retention pond leak

[brnt]

I am a site inspector and I have been working on a large stormwater retention pond. A clay liner was built to seal the pond and unfortunately it has a slow leak.The leak is a problem because when the pond reaches normal water level, water flows out into a wetland area.We need water in the wetland.

Anyone have a suggestion on how to find a leak?

Installing the clay liner was difficult because the bottom of the pond is below the groundwater.The contractor had 4 8 inch pumps working to control the incoming ground water. It appears that when the pressure of the pond water is greater than the ground water pressure the water flows out through the leak.Any suggestions on overcoming this difficulty?

 

[Prepakt1]

You could pour a concrete mattress over the leaking area which can be done underwater.You would have to install grout tubes through the mat and grout below the mat once the mat was filled.

Intrusion Prepakt /marineconcrete.com

 

[beej67]

Do you know it's one particular spot that's leaking, or could it be leaking all over? Could it be the clay wasn't clayey enough? That'd be my first worry. Where's the job at and how are the soils? Can you test the borrow pit location for permeability?

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[brnt]

Permeability tests were done on the clay used in the liner and it met specs. In fact the clay was hauled from another site because there was no suitable clay on site.

I suspect the water is leaking in specific areas where the ground water undermined the liner before the pond filled. I was just wondering if there are technical or scientific procedures for locating leaks in a large pond.

 

[beej67]

If the leak is large enough to provoke an actual velocity profile, however small, in the pond, then you could find the leak with dye tracers. I'm basically grasping at straws though. I'll be interested to see what some others come up with.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[cvg]

I have heard that sealing a pond with bentonite broadcast into the water can be done relatively easily (but never tried it myself). I believe I have seen this discussed in the forum before.

 

[MikeHalloran]

If the water is clear enough to support fish, add some if needed, and follow them around, e.g. by diving.

Watch closely. Fish are very sensitive to lateral currents, and hardly at all to vertical currents. So they will tend to swim away from the vicinity of a leak, but if they happen to pass over it, they may be displaced downward with the flow.



Mike Halloran
Pembroke Pines, FL, USA

 

[Ryb01]

Try monitoring the pond for a duration to find out what water elevation the pond is leaking from. This may take some time, however, it may be beneficial to determining the location of the leak without spending money on an unnessecary solution.

For a starting point, brnt, you had mentioned that the bottom of the pond is located within the ground water table. Where's the ground water elevation in relation to the normal water elevation? If the ground water elevation remains consistent throughout the year(doubtfull but none the less), a leak in the pond bottom may not be noticeable. However, the elevation difference between the two may be where the leak is. Given time, the leak will reveal itself. If the pond is leaking around the water's edge at a particular elevation a simple visual inspection may determine the location of the leak. Constantly moist soils areas (pockets) around the perimter of the water level are a result of capillary suction. The pond banks would have to be relatively dry to see this happening (i.e periods when rainfall has not occured, allowing the soils to dry).

Sealing the pond with benonite clay as cvg suggested, once the leak has been located is a good method of sealing the pond. Benonite clay, as a material, expands up to 18X it's original size when in contact with water, thus producing an excellent seal within ponds.

Hope this helps.

 

[brnt]

After periods of no rainfall, the pond water level holds at a particular elevation, approx. half way between the bottom of the pond and the desired normal water level (total depth of pond is 6m). You would think that the leak must be occurring at or above the stabilized water level. Someone suggested this may not be the case. They said this could be the level where the pond water pressure is equalized with the ground water pressure.Is that a possibility?

 

[beej67]

Yes.

Here's how that "generally" works,

If the bottom of your pond is at 100, the top (designed control elevation) is at 120, and the adjacent groundwater table is at 110, then the leaky pond will drain down to 110 and stabilize at 110 regardless whether the leak is at 110, or 105, or 100. If the adjacent groundwater is at 110, and the leak is at 115, then the pond will drain down to 115 and stabilize there. Make sense?

So if your pond is holding at "half way," but "half way" is still higher than adjacent ground water, then it's a good bet your leak is at that "half way" elevation as well, likely somewhere around the edge of the "half way pool." If "half way" also corresponds to the adjacent groundwater table, though, then all you know for sure is the leak is at or below the half way mark in the pond.

Now there's all sorts of possible exceptions that are terribly complicated, depending on soil properties and pond size and confined aquifers and blah blah, but that's "generally" how it works if you stick a clay lined pond into reasonably sandy soils.

You can eyeball what the adjacent groundwater is by digging a test pit or two near the pond, deeper than the water table. If over time the water in your test pit rises to the water level of the pond, then your leak could be anywhere in the pond. Could be the whole liner is cracked. If the water in the test pit is lower than the water in your pond, then the leak is probably somewhere along the water's edge in the pond.

I'm a little concerned about your wetland. Is it a constructed wetland or an existing one? Existing wetlands are typically down near the water table, and if this was intended to be a constructed wetland fed by the pond, did they put a clay liner or something in the wetland as well? If not, it probably won't stay wet even if you do get your pond fixed.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[brnt]

Ok--I am going to throw an idea out there for feedback.
You have a dead calm pond where water is leaking at a significant rate from a relatively small hole. Would there not be sound coming from the leak? I remember working on a job where there was a leak in a sanitary force main and a specialist was brought out with sensitive sound detection equipment to see if he could hear the water rushing from the pipe ( which was 3m below a road). Maybe someone could work around the perimeter of the pond listening?

 

[letrab]

My first problem is with the question "A clay liner was built to seal the pond and unfortunately it has a slow leak.The leak is a problem because when the pond reaches normal water level, water flows out into a wetland area.We need water in the wetland."
It sounds as if the slow leak is the solution to keep the water in the wetland rather than a problem. It is difficult for me to visualise the problem.

You can use a plastic sheet 1mx1m and float it 100mm or so fom the clay liner. At leak positions the sheet will move towards the clay liner. It works quite effectively in concrete reservoirs, you need scuba eqmt and divers to do so. I find it quicker than dye.

The long term problem with a relatively thin impermeable layer below the water table is the external water pressure when the dam water level is low. This external pressure is higher than the internal pressure and it cracks the clay liner and flow between the in situ soil and the caly liner. These cracks can grow unrestricted as long as the pressure differential exist. When the dam fills the water leaks out through these cracks and spreads to move into the soil.
It is therefore possible that the leaks may seal and re-appear at various locations as the water level moves in the dam