Saltwater Upconing Near the Bay
A Chesapeake Bay-area homeowner has a 300-ft, 6-inch well screened from 270 to 300 ft in a confined sand. Static water level is 18 ft. At 20 gpm the pumping level stabilizes at 73 ft, and chloride has risen from 120 mg/L at completion to 850 mg/L over two years. A 150-ft-distant tidal creek is at high tide most of the day. A neighboring irrigation well pumps 100 gpm from the same sand during summer. The owner wants to deepen the well to 400 ft 'to get away from the salt.' A lab also reports sodium at 480 mg/L and iron at 0.1 mg/L. The well is the sole supply for a household of four; the pump is set at 250 ft.
1. How would you determine whether the chloride is coming from upconing, direct creek intrusion, or a saline zone in the formation?
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The pattern points strongly toward upconing or induced intrusion rather than a simple shallow-to-deep salt layer. Chloride rose over two years while the well was pumped, the well is only 150 ft from tidal water, and a nearby irrigation well is pumping 100 gpm from the same sand. The pumping level of 73 ft is 55 ft below static, which can draw deeper saline water upward in a confined sand.
Confirm it with a chloride profile: sample at several depths or intervals in the open screen, and compare chloride while pumping versus after a long recovery. If chloride falls after the well rests and rises during pumping, upconing is likely. If chloride is high in the lower part of the screen even at rest, a formation salt layer may be present. Also check the neighboring irrigation well’s pumping schedule and chloride trend. Deepening to 400 ft without that profile is a gamble; the deeper sand may be saltier.
2. What completion or operational changes would you recommend, and why is deepening not automatically the answer?
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First, reduce the stress on the aquifer: lower the pumping rate, add storage, and spread irrigation or household use over time so the pumping level does not stay 55 ft below static. If the chloride profile shows a freshwater sand above 270 ft, the better construction fix is to set a new screen in that upper sand and grout off the lower saline interval, rather than deepen into unknown water. If the deeper formation is fresher and the upper sand is the source of salt, deepening could work, but only after test-hole sampling and a proper chloride profile.
Deepening is not automatically the answer because saltwater is usually below freshwater in a coastal aquifer. A deeper screen can intersect the saline water that is already moving upward. Grouting the annulus and sealing the old screen interval are also important to prevent vertical migration outside the casing. At 850 mg/L chloride and 480 mg/L sodium, the water exceeds aesthetic and possibly health-based limits; treatment such as reverse osmosis may be needed, but it should not substitute for correcting the upconing cause.