The Surface Impoundment Modeling System (SIMS) estimates volatile organic compound (VOC) emissions and toxic air pollutant emission from surface impoundments and collection system emponents individually or in series. It can be used to estimate emissions from wastewater sources at hazardous waste treatment works, storage and disposal facilities (TSDFs), publically owned treatment works (POTW's), and industrial wastewater treatment facilities. The minimum information required to use SIMS is the type of device to be modeled, the total flow to the device, the total surface area of the device, and the type of industry discharging wastewater to the device. Default values for water discharged from typical industries are supplied by the program. The user can adjust these default values to match their particular system. The SIMS is a personal computer based program designed to estimate the air emissions from surface impoundments. The emission estimates are based on mass transfer models developed by the Emissions Standards Division (ESD) of EPA during the evaluation of hazardous waste treatment, storage, and disposal facilities (TSDF's). SIMS allows the user to specify all the required inputs to these emission models when the information is available, or when only limited information is available, provides default values for most of the model inputs.