A curtained stormwater storage and treatment facility was constructed in Lake Storsjön (Sweden) to treat urban stormwater runoff discharges. Over two and a half years, sediments were sampled four times across four facility zones (from inlet to outlet) in two depths to inform facility operation and polluted sediment management. Sediments were analysed for particle size distribution, metals, polycyclic aromatic hydrocarbons, and organic matter content to assess the spatial distribution of deposited sediment characteristics. Finer sediments (<0.016 mm) prevailed near the outlet, with a higher presence of fine particles in the deeper layer (10–20 cm). Substantial variations in chemical concentrations were observed, differing by up to three orders of magnitude. Higher concentrations were consistently found in the inlet zone and deeper layers (10–20 cm depth) throughout the sampling program. Combined horizontal and vertical distributions of sediment characteristics indicated a predominance of historical deposits in the sediments collected. Frequent exceedances of Predicted No-Effect Concentrations of chemicals, particularly in the inlet zone, highlighted potential risks to the aquatic environment that would result from dredging activities. This underscores the importance of considering broad mitigation strategies, including the capping of contaminated sediment, to control the environmental impacts of contaminated sediments on the lake ecosystem.