A Postscript on Groundwater Modelling : Daubert, Good Grounds, and the Central Role of Cross-Examination

Groundwater modelling is used to establish liability in complex environmental litigation, where subsurface contamination is at issue. As in all complicated legal actions experts are called upon to establish the validity and the accuracy of a groundwater model. The end result of most, if not all, modeling studies - whether groundwater, weather, air transport or statistical - is usually a predictive simulation. However, the modeller must be prepared to defend the procedures and judgments used in all steps of the modeling process, particularly the validity and the calibration of the model. In Daubert v. Merrell Dow Pharmaceuticals, Inc., 113 S.Ct. 2786 (1993) the Supreme Court noted that scientists typically distinguish between 'validity' (does the principle support what it purports to show? And 'reliability' (does the application of the principle produce consistent results) . . . Although 'the difference between accuracy, validity, and reliability may be such that each is distinct from the other by no more than a hen's kick,' . . . our reference here is to evidentiary reliability-that is, trustworthiness. Thus, all that a trial lawyer has to do, pursuant to Fed. R. Evid. 702, is to employ the oldest and most venerable tradition available to the litigator, the art of cross-examining the expert. Since the appropriate validation of the model by an expert's testimony meets the good grounds or general acceptance rule the inquiry requires only what is known and can be validated. This inquiry does not mean that the model is perfect - it can not be as natural phenomena have more variables then most models can account for - it only requires an approximation that can be supported