I am unwittingly an owner of a property with a seasonal pond. My yard is sloped up, but there is a flat area with a recess where water appears to pool during the wet season(I live in the PNW).
The previous owner installed a 4in drainage pipe to this pond and it naturally flows out into the storm drain, but this pipe isnt keeping up and we get the pond. The pond is shallow, maybe 6in, but quite wide, Ive seen it up to 20 ft at the widest point. This is all in a forested back yard.
I am looking to drain the pond further, curious if a swale or a french drain would be ideal here?
Theres been no flooding to my house, but I do notice higher than normal groundwater around my foundation, which is a good 40 feet down the slope(5ft lower elevation). There is groundwater 4ft into the ground so my sump pump is working hard.
What is the relationship is between the groundwater near my foundation and this pond? If I drain the pond, would it help lower pooling of groundwater? I know groundwater is difficult to track but given the proximity and the slope, wouldnt there be some sort of connection between the pond and the groundwater near my house?
Hoping to hear from the experts here your thoughts and the best way to approach draining the pond to minimize groundwater collection. thanks!
Would need to have an understanding of the soils between the seasonal pond and the house. It is possible that water is getting trapped between the house/sump and the pond but without knowing what the soils are there's really no way to say if that is occurring.
I am no expert though I have worked on a couple of projects where water was getting in someplace it wasn't supposed to.
It is difficult to diagnose underground problems over the internet, but that has never stopped us before!
1.) Pooling water is much easier to mitigate when the ground is sloped and you have more ground available significantly downslope of the pooling water. In those circumstances, french drains and similar devices can be very effective.
2.) Do you have a pump available for testing? Either a gas-powered trash pump, or an electric sump pump (if the forest pool can be reached with an extension cord). If so, I would test the forest pool before spending a lot of time and effort on a fix. Dig a little sump in the pool when it is dry. After the next rain, pump the pool out. If it pumps down quickly, and remains dry, then it is a depression that collects surface water. If you pump it mostly dry, and then the next morning it is full again, then you have confirmed it is a groundwater pool. (And you have confirmed that you have permeable subsoils in your area, AND that there is water being retained in the pore network at a higher elevation than your forest pool.)
If that is the case, then there is a very good chance that your sump pump is working overtime because there is a lot of groundwater (including your forest pool) that has a hydraulic connection to your foundation.
3.) Conveying any portion of that water farther down the slope can only help your house foundation! You can do perforated drain tile adjacent to your foundation. You could do a hidden project out at the forest pool to drain the groundwater if the property slopes away from that location.
4.) Can you get a soil profile at the forest pool? (Post hole digger, mini-excavator, rambunctious teenagers with a shovel?) If you find sand and gravel down to 4' (for example), and then clay, that indicates how deep you need to cut to convey away the groundwater. You could just cut a 4' deep ditch out of the forest pool to drain the groundwater downslope. It does not have to be straight - it could meander through the trees in your forest so you don't have to cut as many roots.
Just throwing out some ideas. Your eyes on the specific situation will be the best final judge of the optimal solution.
Good luck on your drainage project!
Of course, since it is "Pond Boss", do you have room on your property at a lower elevation than the floor of your basement to create a groundwater pond? That would certainly be an elegant solution!
thanks for the help FishinRod. I was suspecting that this pond would be a groundwater pool. The drain that currently connects to it runs weeks after a storm continuously, I just cant imagine that much surface water being there, but good idea on that test, I can give it a try. My area is known for the glacial till, so I suspect that contributes to this.
Could you explain why you think there could be a good chance there is a hydraulic connection between my foundation and the pool? Is this something that you've seen before? This is my hopeful wish ofc.
Unfortunately my foundation level is relatively flat, slightly sloping down, not enough to make a pond, I am lucky that I have the slope uphill to drain this water in the first place.
I am a geologist, but not an expert in unconsolidated sediments.
My analysis of the chance of connection to your foundation is solely based on the apparent presence of an adjacent aquifer of sufficient extent to keep a groundwater pool full of water for extended periods. If they put gravel outside your concrete pours, but THAT permeable gravel was surrounded by impermeable clay, then your sump pump would pull out all of that water in a few hours after the rains ceased.
Your foundation might not be hydraulically connected to the pond, but may be connected to a separate aquifer. (However, I speculate that a connection to your forest pool is more likely.)
Glacial tills are notoriously heterogeneous, since they are not initially deposited in moving water like most sediments. However, most glacial tills are subsequently re-worked by more typical surface factors after the glacier recedes. This process tends to form layers of sand and gravel that make excellent aquifers.
As to your dye question, it is easy to observe dye when you are drawing directly from the dyed source. For example, if you dyed a pond red, and someone stuck a 3/4" hose into the pond and pumped water into their fish tanks, they would immediately notice the color.
When you are drawing groundwater, you would be first drawing from all of the connected pores BEFORE you started drawing from your dyed forest pool! However, there are dyes that were created for that situation. Unfortunately, it has been over 30 years since I have pulled dyes to monitor groundwater wells that were supposed to be pulling pollutants from contaminated groundwater. I suspect the technology has improved since that time!
Here is a recent copy/paste for you.
Why use fluorescent dyes?
Groundwater tracing using fluorescent dyes is the best method to determine the connectivity of recharge and discharge points and allows for determination of direction and velocity as well. Dye tracing is a vital tool in understanding groundwater flow and the resulting information is used in a variety of ways to protect groundwater and surface water quality.
Fluorescent dyes make excellent tracers for a variety of reasons, including:
They are water soluble. They are highly detectable (some below 10 parts per trillion). They have extremely low toxicity ratings. They are inexpensive. They are fairly stable in natural waters. They have been used successfully for tracing for over one hundred years.
