An investigation of sedimentary matrices in a polluted Irish river and estuary was undertaken to determine the distribution both spatially, and more importantly seasonally, of metal and organometal contamination on a monthly basis over 2 years. No previous investigations in Ireland have undertaken such a detailed analytical regime. The Tolka River and Estuary, Dublin, Ireland, is a typical Irish urban river and estuarine system, and it was chosen for this study as it has significant metal and organometal contaminant loading arising from the urban environment in general but also specific sources including a closed landfill. A 1-month sampling and analysis regime was devised to optimize riverine and estuarine analysis to determine seasonality of contaminant distribution. Between August 2003 and January 2005, contaminant concentrations within the surface sediment of the river and estuarine system were analyzed monthly from 20 sampling points. Six metals (cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn)) and an organometal (tributyltin (TBT)) were analyzed using differential pulse polarography. This assessment created an extensive database to determine spatial and seasonal distributions of contaminants over time and various in situ environmental and anthropogenic parameters that influence these distributions. It was found that contaminant concentrations within surface sediment are highly variable seasonally. Contaminant concentrations were observed to gradually increase (30–120 %) in spring to a maximum at the end of summer, which was followed by a decrease in winter months (30–60 %); e.g., at one estuarine site in February 2004, lead concentrations of 148.5 ppm were recorded, followed by a gradual increase to 312.1 ppm in July and a subsequent decrease to 131.1 ppm in January 2005. This paper presents the final 12 months of contaminant analysis as this period provided the clearest illustration of contaminant spatial and seasonal distributions.