A surface seal closes the top 3 m of a well against surface water: the lining of a dug well is plastered watertight inside and packed or grouted behind with clay or cement, and the annulus of a drilled well is filled with clay or cement above the gravel pack. Without it rain and spills run down the gap outside the lining straight into the aquifer, so the seal must be continuous from the apron down, and a well whose water clouds after rain has a failing seal.
Part of Deep Wells
A surface seal, also called a sanitary seal, is the watertight lining and packing in the top few metres of a well. It stops surface water short-circuiting down the outside of the lining or casing to the aquifer.
The Short Circuit Runs Down The Outside Of The Lining
A lining or casing is never a perfect fit in the ground. Between it and the soil is a gap, and soil near the surface is the most permeable, the most disturbed by digging, and the closest to everything that soils water. Rainwater that soaks in beside the well finds the gap and runs down it to the aquifer with almost no filtering. The seal makes that gap watertight over the top 3 m, the depth the Deep Wells guide specifies, so that water must travel through undisturbed soil before it can reach the well.
Different Wells, Different Seals
| Well | Seal outside the lining | Seal inside |
|---|---|---|
| Dug, brick or ring lined | Clay or grout in the gap behind the top 3 m | Cement mortar plaster on the inside of the top 3 m (Brick Lining, Concrete Rings) |
| Drilled or augered, cased | Clay or cement in the annulus above the gravel pack, up to the surface | Solvent-welded or sealed casing joints (Gravel Pack) |
The Top Three Metres Of Seal Take About A Cubic Metre In A Dug Well
Dug well. A ring 1.0 m across in a shaft 1.2 m across leaves a gap of 0.1 m all round. Per metre of depth the gap has an area of π × (0.6² − 0.5²) = 0.346 m², so the top 3 m takes 3 × 0.346 = 1.04 m³, about a cubic metre of clay or grout.
Drilled well. A hole of 160 mm round 110 mm casing leaves an annulus of π ÷ 4 × (0.160² − 0.110²) = 0.0106 m² per metre. The top 3 m takes 3 × 0.0106 = 0.032 m³, which is 32 L.
What You Need
- Puddled clay or cement grout — clay kneaded with water until smooth and plastic, or neat cement mixed to pour
- Cement mortar for plaster — the cistern plaster recipe applies: two coats of 1 part cement to 2 parts sand, each 10-15 mm thick, cured damp for at least 7 days (Cement Plaster, Curing)
- A tamping rod — for packing clay in layers
- A tremie pipe — a small pipe to place grout from the bottom up
Method: Pack In Layers, Pour From The Bottom, Plaster The Inside
Step 1: Pack clay behind a dug lining in thin layers as the lining rises, ramming each layer firm. Do not drop clay in as a loose heap; it bridges and leaves voids. The Phase 0 Well Lining guide packs puddled clay 10-15 cm thick behind the lining in the top section.
Step 2: Or grout. Pour cement grout into the gap through a pipe lowered to the bottom of the section, raising the pipe as the grout rises, so no voids are trapped.
Step 3: Plaster the inside of the top 3 m of a dug lining watertight in two coats, curing each one damp.
Step 4: Seal a drilled well’s annulus with clay or cement above the gravel pack up to the surface (Gravel Pack).
Step 5: Join it to the apron. The seal must meet the apron without a gap (Sanitary Apron).
Step 6: Test it in use. Take a sample before a rain and another a few hours after. Use the turbidity tube in Community Water Testing: a black mark still visible at more than 30 cm of depth is good water, and a mark that vanishes at under 10 cm is bad. If the water gets cloudier after rain, the seal is leaking, or the aquifer is connected to the surface directly.
Common Mistakes
- Dry clay seals. Clay left in dry ground shrinks and cracks; keep it moist and covered, or use cement.
- A seal that stops short. It must run continuously from the apron down.
- Loose backfill. A gap filled with loose soil is a channel.
- Trusting the seal in fissured rock. In limestone or fractured rock water enters through cracks far below the seal; a seal does not protect against that, and the water must be purified.
Safety
Work at the open shaft and with wet cement
Any worker in the shaft follows the ventilation, harness and two-above rules (Safety Protocol); no one is below when clay or buckets are lowered. Wet cement and lime burn skin and eyes: wear gloves and eye protection and rinse splashes at once. Flush the well after finishing the new plaster before the water is used.
What’s Next
- Learn next: Sanitary Apron — the slab that meets the seal
- Combine with: Well Protection — the whole set of barriers