Water Supply and Sewage Disposal

by Philip J. Stone

In addition to providing a portion of the natural hydrologic cycle by moving precipitation which falls on the land down to the sea, rivers perform a number of useful functions for mankind. Most of these are non-consumptive uses which utilize water without using it up -- recreation (swimming, boating, fishing, water skiing, ice skating), navigation, wildlife habitat, and water power. One other use is consumptive -- water supply for municipal and industrial use and for irrigation. Much of this water withdrawn from streams ultimately finds its way back to them as sewage, treated and untreated. Textbooks on sanitary engineering not many years ago listed "transportation of wastes" as a legitimate use of rivers. Fortunately public attitudes have changed.

The purpose of this article is to discuss water supply and sewage disposal in the Potomac Valley near Washington. Observers along the river and C & O Canal will notice some of the structures referred to from time to time.

Water Supply

In the early history of the Nation's Capital springs and wells afforded adequate water for the relatively small population. First it was brought by hand to residences and places of business; later pipes were laid under streets to hydrants, houses, and businesses. Originally made of bored logs, the pipes were later made of cast iron.

By mid-19th century it was apparent that a larger, more reliable, and safer water supply was needed. Beginning in 1850 Congress authorized tapping the Potomac River above Great Falls for a water supply for Washington and construction by the Army Corps of Engineers soon began under the leadership of (then Lieutenant) Montgomery C. Meigs. The project, known as the Washington Aqueduct, comprised a dam across the river just above Great Falls, a conduit parallelling the Potomac from Great Falls to Dalecarlia at the D.C. line, storage and settling reservoirs at Dalecarlia and west of Georgetown, and various tunnels and mains to bring water into the city. The Great Falls Dam was 2,877 feet long, 8 feet wide at the top, and 10-15 feet high, with intakes at the Maryland end. The original conduit was 9 feet in diameter and 11 miles long, conveying water by gravity flow. It parallelled the C & O Canal rather closely and was carried over Cabin John Creek by the Union Arch, or Cabin John Bridge. For many years this 220-foot stone arch span was the longest such span in the world. The project first delivered water to Washington in 1859 and was considered one of the great engineering feats of its time.

Conduit Road (later renamed MacArthur Boulevard) was built atop the aqueduct, except for the stretch between Old Anglers Inn and Great Falls, where an unimproved road known as "Berma Road" affords passage to non-motorized traffic.

Shortly after the turn of the century a typhoid fever epidemic in the Capital brought demands for cleaner water than the often turbid Potomac provided. Congress responded by authorizing the construction of the MacMillan Filtration Plant and Reservoir at First and Douglas Streets, NW (east of Howard University) and the 4-mile Washington City Tunnel to carry water under the city from Georgetown to the new site and filter it. This was completed in 1905.

Increasing population required expansion of the water system and in 1926 a second conduit from Great Falls was completed, together with another filtration plant at Dalecarlia, finished in 1928. The new conduit was placed under the south shoulder of MacArthur Boulevard and crossed Cabin John Creek in a siphon instead of a bridge. The more modern and compact Dalecarlia Filtration Plant uses rapid-sand filters (instead of the slow-sand filters in the McMillan Plant). Aluminum sulphate, chlorine, and flouride are now used to purify water at both plants.

In addition to the primary source of water at Great Falls, two additional sources are now available. In 1959 the Washington Aqueduct built the 1500 foot long Little Falls (Brookmont) Dam just upstream from the old Little Falls Dam used to divert water into the C & O Canal. This plant can pump water through a 10-foot tunnel 4,560 feet up to the Dalecarlia Reservoir, providing an independent, supplementary supply. This is used only occasionally in summer and fall droughts when the flow through the conduits does not meet the system's needs.

Further concern about adequacy of the water supply due to population growth and dry periods was allayed in the 1970's by construction of an emergency intake from the Potomac at the head of its estuary, just below Little Falls. Great improvement in the quality of the water in the estuary in recent years, due to secondary and tertiary treatment at the Blue Plains Plant, have made it feasible to utilize this large reservoir during emergency drought conditions. This estuary water can be pumped to the Dalecarlia Reservoir without lowering levels in the Potomac above Little Falls to the point where the scenic beauty of Great Falls would be impaired.

