Aquatic Toxicity Tests in the Context of Science and ...

AQUATIC TOXICITY TESTS IN THE CONTEXT OF SCIENCE AND MANAGEMENT

1.0 Understanding the purpose of the tests 1.2 The scientific context 1.3 The management context

2.0 Test Procedures 2.1 Basic Principles 2.2 Factors in test design 2.3 Selecting test species 2.4 Data reporting

3.0 Waste Sampling

4.0 Setting and Applying Toxicity Limits

1.0 Understanding the purpose of the tests

From time to time misconceptions arise concerning the use of toxicity tests. For example, there has been criticism that the tests do not simulate conditions in the receiving environment either in terms of the species employed or the characteristics of the medium. Thus it is felt that the tests have no ecological relevance and are of little value for purposes of environmental protection.

It is important to understand that toxicity tests are specifically designed to measured compliance with toxicity limits set by the regulatory authority and included as conditions of the discharge licence. Toxicity limits are equivalent to emission standards for chemical and physical parameters. They are based on the current views of the regulator concerning the levels of toxicity achievable by Best Available Technologies (BAT) for the processes concerned and are applicable nationally i.e. they ensure conformity among similar industries at different locations. The values applied in Ireland for the past thirty years correspond to extremely low levels of toxicity and, both in principle and in reality, represent minimal hazard to surface waters in practically all circumstances.

It should be clear from this that toxicity tests are not intended to replace assessments of the biological impacts in the natural environment. Where such assessments are necessary, there are many other suitable techniques that can be applied. To Top

1.2 The scientific context

Aquatic toxicology is the science that deals with the effects of substances and physico-chemical conditions on plants and animals that live in water. This includes the study of mechanisms by which changes in the quality of water or food supply affect the growth, reproduction, behaviour or survival of aquatic organisms.

As a branch of eco-toxicology, aquatic toxicology has made an immense contribution to our understanding of how natural and man-made substances affect the living environment. Consequently, it has also played a central role in the development of policies and strategies for environmental protection, providing the scientific basis for many of the standards and quality objectives now applied in waste management and water pollution control.

There are various ways in which aquatic toxicology can contribute to the management of individual waste discharges. In many cases, a detailed analysis of the chemical and physical properties will enable the toxicologist to predict the degree of toxicological hazard that the material will present to aquatic organisms. This is fairly straightforward for wastes composed of well-researched chemicals, and predictions can be made with a high degree of reliability. In the case of new or poorly researched substances, a comparison of their physico-chemical structure with related chemicals can provide good indications of the likely hazard profile.

However, chemical characterization of complex wastes is often difficult, providing strong justification for the inclusion of toxicity limits in waste licenses. Used in

conjunction with limits on chemical and physical constituents, toxicity limits afford a safeguard against the presence of unknown or unanticipated contaminants. Nevertheless, it is essential to understand that toxicity tests can only measure the toxicological properties they are designed to detect. The tests currently used in Ireland are designed to detect acute (rapid), easily observed and unquestionably harmful properties such as lethality and immobilisation. The same tests may not detect substances that can, for example, affect fertility or growth. To Top

1.3 The management context

As we have indicated, the toxicity tests routinely carried out by Enterprise Ireland on behalf of Irish industry have a very specific function within a comprehensive framework of waste management. This framework is illustrated in Figure 1. It encompasses characterization of the effluent source, treatment process and receiving environment as well as licensing and monitoring. The role of the test laboratory is confined to sampling, testing and data reporting. The choice of sampling and test procedures, setting toxicity limits and determining compliance with these limits, are the responsibility of the regulator. Nevertheless, it is implicit that test design, the way results are expressed and the interpretation and use of test data depend to a great extent on the values and approaches applied to other elements of the framework. To Top

2.0 Test Procedures

2.1 Basic Principles Scientists describe the toxic properties (toxicity) of substances and wastes in

terms of the dose (amount or concentration administered) that causes a particular effect (response) in a specified population.

If a group of fish of the same species, age and weight (the population) is exposed to a known concentration of a substance for a fixed period of time (the exposure), and one by one they start to die (the response), the toxicity, (in this case the lethality) of the substance could be assessed simply by recording the time it takes for half of the fish to die. In other words, the concentration (C) of the substance which is lethal (L) to 50% of the fish in x hours is, in this case, the concentration in the fish tank; in technical terms it is the

x-hour LC50

where x is the period of exposure. This is one type of median Effective Concentration or EC50. Equivalent expressions for use with data derived from tests involving the ingestion (intake) of substances are the Effective Dose (ED50) and the lethal Dose (LD50).

