NEPA Process and Specific Requirements - Environmental Impact Analysis: Process and Methods - page 78

However as the population and demand for water grew, another water
source was necessary, and the adjacent Potomac River was identified and
put into service as the obvious and plentiful potable water source. Currently
the Washington Aqueduct, a division of the U.S. Army Corps of Engineers,
Baltimore District, operates two water treatment plants (WTPs) processing
the Potomac River water. These facilities, the Dalecarlia and the McMillan
WTPs, both in Washington, D.C., provide potable water in adequate quan-
tity and quality for over 1 million people in the D.C. and Northern Virginia
areas. When these facilities were first constructed at the beginning of the
twentieth century, they represented the state of the art technology in water
treatment and the processes were more advanced than virtually any in
the country. The treatment removed solid particles (e.g., river silt) from the
Potomac River raw water source, treated and disinfected the water, and
distributed it to the metropolitan service area. The water treatment system
historically consisted of a series of reservoirs and treatment facilities, with
raw water diverted from the Potomac River and collected in the Dalecarlia
Reservoir. River silt was naturally deposited in the forebay of the Dalecarlia
Reservoir and the silt (forebay residuals) was periodically dredged and
trucked offsite, or utilized onsite. The water treatment operations achieved
an additional level of silt removal by adding aluminum sulfate (alum) prior
to the sedimentation basins at the Dalecarlia Water Treatment Plant and
the Georgetown Reservoir where the sediment was removed. Periodically
flushing the settled residuals from the basins to the Potomac River was the
historic residual management practice.
Thus the solids removed during the treatment process were returned
to the Potomac River under the logic that they came from the river and it
made sense to return them to the river. However, when the existing National
Pollution Discharge Elimination System (NPDES) issued under the Clean
Water Act expired a new permit was issued which effectively precluded the
discharge of water treatment solids, or residuals, to the river. Since chang-
ing the treatment process to eliminate the discharge of solids to the river
was a federal action with potentially significant impacts, NEPA applied and
because of the magnitude of the project the U.S. Army Corps decided to pre-
pare an EIS.
3.3.2 Washington Aqueduct Water Treatment
Residuals Purpose and Need
Consistent with the discussion above regarding the relationship of the pur-
pose and need statement and evaluation of alternatives, the Washington
Aqueduct purpose and need statement was wisely and narrowly constructed.
The forcing function for the action was compliance with the new NPDES
permit requirement to minimize or eliminate discharge of solids to the river,
and thus this was central to the purpose and need for the action. The pur-
pose and need statement was included in the notice of intent, published in