Die vier Griechischen Elemente: - TOBIAS-lib - Universität Tübingen

The four Greek Elements: Mass balance of polycyclic aromatic hydrocarbons (PAHs) in
small catchments of rural areas
Abstract: The study is structured according to the four Greek Elements (earth, water, air and fire):
Emissions of pyrolytic generated polycyclic aromatic hydrocarbons (PAHs) into the atmosphere reach
the earth surface due to atmospheric deposition even in rural areas. Thus, PAHs accumulate in the
soils over time, which may pose a threat of groundwater contamination due to transportation with
seepage water. In general, deposition rates of PAHs are higher in the winter season than during
summer. Nevertheless, annual deposition rates display constant input values of these persistent organic
pollutants into the soils. The heterogeneity of the long-term atmospheric deposition (annual basis) due
to relief-depending exposure is negligible. On this time scale, the variation between open-field
deposition and deposition in forests (via throughfall and litterfall) is within a factor of 2 depending on
the amount of litterfall. Similar distribution pattern of PAHs in atmospheric deposition and soil
samples indicate a close relationship between atmospheric input and accumulation of PAHs in soils in
the terrestrial environment. Taking historical deposition rates into consideration the actual soil burden
can be linked to this pathway even quantitatively. This is a sign for the persistency of the PAHs in the
soils. Relatively high concentrations of perylene in the subsoil of a dystric Planosol supports the
hypothesis of in-situ formation of this compound even in terrestrial environments. The PAHconcentrations
in groundwater, seepage water as well as in surface water in remote areas are very low.
The mass balances on catchment scale indicate that more than 90% of the deposited PAHs remain in
the soils. Thus, the buffer capacities of the soils are not yet exceeded due to adsorption of PAHs on the
soil organic matter. Even leaching tests on representative soil samples support this conclusion.
Organic particles (soot, char, charcoal) act as carriers for the PAHs. They are abundant in the
atmospheric deposition as well as in soil samples. Based on morphological studies of organic particles,
traffic soot seems to be the major source for the actual PAH-deposition in rural areas. Due to
atmospheric deposition of such particles, sorption capacity of the soil can rise over time. This should
affect the mobility of PAHs in the soils. Based on a simple model the time was calculated, when PAHs
should break through the unsaturated zone under equilibrium condition. Following this model
calculation, legal limits defined for seepage water in the German Federal Soil Protection Act and
Ordinance will not be exceeded in the next 1000 years. However, this model doesn’t take into account
preferential flow, even though preferential flow controls the actual PAH-dynamics on catchment scale.