Entry of heavy metals from anthropogenic discharges into small streams: Where can they be found again? Viewing samples of water, sediment and Gammarus spp. (Crustacea: Amphipoda)

The entry of heavy metals into rivers affects both water quality and aquatic biocoenosis. Discharged with wastewater, heavy metals are transported downstream, where they accumulate in sediments and are absorbed by aquatic organisms. This study investigates whether anthropogenic heavy metals can be detected in water, sediment and biota samples and which medium is best suited to assess different aspects of riverine contamination, including the role of gammarids as bioindicators. Several smaller rivers in Hesse and Bavaria (Germany) were examined in connection with wastewater discharge points, covering pollution scenarios such as contaminated sites, technical defects and sewage treatment plants. Thirty gammarid, 100 sediment and 30 water samples were collected from 50 m river sections. ICP-MS was used for lead, cadmium, total chromium, copper, nickel and zinc, while mercury was analysed via CV-AAS. Results show that sediment concentrations of heavy metals are about 10,000 times higher than in corresponding water samples, while concentrations in Gammarus spp., containing G. fossarum, G. pulex and G. roeselii, are approximately 1,000 times higher than in water. No linear relationship is observed between gammarids and sediment, refuting such a hypothesis. Gammarids inhabit the riverbed and banks, accumulating heavy metals through their diet and serving as reliable accumulation indicators due to their abundance. However, their collection is labor-intensive, requiring sufficient biomass and the separation of other macrozoobenthos species. Sediment sampling is less demanding, while water samples can be obtained rapidly and universally. Case studies illustrate different perspectives. In the Kössein and Röslau rivers (Bavaria), historically contaminated with mercury, concentrations decreased after dredging and securing the contaminated banks. In the Wieseck River (Hesse), untreated sewage discharged for one week resulted in sediment contamination still detectable a year later. At Rosbach (Taunus region), a combined sampling approach proved effective. Water analysis revealed elevated zinc downstream of a wastewater treatment plant, which was later confirmed in sediment further along the course. In conclusion, water sampling is particularly suited for short-term monitoring of acute contamination events, while sediment and gammarids provide valuable insights into long-term impacts and past discharges. Both matrices confirm increases in heavy metal contamination associated with wastewater inputs.