TOXIWASP is a dynamic model for simulating the transport and fate of toxic chemicals in water bodies. Two state variables are simulated: organic chemical and total sediment. The generalized chemical model can be used for problems requiring dynamic transport and loading capabilities, simple mechanistic sediment predictions, and process-derived chemical predictions. TOXIWASP can be applied to stratified lakes and reservoirs, large rivers, estuaries, and coastal waters. Created for use in exposure assessments, the model allows the user to perform simple dynamic simulations of potentially toxic organic chemicals, such as pesticides. Dynamic simulations allow the consideration of variable loads, the prediction of peak events, and the estimation of time-varying chemical exposure.