Parameter Uncertainty for Capture Zone Delineation in a Complex Hydrogeologic Environment
Capture zones are typically delineated by applying backward (advective) particle tracking to a calibrated model and projecting the encompassed footprint to ground surface. It is recognized that considerable uncertainty exists in the size and shape of these capture zones. To compensate, professional judgment is typically applied to provide a degree of conservatism. This is done to account for features that cannot be explicitly represented in a numeric model such as the degree of heterogeneity and variability of the natural system.
In an effort to address this uncertainty, a backward-in-time advective-dispersive transport approach has been applied as an alternative approach to delineating capture zones. This technique has the advantage of representing local scale heterogeneity through the inclusion of the dispersion term. The resulting capture zones can be delineated within the context of a probabilistic framework.
This paper presents an alternative approach to developing capture zones that encapsulates the uncertainty of the hydraulic conductivity distribution. One hundred statistically equivalent models are generated by randomly sampling log-normal hydraulic conductivity distributions that are centered about model calibrated values. Each model explicitly simulates different flow paths, thereby sampling the variability of the flow system. A composite capture zone is created by combining all these capture zones. The composite capture zone can be likened to a spatial probability distribution of groundwater contribution to a given well. A comparison of the capture zones created using these two approaches is presented.