Dagvattenkvalitet vid Umeå DAC : En analys av samband mellan metallhalter, flöde och vattenkemiska parametrar

Sammanfattning

The purpose of this study was to investigate the elevated metal concentrations in stormwater at Umeå DAC, and how concentrations in the stormwater well “BR5” varied over time in relation to variations in flow and water chemistry. The focus was primarily on cadmium, nickel and zinc, which exceeded environmental permit limits during specific periods, particularly in connection with snowmelt. Water samples were collected from five locations at Umeå DAC during March-April 2025 and analyzed for metal content, pH, alkalinity, conductivity and suspended solids. Flow measurements were conducted at two of the five sites. Historical data from 2021–2024 were also used to identify seasonal trends, and data from 2016-2019 on the leachability and total metal content of slag material were evaluated. Statistical correlations between metal concentrations and water chemistry parameters were assessed. The study revealed strong seasonal variation and a clear link between high flow events and increased metal loads during spring 2025. Metals such as cadmium, nickel and zinc showed strong intercorrelations and were primarily present in fine particulate or dissolved form (<0,45 µm), increasing their bioavailability. During spring 2025, upstream ditches leading to BR5 exhibited high concentrations of several metals, particularly copper, arsenic and chromium, which could be traced to slag material used beneath a nearby asphalt-reinforced area. However, these ditches contributed little to metal loads in BR5. In contrast, stormwater passing through the sedimentation-based treatment system (i.e., stormwater ponds) showed elevated levels of cadmium, nickel and zinc during high spring flows. The variation in metal concentrations is likely driven by water chemistry, the geochemical behavior of individual metals and shared sources. The study concludes that the current treatment system is effective for particle-bound metals, but additional treatment methods are needed to reduce dissolved metal concentrations and meet permit requirements under varying hydrological events.