Robin Oliver and Laurence Hand
Many regulatory risk assessments for chemicals are based on laboratory studies in which the key processes of sorption, hydrolysis, photolysis and microbial degradation are evaluated separately in simple, standardised systems, in accordance with the appropriate guidelines. These studies provide information on the fate and behaviour of the chemical in soil, sludge, sediment and water.
Hydrolysis studies are conducted under sterile conditions in the dark, to exclude microbial and photolytic degradation, respectively. Photolysis studies are also conducted under sterile conditions and at a pH at which hydrolysis is known to be minimal. The most complex laboratory studies are those designed to determine microbial degradation in soil, sludge or sediment / water systems. These are conducted in small vessels that are incubated in the dark under either aerobic or anaerobic conditions. Potential loss mechanisms in these studies include sorption to the solid phase, hydrolysis, and microbial degradation. However, because these studies are conducted in the dark, the contributions of photolysis and metabolism by phototrophic organisms (algae and macrophytes) are excluded. In the regulatory scheme for crop protection products (CPPs) degradation studies under field conditions are a requirement for some compounds. Historically such studies have provided an indication of how different processes combine to influence the rate of degradation of chemicals. However in most field study designs the chemical is applied to recently cultivated, bare soil (which is often not representative of the use conditions), thus excluding the potential contribution of surface dwelling phototrophs and microbes in the rhizosphere. Recent changes in Europe in the guidance for such studies have increased their artificial nature by indicating that degradation by surface processes should be excluded. There are, therefore, no studies in the regulatory suite that provide a realistic integration all of the potential degradative processes in either the terrestrial or aquatic compartments. It is questionable how well the potential persistence of chemicals can be assessed without such studies, as adequately reconstructing potentially complex interactions from processes studied in isolation is extremely difficult.
Over recent years Syngenta has developed test systems to investigate the potential significance of degradation resulting from indirect photolysis and metabolism by phototrophic organisms in soil and sediment / water systems. A semi-field aquatic test system has also been developed to enable the determination of the rate and route of degradation, when multiple processes are acting together.
Indirect photolysis was studied using sixteen waters from corn growing regions of the United States. Six test compounds were selected; two that did not degrade significantly by direct photolysis, two that were degraded slowly by direct photolysis and two that degraded quickly by direct photolysis. The compounds that did not degrade or showed moderate degradation by direct photolysis were degraded significantly faster in all of the natural waters tested. For the two compounds for which degradation by direct photolysis is rapid, one was degraded faster in all of the natural waters tested while the other was degraded more quickly in some natural waters and more slowly in others (but degradation was still rapid in all natural waters). The overall rate of photodegradation in natural waters is a combination of direct and indirect photolysis and, in some cases, light scavenging by constituents of the water can reduce the rate of direct photolysis to a greater extent than is compensated for by indirect photolysis. These findings suggest that this will only be significant for compounds where direct photolysis is very rapid and the overall photodegradation rate in natural waters will still be fast. For those compounds that are not degraded very rapidly by direct photolysis, photodegradation in natural water is likely to be significantly faster than that observed in sterile buffer.
Degradation in aquatic systems by algae and a representative macrophyte (Elodea sp.) was investigated by incubating natural sediment and overlying water under light from fluorescent bulbs (the absence of UV wavelengths means that the contribution of photolysis would be negligible), with or without the macrophyte present. The incubation system enabled any radiolabelled volatile components evolved to be trapped ensuring that a mass balance for the test system could be obtained. For all five compounds tested, degradation in the presence of aquatic plants was significantly faster than in the standard water / sediment system and much closer to the rate obtained in the semi-field study.
The role of soil surface dwelling phototrophs has also been investigated using both sieved soil and intact soil cores incubated under fluorescent lamps (as for the aquatic system). In sieved soil systems the presence of soil phototrophs enhanced the rate of degradation for most but not all of the chemicals tested. The use of intact soil cores further enhanced the degradation rate of the two chemicals tested to date (compared to the rate in the same soil sieved).
These findings suggest that degradative processes that are not currently included in regulatory testing schemes are likely to play a significant part in the degradation of chemicals in the environment. To ensure that assessments of the persistence of chemicals is not biased by a failure to take account of all relevant mechanisms of degradation and their integration, registrants should have the option to include all of these in the estimation of persistence. This can be done cost effectively by the adoption of a tiered testing approach which provides the option to include degradation by all of these processes and, where the degree of complexity requires it, integrated test systems such as semi-field aquatic studies and uncovered field studies.