Updated 2026-10-23
How deep do I need to drill for water under thick clay?
Under thick clay the water-bearing zone often starts right below the clay, which can mean drilling past 100–200 m with a rig able to get through it. In Catalonia, after several failed boreholes, WaterLocation's point gave water at 136 m directly below more than 100 m of clay, and the state pumping test confirmed 648 m³ a day.
What the driller needs
- Capacity to drill past 200 m through clay or saline layers.
- An air-flush rig; cellulose-based fluid only if fluid is unavoidable.
- Airlift or mud-pump flushing, then a flow test.
Clay sites
| Site | Before | Result | Yield |
|---|---|---|---|
| Catalonia, Spain | Several failed boreholes; a 40-year driller said no water | Water at 136 m, static level 39 m | 648 m³/day |
| Latvia, farm | Dolomite over clay, many dry holes nearby | 63 m artesian well, flows 4 m above ground | ≈200 m³/day |
Questions
Why do clay sites fail?
Holes stop in the clay or miss the zone beneath it; surveys looking for shallow aquifers return negative.
Can the depth be known in advance?
Yes, the survey report gives the depth before drilling starts.