The Accumulation of Organochlorines in Biological Systems

Many organochlorine compounds are found in the tissues of fish in concentrations that are orders of magnitude higher than are those in the waters in which they swim. Hydrophobic (water-hating) substances like DDT are particularly liable to exhibit this phenomenon. There are several reasons for this bioaccumulation of chemicals in biological systems.

Bioconcentration

In the first place, many organochlorines are inherently much more soluble in hydrocarbon-like media, such as the fatty tissue in fish, than they are in water. Thus, when water passes through a fish's gills, the compounds selectively diffuse from the water into the fish's fatty flesh and become more concentrated there: This process (which also affects other organisms besides fish) is called bioconcentration. The bioconcentration /actor, BCF, represents the equilibrium ratio of the concentration of a specific chemical in a fish relative to that in the surrounding water, provided that the diffusion mechanism represents the only source of the substance to the fish. BCF values occur over a very wide range and vary not only from chemical to chemical but also, to a certain extent, from one type of fish to another, particularly because of variations in the abilities of different fish to metabolize a given substance.

The BCF of a chemical can be predicted, to within about a factor of 10, for a typical fish from a simple laboratory experiment: The chemical is allowed to equilibrate between the liquid layers in a two-phase system made up of water and 1-octanol, CH3(CH2)6CH2OH, an alcohol that has been found experimentally to be a suitable surrogate for the fatty portions of fish. The partition coefficient» Kow, for a substance S is defined as

^Sjw l®!octanol/EIwater where the square brackets denote concentrations in molarity units. (Since water and fat have approximately the same densities, the ratio of molarities

TABLE 10-3

Selected Data for Some Pesticides

Pesticide

Solubility in HzO (ppm)

log K»w

HCB

0.0062

5.5-6.2

DDT

0.0034

6.2

Toxaphene

3

5.3

Dieldrin

0.1

6.2

Mirex

0.20

6.9-7.5

Malathion

145

2.9

Parathion

24

3.8

Atrazine

35-70

2.2-2.7

Data from K, Verschuerm, Handbook of Environmental Data on Organic Chemicals (New York: Van Nostrand Reinhold, 1996).

Data from K, Verschuerm, Handbook of Environmental Data on Organic Chemicals (New York: Van Nostrand Reinhold, 1996).

in the two phases is identical to the ratio of their masses; consequently, Kow can also be taken as the ratio of ppm or ppb concentrations.) Largely as a matter of convenience, the value of Kow is often reported as its base 10 logarithm, since its magnitude sometimes is quite large. For example, for DDT (see Table 10-3), Kow is about 1,000,000, i.e., 106, and so log Kow = 6. Experimentally, the bioconcentration factor for DDT lies in the range of about 20,000 to 400,000, depending upon the type of fish. The Kow value of a compound is a fairly reliable approximation to the BCF values found for fish. The approximation that Kow = S typically breaks down when the molecules are too large to diffuse into the fish.

In general, the higher its octanol-water partition coefficient Kow, the more likely a chemical is to be adsorbed on organic matter in soils and sediment and ultimately to migrate to fat tissues of living organisms. However, log Kow values of 7 or 8 or higher are indicative of chemicals with such strong adsorption to sediments that they are actually unlikely to be mobile enough to enter living tissue. Thus, it is chemicals with log Kaw values in the 4-7 range that bioconcentrate to the greatest degree.

PROBLEM 10-3

For HCB, log Kow = 5.7. What would be the predicted concentration of HCB due to bioconcentration in the fat of fish that swim in waters containing 0.000010 ppm of the chemical?

FIGURE 10-2 Variation with age of DDT concentration in Lake Ontario trout caught in the same year. [Source: "Toxic Chemicals in the Great Lakes and Associated Effects" (Ottawa: Government of Canada, 1991).]

Biomagnification

Fish also accumulate organic chemicals from the food they eat and from their intake of particulates in water and sediments onto which the chemicals have been adsorbed. In many such cases, the chemicals are not metabolized by the fish: The substance simply accumulates in the fatty tissue of the fish, where its concentration there increases with time. For example, the concentration of DDT in trout from Lake Ontario increases almost linearly as the fish ages, as illustrated in Figure 10-2. The average concentration of many chemicals also increases dramatically as one proceeds up a food chain, which is a sequence of species, each one of which feeds mainly upon the one preceding it in the chain. The food web, incorporating interlocking food chains, for the Great Lakes is illustrated in Figure 10-3. Over a lifetime, a fish eats many times its weight in food from the lower levels of the food chain but retains rather than eliminates or metabolizes most organochlorine chemicals from these meals.

A chemical whose concentration increases along a food chain is said to be biomagnified. In essence, the biomagnification results from a sequence of bioaccumulation steps that occur along the chain. The difference between bioconcentration from water and biomagnification along a food chain is illustrated symbolically in Figure 10-4. The biomagnification of DDT along some of the Great Lakes food chains is shown in Figure 10-3. Notice, for example, the herring gull's higher level of DDT compared with the fish below it in the chains. Fish at the top of the aquatic part of the chain bioaccumulate DDT rather effectively, so that even higher concentrations are found in the birds of prey that feed on them.

As an example of biomagnification, consider that the DDT/DDE concentration in seawater in Long Island Sound and the protected waters of its southern shore at one time was as high as 0.000003 ppm, but it reached 0.04 ppm in the plankton, 0.5 ppm in the fat of minnows, 2 ppm in the needlefish that swim in these waters, and 25 ppm in the fat of the cormorants and osprey that feed on the fish, for a total biomagnification factor of about 10 million. It is by such mechanisms that DDE levels in some birds of prey became so great that their ability to reproduce successfully was impaired. The bioaccumulation of organochlorines in fish and other animals is the reason that most of the human daily intake of such chemicals enters via our food supply rather than from the water we drink.

FIGURE 10-2 Variation with age of DDT concentration in Lake Ontario trout caught in the same year. [Source: "Toxic Chemicals in the Great Lakes and Associated Effects" (Ottawa: Government of Canada, 1991).]

FIGURE 10-3 Simplified food web for the Great Lakes with typical DDT concentrations for some species. [Source: "Toxic Chemicals in the Great Lakes and Associated Effects" (Ottawa: Government of Canada, 1991).I

Bald eagle

Bald eagle

Salmon/Lake trout.

Cormorant Herring gull

Salmon/Lake trout.

Forage fish

Sculpin ▼ Smelt

(\ a Chub Alewife

0.01 ppm

V^tenaDDi

Snapping j turtle

Invertebrates

Plankton

Invertebrates

Plankton v

Waterfowl

Bacteria and fungi

- Mineral nutrients

\ Dead animals j and plants Vegetation

- Mineral nutrients

FIGURE 10-4 Schematic representation of the two modes of bioaccumulation that operate in biological matter present in a body of water,

FIGURE 10-4 Schematic representation of the two modes of bioaccumulation that operate in biological matter present in a body of water,

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

Get My Free Ebook


Responses

  • Futsum
    What would be the predicted concentration of hcb due to bioconcentration?
    4 years ago

Post a comment