Leaks and sealing2026-10-03

Is my pond leaking or evaporating, and how much bentonite seals it?

Drive a marked stake in the pond, read it for a few rainless days, and compare the drop with what evaporation alone would take. Extension figures for hot, dry, windy summer weather run from a quarter inch to half an inch a day, so a pond that drops faster, or keeps dropping in cold, humid, calm weather, is leaking. If you seal it with bentonite, the NRCS rate is 1 to 3 pounds per square foot mixed into dry soil, which for a one-acre bottom is 43,560 to 130,680 pounds, and every source that compares methods ranks sprinkling bentonite over the water as the least reliable.

What the authoritative sources say about evaporation

Evaporation and seepage figures from extension sources
Source	Figure as published
Kansas State Research and Extension	"about half an inch of water can be lost to evaporation each day. Water loss greater than this usually indicates leakage."
Penn State Extension	"a pond may lose over one-quarter of an inch of water in one day" on breezy, warm, low-humidity days
Clemson Cooperative Extension	"Normal evaporation can cause water levels to drop by 0.25 to 0.5 inches per day during hot, dry summer months"
University of Missouri Extension	"rates of one quarter inch per day are common during the hot parts of the summer"; "Seepage loss of 2 inches or less per month should be considered acceptable"

Kansas State measures "the drop in water level with a marked stick" and says "It's recommended this be done during cold or very humid, calm weather", when whatever drops is leaking. Clemson's bucket test runs 24 to 48 hours beside the pond: "If the pond has dropped faster than the bucket, there is a high likelihood of leakage."

Finding the leak

Penn State gives the signs you can read without equipment. A level that stops falling, "a rapid loss of water to a certain point, beyond which it then slows or stops losing water", points to a leak along the bank at that elevation. Below the dam, "Wet areas and growth of water-loving plants (like cattails) at the base of the pond dam or banks may be clues", and "Sprinkling some solid, food-grade dye in the pond near a wet area along the bank may help determine if the pond is actually leaking at that location."

Bentonite rates by method

NRCS Agriculture Handbook 590 gives the rate as "usually is 1 to 3 pounds per square foot of area", mixed "with the surface soil to a depth that will result in a 6-inch compacted layer". It says "A laboratory analysis of the pond area material to determine the rate of application is essential", and warns: "On drying, however, bentonite returns to its original volume leaving cracks. For this reason, sealing with bentonite usually is not recommended for ponds in which the water level is expected to fluctuate widely."

Bentonite application rates, quoted from each source
Method and source	Rate as published
Mixed blanket, NRCS Handbook 590	"1 to 3 pounds per square foot of area", mixed to a 6-inch compacted layer
Mixed blanket, University of Missouri Extension	"one pound per square foot for a soil with some clay, up to 3 pounds per square foot for a sand-gravel material. This rate is for each 4 inch lift."
Blanket, Texas A&M AgriLife Extension	"One-half pound of bentonite per square foot of soil" for fine sand or sandy loam under 6 feet of water; at "6 to 20 feet, increase the rate to 1 pound per square foot"
Mixed and pure blanket, Missouri Department of Conservation	"½-pound bentonite per square foot" mixed into the top 3 to 4 inches, or unmixed and covered "with 3 or 4 inches of soil, sand, or fine gravel"
Sprinkle over water, Texas A&M and Missouri Extension	No rate given. Texas: "The sprinkle method is less effective than other methods." Missouri: "Bentonite should not be broadcast on the water surface."
Holes and crevices, Missouri Department of Conservation	"1 part granular or powdered bentonite mixed with 5 parts soil to fill large holes and crevices"

No source I found gives a pounds-per-square-foot rate for a bentonite pit dug at a known leak; the only figure for a single spot is that one part to five parts mix. On broadcasting over water, Penn State says "The success of this method is low and seems to be most applicable to very small leaks in shallow water areas".

Polymer sealants, compaction and clay liners

Polymer sealants (SoilFloc and similar two-part polyacrylamide products) are the one method applied without draining. No extension or NRCS publication I found gives a rate or evaluates them; the only coverage figure is the maker's label claim of a 110-pound unit for up to 3,000 square feet.

