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Determine 2 Acre Lake capacity by its soil content #166128 - Ask Extension

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Determine 2 Acre Lake capacity by its soil content #166128

Asked February 05, 2014, 5:34 PM EST

Can you tell from a soil sample if a 2 acre lake will hold/not hold water? This lake was full in 2002 but due to drought, possible leakage?? it does not even hold rain water now.   This 2 acre lake is in Smithville, Texas - soil looks like clay and/or sandy loam material.  I am thinking about digging a well to fill the lake but want to ensure the water will stay.  What should I do to have it checked out?  

Bastrop County Texas

Expert Response

The answer is yes and no. A soil sample can tell you the soil type and percentage, such as silt loam, clay loam, or clay and % sand, clay and silt. What a simple soil sample will not tell you is the depth and profile of the soil types. A soil core sample is necessary to answer those questions. Why is this important? Many ponds are built on poor or questionable soil types, and a clay pan (layer) is added on top of them to hold water. A true clay pan only needs to be around 6 inches deep to hold water, but silt loam and clay loams typically need to be 12 inches deep or more to for a suitable pond bottom. An often overlooked component is the organic matter layer. In ponds, the organic layer is decomposing plant and animal matter. This layer has the ability to infiltrate and help plug small areas where for example, the clay loam layer might contain a bit more sand.

The real problem you seem to have is the fact that your pond went dry. The reason clay is used to seal pond bottoms is its unique ability to expand when wet, but it also means that it contracts when dry. That means that when your pond went dry, the clay layer most certainly cracked and experienced fissures to several inches or more. This creates two problems. The first is that the fissures open up the soil beyond the clay layer and allow mixing of sub and surface (runoff) soils with the clay layer. Remember many Texas ponds are built on substandard soil. The second is that oxidation of the organic layer allows bacteria to quickly decompose it effectively eliminating this layer. A pond bottom should never be allowed to dry (I know you don't have much choice in a drought).

Will your pond ever hold water again if you do absolutely nothing? The answer is maybe, but I wouldn't spend money to dig a well when gambling on maybe.  If you want to be proactive, there are three main solutions. The first is to bring in more material that is at least 35-40% clay and put down a layer that is a minimum of 5" deep so the fissures are filled and the pond resealed. This new material must be kept moist and should be covered with straw or mulch to keep it moist if there will be any delay in filling the pond. The second is to lay down a thin layer, at least 1" thick, of high expansion bentonite clay to fill the fissures. Again, the bentonite layer must be kept moist until the pond is filled. The third solution is the easiest and least expensive. Till the soil to a depth of several inches and then compact it. Be careful to till to a depth of no more than 8-10 inches (if your pond was constructed properly to begin with, 5-6" if you are uncertain of the constructed clay depth) because you may go deeper than your clay pan and mix poor quality subsoil with your clay. Then compact the clay soil layer again. This can be accomplish with a small tractor and a disk or shallow shear plow, followed by a drum roller for compaction. This equipment can be rented for a relatively inexpensive price (compared to trucking in clay) provided you know how to operate it or can find someone who does. After compacting the soil, it is best to fill is quickly as possible.

Best of luck to you.
Todd Sink Replied February 11, 2014, 12:04 PM EST

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