On-Site Wastewater Treatment and Disposal Systems - Forced ...

Groundwater, Elevation and Gradients: To accurately determine ground-
water elevations and gradients, observation wells or piezometers are
used. Observation wells are used to observe groundwater flutuations
throughout a year or more. If several are strategically placed about
the area, the local gradient can also be established by measuring the
water surface elevation in each well. Piezometers differ from obser-
vation wells in that they are constructed so that there is no leakage
around the pipe. The water surface elevation measured establishes the
hydrostatic pressure at the bottom of the well. If placed at several
depths, they can be used to establish whether artesian conditions exist.
For construction of piezometers and interpretation of results, see USDA,
"Drainage of Agricultural Land" (26).
The measured or estimated water table elevations for a specific time
period are plotted on the topographic map. By drawing the contours of
the water table surface from these plots, the direction of groundwater
movement is determined, since movement is perpendicular to the ground-
water contours. This helps locate the source of the water and how to
best place the drainage network.
7.2.6.3 Design
a. Selection of Drainage Method
In designing a subsurface drainage system, the site characteristics are
evaluated to determine which method of drainage is most appropriate.
Table 7-11 presents the drainage method for various site
characteristics. In general, shallow, lateral flow problems are the
easiest drainage problems to correct for subsurface wastewater disposal.
Since the use of underdrains for onsite disposal systems has been very
limited, other acceptable disposal methods not requiring drains should
first be considered.
b. Curtain Drains
Curtain drains are placed some distance upslope from the proposed soil
absorption system to intercept the groundwater, and around either end of
the system to prevent intrusion. On sites with sufficient slope, the
drain is extended downslope until it surfaces, to provide free drainage.
The drain is placed slightly into the restrictive layer to ensure that
all the groundwater is intercepted. A separation distance from the soil
absorption system is required to prevent insufficiently treated waste-
water from entering the drain. This distance depends on the soil perme-
ability and depth of drain below the bottom of the absorption system;
however, a separation distance of 10 ft is commonly used.
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