The definition of contaminant source release is a necessary element of contaminant mass transport simulation in the saturated zone. At many sites the release history is unknown, and there has been a significant body of research to develop inverse models to define finite release histories. Instead of rigorously defining the source history, a mass balance-based approach is used to explicitly account for uncertainty in time-variable rectangular pulse release variables. The approach has been incorporated into a spreadsheet model which uses a one-dimensional solution to the advection dispersion equation. The spreadsheet model readily lends itself to Monte Carlo applications. Concentration datasets collected at a Superfund site were used for model calibration and verification. The model provided reasonable results when the source time parameters were assumed to be random, and the results improved when the mass of contaminant in the aquifer was also assumed to be random. However, the improvements came at the cost of degraded calibration parameter values.