Water loss in a pond, known as seepage, occurs when water filters downward or outward through porous soil instead of being held within the basin. This challenge is common in areas where the native soil composition is sandy or gravelly, lacking the necessary clay content for natural water retention. Many pond owners prefer natural sealing methods, opting for earth-based materials over synthetic liners or chemical sealants. These solutions focus on physically modifying the soil structure to create a dense, low-permeability layer that retains water.
Identifying the Cause of Water Loss
Before undertaking any sealing project, determine if the water loss is due to general soil porosity or a specific structural defect. Evaporation is a natural process that can account for a loss of up to a half-inch per day, especially during warm, windy weather. Distinguishing this normal loss from actual seepage is accomplished using the bucket test.
The bucket test involves placing a container filled with pond water on a submerged step or floating it so the water level inside matches the pond’s surface level. After 24 to 48 hours, the water drop inside the bucket indicates loss solely from evaporation. The drop in the pond itself reflects both evaporation and any leaks. If the pond water level drops significantly more than the water level in the bucket, a leak is present. If the bucket test confirms a leak, a simple dye test can help pinpoint a localized flaw, such as a crack or a leak around an overflow pipe. Introducing a non-toxic dye near a suspected leak point will show the dye being drawn out if a structural flaw exists.
Sealing the Pond Bottom with Soil and Compaction
For ponds constructed in soils that contain a moderate percentage of clay, mechanical compaction is an effective method for sealing the basin. This technique involves physically manipulating the soil to increase its density and reduce the size of the pore spaces that allow water to escape. Preparing the pond bottom requires removing all vegetation, rocks, and debris. Then, scarify the existing soil to a depth of eight to twelve inches to break up the original structure, preparing it for re-compaction into a dense, uniform layer.
Water is then added to the loosened soil to achieve the optimum moisture content, allowing the soil particles to be most tightly packed together. The soil must be damp enough to be pliable but not saturated into a slurry. A sheepsfoot roller is used to compact the soil, often requiring a minimum of six passes over the entire area. This process, sometimes referred to as puddling, forces the air out and aligns the clay particles to create a low-permeability layer. The resulting compacted soil layer, ideally at least twelve inches thick, acts as a dense barrier to impede water infiltration.
Application of Bentonite Clay for Sealing
When the native soil lacks sufficient clay content for effective compaction alone, sodium bentonite clay provides an effective sealing solution. Bentonite is a natural clay derived from volcanic ash that swells significantly, up to fifteen times its dry volume, when it absorbs water. This expansive property allows the clay to fill the voids and microscopic pores in sandy or gravelly soil, creating an impermeable barrier. Calculating the required amount of bentonite depends heavily on the soil type, as highly porous sandy soil needs substantially more material than silty soil.
Two primary application methods incorporate bentonite into the pond bottom.
Mixed Method
The mixed method involves spreading granular bentonite evenly over the prepared soil and mechanically mixing it into the top four to six inches using a tiller or disk harrow. This blended layer is then compacted to create a dense mixture of native soil and bentonite, integrating the sealant throughout the upper layer.
Blanket Method
Alternatively, the blanket method requires placing a uniform layer of pulverized bentonite directly on the prepared pond floor. This layer is then covered by a six to twelve-inch layer of clean soil and compacted.
The blanket method is considered more reliable for achieving a consistent seal, as it creates a distinct, continuous layer of the sealing material. Regardless of the method chosen, the newly sealed area must be protected during the refilling process to prevent erosion of the bentonite layer. Water should be introduced gently, often through a pipe, to avoid washing away the fine clay particles before they can fully hydrate and swell. Proper hydration and settling can take several days, after which the expansive bentonite forms a durable seal that significantly reduces seepage loss.
Shoreline Stabilization Through Natural Vegetation
Seepage can also occur horizontally through the banks, particularly if the pond edges are eroded or poorly structured. Shoreline stabilization is an important consideration, and biological solutions utilizing strategic planting are an effective way to address this perimeter water loss. Establishing a dense buffer of marginal aquatic plants along the pond’s edge works to mechanically bind the soil particles together.
The root systems, especially the horizontal underground stems known as rhizomes, of native plants like cattails, rushes, and sedges form a thick, interlocking mat. This dense root network acts as a natural reinforcement, increasing the soil’s resistance to erosion and preventing the formation of channels that allow water to escape. These plants are strategically placed at the water’s edge and up the bank slope to create a living barrier that reduces horizontal seepage. The resulting vegetated zone promotes a stable bank structure while filtering runoff.