Phosphorus is a contaminant that can enter a lake due to natural or human activity (effluent from a wastewater treatment plant, non-point runoff from farmlands, etc.). Once in the lake, it may enter the sediments at the bottom of the lake, where it is stored and becomes availableas a further source of contamination to the lake waters. In this study, wemodel the deposition of phosphorus in the solid phase to lake sediments, the subsequent conversion (diagenesis) to a liquid form, and the diffusion of the liquid phase phosphorus back into the lake water. We develop a system of twopartial differential equations involving two dependent variables (solid and liquid phosphorus concentrations) and two independent variables (time and depth in the sediment). We then show the numerical solution of the system and compare it with observed data. The ultimate goal is to be able to accurately predict future rates of release of phosphorus from the lake sediments if restrictions are placed on the level of human input to the system.