How uncertain is our estimate of a wellhead protection zone?

Protection zones for wells are defined through the use of pathline tracing in ground water flow models. Traditional approaches to ground water now modeling focus on obtaining a single best model, occasionally supplemented by the use of sensitivity analyses, In most situations this approach is inappropriate because the ground water flow model is so poorly determined that a variety of different boundary conditions and parameter values would give similar predictions of heads, However, the range of feasible models may well give radically different predictions of the variable for which the model has been built. For example, the area, orientation, or many other descriptive variables of a catchment may be much more sensitive to the parameters used in a ground water now model than are the values of head commonly used to calibrate the model, This paper outlines a procedure to determine the range of predictions of catchments which would arise from alternative calibrations of a model, The range of catchments is used to identify zones of certainty and uncertainty, leading to alternative definitions of the protection zone for differing purposes, An example is presented, based on Bestwood Pumping Station, Nottinghamshire, which is located in the Triassic Sandstone Aquifer of the northern East Midlands of the United Kingdom, A trial and error calibration of a ground water flow model is used to determine an acceptable level for automatic calibration in the subsequent step. Only 12 measurements of ground water head were available, and these were used to determine the fit of a variety of model structures. A search routine revealed a range of feasible models and superposition of their catchments delineated the zones of certainty and uncertainty, which had areas of 7.4 and 15.2 km(2), respectively.