Using One and Two-Dimensional Models as Screening Tools: Are They Useful for Decision Making?

Hydrogeologists are often called upon to evaluate environmental settings prior to implementing site investigations.  This request is especially true when evaluating sustainability issues such as military range encroachment.  In years past, one would approach these problems through a variety of conservative calculations that could be used to show if conditions warranted further investigation.  With the advent of groundwater models, a logical progression is to use screening-level models that can account more fully for the environmental conditions, but be conducted with a limited amount of information.  The criticism of this approach from lay stakeholders and regulators is that the modeling may not be as conservative as expected and that Type II errors (underestimating arrival) may occur.  It is our contention that appropriately constructed screening-level models can be used to conservatively approximate field conditions.  Such screening-level models, if constructed appropriately and in conjunction with a robust conceptual site model, are more likely to result in Type I errors (overestimating arrival) and hence provide a factor of safety.  For this reason such models are a very useful tools, especially when multiple sites are under consideration and resources need to be applied the most onerous sites first.  This approach was successfully employed in a program with the US Marine Corp to evaluate operational military ranges where high explosives are used in munitions.  Screening-level models were used to evaluate multiple ranges to assure additional efforts were expended on the ranges that most needed attention.   Later investigations of ranges that were not screened confirmed the conservatism of the screening models and the usefulness of the approach.