ASTM E1624-94(2008)

The fate of chemicals released to the environment may be evaluated in the field or in laboratory studies. This guide provides direction on the development, use, and evaluation of microcosm studies that simulate a specific aquatic ecosystem and include sediment and relevant biota. A key objective in the use of site-specific microcosms is the ability to extrapolate information obtained in the laboratory system to field situations with a reasonable degree of confidence.

Field studies can obtain important information about the fate of chemicals in a particular ecosystem but have many disadvantages. In field studies, environmental variables, in general, cannot be controlled and the study may be subject to wide fluctuations in variables such as temperature, rainfall or sunlight. Introduction of a chemical into an ecosystem may produce an unacceptable environmental risk. Furthermore, field studies often are prohibitively expensive.

Some environmental fate studies use structural or synthetic communities (not site-specific microcosms) created by placing water, soil or sediment, plants, animals and microbiota in a container according to an established protocol. Some synthetic communities have been specifically designed to examine the fate of chemical substances in aquatic environments (that is, Metcalf et al. (1) and Isensee and Tayaputch (2). These synthetic communities provide reproducible environments in which to evaluate and rank chemicals according to their fate but extrapolation to specific ecosystems is difficult. This is because they lack complex population structures and processes analogous to specific natural ecosystems. In addition, they frequently contain a biomass of organisms that is not scaled to the volume of water or sediment, thereby giving exaggerated rates of chemical metabolism.

A microcosm replicates many of the processes affecting the fate of a chemical in a complex ecosystem. A microcosm can be examined under controlled laboratory conditions in the absence of certain variables that might interfere with an understanding of a particular process. Microcosms provide an opportunity to manipulate variables and to study their effects and interactions. Microcosms also offer replication possibilities for assessing environmental variability, an advantage that is not available from field studies.

Microcosms can be used to examine the significance of various fate processes. By examining test compounds in microcosms it is possible to determine the relative effects of various fate processes (for example, biotic versus abiotic). This makes it possible to focus on critical processes and consider site-specific environmental situations where these processes predominate or are absent. Although some fate processes such as hydrolysis or partitioning to sediments may be quantified adequately in simpler studies (for example, shake-flask or aquaria tests) others such as bioturbation may require the complexity of a microcosm for adequate assessment. An important aspect of microcosm testing is determining the significance of biological processes in environmental fate. By studying test compound fate in sterilized microcosms, the role of bioturbation (that can distribute a chemical deep in sediment beds) can be assessed along with biodegradation.

The following are examples of chemical fate information that might be obtained in microcosm studies.

How long a chemical substance will persist in its parent form in a particular environment,

Whether the fate of a chemical is primarily dependent on biotic or abiotic processes,

The effect on the fate of a chemical by the presence of plants that may take up the chemical and store or metabolize it and that provide additional surfaces for microbial colonization,

The effect on the fate of a chemical by the activity of benthic organisms that move water and sediment, and

The effect of nutrient flux at the water sediment interface on the biodegradation of chemicals in the water column and in the sediment.

1.1 This guide provides procedures and criteria for the development and use of sediment/water microcosms for laboratory evaluations of the fate of chemical substances in the environment. It does not specify specific microcosms but it establishes minimum criteria for distinguishing acceptable microcosms from those that may be incomplete or inappropriate for site-specific extrapolation (see 5.1 and 10.1).

1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.