THE WORTH OF DATA IN PREDICTING AQUITARD CONTINUITY IN HYDROGEOLOGICAL DESIGN

A Bayesian decision framework is developed for addressing questions of hydrogeological data worth associated with engineering design at sites in heterogeneous geological environments. The specific case investigated is one of remedial contaminant containment in an aquifer underlain by an aquitard of uncertain continuity. The framework is used to evaluate the worth of hard and soft data in investigating the aquitard's continuity. The analysis consists of four modules: (1) an aquitard realization generator based on indicator kriging, (2) a procedure for the Bayesian updating of the uncertainty with respect to aquitard windows, (3) a Monte Carlo simulation model for advective contaminant transport, and (4) an economic decision model. A sensitivity analysis for a generic design example involving a design decision between a no-action alternative and a containment alternative indicates that the data worth of a single borehole providing a hard point datum was more sensitive to economic parameters than to hydrogeological or geostatistical parameters. For this case, data worth is very sensitive to the projected cost of containment, the discount rate, and the estimated cost of failure. When it comes to hydrogeological parameters, such as the representative hydraulic conductivity of the aquitard or underlying aquifer, the sensitivity analysis indicates that it is more important to know whether the field value is above or below some threshold value than it is to know its actual numerical value. A good conceptual understanding of the site geology is important in estimating prior uncertainties. The framework was applied in a retrospective fashion to the design of a remediation program for soil contaminated by radioactive waste disposal at the Savannah River site in South Carolina. The cost-effectiveness of different patterns of boreholes was studied. A contour map is presented for the net expected value of sample information (EVSI) for a single borehole. The net EVSI of patterns of precise point measurements is also compared to that of an imprecise seismic survey.