Well Development, Testing, and Disinfection
Well Development, Testing, and Disinfection
This domain covers the transition from a drilled hole to a usable well: removing drilling damage, proving performance with a test, and delivering a bacteriologically safe supply. Items mix calculation with sequence questions.
Development
- Purpose: remove drilling fluid, filter cake, and fines from the screen and filter pack, and increase specific capacity.
- Methods include surging and swabbing, air-lift or air-jetting, high-velocity jetting, brushing, and overpumping. Match the method to the completion, and never surge hard enough to pull formation sand into the pack.
- Acceptance: sand-free water, stable or improving specific capacity, and clearing turbidity.
Testing
- Measure static water level, pump at a constant rate while recording pumping level and time, then measure recovery.
- Drawdown = pumping level − static level. Specific capacity = yield ÷ drawdown (gpm/ft), a comparative efficiency index, not a safe yield by itself.
- A step-drawdown test separates formation loss from well loss; a constant-rate test with recovery data supports a sustainable-yield determination.
Disinfection and sampling
- New, repaired, or redeveloped wells are chlorinated with a strong chlorine solution that gives a residual throughout the water column, held for the required contact time (commonly 24 hours), then pumped to waste until chlorine is gone before the well goes into service.
- Take the bacteriological sample only after chlorine is fully purged, from a clean sample point, and submit it for total coliform and E. coli.
Worked example
A 6-inch well has a static level of 40 ft. Pumping at 25 gpm stabilizes the level at 90 ft. Drawdown = 90 − 40 = 50 ft. Specific capacity = 25 ÷ 50 = 0.5 gpm/ft. If a later test after development shows 30 gpm at 50 ft drawdown (0.6 gpm/ft), development improved the well by about 20 percent. Neither number proves a long-term 25 gpm yield - that requires a duration test showing stable drawdown.
Traps
- Reporting drawdown as the pumping water level.
- Sampling while chlorine residual remains, then retesting without investigating.
- Calling specific capacity a safe yield.
- Overdeveloping a screened well and pulling formation sand into the pack.