Pondology
news from the waterfront by Tim Matson

PONDS AND CLAY

Earth ponds have long depended on soils with good clay content to hold water. Sometimes the clay is present in the soil on site, and the lucky pond owner winds up with a watertight pond. But if the clay has to be trucked in from a local source, how well the pond holds water will depend on the quality of imported clay (all clays are not alike), and how well it is applied. Sometimes imported local clay might not do the trick. Perhaps the clay is not good quality, perhaps not applied properly, or both.

Here are some basic factors pond builders should consider when using clay. If the clay is imported,
can it be located within a reasonable trucking dictance, in order not to bust the budget with transportation costs? The clay should be a mixture of soil and at least 10 percent clay, 20 percent is preferable. If you're not sure of quality, a simple soil test should be done. Screen the material  through ½" mesh to remove pebbles and gravel, then add to a one quart glass canning jar. Add water to fill, cap and shake. Allow 24 hours to settle. The clay will settle last. The visible top layer above sand/silt should be at least 10 percent clay, better if 20 percent. When adding clay to a constructed pond, the pond should be empty. If ground water or springs have the potential to compromise a clay lining, drainage should be installed. The clay should be added in the proper moisture condition so it can be spread and packed. Often one foot of clay is considered sufficient, and is added in two 6 in. layers. The first layer is compacted after application, then the second.

Pond builders lacking good local clay to beef up porous soil might order bentonite clay to add to the soil. Bentonite is a manufactured water retaining clay sold in dry form, and when applied properly can help keep a pond full of water. The downside to bentonite is that it's not cheap, and it can require skillfull application or it won't work. It can also cause turbidity. Contrary to some thinking, just throwing some bentonite over the side of rowboat into a half empty pond is not likely to solve leak problems. Bentonite works best when worked into dry soil by disc or tiller. I have seen bentonite spread by hand on the pond bottom before filling. With luck and a good water supply that might work.

 

 

 

 

 

Pond builders lacking good local clay to beef up porous soil might order bentonite clay to add to the soil. Bentonite is a manufactured water retaining clay sold in dry form, and when applied properly can help keep a pond full of water. The downside to bentonite is that it's not cheap, and it can require skillfull application or it won't work. It can also cause turbidity. Contrary to some thinking, just throwing some bentonite over the side of rowboat into a half empty pond is not likely to solve leak problems. Bentonite works best when worked into dry soil by disc or tiller. I have seen bentonite spread by hand on the pond bottom before filling. With luck and a good water supply that might work.

Paradoxically, building in a clay rich soil that does hold water can backfire. The pond will fill, but the water clarity is poor: cloudy, turbid. The pond looks like a gray or brown blob. You can't see below the surface.

It's possible that the clay will settle as the new pond ages a bit. Sediment will form a coating over the clay and keep it from clouding the water. As the shore around the pond supports new grasses, runoff will be less silty and flow in slower. The pond may clear up. But some clays are pesky and want to stay in suspension. Perhaps there are springs or inflows or a fish crop keeping the clay stirred up. Ponds used for dog training often tend to be turbid, but their trainers expect that.

If you suspect you have a clay soil that will cause turbid water (perhaps it's characteristic of your region; your neighbors' ponds are turbid), pond construction techniques can be employed to offset the problem. The pondbed can be covered with a woven landscape fabric topped with gravel to blanket the clay. Keep in mind that gravel or sand on liners of any kind may slide if the slope is too steep. If it's too late for lining, there are a variety of water additives that may help settle the clay. They include gypsum, alum, lime, barley/grasses, phosphorous, and more. The troubled water will need to be analysed and treatments tested for effect.

Clarifying pond water can be a hit or miss proposition, but while you're working on a solution, consider yourself fortunate at least to have a pond full of water, cloudy or not.

July 1, 2014

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