Groundwater and Hydrogeology
The Groundwater and Hydrogeology domain tests whether you understand where the water is, how much the aquifer will give, and what your pumping does to the well and to neighbors. Connecticut aquifers are glacial till (low yield), stratified drift in river valleys (high yield) and fractured crystalline bedrock, where yield depends on intersecting fractures.
Key concepts
- Porosity vs permeability: till has high porosity but low permeability; sand and gravel have both; bedrock stores little but transmits water along fractures.
- Water table vs confined: a confined aquifer rises to its potentiometric surface, which may stand above the top of the aquifer, producing flowing wells in valleys.
- Drawdown and cone of depression: pumping lowers the water level around the well; the cone grows with rate and time and can reach a neighboring well (interference) or a nearby stream (induced recharge).
- Specific capacity = pumping rate divided by drawdown, in gpm per ft; it declines as pumping continues, so treat it as a snapshot, not a constant.
- Safe yield is what the aquifer sustains long term, not what the pump can draw in an hour.
Worked example
Static level 40 ft; pumped at 15 gpm the level stabilizes at 100 ft. Drawdown = 60 ft, so specific capacity = 15 / 60 = 0.25 gpm per ft. If the pump must stay 20 ft off the bottom and the well is 200 ft deep, available drawdown to that limit is 200 - 20 - 40 = 140 ft, which would suggest about 35 gpm. That extrapolation is not a tested yield: a step test and a longer constant-rate test are what justify the number, because specific capacity falls non-linearly as drawdown increases and the fractures dewater.
Traps that cost marks
- Treating drilled depth as usable drawdown.
- Ignoring seasonal decline and neighboring pumping when setting the pump.
- Assuming bedrock is uniformly productive; two wells 100 ft apart can differ tenfold.
- Confusing recovery after shutoff with the true static level.