Well Siting, Geology, and Hydrogeology
Well Siting, Geology, and Hydrogeology
This domain tests whether you can place a well where it stays sanitary and yields usable water. New York’s well construction standards (10 NYCRR Appendix 5-B) set minimum separations: keep a well at least 50 ft from septic tanks and sewer lines and 100 ft from septic absorption fields, manure storage, and animal yards; local health departments can require more. Site upslope and upgradient of contamination, avoid floodways, and verify property-line and easement constraints before drilling.
Core concepts
- Aquifer type: unconsolidated sand/gravel yields water from pore spaces; bedrock yields from fractures. Glacial till and lake clays can be low-yield or confining layers.
- Water levels: static water level is measured before pumping; pumping water level is measured during pumping; drawdown is the difference.
- Specific capacity: yield divided by drawdown, expressed in gpm/ft. It is a quick productivity indicator, not a substitute for a longer aquifer test.
- Cone of depression: pumping lowers the water table or potentiometric surface around the well; overlapping cones from nearby wells cause interference.
Worked example
A 6-in well pumps 30 gpm at 12 ft of drawdown. Specific capacity = 30 ÷ 12 = 2.5 gpm/ft. If the pump intake is 40 ft below static water and you want 10 ft of submergence, allowable drawdown is 30 ft. Estimated yield at that drawdown = 2.5 × 30 = 75 gpm. Reason it through: the arithmetic gives a target, but verify with a stepped-rate or constant-rate test because specific capacity changes with pumping rate and aquifer boundaries.
Traps
- Using static water level instead of pumping level in pump-setting calculations.
- Assuming a high specific capacity means the aquifer is sustainable; nearby pumping and drought can change it.
- Ignoring county setbacks that are stricter than state minimums.
- Siting a well downslope of a septic field because the ground “looks clean.”