Materials and methods Sampling location Sediment samples were collected in the Mtoni estuary (Figure 1), located 3 km south of Dar es Salaam, during low tide. Samples (E1-E7) were taken from the mixing zone in the Mtoni estuary, including samples from the Mzinga River, the Kizinga River and their confluence in 2 different campaigns (wet and dry seasons). The Kizinga River drains urbanized areas of approx. 700,000 inhabitants whereas the Mzinga River drains more rural areas (approx. 110,000 inhabitants). Both rivers are suspected to carry waste from industrial and residential areas 6. Important to consider is that a textile factory and a solid waste dumping site are located near the rivers, respectively near the Kizinga river and in between the rivers. For this reason, additional end-member samples (F1-F8) were taken upstream (fresh water) of both rivers and at the mouth of the estuary (marine water).

Sampling was performed according to EPA standards 7, using a hand corer (30 cm height and 6 cm internal diameter). Cores were cut into three intervals (0-3, 3-6 and 6-9 cm deep), stored at-20 C and a total of 50 samples were transported for analysis.