Natural ponds can spring leaks, and repairing those leaks can be troublesome, time-consuming and expensive, depending on how large the leak is and where it is located. Small leaks located near the upper rim of a pond may not be too difficult to deal with, but larger leaks near the bottom of a pond will require draining the pond and, potentially, spending a great deal of time and expense.
Pour a small amount of colorful food dye into the water near a suspected leak and watch for the color to seep out to determine if you actually have a leak in your pond. Ponds can lose up to 12 inches of water per month simply from evaporation during hot, windy, drought conditions. This method only works with leaks in which the escaping water can be seen outside the pond.
Use pumps and hoses to drain the water until the leaking area is revealed. If the leak is in the bottom, this can mean draining the entire pond.
Remove all vegetation, tree stumps and large rocks in the area of the leak. If tree roots are involved in the leaking, remove the trees and dig out their roots, or move the trees to a different location.
Allow the exposed portions of the pond to partially dry. Do not allow the exposed portions to dry completely--they should become damp, but not soggy.
Rototill the leaking area to a depth of 12 to 16 inches. Remove all roots and other debris that is turned up by the rototiller.
Cover the affected area with bentonite clay, adding approximately one pound per square foot and then mixing the clay into the damp soil to a depth of 6 inches with a rototiller. Bentonite will swell to many times its dry mass when water is added to the pond.
Roll a compactor over the area to compact the soil. Use as much weight in your compactor as you can, as the deeper the compaction, the better. Soil must be damp but not soggy for the best compaction.
Refill the pond immediately after compaction of the affected area. Do not let the ground you have compacted dry out completely before the pond is refilled. Cover the compacted area with straw, if necessary, to keep the soil damp if the pond cannot be refilled immediately.
Check the water level periodically after refilling to make certain that your leak is holding.