The objective of this study was to develop quantitative stochastic methods to assess effects of land-disposed E&P waste on groundwater quality. A computer model was developed to assess the probability of groundwater contamination from salts and oily waste constituents disposed by landspreading or in waste pits. The model considers leaching of chemicals from the waste zone over time until the constituent mass is depleted. It considers volatilization of organics for landspreading. Water flow is controlled by the net infiltration rate (rainfall minus evaporation and surface runoff), except for lined pits, in which flow may be limited by a low permeability layer. The program models one-dimensional convective-dispersive transport of dissolved constituents through the unsaturated zone and three-dimensional transport in groundwater zone. Organic constituents are subject to adsorption and biodecay. A Monte Carlo method is used to determine the probability distribution of maximum constituent concentration at receptor locations, given probability distributions of hydrogeologic and waste properties.