Development and Disinfection After a Turbid, Sand-Producing Completion
A new 8-inch gravel-packed well is 140 ft deep with 60 ft of stainless-steel screen from 75 to 135 ft. Static water is 50 ft. After 6 hours of surging and bailing, discharge is still 20 NTU with fine sand; specific capacity improved from 0.6 to 0.9 gpm/ft but is below the 1.5 gpm/ft expected from the test hole. The screen slot is 0.020 in. The contractor proposes to disinfect with 50 mg/L chlorine for 2 hours, pump to waste for 30 minutes, and turn the well over. The county requires a 24-hour pump test and a coliform sample before use. The owner needs water within a week. The driller has a surge block, airlift, and jetting tools.
1. Is the well developed enough to disinfect? What development steps would you take next?
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No. A specific capacity of 0.9 gpm/ft, only 60% of the test-hole expectation, plus 20 NTU and fine sand after 6 hours, means the gravel pack and formation still contain drilling fines and the sand pack may not be stable. Disinfecting now would mask the problem and the 24-hour pump test would likely fail. Continue development with a surge block and simultaneous bailing, working the screen interval in stages from bottom to top; then airlift or jet with clean water if the surge block stalls. Stop when the discharge is sand-free or nearly so and turbidity has stabilized at a low value, not when a fixed number of hours has passed. If sand persists, check the gravel pack size and screen slot against the formation sample; a pack that is too coarse for the sand will keep producing.
2. Critique the proposed disinfection and sampling sequence. Give a defensible sequence.
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The 50 mg/L for 2 hours is too weak and too short for a new well; standard practice is a stronger solution, commonly about 100 mg/L available chlorine, with at least 24 hours contact time. The sequence is also backwards: the well must be developed first, then disinfected, then flushed, then sampled. Disinfect before the final pump test only after development is complete. Calculate the water volume in the casing and screen, add the chlorine solution, circulate so the whole column is treated, and cap the well. After 24 hours, pump to waste until the free chlorine is gone and the water is clear. Then run the 24-hour pump test, collect the coliform sample after flushing, and do not turn the well over until results are satisfactory. Record chlorine strength, volume, contact time, and flush volume.
3. What field evidence should be in the completion report before the owner uses the well?
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The report should show final developed yield and specific capacity, stabilized drawdown, pump setting and rate, sand content, turbidity trend, and the disinfection record. It should also show the as-built screen and gravel-pack depths, seal depth and material, and the pump-test results. Before use, require a sand-free or acceptable sand count at the design rate, a coliform sample that meets the standard, and no unresolved turbidity. If the specific capacity remains below the test-hole expectation, state that in the report rather than reporting only the best short-term number. The 24-hour test should be at a constant rate, with water-level measurements, and the sample should be collected after the lines are flushed, not immediately after chlorination.