Start here: mark the water level, shut off waterfalls or external plumbing if it is safe to do so, and record the drop for several days. The level where loss slows or stops is often an important clue.
1. Diagnose before adding material
Water loss can come from evaporation, splash, a leaking pipe, a liner puncture, an unfinished overflow, or permeable soil. A treatment aimed at the bottom will not repair a leaking return line, and adding bentonite blindly can become an expensive experiment.
Clues worth recording
- Daily water-level change and recent weather
- Whether loss changes when pumps and waterfalls are off
- The elevation where the water level stabilizes
- Wet ground, animal burrows, exposed roots or visibly sandy bands
- Whether the pond ever sealed naturally after filling
2. Understand what sandy soil changes
Sand has large connected pore spaces and generally does not seal like a well-compacted clay soil. If the basin intersects a permeable sand or gravel layer, water can move laterally or downward even when the rest of the pond appears sound.
Field check: collect soil samples from several depths and locations. A single clay-looking surface sample does not rule out a deeper sand seam.
3. Match the repair to the failure
| Approach | Best fit | Main limitation |
|---|---|---|
| Rework and compact existing soil | Soil contains enough suitable fines and can be worked at the correct moisture | Weak choice when the basin is dominated by sand or gravel |
| Compacted clay blanket | Suitable clay can be imported, mixed and compacted in controlled lifts | Requires access, equipment and careful construction |
| Bentonite treatment | Defined seepage problems where the soil and application method are appropriate | Rates and success depend heavily on soil; random broadcasting is unreliable |
| Flexible liner | Small or moderate ponds where predictable sealing matters most | Cost, underlayment, anchoring and puncture protection |
| Professional investigation | Large ponds, dams, changing groundwater or uncertain geology | Upfront cost, but often cheaper than repeating failed treatments |
A sensible order of operations
- Measure and document the loss.
- Eliminate plumbing, waterfall and overflow causes.
- Identify the elevation and area associated with the leak.
- Sample the soil rather than assuming the entire basin is the same.
- Price one properly designed repair against repeated temporary treatments.
For an embankment pond or a repair that could affect neighboring property, drainage or dam safety, involve a qualified pond contractor or engineer.
Further reading: Penn State Extension explains diagnosis and several pond-sealing methods in Fixing a Leaking Pond. Local NRCS or Soil and Water Conservation District staff may also help interpret soil and site conditions.