Wellhead Protection, Sealing, and Grouting
This domain tests the seal that stands between surface contamination and your water supply: the annular grout, the sanitary seal, and the wellhead details that keep runoff out for the life of the well.
Core rules
- The annular space between borehole wall and casing must be sealed continuously from the bottom of the casing to the ground surface. A shallow plug leaves a vertical pathway for surface water.
- Grout is placed by tremie - pumped from the bottom up through a pipe kept submerged in grout - never poured from the top, which bridges and voids.
- Approved materials include neat cement, bentonite slurry, bentonite-cement blends and sand-cement mixes. Neat cement gives about 1.18 cubic feet per 94-lb sack.
- The finished wellhead carries a sanitary seal or vented cap, sits at least 12 inches above finished grade (local rules often require 18), and is surrounded by a graded, impermeable pad that sheds water away.
- MassDEP wellhead protection uses a Zone I radius of 400 feet for wells in unconsolidated deposits and about 250 feet for bedrock wells, controlled by the water supplier.
Worked example
Grout an 8-inch borehole around 6-inch casing to a 100-ft depth. Annular area = 0.7854 × (0.667² − 0.50²) = 0.7854 × 0.1944 = 0.153 ft². Volume = 0.153 × 100 = 15.3 ft³. Add 15% for voids and losses = 17.6 ft³. At 1.18 ft³ per sack, order 15 sacks of cement and about 78 gallons of water. Ordering bare theoretical volume is how holes get backfilled late and badly.
Traps
Top-pouring grout; skipping the excess factor; sealing only the upper 20 ft; leaving a below-grade well pit that floods; and forgetting that the grout seal is what protects the Zone I.