A Mixed-Order Model to Assess Contaminant Declines

We introduce a generalized form of the common first-order (exponential) decay model, that has potential utility for describing contaminant declines in environmental applications, particularly when declines are a mixture of many underlying processes. The exponent on contaminant concentration is left as a free parameter allowing the order of the reaction to be determined by the data. The mixed-order model is more flexible than models with the exponent determined a priori, facilitating an improved fit to observed behavior. We demonstrate the utility of this model, and compare it to two other models, by estimating PCB concentration declines in Lake Michigan fishes.