On the Need for Uncertainty Assessment in TMDL Modeling and Implementation

Will TMDL decisions be improved with knowledge of the uncertainty in outcomes from proposed pollutant load reductions? That is, will our decisions generally be better if we have some idea of the range of possible outcomes that might result? I believe that the answer is yes; and yet current practice in water quality assessment and management suggests that others may believe that decision making may be undermined with full disclosure of uncertainties, or perhaps believe that uncertainty is small enough that it can be safely ignored. Despite these reservations, it is noteworthy that the U.S. EPA also believes the answer is ‘‘yes,’’ although their reasoning is unclear. EPA’s perspective is implicit in their technical requirement for an uncertainty-based ‘‘margin of safety’’ ~MOS! in a TMDL application; however, absent from EPA guidance is an explanation as to why decisions improve with an uncertainty analysis. Despite the requirement for an uncertainty-based MOS estimate, few TMDLs are accompanied by actual estimates of forecast uncertainty. Instead, TMDLs are typically proposed with either ‘‘conservative’’ modeling assumptions or an arbitrarily chosen MOS. Neither approach explicitly links the MOS to TMDL forecast uncertainty. However, by hedging the TMDL decision in the direction of environmental protection, the MOS effectively increases the assurance that water quality standards will be achieved. This may seem reasonable and even desirable, but it must be noted that this hedging comes at a cost, and the basis for the hedging cost is totally arbitrary in most cases.