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Plant at costs shown in Table 2. The number of days of pomping, as shown in Table 3, would also increase somewhat.

The pumping cost which would stem from maintenance of a flow of 100 mgd in the Potomac River from Great Falls to the Estuary should, it appears, be allocated to all of the water systems that are supplied water by the Wash

ington Aqueduct, in proportion to the quantity of water taken. This is appropriate, since the need for pumping 100 mgd from the Estuary is the result of their usage of Potomac River water.

The possibility of producing hydroelectric energy from these units at times when they are not needed for pumping has been investigated and found to be uneconomic.

9. The Potomac River Estuary

The Potomac River Estuary extends for 117 miles to its junction with the Chesapeake Bay. Water is added to it by many tributaries, the largest of which is the Anacostia River, which enters at Washington. The total contribution of the tributaries in the reach of most interest in this study, which is that part of the Estuary between Little Falls and the vicinity of Indian Head, Maryland, is not sufficient however to add appreciably to the water in storage in the Estuary, as their drainage area is only 4 percent of the total drainage area of the Potomac River above Indian Head.

The width of the Estuary varies from a few hundred feet at the head to almost 6 miles at the mouth. Shallow depths are found throughout most of the estuarine reach except in the channel. The reach from Little Falls downstream 2 miles to Three Sisters, in which section it is proposed to locate the Palisades Pumping Plant, is by contrast fairly deep in most places.

The Estuary is a drowned valley where the fresh waters of the Potomac River and the salty waters of the Chesapeake Bay mix. This mixing extends up the Estuary to the vicinity of Indian Head, where a brackish-water front exists. Although some salty water intrudes beyond this front, the chloride content is within the limits considered desirable for municipal water.

The ocean tides which reach to Little Falls, where the river makes its last descent over the fall-line rocks into the Estuary, are a major contributor to the mixing of the waters. The mean tidal range in the Washington Channel is 2.9 feet.

The upper reach of the Estuary, which contains fresh water, is a relatively narrow section extending for a distance of about 24 miles to the brackish-water front. The width of the Estuary in this reach does not exceed about 134 miles, as may be seen in the back-cover illustration.

The total volume of water discharged by the Potomac River in every water year exceeds by many times the 100 bg of fresh water that is held in natural storage by the upper Estuary. As a result, this reach of the tidal river is thoroughly flushed several times every year. For example, the total discharge of the Potomac River into the Estuary in the low-water year of 1965-66 was about 1,300 bg, excluding the water taken for municipal use; or 13 times the 100 bg of fresh water in natural storage. In the high-water year of 1948-49, the total discharge of the Potomac River into the Estuary was 3,800 bg, or 38

times the volume of fresh water in natural storage. Not all of the discharge of the Potomac River is effec tive in flushing the upper reach of the Estuary, as the rate of discharge must be over about 10,000 cubic feet per second for perhaps as long as a minimum of 10 days in order to reach the brackish-water front. Such conditions occur several times in every year, however, even in years of extremely low water.

In addition to the water discharged into the Estuary by the Potomac River and its tributaries, waste water will also be discharged into the Estuary. The annual amount of this forecast for the years studied, is as follows:

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The amounts of waste water shown above will be de rived from used municipal water supplied from the Potomac River, water imported into the Potomac River Basin from the Patuxent River Basin, water taken from small streams, wells and springs, and, starting with 1980, water pumped from the Potomac River Estuary. The volume of sanitary waste water is fairly uniform throughout the year. This will make it easier to dispose of the sanitary waste water if it is decided to transport it any distance from the places where it is treated.

The increasingly large volume of waste water will, if properly treated and discharged at the right places in the Estuary, have little or no adverse effect upon the environment. If these things are properly done, the continued availability of an adequate amount of fresh water in natural storage to supply the emergency requirements of the Region can be assured.

By installing the Palisades Pumping Plant it will be possible to meet all of the emergency municipal water requirements of the Region until about the end of this century, without taking more than a small amount of water from the Estuary. Thus there will be adequate time to make the needed changes in the treatment of waste water and its discharge to the Estuary, so as to bring about an improvement in the estuarial water which, it is expected, will be required by the Federal Water Pollution Control Act.

