news from the waterfront by Tim Matson


As we discussed in the April Pondology,
there are numerous ways to remedy low water ponds. Add water, from a variety of existing and potential sources. Line the pond with local clay, which requires a good source at a reasonable trucking distance, and a savvy installation technique; and comes with some uncertainty about how well it will work, and for how long.

This month let's take a look at some other solutions.


When good local clay is unavailable, some contractors use bentonite, a mined clay often packaged in bags. Bentonite has sealing and application properties that may outperform local clays. It comes in powdered
form, and is usually applied dry to the dewatered pond basin. After application, bentonite particles swell up to 20 times in size, and create an effective water seal.

To be most effective, it is recommended that
bentonite be worked into the pond soil. It might be disked in by tractor, or perhaps with a roto tiller. This may be problematic in the rocky soils of New England. And in ponds where surface ledge rock is the leak source, surface application of bentonite may be ineffective. Bentonite has a gooey consistency that some people find unappealing to step on; and when stirred by aquatic critters or swimmers or springs or runoff, it may cause turbid water. As with native clay, a protective layer of small stone on the bentonite may supress turbidity.

I did hear recently about the owner of a leaky pond who applied bentonite directly in the water. He broadcast it from a rowboat using a fertilizer type spreader, building up layers of bentonite over time. Apparently it worked. However, just tossing scoopfulls helter skelter is notorious for not working. I've also seen bentonite spread by shovel on the face of a new dam, but the soil was already clay rich, and the pond had a good water supply, so the success of the pond may or may not have had much to do with the bentonite.



If you're looking for a sure fire cure for pervious soil or poor water supply, or both, the answer is a membrane liner.

Lining with a sheet of pvc or epdm or other water tight material stops all leakage. The liner may be one piece, or pieces assembled and glued or heat welded together. Liners are thick enough to be durable, perhaps 30-40 mil. They are often UV resistent. And best practice installation usually requires several criteria be met.

Pond basin slopes should not be as steep as often found in earth ponds. That's because a protective layer of small round stone or sand is usually applied on top of the liner, and would slide down big inclines. The protective layer is needed to prevent animals from stepping into the pond and puncturing the liner.

Beneath the liner there is usually a protective underlayment, too. This is to prevent sharp stones from puncturing the liner under the weight of the water. And finally beneath the whole thing, there may be a need for drainage to prevent ground water (if there is any) from lifting the liner. An underdrain often runs through
the base of the dam and discharges downhill,
possibly into a recapture system where the water can be pumped back into the pond. In an excavated pond, the drain might need to run into a cistern, for recapture or discharge.

Pond owners often ask about the lifespan of liners. I believe 20 years is a standard specification. But liners are a relatively new pond feature, and not many are that old. I'd guess that a good quality, properly installed liner would last longer.

I've also seen product descriptions of alternatives to single sheet membranes. Bentonite is sandwiched between two porous membranes, creating a good seal that may not require underdrains, watertight seams, or extra protective layers. Not having had any direct experience with such liners, I'll withhold judgement for the time being.

But as these innovative new liner types suggest, the need for new methods to leakproof ponds is a continuing challenge.   


May 1, 2016

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