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Sodium bentonite is how a seeping pond gets fixed on Texas ground. Here is how much clay a pond takes by soil type, why it comes by the truckload, what our trucks and machines charge, and a calculator that turns your pond into loads and a starting number.
A pond seals with sodium bentonite at somewhere between one and five pounds per square foot of wetted floor and bank, depending on how sandy the soil is: about a pound and a quarter on clay, a little over two on sandy silt, three and a quarter on silty sand, over four on clean sand. The clay comes to the site 21 tons to the truckload at $6,500 a load, and a job takes whole loads. A quarter acre pond on sandy silt fits in one load; a half acre needs two. It is spread with a skid steer, worked into the top four to eight inches with a dozer and compacted, and the dozer starts at $3,000 a day plus transport. Put your pond into the calculator below and it gives you the tons, the loads and a starting figure for the material before anyone drives out.
Every bentonite page on the internet is written by someone selling bentonite, so the numbers stop at the bag. This one is written by the outfit that spreads it, so it goes through to the machine days and shows you a pond we sealed in September.
Enter the pond as it will be when it is drained and shaped: the surface it covers when full, the depth at center, the bank slope and the soil in the basin. The calculator applies the supplier’s published rate for that soil, adds the wetted area on the banks, rounds up to whole truckloads and prices the clay at our load price. Machine time is shown separately at the published dozer day rate so you can see both halves of the job.
Rates are the supplier’s published application guide for ponds to eight feet deep, plus one pound per square foot for each further eight feet. The skid steer that spreads the clay, transport, draining and any reshaping or dam work are priced on the site walk and are not in this figure. A pond that is also too shallow should be priced as a rebuild rather than a seal.
Sodium bentonite is a clay that swells to many times its dry size when it gets wet. Worked into the top layer of a pond basin and compacted, it fills the pore space between soil grains that water was escaping through. How much you need depends almost entirely on how much pore space there is to fill, which is a question about the soil.
| Soil in the basin | Rate per square foot | Quarter acre pond | Half acre pond |
|---|---|---|---|
| Clay | 1 to 1.5 lb | 1 load | 1 load |
| Sandy silt, at least a fifth clay | 2 to 2.5 lb | 1 load | 2 loads |
| Silty sand | 3 to 3.5 lb | 1 load | 2 loads |
| Clean sand | 4 to 4.5 lb | 2 loads | 3 loads |
| Rock or gravel | 5 to 6 lb | 2 loads | 3 loads |
Those rates are the supplier’s published application guide for ponds to eight feet deep, and the load counts assume the banks add a few percent to the plan area and a job takes whole 21 ton loads. Deeper ponds carry more water pressure on the floor and take about a pound more per square foot for every extra eight feet. The quick way to read the soil is to wet a handful from the floor of the basin and squeeze it. If it holds a ball, it is clay and may only need a light rate. If it crumbles, it is sand and it needs the heavy end.
Note what the table does not say. A clay pond that is losing water is usually not losing it through the clay. It is losing it through a gravel seam, a sand lens, an old root channel or the dam, and spreading a light rate of bentonite across the whole basin will not fix a leak that is coming from one place. The bucket test and the signs that tell a basin leak from a dam leak are on their own page, and they come before the clay.
Bentonite has to be mixed into the soil, and that only happens on a drained pond. Owners usually let a leaking pond go down over a dry spell, which it will do on its own, or pump it before we arrive. The basin needs to be firm enough to carry a dozer without rutting.
Any reshaping happens before the clay goes down: deepening the center, cutting the banks to a consistent slope, stripping out silt and old growth. At Lockhart the old quarter acre pond was cut to a half acre and taken from six feet to ten before any bentonite was spread. If you seal a pond and then dig in it, you have dug through your seal.
The bentonite is dumped on the bank and spread across the floor and banks with a skid steer. The white stripes in the photo above are what that looks like: rows across the basin at the rate the soil calls for, laid out so the dozer can blade them into an even blanket.
This is the step the bag instructions gloss over and the one that decides whether the seal holds. The dozer blades the bentonite into the surface soil until the top four to eight inches is an even mix rather than a layer of clay sitting on top of the dirt. A layer on top cracks when it dries and washes when it wets. A mix becomes the soil.
The whole basin, floor and banks, is track packed with the dozer until it is tight. Compaction is what gives the swelling clay something to swell against. Then we leave, and the next big rain does the rest.
The finished pond is dry the day we hand it over. That is the honest state of every seal and every new pond: we build it to hold water, and the weather fills it. On sandy ground the basin can keep losing a little until the bentonite has fully wetted and swelled through the first fill, and after that it settles down.
There are two halves to the price and we publish both. The clay comes 21 tons to the load at $6,500 a load, and a job takes whole loads because a truck does not bring half of one. The machine time is a skid steer to spread and a dozer to work in and compact, and the dozer starts at $3,000 a day plus transport, depending on the size of the machine the job needs. How many days is set on the site walk, because a pond that needs reshaping before the seal takes more than one that is already the right shape.
The Lockhart pond is the reference. A quarter acre pond another contractor built would not hold water, so we rebuilt it as a half acre, ten feet deep with three to one banks, spread and worked in bentonite, and compacted it. Four days on site with a Cat D3, $20,000 to $22,000 all in, and that price carried the rebuild as well as the seal. A seal only job on a pond that is already the right shape and depth is a smaller number than that, and the calculator above will get you close to it.
For comparison, our pond price list starts a new half acre pond at $19,200 for six feet of depth on a four to one bank. When a leaking pond is also shallow, the rebuild and the seal together cost about what the new pond would, which is why we usually recommend doing both at once rather than sealing a pond you will want deeper in two summers.
On sandy silt, about 24 tons, which is two truckloads. On clay, one load. On clean sand, three. Put your own numbers in the calculator above.
It can be done and some of it will settle into the leak, but it takes the most clay and gives the least reliable seal. We seal drained ponds where the clay can be worked into the soil and compacted.
Once it is mixed into the soil and compacted it becomes part of the basin and does not wash out or wear away. The things that break a seal are digging through it, letting the basin dry out and crack for a long stretch, and cattle cutting up the banks.
No. It is a natural clay, the same material used to line stock tanks and drilling pits. The pond will be cloudy after the first fill and clears as the clay settles.
Not by itself. A dam that leaks through the fill or under it needs to be opened and rebuilt with a compacted clay core. That is dirt work, and it is covered on the leaking pond page.
You can buy it by the bag or the load and many owners have tried. The part that fails is the working in and the compaction, which take a dozer. Clay spread on top and left there cracks and washes.
Tell us the pond size, the soil, and how far it drops in a week, and you get back the load count, the machine days and a written number.