a. Water and Sewer Lines in Close Proximity. Water and sewer lines are both
subject to infiItration, even though joints are properly made and are tight. The infiItration
into sewer Iines laid below the ground water level may be as much as 10,000 gallons
per miIe of sewer per day. Sewer Iines above the ground water level wiII have much
less infiItration; but, during periods of heavy wastewater flows, seepage into the ground
surrounding the sewer pipe wiII occur since the pressure inside the sewer pipe wiII be
greater than outside except during periods of heavy rainfall when the earth is saturated.
Since water Iines are normally under pressure, there should be no infiItration into the
system; however, small leaks may permit some seepage into the surrounding earth.
Under a combination of unusual conditions (such as a leak in a potable water Iine, a
reduction in water pressure caused by such a leak, or by shutting down a portion of the
system for repairs, and an unusual amount of water in the soiI surrounding the leak),
nonpotable water could be drawn into the potable water pipe. If the water in the soiI
were from a leaky sewer pipe, septic tank, or disposal field, the results could be
disastrous. In order to prevent such possibiIities, water and sewer pipes should always
be separated by as much distance as possible, and never in the same trench if it can
possibly be avoided. In any event, the sewer Iine should always be laid at a lower
elevation than the water Iine. In acid soiIs or in cinder fiII, both the sewer and water pipe
should be enclosed in a 3-inch concrete casing to prevent see page and infiItration (see
Figure 6-10. Concrete casing in acid soiIs or cinder fills.