Well Siting, Geology, and Hydrogeology
This domain tests the decisions made before the bit turns: where to put the well, how deep to finish it, and what yield and water quality you can honestly promise. Massachusetts questions usually ask you to connect glacial geology to a drilling choice, so learn the deposits, not just the vocabulary.
Reading the setting
- Till is dense, unsorted, low-permeability material. A well finished in till needs a large saturated thickness and normally produces modest yield.
- Outwash and other sorted sand and gravel transmit water readily and are the classic high-yield aquifers, but they are also the most vulnerable to surface contamination.
- Bedrock yields water only from fractures. A dry hole in granite is a siting problem, not a drilling problem.
- Separate the static water level (non-pumping) from the pumping level (static minus drawdown). Every pump-setting decision uses the pumping level.
Siting rules that carry marks
Place the well upgradient of septic systems, fuel tanks, road-salt piles and animal waste, and respect Title 5 and local board of health setbacks. Avoid floodplains, ponded ground and drainage ways. In coastal communities, keep pumping rates modest so the cone of depression does not pull salt water toward the screen.
Worked example
A test shows 12 gpm at 24 ft of drawdown. Specific capacity = 12 ÷ 24 = 0.5 gpm per foot. If the household needs 8 gpm, expected drawdown is 8 ÷ 0.5 = 16 ft. With a static level of 45 ft the pumping level is about 61 ft. If total depth is 85 ft, set the pump near 72 ft: roughly 11 ft above the pumping level and 13 ft off bottom. That reasoning, not a rule of thumb, is what the item is testing.
Traps
Drilling in the lowest, wettest spot on the lot; reading a perched seep as a producing aquifer; quoting static level as if it were the pumping level; ignoring an old buried tank or a neighbor's leaching field upslope.