Hidden hazard or safe sink? Approaches to consider non-extractable residues in the regulatory assessment of chemicals 1

Caren Rauert,

Andreas Höllrigl-Rosta, Elisabeth Thumm Umweltbundesamt, Germany

Formation of non-extractable residues (NER) is regularly observed in studies on the fate of organic chemicals in soil. NER formation may be interpreted as a specific form of compound persistency (‘hidden hazard’) or as a detoxification step (‘safe sink’). Despite the considerable scientific progress made in analysing NER and identifying their binding types, these insights have not yet been utilised in regulatory risk assessment.

In a 2010 workshop held at the German Federal Environment Agency (UBA), it was agreed that three main types of NER should be considered in regulatory schemes: Fixation of substance molecules by physical entrapment in the soil matrix can be reversed under certain environmental conditions. Those ‘Type 1’ NER must be considered as a reservoir for remobilisation of a chemical over prolonged times. In contrast, formation of strong chemical bonds between substance molecules and soil matrix will produce ‘Type 2’ NER, which are unlikely to be released in their original structure under environmental conditions. Finally, NER can also be formed via incorporation of single labelled atoms or small fragments from the original substance into biomass. These ‘biogenic’ NER are no longer structurally related to the original substance. While the formation of Type 2 and biogenic NER can be considered a ‘safe sink’, Type 1 NER would constitute a ‘hidden hazard’.

A generic extraction scheme was proposed for residue analysis in the standard studies on the fate of organic chemicals in soil. Specific methods are required to determine the amount of biogenic NER. Extraction with non-destructive methods allows conclusions to be drawn on the availability of residue fractions.

To differentiate between Type 1 and Type 2 NERs, a set of destructive extraction methods differing in strength is available, which may be complemented with sophisticated spectroscopic techniques. Where no information on their nature is available, NER should in principle be assumed to belong to Type 1 (i.e. worst case scenario).

Formation of Type 1 NER will have different implications on the environmental risk and hazard assessment. In particular, their potential for substance remobilisation will significantly impact groundwater risk assessment and persistence assessment. Existing trigger values and decision criteria for NER formation were deemed inappropriate for addressing those concerns; hence, a need for developing new criteria was identified.

1 A precursor of this talk was held by Andreas Höllrigl-Rosta at SETAC in Milan 2011.