ffers some hope for the future,
especially if it is fostered and strengthened by overall protective
measures.

Techniques for cleaning up

Two main general approaches to water quality improvement exist:
treatment of pollution at its source or occasionally after it has
entered a stream, and augmentation of the stream's flow to help it
assimilate loads of waste beyond its natural capacity. A third
possibility in certain situations is the diversion of wastes out of a
stream's drainage entirely. In practice, these methods can be varied and
combined in any number of ways to fit a need.

To take the last one first, diversion of whole wastes as received from
their sources is a total and dramatic means of coping with a pollution
problem stemming from collectable wastes, but it often has
disadvantages. One of these, of course, is the possibility that the
pollution problem may be simply transplanted elsewhere--that the water
in which the wastes eventually end up will suffer. Another is loss of
water from the stream system. If, as is usual, a town gets its water out
of the local river or a tributary and does not give it back after
use--preferably well cleaned up--other users downstream are not going to
have as much water available to them, and the essential processes and
ecology of the river itself may suffer.

The only place such wholesale diversion of wastes has been seriously
considered in the Potomac Basin is at metropolitan Washington, whose
sewage could feasibly be piped across Chesapeake Bay and the Delmarva
peninsula and well out into the Atlantic--possibly, as has been
suggested, in combination with sewage from Baltimore. It would be a
permanent means of disposal, but very expensive in terms of both
investment and operating costs. Furthermore, though in the estuary no
downstream users would suffer a loss of water supply, the water content
in metropolitan sewage has at times risen as high as 80 percent of the
flow of the river above the upstream intakes. The effects of such a
subtraction of fresh water on the estuary itself--changes in flow, and
in the penetration of salt water upriver, with an inevitable alteration
in valuable fisheries and the whole balance of aquatic life established
through millennia--could easily turn out to be disastrous.

Standard treatment of pollution at its source consists of the primary
and secondary processes we have glanced at, sometimes adjusted to
specific industrial wastes. It has t