In the 40 or 50 miles below that point, the North Branch accumulates great quantities of more usual kinds of pollution as it runs down a broadening valley past towns and industries that have grown up because of the conjunction of coal, timber, water, and railways—and in the old days water transport, for flatboats used to shoot the river at high water, and later the C. & O. Canal operated upriver as far as Cumberland. Treatment of wastes in this reach is spotty and mainly inadequate. Some industries and towns sluice them raw into the dark, sad water, and others give only perfunctory primary treatment; the city of Cumberland releases the equivalent of about 18,000 persons' body wastes each day as effluent, besides extra raw wastes whenever storm runoff overloads its combined storm and sanitary sewer system and causes it to overflow. Where major efforts have been made, as at the Upper Potomac River Commission's Westernport plant below the big Luke, Maryland, pulp and paper mill, the wastes are so voluminous and complex that some of them still have to be dumped, and the effluent from even highly efficient treatment further degrades the river.
Fortunately, the North Branch, acid above, deprived of oxygen and overenriched and septic below, is not typical of the flowing parts of the Potomac river system, but it stands as a good grim example of what pollution can mean, and as a foretaste of what may be expected to happen elsewhere in the Basin if it is not stopped soon. Mine acid is not a significant problem in any streams outside of that region, but untreated or inadequately treated wastes are badly blighting many streams and rivers or stretches of them. Some smaller watercourses, like historic Antietam Creek below Hagerstown, Maryland, have deteriorated under the influence of discharges from single or limited sources, while larger ones suffer from a cumulative waste buildup in areas of concentrated population or industry. Some twenty miles of both industrial and municipal pollution in the South River Branch of the Shenandoah's South Fork below Waynesboro, Virginia, have done much damage to that legended river for a good distance downstream, a situation that is worsening with the area's growth. On the North Fork of the Shenandoah similar effects have been wrought by heavy organic loads from poultry processing and other things. The list could be extended: aside from a few happy exceptions like the prized Cacapon, draining rugged, forested, thinly peopled hill country, nearly all the Basin's flowing streams of any size receive damaging loads of waste from towns and industries.
WATER TREATMENT STEPS
| (1) River water enters here |
| (2) Water chlorinated |
| (3) Water settles. Heavy particles sink |
| (4) Water pumped to Pretreatment Building |
| (5) Various chemicals (chlorine, alum, lime, carbon) added. Chemicals |
| and water stirred in rapid mixing basins |
| (6) Slow mixing to form "floc" (see Alum below) |
| (7) Water settles for 2½ hours. "Floc" carries impurities to bottom |
| (8) Water filtered through 94 rapid sand beds |
| (9) Final chemical treatment (chlorine, lime, fluoride, phosphate) |
PURPOSE OF CHEMICALS
| CHLORINE: | Destroys organic materials |
| PHOSPHATE: | Lessens pipe corrosion |
| FLUORIDE: | Lessens tooth decay |
| CARBON: | Controls taste and odor |
| ALUM: | Forms "floc" (snowflakes) to trap impurities |
| LIME: | Helps "floc" formation; lessens pipe corrosion |
The basic and usual damage comes from oxygen depletion. A stream has a natural capacity for hastening the decay of organic wastes, which is determined by such things as the volume of its flow, the pollution already in it, its velocity and depth, and its temperature. When that capacity is exceeded, as we have noted, too much of the stream's oxygen is used up by the process of decay and the stream, which is an intricately complex work of living things, begins to die. Under really bad conditions, the waste solids themselves cannot all be assimilated, and hence may build up in layers of stinking sludge at the bottom of the stream and continue to seize available oxygen for a long time thereafter.
Conventional waste treatment, in plants built by towns or by industries whose raw materials are animal or vegetable in origin, is aimed at removing the solids in the wastes and reducing the bio-chemical oxygen demand—called B.O.D. It is a speeded-up version of the same process of purification that goes on normally in any stream when loads are not too heavy. "Primary" treatment removes such solids as will readily settle out and passes the rest on back to the stream as part of the effluent. "Secondary" treatment plants, after settling out the gross solids, speed up decay by furnishing air to the bacteria that eat up dissolved and finely suspended materials; a good secondary plant, under much more skillful supervision than is usual, can get rid of 85 or 90 percent of the organic materials and the associated B.O.D. by the time it turns its effluent into a stream. How damaging that effluent will be depends on a number of things, chief among them being the size and condition of the receiving stream and the volume of organic materials that went into the treatment plant in the first place. A riverside town of 1000 with a secondary treatment plant operating at 75 percent efficiency is going to inflict on its river a daily load roughly equivalent to the raw sewage from 250 people.