Trace metal geochemistry of organic carbon-rich watercourses draining the NW German coast

Numerous small watercourses are draining the hinterland of the NW German coast. The waters mainly originate from marsh and fen areas and have yellow to deep brownish color. During their flow path, the waters exhibit gradients in salinity (0.2–3), pH (6.2–8.8), particulate organic carbon (5–25%), and iron oxides (7–12%), which alter the concentrations of most dissolved and particulate trace metals. For example, dissolved Fe is rapidly removed from solution at increasing salinities by flocculation, whereas dissolved U is removed in the very low-salinity zone by Fe- and organic-rich colloids. The waters at the flood-gate of Neuharlingersiel, where a composite sample of the entire study area is collected before the freshwater is discharged into the marine-dominated tidal flat area in front of the mainland dike, have the following average trace metal concentrations: dissolved Fe 11 μM, Mn 4 μM, Mo 10 μM, U 1.8 μM, V 75 μM and particulate Fe 7 mg kg−1, Mn 1200 mg kg−1, Mo 2 mg kg−1, U 1.7 mg kg−1, V 140 mg kg−1. After passing the flood-gate most Mo- and U-salinity distributions fit well to the conservative mixing line that connects the seawater of the Wadden Sea to the low-salinity river water entering the tidal flat area. In contrast, dissolved Fe, Mn, and V are removed from solution, especially at intermediate salinities, which may be due to mixing of freshwater with seawater depleted in these elements, flocculation and/or scavenging by particulate matter. In general, processes similar to those in estuarine systems alter the geochemical signatures on transects from land to sea. Compared to average river water, the watercourses are enriched in dissolved and particulate Fe, Mn, Mo, U, and V. Due to the fast removal of dissolved Fe in the salinity gradient and the high concentrations of Mo and U in seawater, the watercourses only form a source for Mn, V, terrestrial organic carbon, and particulate Fe for the Wadden Sea.