Groundwater and Well Hydraulics
Groundwater and Well Hydraulics
This domain tests how aquifers store and transmit water and how well design and pumping affect drawdown, yield, and interference. Expect porosity, specific yield, transmissivity, storativity, Darcy's law, cone of depression, specific capacity, and pumping-test interpretation.
Core concepts
- Porosity is the fraction of void space; specific yield is the drainable fraction. A high-porosity clay can still yield little water because permeability is low.
- Transmissivity (T) describes aquifer transmission: T ≈ K × saturated thickness. Storativity describes release from storage; confined aquifers have much lower storativity than unconfined aquifers.
- Pumping creates a cone of depression. Drawdown increases with pumping rate and time, and decreases with distance and higher T.
- Specific capacity = pumping rate ÷ drawdown. It is a useful local index, but it is not constant: drawdown is nonlinear and well efficiency changes with rate.
- Well interference occurs when cones overlap; in Oregon, limited-area rules may restrict new groundwater appropriations where interference or depletion is a concern.
Worked example
A well pumps 400 gpm and stabilizes at 10 ft of drawdown. Specific capacity = 400 ÷ 10 = 40 gpm/ft. If the pump is increased to 600 gpm, do not simply predict 15 ft drawdown by linear scaling; turbulence, well loss, and aquifer response make actual drawdown higher. Use a step-drawdown or pumping test, not a single-point ratio, for design.
Traps
Confusing porosity with permeability; using specific capacity as a guaranteed well yield; ignoring well loss; and interpreting a short test as a long-term sustainable yield.