10. Quality of the Water in the Potomac River Estuary

In considering the adverse effects of pollutants in the Potomac River Estuary, it might be helpful to understand how impurities get into the water and how they can be removed. Water is used over and over again in the natural hydrologic cycle of the world. In this cycle atmospheric

moisture that is in transportation and storage, almost free of pollutants, eventually falls to the earth as precipitation. There it is disposed of by surface runoff and evaporation, by infiltration into the ground, and by transpiration from trees and other vegetation. All this water remains in the

hydrologic cycle. Part of it is initially returned to the atmosphere by evaporation and transpiration, and the remaining portion goes into storage in the ground or in lakes, ponds, and the oceans. From these natural reservoirs it is evaporated to repeat the cycle.

The waters in the natural hydrologic cycle become pol. luted to a certain extent as they move over the surface of the earth and through the soil and rocks, until again taken into the atmosphere by evaporation or transpiration, or after reaching the oceans from where they are evap orated. In either case they leave the pollutants behind. The pollution of the portion of these waters that is used by man for municipal and industrial purposes is increased, because of use primarily for transporting unwanted waste substances. These substances are usually fecal material, soap and detergents, chemicals, heat and other materials. The water itself is not changed by addition of these pol lutants. Upon their removal by the natural process of flowing streams, and while in storage or in man-made treatment plants, the water becomes suitable for reuse. The effectiveness of these natural and man-made purification processes is indicated by the large extent to which water, during its stay on earth, is being reused today for munici pal and industrial purposes in many parts of the United States.

Pollutants entering the Potomac River Estuary from the Region are chiefly from the following sources: 1. Municipal Waste Water

2. Storm Waters

3. Boats and Ships

4. Dredging Materials

All of these pollutants can be reduced to a satisfactory level using means now available.

A comprehensive estuarine pollution study by the Department of the Interior to develop recommendations for a national action program, was directed by the Congress in the Clean Water Restoration Act of 1966. Under the Act the Secretary of the Interior must report on the matter by November 1, 1969; but he has indicated that recommendations for action may be made earlier. The study is needed; for, while we may in the past have planned well in our separate compartments for a single purpose there is now a need for consideration of all phases of the Estuary problem in the full perspective of their inter-relationship. It seems that, so far, no group of planners has considered the ecological impact of their proposals on the Estuary, even when confronted with a certain threat to its natural values. What is needed is better Estuary management.

Processes commonly used for many years can be employed to remove pollutants from municipal waste water of the Region. Filtration (usually through beds of sand), sedimentation (by precipitation and settling) and coag. ulation (by thickening or curdling) can be employed to remove up to 95 percent of the suspended solids. Oxida tion by forced aeration can make oxygen-demanding substances inert by removing up to 95 percent of the biochemical oxygen demand (BOD). These are usually referred to as primary treatment (filtration and sedimentation) and secondary treatment (oxidation) and use what is called the activated sludge process.

Nutrients (nitrogen and phosphate) remaining in the

effluent from the treatment plants, which result in the biological enrichment or eutrophication of the Estuary, stimulate the growth of aquatic plant life such as algae. These die and sink to the bottom, and as they decompose will absorb oxygen from the water. This shortage of oxygen may kill animal life, which in turn decomposes. This however does not make the water unusable for municipal purposes when properly treated. These nutrients can however be removed by additional treatment such as adsorption, or perhaps by a modification in the conventional primary and secondary treatment to increase the aeration. An increase in aeration can, at least in some treatment plants, remove most of the phosphates which will drastically reduce the growth of aquatic plant life. A joint Government and soap and detergent industry task force has recently been established by the Secretary of the Interior to investigate eutrophication, including the role that phosphates and detergents play in the process, and their possible replacements.

The effluent from waste water treatment plants is nor mally treated with chlorine to sterilize it. When this is done adequately, the coliform bacteria count can be kept low. Effluent from the Blue Plains sewage treatment plant of the District of Columbia, which treats most of the waste water in the Region, is not well treated. As a result, the effluent pollutes the Estuary more than it should. Instead of the Blue Plains plant removing up to 95 percent of the contaminants, as is possible with a well-designed and operated treatment plant, the removal has been as low as between 60 percent and 70 percent in the summer. In addition the plant does not remove an adequate amount of nutrients, with the result that algae flourishes in the Estuary. These are matters which must be corrected under the Federal Water Pollution Control Act. Plans have recently been initiated toward this end.

