Geologic And Hydrologic Factors Governing ... - Gunnison County

Geologic And Hydrologic Factors Governing Impacts Of Development
On The Crystal River Near Marble, Colorado
Gunnison County, Colorado
Inadequate Distances Between Wells and Septic Systems. If a well is placed too close to a
septic system the effluent from the leach field will not have been adequately treated before it flows
into the well and is pumped to the surface. Both the CDPHE and Gunnison County require a 100-
foot horizontal distance from the well head to the nearest point of the leach field.
Soil Thickness. To assure adequate treatment of septic tank effluent, the soil profile should be
thick enough and have adequately-developed horizons to allow complete filtration and digestion of
the waste by bacteria before the bedrock or the water table is encountered. Normally, this requires at
least 4 feet of adequate material below the bottom of the leach field and above bedrock or the water
table.
Permeability. The soil beneath and around the leach field must be permeable enough to allow the
effluent to pass through it but not so permeable that waste passes through it too quickly and without
complete treatment. Permeability is measured with percolation tests and is usually rated as the num-
ber of minutes it requires for the water level in the percolation hole to drop 1 inch. If the water level
drops 1 inch in less than 5 minutes, the soil material is too permeable to allow for adequate treatment
of the effluent. Normally, this occurs in gravels, coarse sands, or highly fractured bedrock. If the
percolation rate is slower than 60 minutes for a 1-inch drop in the water level of the percolation hole,
the soil is not permeable enough to allow the leachate to pass through it and the leaching system will
plug. Normally, these are clay soils or impermeable bedrock.
Slope. If the slope of the surface of the ground at the septic tank location is too steep, the effluent
from the leach field will percolate sideways as well as downward and come to the surface forming a
leachate spring. Normally, this occurs on slopes greater than 20 percent.
Shallow Water Table. A shallow water table can interfere with proper functioning of the leach
field. If the water table is too close beneath the leach field, then the effluent will not be adequately
treated and the effluent will pollute the groundwater zone. Normally, this problem occurs when the
water table is within 4 feet of the base of the leach zone or approximately 8 feet from the surface of
the ground. Whereas, the water table in most areas fluctuates between the wet season and the dry
season, one should be certain that these water levels relate to the water table during the wet season or
951-110.000 Wright Water Engineers, Inc. Page 42