Bentonite Clay for Pond Sealing

Granular Sodium Bentonite Clay, when used appropriately, can be one of the most effective pond soil sealers available.

There are two significantly different types of bentonite clay - Sodium Bentonite and Calcium Bentonite. Sodium bentonite clay is known as "high-swell" clay. Sodium Bentonite is preferred to Calcium Bentonite Clay for pond sealing applications. Calcium bentonite has limited swelling capability and is only effective for relatively short periods of time as a sealant when it is treated with other additives. However, the additives improve the swell of the Calcium Bentonite Clay for a limited time before they start to fail. Sodium Bentonite Clay has a naturally high swell capacity and does not require additives to maintain the swelling characteristics indefinitely.

Swelled Bentonite Clay (top) prevents seepage in blanket method

1. The blanket method. The blanket method of bentonite clay application is simply placing a uniform layer of granular bentonite between 2 confining layers of soil. In new pond construction, the pond bottom is over-excavated by 6 to 8 inches. The bentonite clay is then spread in a uniform layer before the over-excavated soil is then placed back over the bentonite clay blanket. The top layer of soil is compacted with a pad-foot roller to ensure proper sealing. When using the blanket method for bentonite the pond side slopes should less than 33 degrees (3:1) or the bentonite material will slump toward the pond bottom. This is an important detail because as the blanket of bentonite becomes saturated it takes on a gel like consistency and the bentonite can very easily slide down the pond slope.
2. The mixed method. The mixed method for bentonite clay application provides some advantages over the blanket method. Using the mixed method, bentonite is spread at the desired application rate and then mixed with the existing pond soil using an agricultural disc harrow, rototiller, or other mixing method to be sure that the granular bentonite clay is completely mixed with the top 6 to 10 inches of pond bottom soil. Once complete, the soil mixture must be thoroughly compacted with a pad-foot roller. Very dry soil will need to have water applied to the soil/bentonite mixture before rolling to ensure complete compaction. The mixed method for applying bentonite clay does allow application on steep slopes up to 45 degrees. It is typically best to apply the bentonite at a higher rate near the top of slopes since it will tend to work down the slope when it is tilled into the soil.

The USDA Web Soil Survey is an excellent resource for learning about the soi types at your pond site. We often use it to select the best site to the build a pond. This DIY tool is free to use and gives access to a wealth of pond soil data. You can watch a short tutorial video that shows how to get the right soil data for a clay lined pond.  You will also learn how to interpret the data.

We recommend that you test application rates with your particular soils by treating a small area at the anticipated bentonite clay application rate. Drive a 12" or larger diameter smooth wall pipe at least 8" into the soil column. Manually compact the soil-bentonite mix within the pipe and fill with 18" or more of water. Measure the rate of decline in the pipe and adjust application rate for your soils as needed. 45 lb pails of sodium bentonite are available for testing application rates or treating small areas.

Another simple bentonite sealing test can be completed using a 5 gallon bucket.

The bentonite bucket test is completed as follows:

1. Drill 15 - 1/4" to 3/8" holes in the bottom of a 5 gallon bucket
2. Place 6-8 inches of soil from the pond to be sealed in the bottom of the bucket
3. From the application rate table, select the bentonite application rate for the soil that matches the pond soils
4. Mix soil with bentonite at that rate in lbs/square foot
5. Thoroughly blend the soil and dry granular bentonite until the mixture is uniform
6. Add 3 cups of water to the soil bentonite mixture and mix again until moisture is evenly distributed
7. Firmly tamp the soil/bentonite mixture into the bottom of the bucket
8. Using a permanent marker or paint stick mark a fill line roughly 2 inches from the top of the bucket
9. Gently (so that the soil isn't disturbed) add water to the fill line
10. Monitor water loss over a minimum 48 hour period
11. If seepage continues, repeat at an increased bentonite application rate until desired results are achieved
12. Calculate the bentonite required for your pond area as follows. Multiply the number of pounds of bentonite used in the successful bucket test by 1.75 to determine the application rate in pounds per square foot. (If using our 5 gallon pail for the test. The area of the bottom of our standard 5 gallon bucket is 0.57 square feet) If using a different bucket, determine the bottom area by the radius of the bottom (in inches) times 3.14 and divide by 144 to convert the answer to square feet.

For more precise analysis, use our Pond Soil Testing to determine the hydraulic conductivity of the pond bottom soil on your site. This laboratory measurement can be incorporated into the water budget to determine 1) whether additional clay liner material is needed and 2) if clay is needed, the application rate necessary to construct the impermeable clay liner.