Leaky ponds are one of the most common problems on acreages and one of the most difficult to fix. But there are solutions. First, remember that all dirt leaks. If it didn’t, there would be permanent puddles with every rain.
Before you build a pond, know the soils. There are tens of thousands of soil types – from sand to clay tight enough to make bricks. Work with your engineer, pond designer, or contractor to figure out soil types. You will have more than one because most soils are layered. Some layers are good soil, some are marginal, and some are downright off limits to build a dam or coat the bottom of a pond. Look for clay. Clay is good.
Seasoned earth movers should be able to tell good soils from bad. If there’s a question, seek help. Civil engineers can help define the plasticity index, or how well a soil type can compact. You want compaction. (That doesn’t mean sandy soil can never work. If you have more water flowing in than flowing out, don’t worry too much about a leak unless it’s undermining the structural integrity of your dam.)
Your best chance to stop a leak is to prevent it in the first place and that’s during construction
A leak or evaporation?
If you didn’t have the opportunity to build your dream pond and the one you have is leaking, you have some decisions to make. First, quantify the leak. Keep in mind that 1 acre, 1 inch deep is 27,000 gallons of water. Depending where you live, evaporation can be 1/2 inch a day in summer.
Don’t misconstrue evaporation as leakage. Expect some evaporation. Learn about your evaporation rates and compare those rates to your pond’s loss. If your pond level drops more than 1/2 inch daily, investigate. Your dam could be leaking.
If water seeps through or under the dam, the first thought is that the dam wasn’t properly cored when it was built. A properly built dam sits on a firm foundation and is physically tied to clay subsoils to create an underground, waterproof bond.
Water could be escaping around an overflow or drain pipe. If a pipe isn’t installed properly, it can be a source of a leak.
The best dam in the world can’t always compensate for porous soils in the bottom of the pond. There’s an area in west Texas, near the Caprock, where big veins of crystal-looking gypsum follow serpentine courses through dry creek beds and beyond. A dam was built to impound 15 surface acres for a fishing lake. The water was beautiful, a pristine, green-tinted clear. About four years after it was built, the lake disappeared, leaving just a puddle. Careful investigation found a gaping hole as big around as a cedar fence post. The dam builder had overlooked a vein of gypsum. It took several years, but the gypsum dissolved like sugar and left a snake-shape void that let the water escape. The earth mover was recalled and he packed the voids with thick, heavy clay. Today, the lake holds water well for an arid area.
One new lake in Tennessee suffered a similar fate. The owner created it by building a levee around a soybean field. A trench was dug around the site and clay was brought in from a nearby hillside. Water was pumped in and before long, the lake was full and stocked with fish. The lake leaked a little bit at first, then became unmanageable. Turns out the water chemistry was acidic enough to absorb minerals from the lake bottom. Once the lake went dry, you could see 10 holes in the sandy bottom.
Most leaks aren’t that dramatic. The pond goes down, rains fills it back up, and then the water slowly drains again. The problem still tracks back to porous soils. The hard part is trying to figure out which soils, under water, are porous and where the leak is.
Bentonite and other products
There are products that purport to solve this leaky puzzle. Sodium bentonite, also called drilling mud, is the most popular. But if you don’t put the stopper in the bathtub drain, the tub won’t hold water. You can fill a bathtub full of stoppers, but until you get that little hole plugged, forget it. The same holds true with a pond. You can add bentonite, clay, hay, organic matter, tires. But until you plug the hole, you’ll have a leak.
You must figure out where the pond leaks. Drain the pond and examine the entire surface area of the bottom. If the leak isn’t obvious, you will need to coat the entire bottom of the pond with good clay or bentonite. A product called ESS-13 is designed to assist mildly porous soils to naturally compact. There are also polymers on the market.
In some cases, a full rubber liner over the entire pond is the only sure answer to prevent leaks, but with materials and the cost of applying the liner, this is a costly option.
There is no easy fix, and most fixes are expensive. The best approach is to learn about your soils early – before the pond is built – and then make sure the contractor does everything possible to prevent leaks.
Beyond that, seek help from earth movers or engineers to figure out the source of the leak. It may be practical to repair or it may not. If you can find the leak, you stand a good chance of fixing it. Do your homework, seek the best help, and then make good choices.