Coastal Bedrock Well Siting With a Saltwater Interface
On Cape Cod, a driller is asked to site a 6-inch bedrock well for a four-bedroom seasonal house on a 0.4-acre lot. The proposed location is 150 ft from a coastal pond, 80 ft from the septic system's leaching area, and 35 ft from the property line. The lot is 18 ft above mean sea level. The existing dug well is 12 ft deep and has chloride 210 mg/L. A nearby 400-ft bedrock well produced 12 gpm but showed chloride rising from 45 mg/L at 180 ft to 900 mg/L at 320 ft. The driller proposes casing to refusal at 55 ft, drilling to 300 ft, and setting the pump at 240 ft. Static water level in nearby wells is 6-10 ft. The client wants 'the deepest well possible' to avoid running dry in summer.
1. What siting and setback issues must be resolved before the local board of health will issue a well construction permit, and how would you resolve them?
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The 150-ft distance to the pond may satisfy some local wetland buffers, but many Massachusetts wetland bylaws require 100 ft and some require more; verify the actual wetland line and bylaws rather than assuming. The 80-ft distance to the leaching area is the larger problem. Title 5 and typical board-of-health rules require greater separation from a soil absorption system, often 100 ft, and the well should be uphill and outside the septic system's zone of influence. The 35-ft property-line distance may need a local waiver or relocation. Before the permit, plot the septic tank, leaching area, property lines, wetland line, and surface drainage. Relocate the well upslope and as far from the leaching area as the lot allows, or obtain written board-of-health approval or a variance. Also check for fuel tanks, floor drains, road-salt storage, and fill. The board will want the proposed casing depth, grout method, and anticipated water quality, not just a stake in the lawn.
2. Is the proposed 300-ft depth and 240-ft pump setting defensible? Explain your reasoning.
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Not as proposed. The nearby well shows a chloride transition between 180 and 320 ft, so drilling to 300 ft and pumping at 240 ft risks pulling the saltwater interface upward. A 240-ft pump setting also leaves little margin above the open fracture zone and can increase upconing. The correct approach is staged drilling: collect water-quality samples at 20- to 50-ft intervals or at each major fracture, and stop deepening when chloride rises above a trigger such as 100-150 mg/L or shows a clear upward trend. Case off any brackish zone. The final pump setting should be above the saltwater transition while staying submerged at the design drawdown. 'Deepest possible' is the wrong objective on a coastal aquifer; the goal is the shallowest sustainable fracture zone with acceptable chloride. If high yield is needed, a constant-rate test with chloride sampling at the start and end is essential. If chloride rises during pumping, reduce the rate or relocate.
3. Design the yield and water-quality test plan, including acceptance criteria.
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After development, measure static water level and pump at a constant rate near expected demand-for a private four-bedroom home, typically 4-6 gpm-for at least 4 hours, with readings every 15 minutes and recovery until 90% recovery or 2 hours. Record pumping level, drawdown, specific capacity, and sand production. Collect chloride at the start, middle, and end, plus a private-well suite: pH, iron, manganese, arsenic, nitrate, bacteria, hardness, and sodium. Acceptance criteria should include: drawdown stabilizes rather than trending downward; the pumping level stays at least 10 ft above the pump intake; chloride remains below 250 mg/L and does not rise during the test; and no visible sand remains after development. If the well will serve a public water system, use a 72-hour test and approved laboratory methods. If chloride trends upward, reduce the pumping rate or relocate the well. Submit results to the board of health with the well completion report.