the healthy functioning of soil as the productive base for sustainable agriculture. As well, these fluxes are critical determinants of the quantity and quality of our underground and surface reserves of water. Our understanding of transport and fate processes in the vadose zone has been enhanced thanks to two developments. New measurement devices and remote monitoring techniques are providing better observations, at the local scale, of the state and fluxes of water and solute into, and through, the vadose zone. Here I outline some of the new observations that are improving the acuity of our vision of rootzone processes. In tandem, better theoretical understanding and new modelling techniques are being developed to extend and extrapolate these still-meager observations to realize improved understanding of vadose zone processes at larger spatial and temporal scales. Water is now known as "blue gold". "Blue gold" will be this century's most urgent environmental issue. We need to use our scientific understanding of the functioning world's largest water filter - the vadose zone - to protect the quantity and quality of our reserves of "blue gold".
http://www.hortresearch.co.nz
H32A-03 11:05h
Reflections on Dry-Zone Recharge
Quantifying recharge in regions of low precipitation remains a challenging task. The design of permanent nuclear-waste isolation at Yucca Mountain, Nevada, the design of arid-site landfill covers and the pumping of groundwater in desert cities, like Las Vegas, are examples where accurate recharge estimates are needed because they affect billion-dollar decisions. Recharge cannot be measured directly and must rely on estimation methods of various kinds including chemical tracers, thermal profiling, lysimetry, and water-balance modeling. Chemical methods, like chloride-mass-balance can significantly underestimate actual recharge rates and water-balance models are generally limited by large uncertainties. Studies at the U. S. Department of Energy's Hanford Site in Washington State, USA illustrate how estimates of recharge rates have changed over time and how these estimates can affect waste management decisions. Lysimetry has provided reliable estimates of recharge for a wide range of surface condittions. Lysimetric observations of reduced recharge, resulting from advective drying of coarse rock piles, suggest a way to avoid costly recharge protection using titanium shields at Yucca Mountain. The Pacific Northwest National Laboratory is funded by the U. S. Department of Energy under contract DE-AC05-76-RL01830.
H32A-04 11:20h
Vadose Zone Hydrology Across Scales: Modeling Versus Experimental Challenges
Vadose zone hydrology faces both experimental and modeling challenges, however, they occur in different spatial domains. Both experimental and modeling advances are naturally coupled through use of parameter optimization techniques that estimate scale-appropriate flow and transport parameters. The presentation will demonstrate that state-of-the-art experimental techniques are becoming increasingly available that apply to the pore scale, but that knowledge of the relevant mechanisms at the pore scale hinder modeling advances at that scale. In contrast, vadose zone hydrology at the landscape scale seems more advanced in the modeling aspects than experimentally. In fact, scale-appropriate measurements are increasingly required to validate multi-dimensional flow and transport at the landscape scale. Both numerical and experimental examples will be presented.
H32A-05 11:35h
From Laboratory to the Field: Intermediate Scale Testing, a Necessary Step.
Fundamental processes associated with water flow and transport and fate of chemicals, both dissolved and in separate phase, that occur at the microscopic pore scale will affect the large-scale evolution of chemical plumes in the subsurface. However, the ultimate transport and fate that define the spatial and time distribution of plume concentrations are significantly dependent on both the physical and chemical heterogeneity of subsurface formations. In attempting to understand, study and model the field behavior, the question always arises on how to transfer the pore-scale or the representative elementary volume scale observations and characterization data to the large field scales, incorporating the information on multi-dimensional flow fields and heterogeneity. In most cases, field systems are difficult to study due to their inherent complexity, inadequacy of characterization data, expense, and infeasibility in conducting controlled experiments. The author will make an argument that controlled experiments conducted in intermediate-scale laboratory test tanks even though difficult, is a necessary step in up-scaling the information from the laboratory to the field in studying a class of complex subsurface problems. Intermediate scale tank testing offers many advantages over complex and generally expensive field-testing. Testing conducted in two and there-dimensional test tanks in laboratory settings provides for better control, accurate characterization and higher precision achievable in data collection. The art and science of designing successful intermediate scale experiment require in-depth understanding of fundamentals, good engineering, understanding of capabilities and limitations of modeling tools, careful planning and patience. The author will share the knowledge gained and lessons learned from more than twenty years of experience in conducting such experiments involving water flow, solute transport, non-aqueous phase liquid behavior, site characterization and remediation. New advances made in sensing technologies, monitoring and data acquisition provide improved capabilities to design intermediate scale test systems. This allows for the study of many evolving and challenging problems related to the behavior and remediation of toxic chemicals in the subsurface.
http://cesep.mines.edu