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

periods, particularly on level sites, are not practical to drain. Other
disposal methods should be investigated for such sites.
Because each of these drainage problems require different solutions, it
is important that the site 'evaluation be done in sufficient detail to
differentiate between them. Where the need for subsurface drainage is
anticipated, topographic surveys, soil profile descriptions and estima-
tion of the seasonally high groundwater elevations and gradients should
be emphasized. Evaluation of these site characteristics must be done in
addition to other characteristics that are evaluated for subsurface dis-
posal (see Chapter 3).
Topographic Surveys: Topographic maps of the site with 1 to 2 ft (0.3
to 0.6 m) contour intervals are useful as base maps on which water and
soils information can be referenced. Water table elevations, seep areas
and areas with vegetation indicative of seasonal or prolonged high water
tables should be locat* In the map. Elevations of ridges, knolls, rock
outcrops and natural drainage ways should also be noted. This informa-
tion is useful in establishing the source of the groundwater, its direc-
tion of flow, and the placement of the drainage system.
Soil Profile Descriptions: The soil profile must be carefully examined
to identify the type ot drainage problem and the extent of seasonal
water table fluctuations. Soil stratification and soil color are used
to make these determinations.
Soil stratification or layering may or may not be readily visible. Soil
texture, density, color, zones of saturation and root penetration aid in
identifying layers of varying hydraulic conductivity (see Chapter 3).
The thickness and slope of each layer should be described. Deep uniform
soils indicate that the drainage problem must be handled as a free water
table problem. Stratified soils indicate a perched or lateral flow
groundwater problem.
The soil color helps to identify zones of periodic and continous satura-
tion. Soil mottling occurs when the soil is periodically saturated, and
gleyed soil indicates continuous saturation (see Chapter 3). The high-
est elevation c$ the mottling provides an estimate of the seasonally
high water table, while the top of the gleyed zone indicates the season-
ally low water table elevation. It is particularly important to estab-
lish the extent of the seasonal fluctuations to determine if drainage is
practical. If the seasonally low water table is above the elevation to
which the soil must be drained to make the site acceptable, drainage
must be provided throughout the year. If pumps are used to remove the
water, costs may be excessive and other alternatives should be
investigated.
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