Pond Boss Forums POND CONSTRUCTION Renovating a Pond or Lake? Recently renovated pond seeping where dam was cut

I bought a property with an existing 1-acre pond that was neglected for 20 years or more. I literally couldn't walk across the dam, because it was so overgrown and had a dozen or more trees growing on it, and it was seeping water all across the base of the dam. I hired a local excavator/lagoon/pond builder who removed all the trees, cut a gap in the damn wide enough to drive a bulldozer through to drain it and push out all the old muck, got the entire pond back down to hard pan clay, rebuilt the gap they cut to drain it, rebuilt the inside of the dam and cored it. He finished the 1st of November 2023. I live in Mid Missouri, USA where we have a lot of clay. I watched it closely as it filled over the course of 2024. It was late in the year, but I sowed winter wheat on the top and back of the dam and rye grass inside the pond, and it all actually came in nicely. We didn't get much snow/rain in the winter or spring but received record amounts of rain in May/June that filled the pond completely. So much so, the excavator had to come back out and lower the spillway and added dirt to the top of the dam. I watched it closely as it filled and observed no leaks on the back of the dam anymore...that is until it got to about 3-5 feet from the top of the dam. The dam leaks (or seeps) very slowly in two spots at the base of the dam in the location of where the dam was cut. The seep is enough to wet the surrounding area behind the dam, but not enough that I can actively see it flowing or cause a noticeable drop in the water level. I have not been able to get the excavator to come back out and take a look since he redid the spillway. My guess is that the dam cut didn't get properly compacted near the top of the dam where it is very steep. My solution so far has been to apply 700lbs of bentonite clay in granular form to the area of the dam where the cut was made in two different applications, the first 200 lbs of which was late July and the last 500 lbs yesterday.

Finally to my question, would it be normal for a pond to leak/seep where the dam was cut and will it stop eventually as it silts in? I don't plan to add anymore bentonite this year, but should I continue applying it if the dam is still leaking next year? The lagoon I had dug (by the same excavator) also had a small leak, but it did eventually stop leaking after about a year.

Thank you for any thoughts or advice.

Welcome to Pond Boss!!

Let's see what others have to say.

Not an expert, but several reports on Pond Boss of leaks in the dam after it was breached and subsequently repaired. I believe it is very difficult to seal the "repairs" against the old core trench.


You said, "... rebuilt the gap they cut to drain it, rebuilt the inside of the dam and cored it."


Did they core the whole dam, or just the gap that they had cut?

How were they compacting the repaired core, especially at the top where it was steep? Were they wetting the clay while compacting?

I do like it that your pond only started leaking with the last 3-5 feet of water in the pond. That PROBABLY means your leak is shallow. However, it could still be a bottom leak. Sometimes the extra bit of hydrostatic pressure due to a higher water elevation can push water through tight material that otherwise does not leak.

If it is a shallow leak, then it is likely due to the breach repairs. However, there could be other causes. Do you currently have muskrats at the pond? Did the old pond have muskrat damages. Those b*astards can burrow back a long way into a dam. Water could be entering an old burrow and then working downwards beyond the core trench to exit at the toe of the dam. Likewise, some of the old tree roots in the dam could still be rotting out. They could create a water leak pathway similar to a muskrat burrow. Either would present as a shallow leak.

Did the first bentonite treatment have a noticeable effect on the leak rate? Will you be able to tell if your second (larger) treatment does any good?

If the leak is not affecting your water level much, you might just leave it as is. Most leaks do NOT get better over time. However, one of the common exceptions is heavy clay soil. As the leak continues to hydrate more clay over a larger area, you may get enough swelling to reduce the leak further.

How deep is your pond at full pool? If you get an extended drought and your pond goes down 3', and the leak ceases at that point, then your fish are not really in much danger if your pond has sufficient depth.

I hope you enjoy your renovated pond ... despite the leak. It sounds like you paid good money to do it right the first time. It also sounds like your contractor got it 99% right, which is pretty good.

Fingers crossed for your second bentonite treatment.

If that works a little bit, then came back and post some more. There is another product called Soilfloc that may work a little better if you are getting partial success with the bentonite.

Hi FishinRod, thank you for your reply. My responses are below your questions which I copied below:

Did they core the whole dam, or just the gap that they had cut?
They cored the whole dam

How were they compacting the repaired core, especially at the top where it was steep? Were they wetting the clay while compacting?
They compacted with a skid steer and bucket. I did not see them wetting it.

I do like it that your pond only started leaking with the last 3-5 feet of water in the pond. That PROBABLY means your leak is shallow. However, it could still be a bottom leak. Sometimes the extra bit of hydrostatic pressure due to a higher water elevation can push water through tight material that otherwise does not leak.
This has crossed my mind, thank you for mentioning it.