Storm waters from the Region should be treated before release to the Estuary to prevent pollution from this source instead of overflowing without treatment when the capacity of sewers is reached as is presently the case. These waters contain pollutants deposited on streets and highways and from other sources, such as garbage, animal and vehicle droppings, newspapers, dirt and pesticides, and all manner of debris. When storm waters overflow, they also contain sewage picked up in the sanitary sewers. In order to treat the polluted storm waters it will probably be necessary to enlarge the sewers leading to the treatment plants, and increase the capacity of the plants. As an alternative, provision might be made for temporary storage of storm waters so they can be subsequently transported to treatment plants through existing sewers a little at a time, to avoid need for increasing capacity of the sewers and the treatment plants.

The District of Columbia has, for a number of years, gradually been separating storm sewers from sanitary sewers so that storm waters will discharge directly into the Estuary without treatment. This does not solve the basic problem of keeping pollutants out of the Estuary, and in addition is costly. Also unfortunately, there is usually considerable precipitation in the Region during the low-flow season, and storm water flushes pollutants into the Estuary when the river is not high enough to flush them out.

Storm waters can be stored temporarily in a number of ways, depending upon circumstances. Among these are

storage in ponds and sewers, and storage in tunnels and caverns beneath city streets. The City of Chicago is presently boring a tunnel beneath streets, near the northern city limits, for use as a waste-water storehouse. Waste water after a storm can be gradually pumped from storage into the existing sewer system leading to treatment plants. It has been estimated that this method of handling storm waters is less costly than separating storm and sanitary sewers, and in addition provides for treatment of the storm waters, not usually contemplated in the separation of the sewers. Several other major cities in the United States either have started construction of projects to eliminate pollution from storm water or are studying the matter. Pollution released from boats and ships in the Estuary can be controlled by promulgating proper regulations. Major sources of pollution are sewage, oil, garbage and ballast and bilge water. Steps have recently been initiated to control these by Federal legislation.

Pollution of the Estuary by dredging materials is being studied and it is expected this will be adequately controlled in the foreseeable future.

Although the average citizen of the Region is likely to call the Potomac River a cesspool or an open sewer this is to say the least incorrect. The quality of the water entering the Region is reasonably good and is improving. The quality of the water in the upper Estuary is determined by the Region and not by upstream abuses, as pollution in the Estuary remains near its originating source. Primary cause of the difficulties in the Estuary is growing population and lack of adequate treatment for waste and storm waters that are discharged directly into the Estuary. If the steps previously outlined in this study are taken, quality of the water in the Estuary should improve considerably. The cost of keeping pollution out of the Estuary will be relatively small in comparsion with the increasingly adverse effects that may be otherwise expected if this is not done. Among other things, it will be less costly to take municipal water for emergency use from a clean Estuary than to construct 16 large dams and reservoirs upstream to dilute pollution and supply the water.

The upper Estuary of the Potomac River would in each month of a low-water year receive more water from the discharge of the river than would be taken from the Estuary for municipal water in each of the years studied, until July in the year 2000. Starting in that month under these emergency conditions, more water would be taken from the Estuary than would be supplied by the Potomac River discharge in the months of July and August, as may be seen in Table 4. This condition could first occur in a low-water year between the years 1990 and 2000, and might occur in any year thereafter under these emergency conditions. Such emergency conditions may be expected only occasionally, as previously noted.

By the year 2010 the amount of water taken from the Estuary would in a low-water year slightly exceed the discharge of the Potomac River in the month of November as may be seen in Figure 3. The net amount of water taken from the Estuary would however be relatively small and not of significant importance.

It will be observed that the net amount of water taken from the Estuary for municipal water is relatively small in all cases compared with the 100 bg of fresh water held in natural storage.

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All the water that enters the municipal water systems of the Region will be discharged into the Estuary as effluent from waste water treatment plants, except for losses by leakage from the water and sewer systems, use losses such as evaporation, transpiration from watered plants and trees, or passage into the ground after use. These losses will be about 25 percent of the total entering the water systems.

In the year 2010 for example the following quantities of waste water from municipal systems are expected to be returned to the Estuary during the summer months.

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These amounts are substantially greater than the net amounts it is proposed to take from the Estuary for municipal water supply shown in Table 4. As a result, the use of water from the Estuary for municipal water supply will not decrease the total volume of fresh water in natural storage, and the brackish-water front will not move upstream because of this use.

It may be desirable at some time in the future to transport part or all the municipal waste water of the Region to a point near the brackish-water front before discharging it into the Estuary. This would, to a considerable extent, prevent mixing the waste water with the fresh water in natural storage. The need for such transportation will not develop, however, until about the end of this century, as there is about 13 bg of fresh water of good quality in natural storage in the upper Estuary between Little Falls and the Blue Plains Waste Treatment Plant.

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