Sealing Leaking Ponds and Reservoirs
Excessive seepage in farm ponds is usually due to soils in the pond site that are too permeable to hold water. Thorough site investigation in the planning stage usually spots problem pond sites and allows landowners to avoid them. Where no satisfactory site is available, however, the need for water may be important enough to justify selecting a site somewhat less than satisfactory. For such sites, plans for reducing seepage by sealing should become a part of the original pond design.
There are cases where permeable areas cannot be readily detected before construction. Sometimes the amount of seepage cannot be accurately forecast without an excessive amount of investigation. It is not unusual to proceed with pond construction knowing that there is a calculated risk that some corrective measures may be needed later.
Seepage may be reduced by one or more of several different methods, the choice of which depends largely on the proportions of coarse-grained sand and gravel and of fine-grained silt and clay in the soil. A thorough investigation of the materials in more the area to be sealed should be made by a trained soil scientist or engineer before any method of sealing is selected. In some cases it may be necessary to have a laboratory analysis.
The most common methods of sealing leaking pond and reservoir sites are discussed in this publication. Local soil conservation districts can assist in choosing a practical one.
For all sealing methods discussed, the pond site should be prepared by draining, and the area to be sealed permitted to dry. Vegetation, stumps, debris, and loose rocks should be removed, the site should be smoothed; and gravel or coarse sand to fine sand, silt, and crevices, holes, and areas of exposed gravel clay or rock should be filled or covered with suitable soil material and compacted. After sealing, points where water flows into the pond should be protected from erosion by rock riprap or other suitable means.
SEALING BY COMPACTION ALONE
Sealing by compaction alone is the least expensive method of those presented in this publication. Its use, however, is limited to sites that have a wide range of particle sizes--from small gravel or coarse sand to fine sand with enough clay and silt, usually10 percent or more, to effect a seal. Sealing by compaction is relatively simple. After the site is prepared for sealing, the soil should be scarified to a depth of 8 to10 inches with a disk, roto-tiller, pulverizer, or similar equipment. With moisture conditions at optimum, the loosened soil should be rolled to a dense, tight layer with four to six passes of a sheepsfoot roller.
Where the depth of impounded water is 10 feet or less, the thickness of the compacted seal should be not less than 8 inches. Where it exceeds 10 feet, the thickness should be increased proportionately since seepage varies directly with the depth of impounded water. Seals thicker than 8 inches must be compacted in two or more layers each no more than 8 inches thick. Thus where the water depth exceeds 10 feet, the top layer or layers has to be removed and stockpiled while the bottom layer is being compacted.
SEALING WITH BLANKETS OF EARTH
Sites with too little clay to prevent excessive seepage can be sealed by an earth blanket if there is a suitable borrow area close enough to permit hauling at a reasonable cost. The blanket material should have a wide range of particle sizes-- from small gravel or course sand to fine sand, silt and clay in the desired proportions. The clay particles should make up about 20 percent of the weight. The adequacy of the material sealing and the required thickness should be based on laboratory tests made in the area, or on local experience.
After the site is prepared for sealing, the earth material is hauled in from the borrow area in tractor-pulled wheeled scrapers or similar equipment and spread uniformly in layers of 6 to 8 inches thick. With moisture conditions at optimum, each layer is thoroughly compacted by four to six passes of a sheepsfoot roller before the next layer is placed.
Earth blankets may require protection from the cracking that results from drying and from rupture caused by freezing and thawing. For this, a cover of gravel 12 to 18 inches thick is often placed over the blanket.
SEALING WITH BENTONITE
If there is a source near enough to the pond site that freight rates are not- so high as to prohibit its use, adding bentonite is a satisfactory way of sealing a leaking pond that does not have a widely fluctuating water level. Bentonite is suitable for use on having a high proportion of coarse-grained particles and insufficient clay. It is a hydrous silicate of sodium composed chiefly of montmorillonite. A representative chemical analysis of dry bentonite is as follows:
|Percent||56 - 66||18-24||2.5-5.0|
Bentonite absorbs several times its own weight of water and at complete saturation swells as much as 8 to 20 times its original volume. When mixed in correct proportions with well-graded coarse-grained material, thoroughly compacted, and then saturated the particles of bentonite fill the pores to where the material is nearly impervious to water. But upon drying, it returns to its original volume, leaving cracks. It is for this reason that sealing with bentonite is not usually satisfactory for ponds with a wide fluctuation in the water level.
Rates of application range from 1 to 3 pounds per square foot depending on the site material and to a lesser extent on the depth of water to be impounded. A laboratory analysis of the material to determine the rate of application is essential.
Bentonite should be uniformly spread over the area to be treated at the rate determined by the laboratory analysis. It is then thoroughly mixed with the soil to a depth of at least 6 inches. A roto-tiller is best for this operation but a disk or similar equipment can be used. The treated area should then be compacted with four to six passes of a sheepsfoot roller.
The soil moisture level of the material to be treated should be optimum for good compaction after bentonite is applied. If it is too wet, sealing operations should be postponed. If too dry, water should be added by sprinkling.
Since considerable time may elapse between application of the bentonite and the filling of the pond, it may be necessary to protect the treated area from drying and cracking. A mulch of straw or hay pinned to the surface soils by the final passes of the sheepsfoot roller gives this protection.
TREATMENT WITH CHEMICAL ADDITIVES
Excessive seepage often occurs even in fine-grained clay soils because the clay particles are arranged to form an open, porous, or honeycomb structure. Applying small amounts of certain chemicals to these porous aggregates may disperse them and reduce soil permeability. The chemicals used are called dispersing agents.