If it is a shallow leak, then it is likely due to the breach repairs. However, there could be other causes. Do you currently have muskrats at the pond? Did the old pond have muskrat damages. Those b*astards can burrow back a long way into a dam. Water could be entering an old burrow and then working downwards beyond the core trench to exit at the toe of the dam. Likewise, some of the old tree roots in the dam could still be rotting out. They could create a water leak pathway similar to a muskrat burrow. Either would present as a shallow leak.
No current muskrats or signs of muskrats before the renovation. Old rotting roots is a possibility since the whole of the dam was not rebuilt.


Did the first bentonite treatment have a noticeable effect on the leak rate? Will you be able to tell if your second (larger) treatment does any good?
The first treatment did not seem to make a difference. Yes, I will be able to tell if the second treatment does any good. I've read that it may take up to 12 weeks, but I'm not sure that is accurate.

If the leak is not affecting your water level much, you might just leave it as is. Most leaks do NOT get better over time. However, one of the common exceptions is heavy clay soil. As the leak continues to hydrate more clay over a larger area, you may get enough swelling to reduce the leak further.
This is good to know. I'm not concerned much about the water level. Having the area behind the dam always wet and muddy is annoying. My biggest concern is the leak getting worse over time.

How deep is your pond at full pool? If you get an extended drought and your pond goes down 3', and the leak ceases at that point, then your fish are not really in much danger if your pond has sufficient depth.
At full pool the pond is 15' deep, so yes you are right; the fish would be ok.

I hope you enjoy your renovated pond ... despite the leak. It sounds like you paid good money to do it right the first time. It also sounds like your contractor got it 99% right, which is pretty good.
Thank you! I grew up with a farm pond and have waited 30 years to get back to the country and have a pond again. This contractor has done a lot of work for me: building site excavation, sewer line, water line, final grading, lagoon, septic and has treated me well. Worse comes to worse, I think I can get him to come back and try a repair.

Fingers crossed for your second bentonite treatment.
Ditto!

If that works a little bit, then came back and post some more. There is another product called Soilfloc that may work a little better if you are getting partial success with the bentonite.
I have researched the soilfloc a bit and the cost is prohibitive for me right now. The bentonite is costing me ~$10 per 50lb bag, but its still throwing good money away if it isn't doing any good.

Thanks again!

I have the same situation/seep where the existing side was connected to the built up area. It wasn’t mixed. Not a big problem because I seldom get enough rain to fill it to that point. It might cost me a foot of water if I EVER get enough rain to get to that point. So far, in about 30 years, I’ve gotten there about 2 or 3 times.

"How were they compacting the repaired core, especially at the top where it was steep? Were they wetting the clay while compacting?"

"They compacted with a skid steer and bucket. I did not see them wetting it."

A skid steer is not a compaction machine. Also, you cannot compact dry clay.

I suspect your leak is due to a little water passing through your core trench. I wonder if they were working with moist material down at the bottom and achieved sufficient compaction at that level? Was there a little water left in your old pond basin during the renovation?

I suspect your core trench could be re-excavated in the area of the suspected leak, plus some additional safety margin, and then sealed with a better compaction effort and your clay at the optimal moisture content. However, if your pond is not having a problem staying full, then probably not worth the cost.

Sorry to hear about the wet area. Before the era of concrete mega-dams, the old earthen dams used to be designed with a gravel toe drain to collect the water that would inevitably seep through the dam, which was then routed away.

Do you have some even lower ground beyond your soggy area? You may have to make a collection system (like a french drain) and run some corrugated flexible pipe to your lowest point to get rid of the water from a persistent dam leak.

Hi Chris

Welcome to the forum. I help folks nationwide with pond construction, rehab, and leak abatement and am happy to share my insights on your project after learning a bit more about your situation. My time is always free to my Pond Boss family, feel free to reach out anytime and we can schedule some time to chat. Glad to help however I can.

tj@hudlandmgmt.com

Christopher,

The reason I wanted you to post again after your bentonite treatment, was to give you the contact info for TJ. However, I see that he has now dropped into your thread.

He is the "leak expert" on Pond Boss and can give you more detailed guidance if you contact him directly.

P.S. I had the product name wrong in my previous post. The product that people have had the best success with is called SeekLeak. I know it sounds expensive, but if you "right size" the treatment, it should be significantly cheaper than my mechanical solution of re-doing part of the core trench.

Welcome to PB fellow Miserian.

I'm dealing with a similar situation with my pond.

I recommend you take TJ's offer of consultation services to heart. He won't steer you wrong.