Workshop Report 24

Challenges with assessing degradation and persistence

Thomas W. Federle

Procter & Gamble, USA

The ability to accurately determine the potential for an organic chemical to degrade and the rate at which degradation will occur in environmental compartments where it is released and ultimately resides is critical in evaluating its environmental persistence. Moreover, this understanding is essential for accurately estimating environmental exposure when conducting an environmental risk assessment. Historically, ready and inherent biodegradation tests have been the principle regulatory tools utilised for assessing degradability. However, these tests are ineffective for chemicals that are difficult to test due to their physico-chemical properties, are not used as growth substrates by microorganisms or whose degraders are rare in standard test inocula. While some of these limitations are remedied in simulation tests, these tests come with their own unique issues.

The presentation surveyed some of the challenges commonly encountered in accurately evaluating the degradation and persistence of organic chemicals. These include challenges that are not only scientific and methodological but also financial and practical. Methodological challenges include dosing difficult to handle substances and having sufficient analytical signal above background to quantitatively measure biodegradation at test concentrations, which are not inhibitory to the microbes or at which mass transfer is not a limitation. Scientific challenges include having an inoculum that is of sufficient size and diversity that rare degraders are present and in the case of substances that are co-metabolised rather than used as a growth substrate having a metabolically active microbial community available in the test. The former is complicated by regulatory restrictions on using pre-adapted inocula, which is particularly a problem for chemicals that are new to the world. A consistent scientific challenge relates to having a ratio of test chemical to microbial biomass that is reflective of actual exposure and in situ conditions in the environment.

The use of simulation tests come with their own specific challenges, many of which are of a practical nature. These include not only the cost but the difficulty of obtaining high quality and well characterised radiolabelled test materials with the consolidation and contraction that has occurred in the industry during the past few years. Others relate to the complexity of such tests, the difficulty of successfully executing them, uncertainty about the results themselves and even their regulatory acceptance. This latter uncertainty includes whether the results from scientifically sound but non-prescribed tests (e.g. OECD 314) will even be considered by regulatory agencies, potential variability in how individual regulators or agencies will weigh and interpret such tests and how they will consider bound residues in the ultimate assessment. Unfortunately, such uncertainty can translate into reluctance by business managers to proactively fund testing and research that could lead to more definitive understanding on the fate of many chemicals in the environment. The hope is that by identifying the challenges, whether scientific or practical, and the dilemmas that they pose, this workshop can catalyse the development of improved approaches that will ultimately advance our understanding of chemical fate and result in better environmental protection.