Oceanography

Today, with all kinds of new compounds entering the water supplies, the specification of allowable limits has become almost impossible; new substances are being added faster than their effects can be assessed. In addition, the toxic effect of a compound may be severe when it is tested alone, but when mixed with all the other constituents of drinking water it may be neutralized in one way or another, or it may combine with other substances in such a way as to become more toxic. The poisonous effects of equal amounts of zinc and cadmium are worse in waters also high in calcium and magnesium.

The quality control of water is compounded by the fact that not only does the water have to be safe to drink with respect to its individual components, but also with respect to the rest of the environment. For example, lead limitations set for water take into account other sources of lead-in food and beverages, in the air, in cigarette smoke. The problem is to try to guess how much of this generally unregulated intake can be combined with the intake from water before toxic levels are achieved.

In general, the Health Service bends over backward in its specifications of the maximum tolerable amount of the various elements, but every once in a while comes a rude shock from some unsuspected effect. The kind of complication that is hardest to anticipate is the behavior of various elements during their successive travel through the food chain. Water with a given composition, drunk directly by humans, may be perfectly safe, but one of the substances in the water may be enriched in the tissues of other water users, either plants or animals, and then enriched again when one animal preys on another. If the final predator is eaten by humans, toxic concentrations may take their toll. In a sense, then, the water may be satisfactory for direct use but dangerous when considered in terms of its total utilization. DDT, originally at a concentration of only one hundredth of a part per million, has been reported as one thousand parts per million in fish that have eaten fish that have eaten microorganisms from the water originally containing dissolved DDT.

The amounts of different kinds of dissolved mineral matter that can be safely drunk differ widely. The merest trace of lead or arsenic is dangerous because they are cumulative poisons, and, although the amount taken in each day might be harmless, the buildup through the years can produce chronic illness or death. The early symptoms produced by many cumulative poisons weakness, stomach upset, headache-are so common that they can be attributed to a hundred other causes and are seldom properly recognized. By the time a correct diagnosis is made, internal damage may be irreparable, and there is no way of dispelling lead or arsenic deposited in bones or tissues.

The problems that are beginning to emerge in relation to mercury are typical of the complexities that are encountered today when detailed investigations are made of the occurrence and effects of a toxic element. It has been known for a long, long time that the liquid metal mercury was dangerous. When mercury from a thermometer or barometer is spilled and separates into thousands of tiny droplets, the drops release mercury vapor into the air. If the droplets are left on the floor of a poorly ventilated room, so that the vapor can be inhaled, damage to the liver and kidneys results. Until recently this seemed to be a minor and local peril that could be avoided by a little knowledge and attention to cleaning up mercury spills.

Then two things happened. People began to search for poisonous substances in the environment, and simultaneously a highly sensitive analytical method was developed for mercury detection. It became possible to test for the presence of mercury at hitherto impossibly low levels, and thus to find out just where it occurs and where it is concentrated. During the research on mercury occurrence in the environment, the presence and effects of mercury vapor from liquid mercury droplets were corroborated, and in addition, a new and disturbing relation was discovered. If mercury compounds are discharged into water bodies they accumulate in the sands and muds beneath the water. There, under the influence of microorganisms, mercury compounds can be transformed into an organic mercury compound, methyl mercury. In this organic form the mercury is mobilized and can be utilized and concentrated by marine or fresh water organisms. Moreover, as methyl mercury, mercury becomes far more poisonous than mercury vapor or other inorganic forms of mercury. In man it seeks out the nervous system, where accumulations of no more than a few millionths of total body weight are enough to cause paralysis and death. If methyl mercury enters the body, it takes about seventy days before it is excreted, so a safe daily intake is less than one seventieth of the miniscule amount that causes severe nerve damage.

The current situation with respect to mercury is unclear.