I don't know if you will see dye at your sump, but I don't see any harm in trying. (After you determine the forest pool is in fact a groundwater pond.)
We had injection wells at the outside perimeter of the pollution plumes. I think in one case we had dye show up in a groundwater producing well more than 100 days after it was injected!
Interesting! I am definitely learning a lot here. Ive also become some sort of a sump pump expert through this experience.
I took a stick and stuck it into the pond, theres only 6in of water above the surface but the soil there is all mush, so if you step in it sinks pretty far. Need some high boots to navigate it.
Regardless if its a surface water or groundwater, I would still like to divert it. Would my strategy be different between the two cases?
It looks like given the slope I can dig 2 ft into the pond center and have a frenchdrain there. I would imagine in the case of surface water buildup only, I wouldnt have to go in too far, but for groundwater, wouldnt it be deeper the better?
In a forest pool, there is almost certainly some "muck". This is decaying organic matter - probably leaves and twigs for your pool. You may also have inorganic sediment (soil particles) that has washed into the pool. This is usually called "silt", although that is not always technically correct. The silt would also feel squishy to walk on.
I am still not clear, do you have a lower elevation of land available to drain water from your forest pool? If so, then I believe your project should work, and probably provide some relief.
The muck and silt in your pool probably does provide a poor seal. So yes, a french drain on top of the pond muck layer would convey away surface water. However, if it is truly a ground water pond, then getting below the muck and silt would allow you much more efficient drainage of the surrounding aquifer.
Can you dig down a little ways at the edge of the pond? The muck should be a little thinner there before you encounter your true subsoil. If possible, dig a foot or two below the elevation of the middle of the pond to test your deepest layers. If you only encounter clay, then probably NOT a ground water pond.
Just breaching the side wall deeply enough and letting the pond contents run down the slope could be sufficient to drain some ground water away, especially if you scrape some muck away so more of the sand/gravel layer is exposed.
In a forest pool, there is almost certainly some "muck". This is decaying organic matter - probably leaves and twigs for your pool. You may also have inorganic sediment (soil particles) that has washed into the pool. This is usually called "silt", although that is not always technically correct. The silt would also feel squishy to walk on.
I am still not clear, do you have a lower elevation of land available to drain water from your forest pool? If so, then I believe your project should work, and probably provide some relief.
The muck and silt in your pool probably does provide a poor seal. So yes, a french drain on top of the pond muck layer would convey away surface water. However, if it is truly a ground water pond, then getting below the muck and silt would allow you much more efficient drainage of the surrounding aquifer.
Can you dig down a little ways at the edge of the pond? The muck should be a little thinner there before you encounter your true subsoil. If possible, dig a foot or two below the elevation of the middle of the pond to test your deepest layers. If you only encounter clay, then probably NOT a ground water pond.
Just breaching the side wall deeply enough and letting the pond contents run down the slope could be sufficient to drain some ground water away, especially if you scrape some muck away so more of the sand/gravel layer is exposed.
Sorry for the late response. Yes, I do have a lower elevation to drain the pool. The soil is mostly clay unfortunately, which is why is made for a great pond.
I actually already completed the project. Dug up a trench to bury a 3ft catch basin and routed large pvc pipes downhill to the stormdrain. In the first hour or so, most of the pond disappeared. Now its been a day and theres a smaller pool and a small stream flowing into the catch basin. You can see the attached images.
The clay holds so much water it will probably take a long time before it stops being mush, but its slow to release. There has been no change in the groundwater being pumped by my sump as of yet, it has a timer and has been pumping at the same rate even after the pond mostly went away.
I'm hoping its a similar issue here near the sump, mostly clay and has way to much water collected that it will take a long time for the pond being gone to affect it.
I know you mentioned if we hit clay, its probably not ground water. But I just dont see where else the water can come from?
I know you mentioned if we hit clay, its probably not ground water. But I just dont see where else the water can come from?
I should have clarified that statement a little more to "if you hit dense, hard-packed clay".
However, even with THAT clarification, there still may be little channels of sand within the clay that are capable of both storing AND conveying water! Those are typically very difficult to observe in wet, sloppy clay. Therefore, if you have not had recent rains, then I think you have proved this is ground water that builds up in your pond basin.
What happens to the water after it fills your blue catch basin?
Further, I assume the water flow rate into the catch basin is still less over the course of a day, than what your sump pump is ejecting?
Right now, I suspect you have a gigantic amount of water held in the aquifer surrounding your basement. Running a second little outlet (to your catch basin) is NOT going to drain that ocean any time soon.
However, it may help in the long run. Eventually your sump pump will get caught up with the ground water. After that, the next rain will start to recharge the aquifer. If you keep draining your little pond basin, then you will slow the recharge rate of the aquifer. If you get your drainage outlet at a low enough elevation, you may really pull down the aquifer water level during a hot, dry summer. In that case, the next few rains would have to go to filling back up a depleted aquifer. Hopefully, that will significantly decrease the work required by your basement sump pump.
After the water fills the basin, I have two 6in pipes that leads it down the slope to the storm drain. So none of that water should be refueling my aquifer. There is a constant stream coming out of one of the 6in, its probably only 1/5 full.
My sump pit fills about 3ft before it pumps, and this took about 7min yesterday. Hard to compare against the flow of the catch basin which is flowing constantly. But even before this I had a system using a 3in pipe to drain the pond.
I agree with your suspicion that there's some kind of aquifer around my house, its likely composed of clay too, and retains a lot of water. Draining the aquifer from the top of the hill with the catch basin, should hopefully prevent the aquifer near my house from being recharged as much. But like you said it could take a a season to clear up then.
Ive attached some more pics for clearer view of this setup.
