COMMUNICATING THE RESULTS OF MODELED PREDICTIONS OF CONTAMINANT TRANSPORT IN FRACTURED ROCK VADOSE ZONES

On an annual basis the U.S. Department of Energy (DOE) spends approximately $6 billion to address problems involving contamination of soils and groundwater and about half of this total involves vadose zone contamination. The ability to predict the transport of radionuclides, chlorinated hydrocarbons, and other pollutants has become one of the most important challenges facing the DOE. Moreover, the inability of DOE and its contractors to adequately communicate the results of these predictive models to stakeholders have lead to several situations where the integrity of the models was challenged. The conventional approach to simulating subsurface contaminant transport takes a volume average of the subsurface physical and chemical properties and assumes that these parameters vary in a deterministic manner and obey physical and chemical laws. However, at the field scale, the properties affecting transport are subject to great uncertainty owing to their inherent heterogeneity and to a general paucity of measurements. This uncertainty has lead to numerous surprising "detections" of contaminants. These were unexpected based upon state of the art predictive simulations of transport through the vadose zone and millions of dollars spent on site characterization. Unanticipated detections through vadose zone transport have brought DOE's credibility into question by stakeholders. This paper proposes that recognition of fractured rock vadose zones, as complex systems would greatly ease the burden of generating "perfect" answers in transport models. In the vadose zone, the nonlinear feedback between several parameters (e.g. moisture content, hydraulic conductivity, spatial variability, hysteresis, etc.) generates complexity. It is known from research in biology, economics and other disciplines, that complex systems go through processes of change that are not describable by a single rule, nor are reducible to only one level of explanation, often features occur whose emergence cannot be predicted from their current specifications. It may be possible to explain uncertainty in predictive simulations as being an inherent property of vadose zone transport.