Desalting of sea water, another reality now in arid zones and one of immense importance, has a certain degree of planned scarcity built into it by way of its price, at least at present. Some people believe that in time this process will be refined to the point that it can furnish abundant cheap water to all the world's seacoast cities. Certainly as it develops it may well have a potential for marginal drought-proofing at Washington, an emergency source to be drawn upon if needed. But the day seems distant when it will be truly competitive in price with riverine sources in regions of adequate rainfall.
Inland arid regions and perhaps other places as well are undoubtedly going to find one answer to water shortages in the recirculation of their treated waste waters through municipal systems. In one form or another such recirculation is already working at certain places in the United States on an emergency basis, and its full potential for industrial use has yet to be explored. However, the indications are that towns' and cities' reliance on it during anything but temporary emergency conditions is going to depend on expensive methods of refinement and "fail-safe" overdesign, plus dilution with new water, which means again that it will probably not be competitive in price with natural water where enough good natural water can be had. To this may be added the observation that the consuming public presently has a few definite lingering qualms about the idea involved, particularly if there is other water around.
The underground rocks and sands of the Basin hold huge reserves of water with a fundamental relationship to the whole river system, whose basic dependable sources lie in these aquifers' outflow to the surface. Around the metropolis, some ground water is being taken from wells even now to supplement the overall supply and to satisfy the whole demand of any number of outlying communities. Though locally available quantities are limited and pumping costs rather high, such wells will undoubtedly be highly useful for future extensions of the metropolis, especially into the Coastal Plain.
There is also much promise in studies of the Basin's aquifers being carried out by the U.S. Geological Survey to determine detailed patterns of their contribution of water to the stream system and to see if it can be regulated and made even more useful. Such a possibility has great implications in terms of augmenting river flows both for water quality control and water supply, and could mean much at Washington. So could certain techniques of deliberate drawdown of aquifers to induce recharge with excess surface waters or sometimes treated sewage effluent, also presently under study. Ground water as a source has some unique advantages—among them a minimum of evaporation loss, less need for surface structures, and protection against catastrophic contamination—and it deserves full exploration, though it cannot at present be counted on as a significant part of the answer for the metropolis.
Far out, though possibly not very far off in time, is the likelihood that future water planners will be able to count on some degree of control over a given region's rainfall and snow. Through experimentation, this subject is rapidly being excised from the mists of superstition that once surrounded it, and the Department of the Interior has an active program of research and study in the West, with tremendous implications. But, yet again, present planning cannot take it into account except in the sense that, along with some of the other technologies already mentioned and undoubtedly others that have not yet even emerged to view, it adds to the near certainty that future planners are going to have a much wider range of alternative methods at their disposal, to choose from and mix as may seem best. And this, in turn, reemphasizes the wisdom of flexibility in present planning and the need to keep big irreversible decisions to a minimum.
The upper Potomac estuary from Little Falls down to the vicinity of Marshall Hall and Mount Vernon or below contains a great deal of fresh water, an accumulation made up of inflows from the river above the Fall Line, local storm runoff and tributary flows, and treated sewage returned to the tidal river. The volume of this water that would be available for use without salinity has been variously estimated. At low tide, there would be 9 billion gallons of fresh water in the upper estuary from Chain Bridge to the mouth of the Anacostia River; In the 10 mile stretch from Chain Bridge to the District of Columbia's Blue Plains treatment plant, 15 billion gallons; and, from Chain Bridge to the saltwater front near Indian Head, Maryland, 100 billion gallons. Most of the time now it is afflicted with heavy pollution, as will be detailed in the next chapter of this report. But it does constitute a large natural reservoir of potentially usable municipal and industrial water, whose attractiveness for these purposes, as well as for all others, will grow steadily as the pollution is brought under better and better control. These facts have led some opponents of any and all major reservoirs in the Basin to conclude that the water in the upper estuary is a presently satisfactory reserve with which to face any foreseeable metropolitan shortage of supply from the upper Potomac.
The assumption has strong appeal, but it appears to be too risky to serve as a basis for adequate present planning to meet looming demands. That even now the water in the estuary's uppermost reaches, above the main metropolitan treatment-plant outfalls, would be usable for short emergencies by the installation of relatively simple pumping equipment below the falls, cannot be doubted. That in the long run the major part of the freshwater tidal river at and below Washington is likely to be a valuable source of metropolitan water, maybe a principal source, is quite possible. Its use is and will be a strong consideration in longterm planning—another good reason, in fact, for flexibility. But the truth is that right now enough doubt and ignorance exist in regard to its exact potentiality that it should not be counted on to provide a safe margin of supply under all conceivable conditions during the next twenty years or so, for which planning provisions need to be more rigid and definite.
The doubts and unknown factors have to do mainly with the quality of this water, which comes under discussion later. In abridged summary of relevant facts at this point, it may be observed that unless all sewage and sewage effluents were collected and diverted to points well beyond the limits of the upper estuary, use of its water for periods beyond a few days of emergency would become essentially a form of recirculation of waste waters—with, at this time, the main drawbacks that we noted in regard to that process and certain others besides. For, under the low-flow conditions that would bring about its use, the effluents in the river below the mouth of the Anacostia would penetrate upstream as water was pulled out below the falls and would reach the pumps in fairly short order, probably moving in a tongue up the main channel.
With the radical improvement in the functioning of the metropolitan treatment plants that must be achieved, and other measures to relieve pollution in this part of the river, valid objections to such recirculation will of course weaken and ultimately disappear. But no one can reasonably expect that these things are not going to take a certain amount of time—quite conceivably enough time to run the city up against an emergency it could not handle without other, more standard sources of auxiliary water. Besides the matter of consolidating and improving treatment of collectible wastes, there are certain other diffuse and stubborn sources of pollution, as will be seen, for which good counter measures simply do not yet exist—among them are surface runoff during local storms and overflow from combined sewer systems.
If the collectible wastes were diverted out of the upper estuary and if it proved possible to cope quickly with other pollution or to ignore it, during prolonged use salt water penetration from downstream would take place as fresh water was withdrawn above and not replaced. Studies on a mathematical model of the estuary indicate that under conditions that could materialize, this would make the water at the intake too salty for use. A barrier dam across the entire estuary at one or another point in the freshwater section could prevent such penetration, but would be hugely expensive and undoubtedly more obtrusive on a much-used part of the riverscape than most upstream reservoirs could possibly be.