Modeling and Monte Carlo simulation of TCDD transport in a river.

A one-dimensional water quality model to assess the long-term fate of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in three compartments (water, sediment, fish) of a river has been developed using the literature data on various model parameters. The transient deterministic model with constant or nonrandom parameters is solved numerically by the method of orthogonal collocation, while an analytical solution is developed for the steady-state model. The impact of uncertainty in several model parameters has been studied by means of Monte Carlo simulations assuming that the uncertain parameters are uncorrelated and can be modeled by three probability distributions (uniform, normal and lognormal). For the case of a high TCDD discharge into a small, shallow river, we find that the maximum TCDD contents of water and fish are well below the prescribed safe limits. We also find that the effects of uncertainty on water quality metrics are quite complex or nonintuitive and can be substantial. This is especially true for TCDD in fish, which can be higher by as much as 50-70% than the deterministic predictions, if the parameter uncertainties follow uniform distributions.