A CAUTION ON THE REGULATORY USE OF NUMERICAL SOLUTE TRANSPORT MODELS

Numerical models of solute transport in water-saturated porous media a re routinely used to make regulatory and design decisions. For many co ntaminants, decisions are based on concentrations of 5 parts per billi on (ppb) or less. This ''action level' is usually a small fraction (<0 .01) of the concentrations near a source of contamination. Two one-dim ensional example problems are used to demonstrate that modeling errors using classical numerical methods are largest where concentrations ar e lowest. This implies that in regions of low dimensionless concentrat ion, that is near the action level, large relative errors can be expec ted. Hence, decisions based on numerical model solutions at low concen trations must be taken cautiously. Modern numerical methods for the so lution of transport equations provide better behavior, in this sense, than classical methods.