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
The question to be answered would be the number of leaching fields in series down the slope (steady
state) and in long-term service before the groundwater would reach 10 mg/L of N with 3 mg/L being
due to natural and other sources.
1. A flow of 14,400 gpd which would have a capability of assimilating 0.84 lbs of N per day is
derived as follows:
2. 14,400 gpd x 8.3 lbs/gal x 7 mg/L ÷ 1,000,000 = 0.84 lbs/day of N
3. One EQR contributes 0.10 lbs of N per day. Therefore, a total eight residences extending in
series up slope parallel to the groundwater table would be the limit before the ground water
would reach its regulatory limit of 10 mg/L of N under the bottom-most lot in a series of
eight residences.
4. The rate of water transmission of 10 gpm represents 14,400 gpd. Eight EQRs would repre-
sent a flow of 300 gpd x 8 = 2,400 gpd. Thus, the likely carrying capacity of the shale bed-
rock aquifer would be adequate to transmit the effluent, the pollutional loading of N being
the constraint to development.
5.10.5 Impact on the Colluvial Aquifer
The colluvial deposits on the subject study area vary in thickness, permeability and depth to
groundwater from area to area. For purposes of the evaluation, a condition as follows is selected:
Colluvium Depth 30 Feet
Depth to Water Table 10 Feet
Permeability of Aquifer
50 gpd/ FT 2
Slope of Water Table 15 Percent
For analysis purposes, a 160-acre rectangular tract is tested with 120 residential lots. The lower
edge of the tract is one-quarter-mile-wide (1,320 feet). The following planning parameters result.
951-110.000 Wright Water Engineers, Inc. Page 55