how to seal a pond

I plan to dig a pond (or actually ask some people with the right equipment to dig it). Will be about 20 by 20 meter. The only question is how do I seal that so the water stays in? The earth is rather loose sand on a deeper level. For the rest, the area is slightly sloped (5%). I hope to catch some water from the neighbors who doesn't do any water management and a lot of the water is coming from there.

Finally, just for your information, I live in Thailand. The land is in the mountains in the North near Chiang Mai. So there are 2 to 3 seasons. A rainy season in which it rains a few hours per day. A cold season is about 30C (86F) day time and at night 15C (59F). the last season is the summer. At day it will be around 38C (100F) and at night 25C (75F). Each season takes about 4 month.

Any suggestions how to seal a pond? I wouldn't like to do it with concrete or plastic.

Thanks,

Peter

Three main ideas that I know of for pond sealing are with:
-clay
-compaction
-decomposition

Gleying by itself just means to put a big layer of material somewhere, let it decompose, and create an anaerobic seal. This is best done with animals because it decreases the human workload.

For example, compaction and decomposition are combined to gley a pond with pigs. I imagine that other large or heavy animals could possibly be used to achieve the same results.

Ponds can also be sealed with ducks. The same concept applies but without the compaction. All you need is a build up of manure which collects at the bottom and decomposes anaerobically. This creates the seal.

Sealing a pond with clay or compaction alone are the most tedious and time consuming methods, I think.