For a drained pond, Handbook 590 calls compaction "the least expensive method". It works where the soil has "enough clay (10 percent or more)"; you scarify the bottom, roll it at optimum moisture, and "Make the compacted seal no less than 12 inches thick where less than 10 feet of water is to be impounded." A clay blanket, for soils "lacking enough clay", needs "at least 20 percent clay" and a "minimum compacted thickness is 12 inches for all depths of water under 10 feet."

What does not work

Kansas State says salt "is no longer recommended because of possible water contamination." The Missouri Department of Conservation says penning livestock to trample the basin "is not recommended" because of the nutrients they leave. UF/IFAS says bentonite "is not recommended for sealing ponds with wide fluctuations in water levels".

What pond owners on the Pond Boss forum do

Moderator esshup fills a pan to the brim beside the pond and reads the pond off a yardstick on a fence post rather than the shoreline. Bob Lusk, editor of Pond Boss Magazine, says dye tells you where water comes from: if the seep below the dam runs blue, it came from the pond.

On bentonite the forum is cooler than the extension sheets. Moderator Dave Davidson1 calls it "pretty iffy as a sealing method" after three tries with no result. He credits Hall of Fame member Cecil Baird as the only person known to have sealed a spring leak, by pouring bentonite down a pipe set over it; Cecil also sealed a 25 by 30 foot holding pond with 10 bags by the sprinkle method. Bill Cody confirms the failure the handbook warns about: once the margins dry and crack, the cracks tend to be ongoing.

On polymers, chairman teehjaeh57 (Tom Hudson, who sells SoilFloc, so read him as a vendor too) reports success, but also says multiple applications may be necessary and that one project where the product floated took three to four months. esshup would pick SoilFloc over bentonite, warns that "a quarter pound too little per square foot is about the same as not applying it at all", and says the pond must stay full, because positive pressure holds the polymer in the leak. orgeranyc tried 2 pails of a similar product and it did not work; the seller said he needed 6.

Where the tiers differ and why it matters

The rate spread is wide. The Missouri Department of Conservation and Texas A&M start at half a pound per square foot for sandy soil under shallow water; the NRCS handbook runs 1 to 3 pounds and calls a laboratory test essential. That is a six-fold difference in the bill, and the forum failures cluster around thin or untilled applications, so the higher figure and a soil test are the safer money. Two Missouri agencies disagree about sprinkling: Extension says bentonite "should not be broadcast on the water surface", while the Department of Conservation says it "will generally work if the location of the leak is known and enough bentonite is used." The forum record sides with Extension unless the leak was pinpointed first.

One number circulates without an authoritative basis: the "about an inch a week" evaporation rule on water-garden sites. The extension summer figures are a quarter to half an inch a day, which is 1.75 to 3.5 inches a week, so the inch-a-week rule would diagnose a leak in a healthy pond.

What to do

Set a stake with a ruler on it in the pond and a brim-full pan beside it. Read both daily for a week with no rain or inflow. The pond's drop minus the pan's drop is seepage.
Repeat the readings in cold, humid, calm weather. Whatever still drops then is a leak.
Walk below the dam looking for wet ground and cattails, and note where the level stops falling. Put dye in the pond near a suspect wet spot and watch the seep.
If the leak is local, draw down to the line where the level stabilizes and treat the exposed band with bentonite mixed into dry soil and compacted. If you cannot drain, a polymer sealant is the only method these sources leave open; use the full dose and keep the pond full afterward.
If the whole basin seeps, drain it, get a soil analysis through your NRCS office, and choose compaction, a clay blanket or a bentonite blanket by clay content.
Do not add salt, pen livestock in the basin, or scatter bentonite across deep water and expect a seal.

Bentonite is bought by the square foot of treated bottom, so start with the area. The PondTools volume calculator (also standalone) takes length, width, average depth and a shape factor; the same length, width and shape factor give the surface area, and a traced outline from satellite imagery is more accurate on an irregular pond. A one-acre pond has 43,560 square feet of surface. At the handbook's low rate of one pound per square foot that is 43,560 pounds of bentonite, which is 872 fifty-pound bags or a little under 22 tons. At three pounds per square foot it is 130,680 pounds, about 65 tons. The handbook also treats the upstream face of the dam, so the area you buy for is larger than the water surface.