Residents of the District of Columbia are not the only beneficiaries of the Washington Aqueduct. For many years the Corps of Engineers has supplied water to Arlington County, Falls Church and a small portion of Fairfax County. One pipeline crosses the river to Virginia under Chain Bridge; another crosses in the upstream side of Little Falls Dam.

Burgeoning suburban populations now require new sources of water for the Washington Suburban Sanitary Commission in Maryland and the Fairfax County Water Authority in Virginia. A number of years ago the W.S.S.C. constructed an intake about a mile above Swain's Lock which pumps Potomac River water up to the Potomac Filtration Plant on River Road. More recently the F.C.W.A. has tapped the Potomac across from Violet's Lock to supplement its supply from Goose Creek and elsewhere.

Sewage Disposal

The story of sewage disposal is the story of keeping waste out of the Potomac and its tributaries.

Early Washingtonians made use of privies and waste water drains emptying into the streets. When sewers began to be installed in the 1840s, drainage went to the nearest stream or canal and from there to the Potomac. For some years much waste was absorbed by marshes along the river, but as population and sewage volume increased, a very offensive and unhealthy condition in and near the Potomac developed. After 1890 all trunk sanitary sewers were connected to an interceptor which ultimately discharged to the Potomac near Blue Plains, in extreme southwest D.C.. But that didn't halt the overflows during storms from "combined sewers" into Rock Creek and the Anacostia River.

For a time all this did not affect the stretch of the river above Washington in the Maryland and Virginia suburbs. But as denser communities developed near the river in Montgomery County sewers were built in Cabin John Creek Valley and elsewhere, and to avoid drainage into the Potomac an interceptor sewer was built from the District along the C & O Canal to Cabin John to connect with such drainage about 1930. In Arlington County sewage from neighborhoods near the Potomac was pumped uphill to trunk lines on higher ground.

Between 1935 and 1938 the D.C. Department of Sanitary Engineering constructed its first sewage treatment plant at Blue Plains to process sewage before its discharge to the river through an outfall sewer. The original treatment was "primary," including straining out solids, sedimentation, sludge digestion, and chlorination. Later "secondary treatment" was introduced with a biological process that uses aerobic bacteria, activated sludge, and aeration. Today "tertiary treatment" has been added to remove resistant contaminants such as phosphates and nitrates. Agreements with the Maryland and Virginia suburbs in the 1950s have resulted in a large volume of their sewage being piped to Blue Plains, thus avoiding duplication of expensive facilities.

Shortly after Dulles International Airport was built in the 1960s the Potomac Interceptor was constructed from the airport to Washington, using a route down Broad Run to the Potomac, then parallel to the river on the Virginia side to the Great Falls water supply dam, across the Potomac to Maryland just below the dam, and thence down along the berm side of the C & O Canal to Washington and the Blue Plains treatment plant. This interceptor has three feeders, two from Virginia suburbs and one from the Maryland area around Seneca. In the vicinity of Sycamore Island, on the berm side of the canal, will be seen two types of sewer ventilators: iron pipes about 5 feet high on the earlier Cabin John Interceptor, and smaller stone masonry structures on the Potomac Interceptor.

For some years efforts to treat sewage and maintain the Potomac relatively clean at and below Washington couldn't keep up with the added load of pollution from a rapidly growing population, and the quality of the river deteriorated badly. But as citizens and local officials became aware of the problem they became more willing to spend money for pollution abatement. The availability of Federal grants for this purpose helped greatly. Today great progress has been made in upgrading water quality near Washington. When completed, our Blue Plains Pollution Control Plant will be one of the finest in the country. Conditions above the estuary never were bad, and they have improved as treatment facilities have been installed in small towns upriver.

The remaining problems are the "combined sewer" overflows during storms in older parts of Washington, and the larger question of "non-point" pollution from surface run-off during storms. Much of this is a rural problem as fecal matter from domestic animals washes into streams. The quality of Potomac water near Sycamore almost always deteriorates temporarily after storms, but most of the time it is quite good.

Much credit is due to the Interstate Commission on the Potomac River Basin (INCOPOT), established in 1940, for its efforts on behalf of pollution abatement. Its programs of fact-finding, public education, and gentle persuasion have served as an effective catalyst to bring about a cleaner Potomac which we all enjoy today.