No easily applied method of testing can effectively establish the guaranteed absence of human disease germs. The traditional "Coliform Count" plays safe, as it must. It measures the concentration of certain easily spotted "indicator" organisms that do not themselves make people sick but are always voluminously present in the fecal discharges that can carry harmful germs, and it gauges the danger by the concentration of these indicators.
However, concentrations of coliform bacteria, originating in animal manure or elsewhere, may invade a stream through runoff from rural lands without having any meaningful relationship to human disease germs. Counting them under such circumstances is a little like measuring the depth of the proverbial well by the length of the pump handle. Furthermore, no one really knows how easy or how hard it may be to catch given diseases by swimming. In this country, outbreaks of leptospirosis, an illness common to man and certain animals, have been traced to swimming holes, and other links are obvious. On the other hand, some careful British investigations turned up a good many quite healthy people who habitually splashed about in sea water teeming with pathogenic organisms of one sort or another. Sea water and fresh water have vastly different qualities, but the subject is presently full of confusion, and it needs much research.
Land runoff in general furnishes a large amount of pollution of all classes, and in all parts of the upper Basin except the least-used forest sections. Besides bacteria, heavy loads of organic material may be washed into streams in regions with high densities of livestock or poultry, and some pollution of this sort is found practically everywhere. The wild and domestic animal population of the Basin above Washington has been estimated to produce wastes equivalent to those of about 3.5 million people. Much of this is dealt with by the "living filter" of the soil, but much also reaches the streams, associated with sediment from erosion producing rains. And the sources, particularly in areas such as those along the Shenandoah and the Monocacy and other streams with wide rich valleys, are numerous and diffuse.
Nitrogen and phosphorus nutrients, which foster weeds and slime in quiet stream stretches and contribute to the problems of the estuary downstream, are found in undesirable concentrations in most of the Basin's waters above the Fall Line. Not only are the growths encouraged by these fertilizing agents ugly, but they also upset the ecological balance of streams by favoring certain types of aquatic life over others, and they can cause tastes, odors, and clogging in water supply systems and sometimes, by rotting, a secondary sort of oxygen deficiency. Nitrogen and phosphorus occur in the effluent from waste treatment plants, for they are present in human wastes and in detergents, and in dissolved form are little affected by standard treatment processes. And in the upper Basin a large part of the nutrient load in streams appears to be associated with sediment from the same diffused land runoff mentioned above, for they occur abundantly in manure, in synthetic fertilizers, in certain natural soils, and in decaying organic substances of many kinds. The health and growth of living things is dependent on these elements, of course; it is their excessive release into waters that causes trouble.
From the same farming regions and even more from lawns and gardens and parks in more populated areas, pesticides and other economic poisons accompany sediment into the stream system or are blown into it as sprays and dusts. They seem not to be as great a problem in the Potomac as in some other rivers, but they are present in probably significant amounts; indicator tests hover near Public Health Service drinking-water limits in the river. Their use, here as elsewhere, increases year by year, for they are tremendously effective against many of man's ancient enemies. Being easily available, they are often used in uninformed and careless ways despite government efforts to determine and publicize safe levels of application. Knowledge about their side effects, both immediate and long-term, is still full of gaps. Badly misused, they are obviously dangerous. But information about the precise results of their ordinary use and their buildup in nature accumulates very slowly.
The persistent chlorinated hydrocarbons—DDT and its relatives—last for a long time after being released into the environment, concentrating at various points in the natural food chain and often in man himself. It is said that an average adult Californian's tissues today contain more DDT than is allowed in beef for interstate shipment. But no one is yet certain what this means in relation to that average Californian's physical wellbeing, and in terms of fish and wildlife, though the link between these materials and certain destructive changes can be seen, evidence in other cases—the declining fertility and numbers of bald eagles, for instance, which some investigators believe to derive from pesticide residues—only points toward such a link. Until all the facts are in and the impact of such poisons has been clearly restricted to the pest species at which they are aimed, they are going to continue to be a heavy concern for conservationists and others alarmed about environmental pollution, along the Potomac and elsewhere.
One of the principal Potomac pollutants, silt, not only comes from the land but is the land, most often good topsoil, washing away toward the sea. Even under pristine conditions streams are likely to run somewhat muddy after storms; it is a natural phenomenon, a by-effect of the way climate carves landscapes. On the evidence, however, the Potomac landscape since its colonization by white men has been undergoing a much more rapid carving than anyone could consider to be natural. Most of its streams, particularly in their lower reaches, are thickly opaque for long periods after rain, and gross erosion in the Basin—the amount of soil washed away from where it usefully belongs to somewhere else—averages about 50 million tons per year, a major depletion of the soil resource and a degrading influence on the landscape through erosion. The part of this silt that gets into streams cuts down on the usefulness of the water, creates ugly turbidity, chokes quiet pools and reservoirs, suffocates bottom-dwelling creatures and plants on which the streams' wholeness may depend, and rides down the current to add heavily to the problems of the estuary, into which some 2.5 million tons of it are annually discharged.
Sediment is dislodged from the land by the pounding action of raindrops and the flow of runoff, and sometimes is washed from streambanks during high flows—which may themselves be higher and more frequent because of silt-clogged channels. The bulk of it can be blamed on unsound land use. This may be rural, based in the old use-her-up-and-move-along pioneer outlook that has never died out among us despite wide understanding of better ways of doing things. People in places still overgraze pastures and clean-cut timber so that rain can get at the soil and eat it away, and they still farm land too steep to stay in place without its vegetative cover, or they plow even suitable rolling land in straight rows up and down hill so that water and soil sluice away together down the furrows when it rains. Despite a sharply effective three decades of work and public education by the Soil Conservation Service and other agencies, these old practices continue in some places and cause much erosion.