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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Description of key information

In various small-scale batch experiments using natural sources of water and sediment under aerobic and anaerobic conditions, biodegradation of naphthalene occurred efficiently under strictly aerobic conditions, leading to turnover times of 13 hours in the sediment compartment and 310 hours (13 days) in the water compartment and to complete degradation within 21 days. Furthermore, in one experiment the dependence of the naphthalene degradation on the contamination degree of the water/sediment system has been clearly demonstrated, providing evidence of the acclimatisation ability of the microbial system. It can be concluded that naphthalene is readily biodegradable in the water/sediment compartments under aerobic conditions. 

Key value for chemical safety assessment

Additional information

No useful long-term degradation half-lives or environmental kinetic elimination constants have been located. In none of the four reports which were taken into account, values for half-lives have been determined. By weight of evidence, it can be concluded that naphthalene is easily/inherently degradable without signs of bioaccumulation. The limiting factor is anoxic/anaerobic conditions such as they may prevail in sediment.

Given this fact, it was proposed to use default half-lives of 150 days in surface water and 3000 days in sediment for risk assessment (UK 2007):

These global allocations to temporal categories for aromatic substances were first adopted in the Risk Assessment report for risk assessment of CTPHT (Coal-tar pitch, high temperature) (ECB 2008, pp. 66/67), taken from Mackay et al. 1992: "On the basis of model calculations, Mackay et al. (1992) ranked the 16 EPA PAH according to their persistence in water, soil and sediment in different classes which correspond to a specific half-life in these compartments..."

This not well-founded conclusion should not be taken for granted for a potentially easily degradable substance like naphthalene, even though evaluable long-term elimination data are lacking.

References:

UK 2007: Environment Addendum of December 2007 to the EU Risk Assessment of naphthalene, final approved version. UK Environment Agency, Chemicals Assessment Unit [R020_0712_env].

ECB 2008: European Union Risk Assessment Report: Coal-tar pitch, high temperature. Draft environmental risk assessment report, Rapporteur: The Netherlands. European Chemicals Bureau. [R323_0805_ENV_FINAL_ECB.DOC]

Mackay D, Shiu WY, and Ma KC 1992: Illustrated Handbook of Physical–Chemical Properties and Environmental Fate of Organic Chemicals. Lewis Publishers, Boca Raton, FL.