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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Description of key information

In the study with just marine water, ethane, propane and isobutane showed evidence of degradation with half-lives of 7 to 33 days. In the study with marine water and sediment, propane, butane and isobutane showed evidence of degradation from test initiation, with half-lives of 18 to 139 days but, after a lag-phase of approximtaely 2 weeks, the substances were degraded completely within about 3 days.

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Additional information

In accordance with column 1 of REACH Annex IX, this study does not need to be conducted as the streams wtihin the category have a low potential for adsorption to sediment. In addition column 2 Annex IX and column 2 Annex X is also applicable as simulation testing does not need to be conducted if CSA according to Annex I has not indicated a need to investigate further the degradation of the substance in water.

Experimental data are available on the persistence of the substances in marine water and marine water and sediment systems. Gas exchange-biodegradation experiments conducted in model estuarine ecosystem shows that members of the category are not expected to be persistent. The study is a non-GLP, non-guideline study but is well described and is considered suitable for use for this endpoint.

In the study with just marine water, ethane, propane and isobutane showed evidence of degradation with half-times of 7 to 33 days. Samples from later in the experiment showed much more rapid biodegradation, which the authors interpret as resulting from adjustment of the bacterial populations to the availability of the substances. Although results of >70 days were given for the half-life of methane in marine water, the authors report that the loss of methane could be accounted for by gas exchange. Therefore, results for methane have not been included in the REACH dossier as they do not represent actual degradation of the substance.

In the study with marine water and sediment, propane, butane and isobutane showed evidence of degradation from test initiation, with half-lives of 18 to 139 days. The slower rates of degradation compared to experiment 1 are thought to result from the lower temperature (10°C as opposed to 20°C). A lag-time of approximately 2 weeks was seen for the bacteria in the tank to respond completely to the availability of the gases and develop a population with a strong hydrocarbon utilisation capability. Between 311 and 400 hours, ethane, propane, butane and isobutane showed rapid decreases in concentration, far in excess of earlier degradation, gas exchange and outflow. Ethane, which was degraded completely in about 3 days, was the slowest of the substances. Methane also appeared to be degraded during this period, at a rate of about 2.9% per day, corresponding to a half-life of about 24 days.

Ethane, propane, butane and isobutane are subject to degradation in marine ecosystems on the timescale of a few days to a few weeks. Initial rates of degradation are significantly slower than the predicted loss by gas exchange from typical natural estuaries. After a period of 15 to 30 days of moderate degradation, bacterial populations can adjust and degrade gaseous hydrocarbons at least an order to magnitude more rapidly. Thus, for chronic inputs to estuaries, degradation could become the dominant removal mechanism.