So we’ve proved you can use clumping cat litter to seal a pond because clumping cat litter is made of bentonite clay.
Now, how do you actually do it?
In my first post on this, many people pointed out that actual sodium bentonite clay was available at farm supply type stores. I struck out finding pure sodium bentonite locally, but before you run to Costco and buy $150 worth of kitty litter, go look around and see if plain ol’ sodium bentonite is source-able in your area.
Whether you use pure sodium bentonite or clumping cat litter, know this: once this stuff gets wet, it is gooey. Gooey like the slickest, wettest, slimiest pond edge you’ve ever slipped on. The more water the bentonite clay absorbs, the more gooey it gets, so here’s a few things you shouldn’t do:
Apply this as a surface coating. The clay must be mixed into soil or you’re going to get a pretty nasty layer of clay goop. I know this from experience. Learn from me.
Apply this anywhere you might be walking. The sodium bentonite liner stays soft, like, forever. As long as it’s in contact with water, it has a slickness. See above for comparison to nasty slippery pond edge. You want to fall and break your neck one day? Make a clumping cat litter pathway.
(Related fun fact: you can wet and dry pure sodium bentonite as many times as you want and so long as it doesn’t get contaminated, it’s absorptive properties never go away.)
Ok, now that I’ve firmly emphasized what not to do with clumping cat litter, here’s how you use it to line your pond:
Step 1: Determine how much bentonite or clumping cat litter you need to add to your native soil to create a good seal. There is no exact answer to this because your native soil probably contains some clay already. If your soil is mostly clay or silt already, you will need about 2 pounds of clumping cat litter or sodium bentonite per square foot of pond area. If your native soil is very sandy or mostly gravel, you may need twice this amount.
So you’ll need to do a test.
Sturgis Materials, who markets bentonite clay for pond sealing, describes how to do this. The process is exactly the same with the clumping cat litter:
In the bottom of a 5-gallon plastic pail, drill 20 to 25 eighth inch holes. Gather enough soil from the area to be sealed to fill about 3 inches of the pail. You can either select the most porous soil, (sand) or a mixture of soil taken from several areas of concern in order to present an “average” soil.
In this soil, mix one to two pounds of bentonite and tamp down in the bottom of the pail. Into this pour a gallon or two of water and see if the bentonite provides the necessary seal. If not, repeat the process and increase the amount of bentonite by half a pound until the water is contained within the pail.
The bottom of the pail represents about one square foot. When you know how many pounds of bentonite it takes to seal the pail, then we know how many pounds per square foot to distribute and roto-till into the pond, dam or other earthen structure.
In doing follow-up research for this post, I see that Sturgis Materials is now selling single, 50-pound bags of Sodium Bentonite (from Halliburton, no less!) for $70 including shipping. Like I said, check locally, but in a pinch you could always tell people you contracted with Halliburton to build your pond.
Once you know how much clumping kitty litter or granular sodium bentonite to buy, you have to prepare your pond surface.
Make sure to dig your pond 4-6 inches deeper than the final depth you’re shooting for. After you’ve dug your pond, tamp down the surface very well. If your pond is small, like ours, you might be able to get away with just stomping around in the dry pond for awhile. That’s what we did.
While you are finalizing the shape of your pond, consider the nature of this liner and adjust accordingly. Some of the side walls on our pond were pretty steep. This was a mistake, and it make it far harder to get the cat litter to stay evenly on those sections. Build your pond with gently sloping walls.
When your pond is fine-tuned and tamped down, backfill enough soil plus the required amount of granular sodium bentonite or clumping cat litter to add a 4 inch layer of bentonite-spiked soil to your pond.
Make sure the cat litter is evenly mixed in the soil. We found that backfilling the dirt and bentonite in fairly shallow, 1-inch layers and then mixing each layer together with a grading rake was the easiest way to do this. Obviously, don’t backfill with any large rocks, roots, woody matter or anything that will rot. Now tamp again. Really pack that kitty-litter-soil layer down.
Moment of truth. Time to start filling your pond. Do this very gently so the water pressure doesn’t wear a divot or thin layer in the pond side walls. We used a sprayer attachment to gently wet the sidewalls before beginning the filling process. I don’t think this is necessary, but it’s cool to see the kitty litter swell as it gets wet.
Fill the pond and let it sit for a day or several. Watch for leaks and see if there are any areas where your pond is behaving strangely.
Once you are really, 100% sure this is gonna work, you can add a 2-inch deep layer of protective sand or pea gravel to your pond. This isn’t totally necessary, but it’s a nice additional layer of protection for the bentonite clay and will help keep your pond cleaner.
We ran through about 6 drain-fill cycles on the pond because we wanted extra assurance that the pH and the sodium content of the kitty litter was close enough to neutral to be fine for our ducks and goldfish. I don’t know that this was essential either, but it helped flush the clay fines out of the water too.
That’s pretty much it. The pond’s still holding water like a champ, and the ducks still love it. The male really loves it – did you know ducks prefer to mate in water? Yeah, our drake “prefers” every morning as soon as he and the ladies hit the water.
So what are you waiting for? Build a pond. Duck sex is hilarious.79