Coastal Plain Irrigation Well and Stream Depletion
A municipality proposes a 12-inch irrigation well in the New Jersey Coastal Plain to supply 60,000 gpd (about 42 gpm continuously) for a golf course. The site is 150 ft from a trout-maintenance stream and within 1,000 ft of 15 residential wells completed at 250-350 ft in the same aquifer. The driller proposes a 24-hour pump test at 150 gpm and a single observation point at the pumping well. A previous 6-inch test well produced 300 gpm for 8 hours with 12 ft of drawdown, and the driller concludes the aquifer is “infinite.” The golf course needs 60,000 gpd in July; the stream has a July 7Q10 of 0.8 cfs. The well would be screened from 180 to 380 ft. The owner wants to avoid a Water Supply Allocation Permit because the total is under 100,000 gpd.
1. Critique the proposed pump test and the “infinite aquifer” conclusion.
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The 24-hour single-well test is not adequate. It measures the well’s short-term capacity, not the aquifer’s ability to sustain 42 gpm continuously for a month. The “infinite aquifer” conclusion from an 8-hour test at 300 gpm is unsupported; a high yield can come from a single fracture or induced stream leakage. Require a 72-hour constant-rate test at the proposed rate plus a margin (for example, 50-60 gpm), with observation wells at about 50, 200, and 500 ft, and a recovery period. Measure stream stage and flow before and during the test. The stream’s July 7Q10 of 0.8 cfs is about 517 gpm, so a 42 gpm continuous withdrawal is roughly 8% of low flow - potentially significant for a trout-maintenance stream.
2. What permitting and hydrogeologic issues must be addressed before construction?
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Do not accept the owner’s assumption that 60,000 gpd avoids review. Verify the current NJDEP water-supply allocation and registration thresholds, and include all site withdrawals, not just this well. The project may also need a freshwater wetlands permit or water-quality certification because of the 150-ft stream distance, a flood-hazard review, and a wellhead protection analysis. The residential wells within 1,000 ft create an interference issue: the test must show whether 42 gpm will lower nearby wells below their pump intakes. Screen depth from 180 to 380 ft should match the aquifer and avoid intermixing poor-quality zones. Plan monitoring and a contingency if drawdown or stream depletion is excessive.
3. If the 72-hour test shows 25 ft of drawdown at 42 gpm and 3 ft at 500 ft, what would you recommend and why?
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With 25 ft of drawdown at 42 gpm, specific capacity is about 1.7 gpm/ft; that is modest. Three feet at 500 ft suggests hydraulic connection, not isolation. Compare the projected long-term drawdown to residential well depths and pump settings; if the nearest well has only 10-15 ft of available drawdown, interference is unacceptable. Require monitoring at the residential wells and the stream, reduce the pumping rate, add storage to shift pumping to off-peak periods, or relocate the well farther from the stream. If stream depletion exceeds the permitted limit, the project may need an alternative source or a mitigation plan. Document the test and the decision.