Mobility and storage of hydrophobic urban pollutants in an agricultural river in southwestern Germany (Ammer River)

Hydrophobic pollutants in rivers are transported to varying degrees either in the dissolved or particle-bound phase. For either very turbid rivers or very strongly sorbing compounds, transport dynamics are closely coupled to the cascading sediment movement throught the stream network. Hence, an understanding of sediment storage and mobilisation/movement, including the interaction of suspended and bed sediment phases is the basis for properly discribing contaminant transport.

Suspened sediments from the Ammer River in southwestern Germany show a gradual decrease of polycyclic aromatic hydrocarbons (PAH) loadings in a downstream direction, reflecting urban sources in the headwaters. The decrease of PAH loading is, however, only weak which we attribute to only moderate inputs of more or less clean sediments along the river profile, capable of diluting the contaminant signal. The bed sediments of the river comprise a large fraction of grain sizes < 250 µm and constitute an important storage compartment for hydrophobic contaminants. Transformation or leaching into the water phase may reduce the stored amounts. Furthermore, the particle-bound mobility of the stored contaminants clearly depends on the distribution among grain size fractions. We have, therefore, also sampled the bed sediments < 250 µm along the river profile and separated into four grain size fractions (< 19 µm, 19-54 µm, 54-100 µm, 100-250 µm) using vacuum filtration and fine stainless steel meshes. The fractions have been analyzed for urban pollutants of different origins (PAH, heavy metals and the musk fragrance galaxolide) and particulate organic carbon (POC) content. First results show that POC is enriched in the 19-54 µm fraction and that metal contents are neither correlated with POC nor with grain size. This contradicts the common assumption that compounds typically sorbing to surfaces acumulate in the finest fractions due to the large specific surface area. Final results for PAH and galaxolide concentrations as a function of POC contents and particle sizes will also be presented (analysis still pending). At least for PAH a strong affinity to organic carbon is anticipated, while not much is known to date about the sorption and storage behaviour of galoxolide.

The results will give further insights into the environmental behaviour of the investigated compound groups. Comparison with data from suspended sediment sampling will enhance our understanding of mobile and immobile phase interactions and, hence, the transport dynamics of these sediment-associated pollutants.