Figure 2 Sign found at the city limits of Texas towns whose public water supplies meet state standards for acceptability.

The right dose differentiates a poison and a remedy." The key part of this axiom is dose, the quantity of potential toxicants administered or consumed. The reason results in aquatic toxicology are expressed, as a concentration instead of dose, the quantity administered, is that the dose an organism receives in aquatic studies is often not known due to multiple routes of exposure. What is known is the concentration of a chemical in the water in which the organism(s) is exposed. Therefore, exposure in aquatic toxicology can be defined as the magnitude, duration, and frequency with which an organism(s) interacts with a biologically available toxicant. The concept of biological availability is important. Just because something is measured in the environment at 'high' concentrations does not a priori mean that it is toxic.

For example, the US Lincoln penny shown in Figure 4 was minted in 1982. The composition of the 1982 penny is 97.5% zinc and 2.5% copper. Given the approximate weight of a penny it contains 59 500 mg of copper and 2 420 000 mg of zinc. The US Environmental Protection Agency's National Criteria for Copper states: ''The procedures described in the Guidelines for Deriving Numerical

Figure 3 Woodcut of Auroleus Phillipus Theostratus Bombastus von Hohenheim 'Paracelsus' (1493-1541). Credited with introducing opium and mercury in medicinal use. Also responsible for the often heard, the dose makes the poison.
Figure 4 United States Lincoln Penny minted in 1982. Composed of 97.5% zinc and 2.5% copper.

National Water Quality Criteria for the Protection of Aquatic Organisms and Uses indicate that, except where a locally important species is very sensitive, freshwater aquatic organisms and their uses should not be affected unaccepta-bly if the 1-hour average concentration in (mg/L) does not exceed the numerical value given by the formula

e L v n J more than once every 3 years on average. For example, at hardness values of 50, 100, and 200 mg/L as CaCO3 the safe 1-hour average concentrations are 9.2, 18, and 34 mg/L'' (the formula is solved by entering the hardness of water that is to be protected and solving the equation: if a hardness value of 50 is entered into the equation the answer is 9.2 mgl1 which would be considered a safe concentration for acute exposure in the 50 mgl1 CaCO3 water). The criterion for aquatic life for acute exposure to zinc states, for total recoverable zinc the criterion to protect freshwater aquatic life, as derived using the guidelines is (mgl1) should not exceed the numerical value given by e(°.83[ln(hardness0]+1.95) at any time. For example, at hardness values of 50, 100, 200 mgl-1 as CaCO3 the concentration of total recoverable zinc should not exceed 180, 320, 570 mg/L at any time. Clearly the amount of copper (59 500 mg) and zinc (2 420 000 mg) in a penny minted on or after 1982 far exceeds the safe concentration of these two essential elements if they were present in a liter of water that was otherwise acceptable to aquatic organisms. However, if you place aquatic organisms normally used to test for acute toxicity in a liter of otherwise acceptable water that also contains a penny, what happens.? Nothing happens because the copper and zinc in the penny are not in a biologically available form. There is one additional concept that is important to understand with regards to this example. The criteria presented here represent the state of water quality criteria in the US in 1986 when the criteria for metals were based on Total Recoverable Metal. Current criteria are based on the Dissolved Fraction in the water column. The principle still holds i.e., just because you measure something in the environment it does not mean that it is necessarily bioavailable and toxic!

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