“I have a pond that is leaking. How do I fix it?”

The staff in our office fields this question, on average, two to three times a week. The record is nine calls in one day all asking this same question. There are many reasons that a pond could leak. The following paragraphs will cover some of the more common causes and some solutions.

It is not uncommon for new ponds to leak. I almost always recommend a wait-and-see attitude for the first year, after a pond dam is constructed. Most new ponds will quit leaking once the soil in the reservoir area has had a chance to settle and become saturated. Also the process of filling with water the first time will deposit some silt and sediment in the reservoir area. These silts and sediment are very effective at sealing a new pond.

One of the first steps in new pond dam construction is the installation of the core or cutoff trench. This is the most important step in preventing water from seeping under the dam. A core trench is simply a trench that is cut along the centerline of a new dam. This trench needs to extend, in depth, to an impervious material. An impervious material is any material that water cannot seep through. This material could be rock, shale or even heavy clay. After this material is reached the trench should be filled and compacted with the heaviest clay available on the site.

Many of our older ponds were built without having the core trench installed. This is the cause of most of our older ponds leaking. To correct this problem, it is usually necessary to bring in a track hoe or backhoe with an extendable boom to cut a new core trench. Generally a traditional backhoe can not reach deep enough to cut a core trench through the existing dam and the soil under the dam to an impervious material.

The new core trench should be cut into the front, or reservoir, side of the dam. It should extend to an impervious material as was described above. This is just like the installation of a core trench before building a new dam. The trench should be filled and compacted with the soil that was removed. The track hoe or backhoe should place a layer of soil no more than 8 to 10 inches in the trench and then compact it using the bucket. Due to the increased compaction of the soil in the trench, additional fill material will need to be used. Usually this can be obtained on the site by simply taking soil from the banks of the pond. This soil should be free of rocks, tree roots, limbs or any other vegetation. Heavy clays are the best materials to use. Silt or sediment from the pond should never be used in the core trench.

Cracking rocks

Another cause of ponds leaking is rock. Much of the rock underlying eastern Kansas is limestone with seams and cracks. When this rock is encountered, during the construction of a pond, a minimum of 12 inches of compacted clay should be placed over the area of rock. Many times this will seal the rock ledge and keep it from leaking. If there is a lack of heavy clay on site, bentonite clay, at a minimum rate of 1 pound per square foot, should be added to the material used to cover the rock.

Many times a pond will leak to a certain level and then will maintain that water elevation. This indicates the leak is at approximately the elevation that is maintained. Look for rock outcrops and treat them as described above. If no rock is observed and no obvious other problems are noted, the area about 5 feet above the waterline and about 5 feet below the waterline should be treated with bentonite clay. Again the rate would be a minimum of 1 pound per square foot. The area above the waterline should have the bentonite applied and then lightly incorporated into the top 1 to 2 inches of soil. The area below the waterline will need to have the bentonite simply poured through the water. The waterline around the entire pond should be treated.

If the pond goes completely dry, the leak is in the bottom of the reservoir area. The entire bottom should be treated with bentonite clay. The recommended rate is again a minimum of 1 pound per square foot. If at all possible, the bentonite should be incorporated within the top 1 to 2 inches of soil. This will dramatically increase the chance of sealing the leak.

In the extreme cases where bentonite does not seal a pond, a rubberized liner can be placed in the pond. A liner is very expensive and must be installed by professional crews, trained in this technique. The cost of this option is approximately $12,000-$15,000 per acre.

Other problems

Sometimes burrowing animals such as beaver and muskrat will burrow into a pond dam, creating a leak. These animals should be removed or trapped. Kansas Department of Wildlife and Parks (KDWP) must be contacted before the animals are trapped or destroyed. KDWP will issue a permit to deal with nuisance wildlife. Once the animals have been removed, the burrows will need to be filled in with soil. This soil should be packed into the burrow as tightly as possible.

Bentonite clay is usually available at farm supply and feed stores within the area. Most stores carry it in 50-pound bags and charge between $6 and $9 per bag. A bag of bentonite will treat an area that is 5 feet by 10 feet. Some suppliers also can get bentonite in bulk for bigger jobs.

Sealing a leaking pond can sometimes be quite expensive, and there is no guarantee that the pond will not leak in the future. In extreme cases it is better to fill in the existing pond and build a new one in a different location. Sometimes this is not an option, and in that case landowners will need to decide just how badly they want a pond.

If you have questions concerning any conservation structures, feel free to give our office a call. Our phone number is 843-4260, ext. 3.