urces, 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 environmen