mass transport: 2. Analysis of uncertainty in prediction

The first paper in this series presented a description of a stochastic modeling concept for mass transport. In this paper we extend that analysis to consider a more realistic set of transport conditions in a groundwater basin with geologic layering, hydraulic anisotropy, spatial variations in porosity, and geochemical retardation. Uncertainties in transport predictions can be characterized by frequency distributions formed on the time of arrival of mass at the water table, on the exit location, and on the quantity of mass arriving at the water table as a function of time. Results show that transport is highly sensitive to porous medium heterogeneities. Considerable uncertainty is possible in predicting the spatial and temporal distribution of mass. Porous medium parameters capable of changing both the magnitude and direction of advective transport are of primary importance in influencing predictive ability. Most important in this respect are the arrangement of units with different mean hydraulic conductivities, the standard deviation and spatial continuity of hydraulic conductivity, and the hydraulic anisotropy. In certain geologic systems, patterns of contaminant migration can be predicted with more certainty. Features such as layering and hydraulic anisotropism can constrain the flow of mass to specific directions, thus limiting the size of the region through which mass is likely to spread. Depending upon the relative time scales of the release function and the transport process, lack of information about the timing and concentrations added at the source may cause greater uncertainty than the heterogeneity.