Ponds should be as close to the water source as possible, to avoid long inlet pipes or channels. In addition, there must be enough clay in the earth to prevent leakage. A rule of thumb is that the material must consist of at least 20% clay particles with diameter below 0.002 mm in a 1.5 m deep core taken where the pond is to be established.If the material contains too much sand, it will be porous and water will drain out much faster. The seepage loss in sand is reported to be between 25 and 250 mm/day, in loam 8–20 mm/day and in clay 1.25–10 mm/day. Furthermore, the earth must be free of toxic substances, for instance copper.
There are several methods to prevent leakage from ponds. If the leakage is only slight, a solution is to break down the earth structure, reduce the aggregate size and puddle the bottom. Breaking up the lumps in the surface layer achieves this and is quite commonly done on rice fields. Addition of chemicals may also reduce the aggregate size. Compression of the surface may also be used to reduce the water loss, for example by using a road roller. Several thin layers of compressed earth are better than one thick layer. If the natural soil is unsuitable, a membrane of clay or plastic may be used. A clay layer transported to the site must be about 30 cm thick for a 3 m deep pond.11This, however, represents increased costs for establishing the pond. To avoid the layer of clay crumbling as a result of drying or freezing, a covering layer of sand or gravel may be used; this can be from 30 to 45 cm thick on clay and 15–20 cm thick on plastic. The plastic membrane should also be covered to avoid breakage from plants growing through it. Material used to construct the pond may also be sprayed with plant poison before laying the membrane.
After ponds have been in use for a time, water leakage will normally be reduced because settled materials block the cracks in the earth.