25 years monitoring of PAHs and petroleum hydrocarbons biodegradation in soil

Biodegradation of polycyclic aromatic hydrocarbons (PAHs) and total petroleum hydrocarbons (TPH) in sediment and soil has been monitored on seven experimental fields during periods up to 25 years. With this unique dataset, we investigated long-term very slow biodegradation under field conditions.. The data show that three biodegradation rates can be distinguished for PAHs: 1) rapid degradation during the first year, 2) slow degradation during the following 6 years and 3), subject of this paper, a very slow degradation after 7 years until at least 25 years. Beside 2-, 3- and 4-ring PAHs, also 5- and 6-ring PAHs (aromatic rings) were degraded, all at the same rate during very slow degradation. In the period of very slow degradation, 6% yr⁻¹ of the PAHs present were removed in five fields and 2% yr⁻¹ in two other fields, while in the same period no very slow degradation of TPH could be observed. The remaining petroleum hydrocarbons were high boiling and non-toxic. Using the calculated degradation rates and the independently measured bioavailability of the PAHs (Tenax-method), the PAHs degradation curves of all seven monitored fields could be modelled. Applying the model and data obtained with the Tenax-method for fresh contaminated material, results of long-term biodegradation can be predicted, which can support the use of bioremediation in order to obtain a legally acceptable residual concentration.