In practice, tests usually involve not one but a series of concentrations in separate containers, and exposure times will be pre-selected. The responses at all concentrations are combined in calculating the effective concentration.

The foregoing example illustrates two fundamental principles of toxicology. These are:

a) that toxicity is a function of both dose (or concentration) and time, and

b) that each individual in a population responds to a stress in an individual manner (e.g. some respond quicker than others).

These same principles can also be illustrated graphically. Figure 2 shows the relationships between toxicity (EC50) and exposure for two imaginary substances. Clearly, as exposure times increase the effective concentrations diminish. Note that for both substances there is a concentration below which there is no further effect regardless of how long exposure might continue ? this is the Median Effective Concentration threshold. Note also that whereas the two substances have very different toxicity curves (e.g. effective concentrations ranges and time achieve the threshold) their 48-hr EC50's are, for purposes of this example, identical. The key message here is that the time taken by different substances (including wastes) to achieve the threshold may differ widely and that, because of this, tests run for preselected, short-term periods may not give the complete picture.

Figure 3 shows why it is necessary to use the median (50%) response to determine toxicity. The figure gives typical response curves for imaginary populations of test organism exposed to the same substance. Clearly, more individuals respond as the concentration increases i.e. some are more sensitive than others. Furthermore, one species responds over a much narrower concentration range than the other. The key message here is that sensitivity may differ widely between individuals and between species. To Top

2.2 Factors in test design

Essential considerations in the design of effluent toxicity tests are:

? types of response to be measured, ? length of exposure (i.e. acute or chronic) ? organisms to be used, ? age and size of organism, ? existence of suitable methodologies, and, ? complexity and cost.

As these factors are interrelated, they are not considered independently. The most important factor is the purpose of the tests, described in Section 1. This suggests that what is needed is a rapid and reliable test that is comparatively inexpensive, shows a high degree of precision (repeatability of results) and conforms to internationally accepted standards.

When logistical factors such as the number of tests to be performed, restrictions on the size of samples and practicality (including cost) of sample transport are added to the above, the size and availability of test organisms become all important. To Top

2.3 Selecting test species

In Section 1, we explained that toxicity tests are intended only to assess compliance with regulatory limits on toxicity, that the limits and the tests are applied nationally and therefore should not be expected to simulate local receiving water conditions. Thus, when faced with the choice of species to use for these tests, local presence is not a primary consideration.

In principle, what is needed is a kind of `white mouse', as used in mammalian toxicology. An aquatic version of the white mouse should be small, easily cultured in the laboratory and available worldwide. It would allow one or more readily observable and clearly harmful responses (such as death or immobilization) to be recorded with exposures of 2 days or less; the organism should be relatively sensitive to a variety of chemical types. A standardized and internationally recognized procedure should exist for measuring dose/effect relationships with this species.

Fortunately, there are several species available in Ireland that fit this profile. Examples are the water flea (Daphnia magna), the rainbow trout (Oncoryhnchus mykiss), and the luminescent bacterium (Vibrio fischeri).

In the past, a key question has been whether a single species is sufficient for purposes of a national monitoring programme. Today, few toxicologists or managers believe this is so and current practice is to test four species initially, with subsequent tests using the two most sensitive species.

While there is no need for test species to be part of the local or even national fauna or flora, we do need to recognize that wastes may be discharged to marine and freshwater environments and that aquatic communities tend to consist of many different taxa (e.g. fish, crustaceans, molluscs, algae, bacteria, etc). As we explained in Section 2.1, there can be differences in sensitivity between even closely related species, but differences between taxa may be wider still. For some substances, there is a clear relationship between toxicity and biological complexity. Although experience has not shown any consistent differences in sensitivity (within taxa) related to the salinity of the organisms' natural environment, salinity may alter the chemistry of some wastes and, for those discharged to saline waters, the inclusion of at least one marine species makes good sense.

It is important that decisions on the choice of test species are made by the regulator (see 1.2), but they must inevitably take account of advice from scientists on the options available and any limitations surrounding the use of particular species.

An annotated list of species available to Enterprise Ireland's Shannon Aquatic Toxicity Laboratory is given in Table 1. From the laboratory's experience, as well as from the scientific literature, there are clear indications that crustaceans are among the most sensitive organisms to many different types of chemical. Thus, in developing our testing capabilities, we have given particular attention to extending the range of crustaceans that can be used.

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