Totally agree with Bill.
A clay particle in water is ALREADY hydrated and swelled to its maximum size. They are very unlikely to reduce an existing leak pathway.
In fact, the opposite is true. Leaks due to poor compaction usually get worse over time. Every time the water slowly moving through the leak manages to pluck out a clay particle or sand particle from the walls of the pathway, the leak rate slightly increases. Every time the leak rate increases, it becomes more likely to pluck out the NEXT particle.
The polymer products discussed above by Bill, work by getting into the leak fairly rapidly, and then continuing to swell or continuing to cross-link and build more resistance to shear (water flow).
Here is a good thread about fixing leaky ponds. You can find the contact information to reach TJ on the second page of the thread. If he is not swamped at work, he can usually give you some free guidance about possible solutions for your particular situation. (He is also a supplier of one of the highest rated products.)
Soilfloc - Leak Thread on Pond BossAlso, when you are getting rain in your area, do you have a way to pump water to your pond from a nearby source? If so, you might fill your pond and then monitor the daily drop rate and stabilization point. That will give you some more important data about the nature of your leak.
