The Air Land Water Analysis System (ALWAS) is a multi-media environmental model for describing the atmospheric dispersion of toxicants, the surface runoff of deposited toxicants, and the subsequent fate of these materials in surface water bodies. ALWAS dipicts the spatial and temporal distribution of contaminant concentrations in a watershed and the air above it. Linked in ALWAS are three submodels that provide for independent and partially coupled usage modes--Dispersion and Deposition of Toxics (DiDOT); a modification, called NPSDEP, of the Nonpoint Source Model; and Exposure Analysis Modeling System (EXAMS). ALWAS is appropriate for evaluating multi-media water quality problems for watersheds as large as 10,000 sq km. The full range of ALWAS capabilities may be exercised for hydrophobic organic chemicals of relatively low vapor pressure. By careful selection and linkage of the submodels, ALWAS may be applied to investigate the environmental behavior of a broad range of toxicants including highly soluble or volatile organics and heavy metals. The report incorporates detailed software documentation and a users manual. The sensitivity of model results to uncertainties in key input parameters is investigated for a hypothetical simulation of benzo(a)pyrene behavior in a small watershed.