The report was prepared by a consortium of consultants with expertise in pit lake modelling, and directed by the End Pit Lake Modelling Task Group of the ASG. A peer review of the model and report were conducted and comments were incorporated in the report. The freely distributed CE QUAL W2 (W2) Version 3.6 forms the basis of the OSPLM. Ten processes were added to this default version of the model, housed by the Water Quality Research Group at Portland State University. These processes were identified as key drivers affecting pit lake water quality and include: Tailings consolidation over time, thereby releasing porewater into the overlying water column and increasing water depth; Gases (e.g., methane) produced in the tailings; Release of gases from tailings through bubbling and its travel through the water column; Oxygen consumption in the water column by released gases; and Re suspension of tailings and mixing with the water column. This report and model have been used to develop the 2012 Pit Lake Guide. Next steps include minor model refinement in 2013 and, after this is completed, public release of the model through the Water Quality Research Group at Portland State University.