For chemical treatment to be effective, the soil in the area to be treated should contain more than 50 percent of fine-grained material (silt and clay finer than .074 mm diameter) and at least 15 percent of clay finer than .002 mm diameter. It should contain less than 0.50 percent soluble salts (based on dry soil weight). Chemical treatment is not effective in coarse-grained soils.
While there are many soluble salts that meet the requirement of a dispersing agent, sodium polyphosphates and sodium chloride (common salt) are most commonly used. Of the sodium polyphosphates, tetrasodium pyrophosphate (TSPP) and sodium tripolyphosphate (STPP) are most effective. These dispersants should be finely granular, 95 percent passing a No. 30 sieve and less than 5 percent passing a No.100 sieve. Sodium polyphosphates are usually, applied at a rate of 0.05 to 0.10 pound per square foot and sodium chloride at a rate of 0.20 to 0.33 pound per square foot. These rates are only general; a laboratory analysis of the soil is essential to determine which dispersing agent will be most effective and to determine the rate at which it should be applied.
The dispersing agent is first mixed with the surface soil and then compacted to form a blanket. For depths of water up to 8 feet, the blanket thickness should be at least 6 inches. For greater depths, it should be 12 inches, treated in two 6-inch lifts. A minimum thickness of 12 inches is needed for all areas in the vertical range of water-level fluctuation.
As a part of site preparation, rock out-crops, crevices, and other exposed areas of highly permeable material should be covered with 2 to 3 feet of fine-grained material. This material should then be thoroughly compacted. In cavernous limestone areas, the success or failure of the seal may depend upon the thickness and the compaction of this underlying blanket.
The dispersing agent should be applied uniformly at a rate determined by laboratory analysis and thoroughly mixed into each 6-inch layer to be treated. It can be applied with a seeder, drill, or fertilizer spreader or by hand broadcasting. Mixing can be done with a disk, roto-tiller, pulverizer, or similar equipment. Operating the mixing equipment in two directions will produce best results. Each chemically treated layer is then thoroughly compacted with four to six passes of a sheepsfoot roller.
For good compaction the soil-moisture level should be near optimum down to 12 inches. If the soil is too wet, treatment should be postponed. Polyphosphates release water from the soil and the material could easily become too wet to handle. If the soil is too dry, water should be added by sprinkling.
The seal should be protected from puncture by livestock trampling. The area near the normal waterline should also be protected from erosion by covering with a 12- to 18-inch blanket of gravel or other suitable material.
SEALING WITH FLEXIBLE MEMBRANES
Polyethylene, vinyl, and butyl-rubber membranes are gaining wide acceptance as linings for farm ponds because they practically eliminate seepage when properly installed. Though thin films of these materials are structurally weak, if kept intact they are almost completely watertight.
Black polyethylene films age better than unpigmented vinyl. Vinyl, on the other hand, is more resistant to impact damage and is readily seamed and patched with a solvent cement. Polyethylene must be joined or patched with heat sealing, special adhesives, or tape.
Butyl rubber can be joined or patched with a special cement.
All membranes should be of a quality that meets or exceeds the minimum requirements set forth in the standards and specifications of the Soil Conservation Service.
Polyethylene and vinyl membranes for pond linings should be at least 8 mils in thickness for all material no coarser than sands, either clean or silty, and 15 mils for all gravels-clean, silty, or clayey. Butyl-rubber covers should be at least 15 mils for sands and 30 mils for all gravels.
After the site is prepared for sealing it should be allowed to dry until the surface is firm and will support the men and equipment that must travel over it during installation of the lining. If the material over which the lining is to be placed is stony or of very coarse texture, it should be covered with a cushion layer of fine-textured material. Banks, side slopes, and fills should be uniformly sloped no steeper than 1-to-1 for exposed lining and 3-to-1 for covered lining. Protecting covers tend to slide on the lining when placed on steeper slopes.
Nutgrass, johnsongrass, quackgrass, saltgrass, and certain other plants penetrate both vinyl and polyethylene film. If these are present, it is best to sterilize the sub grade, particularly the side slopes. Several good chemicals for this are available commercially.
Linings are usually laid in sections or strips with a 6-inch overlap allowed for seaming. Vinyl and butyl-rubber linings should be laid smooth but slack. Polyethylene should have up to 10 percent slack. Extreme care in handling is needed at all times to avoid puncture.
To anchor the top edges of the lining a trench can be excavated completely around the area above the normal water level. It should be 8 to 10 inches deep and about 12 inches wide. The top 8 to 12 inches of the lining that must travel over it during installation should be buried in this trench and secured with compacted back fill.
Because of their weakness, all polyethylene and vinyl membranes should be protected from mechanical damage with a cover of earth or of earth and gravel not less than 6 inches thick.
Butyl-rubber membranes need not be covered unless the area will be subject to travel by livestock or to danger of puncture by swimmers or fishermen.
To protect against livestock, swimmers and fishermen a minimum of 9 inches of earth or earth and gravel should cover all types of membranes. The bottom 3 inches should not be coarser than silty sand. All protective covers must be free of large clods, sharp rocks, sticks, and other objects that would puncture the membrane and must be placed without damage to it.
LINING WITH PORTLAND CEMENT
Reinforced concrete, pneumatically concrete, and soil cement are common materials for lining leaky ponds and reservoirs. Literature on the use of these materials is available from the Portland Cement Association and the various cement companies.
PAVING WITH ASPHALT
Asphaltic pavements are also common seals for leaky ponds and reservoirs. Literature on these are available from the American Asphalt Institute and from the various petroleum companies.