3. CONTROLLING WATER LOSSES IN PONDS

3.0 Introduction
1. In a previous manual in this series (Soil, 6), you learned how important it is to select a site with suitable soil for a fish farm. A good choice makes it possible to build ponds with strong impervious dikes and with relatively impervious pond bottoms (see also Pond construction, 20). 2. It may happen that your fish pond is not as good as it should be. Losing much water through seepage, for example as much as 10 cm per day (see Section 2.1, Water, 4), may be attributable to one of the following: the site, although known as less than satisfactory, had to be used; the soil survey was not done well; excessive removal of surface soil for dike construction has exposed highly pervious areas of sand, gravel or rock; the pond was not well designed or constructed.		The selected site
3. As well as losing water, excess seepage results in a continuing loss of fertility. You should aim to reduce seepage to a point at which water losses become at least tolerable, for example less than 5 cm per day.

4. A newly built pond usually loses more water than an older pond. As the pond is being used and well managed, the organic matter produced from fertilization and feeding falls to the pond bottom, where it gradually blocks the soil pores and so reduces the bottom soil permeability.

Note: if seepage losses are due to bad design or construction, e.g. lateral seepage through dikes which are too narrow, the losses can be very difficult to stop.

5. There are several methods you can use:

• either accelerate at minimal cost the natural process of sealing the soil pores; or
• if this is not possible, seal the pond bottom using additional material at extra cost.

6. You will learn more about reducing seepage losses in the next sections. Use Table 3 to select a suitable method according to the quality of the soil to be sealed and the local availability of materials.

7. In the final section of this chapter, you will learn how to repair leaking ponds during periodic maintenance.  

3.1 How to seal the pond with organic matter

Light organic layer  

1. A simple method that greatly accelerates the sealing of a pond bottom is to spread enough cattle manure over it (at least 2 m³/100 m²) before filling the pond with water. Repeat this procedure several times if necessary, at intervals of eight to 12 months, until seepage losses become acceptable. After draining these ponds, refill them with water as soon as possible to reduce the rate of decomposition of the organic matter that seals the bottom pores.

2. If the dikes of a pond have been compacted very well, it may not be necessary to seal them against seepage when you seal the pond bottom. However, if you are not sure that the dikes are sound, it is best to seal them as well.

Gleization

3. If the bottom soil is too permeable to be sealed by this method, an alternative is to create an impervious biological plastic layer in the bottom and on the sides of the pond. Such an impervious layer is called a gley, and the process of its formation is called gleization. Proceed as follows.

(a) Prepare the pond bottom (and if necessary the wet sides of the dikes) by clearing it of all vegetation, sticks, stones, rocks and the like. Fill all cracks, crevices and holes with well-compacted impervious soil.

(b) Completely cover the cleaned surface with moist animal manure, preferably pig dung, spread in an even layer about 10 cm thick.

(c) Cover the manure well with a layer of vegetal material, preferably broad leaves such as banana leaves. You can also use dried grass, rice straw, soaked cardboard or paper, etc.

(d) Cover with a layer of soil about 10 cm thick.

(e) Moisten and compact very well.

(f) Wait two to three weeks before slowly filling up the pond with water.

Gleization

3.2 How to seal the pond bottom by puddling

1. A simple way to reduce water seepage, particularly if the pond bottom is very dry, hard and has open cracks in it, is to break the soil structure of the pond bottom before filling the pond with water. This is common practice in irrigated rice fields, and is called puddling. Refer also to Section 2.1, Water, 4. Proceed as follows.

(a) Saturate the soil of the pond bottom with water.

(b) Let the water soak into the soil just enough to permit working.

(c) Break the soil structure by puddling with a hoe or plough.

Puddling by hoeing		Puddling by ploughing
2. If a puddler is available, flood the pond bottom and puddle the soil under water for best results. A plough can also be used underwater.		Rotary puddler

3.3 How to seal the pond bottom by compacting

1. If the bottom soil contains a wide range of particle sizes, from coarse sand to fine silt and clay, and at least 10 percent clay (Soil, 6), it can be made relatively impervious by good compaction alone. Proceed as follows.

(a) Prepare the pond bottom (and if necessary the wet sides of the dikes) by clearing it of all vegetation, sticks, stones, rocks and the like. Fill all cracks, crevices and holes with well-compacted impervious soil. (b) Loosen the soil to a depth of 20 to 25 cm. If possible, try to borrow a disc harrow or a rototiller from a neighbouring farmer. A plough could also be used.

(c) As necessary, dry or moisten the loosened soil so that it reaches its optimum moisture content for compaction, about 13 percent (see Section 10.2, Soil,6).

(d) Compact the soil surface into a dense, tight layer. You should preferably use four to six passes of a sheepsfoot roller. You may also use the tyres of a heavy tractor or a crawler-tractor.

(e) Check compaction carefully. If necessary, readjust the soil moisture content and improve compaction. Refer also to Section 6.2, Pond construction, 20.

3.4 How to seal the pond bottom with a soil blanket

Laying out a soil blanket

1. If the bottom soil contains a great proportion of coarse particles but lacks enough clay and silt to prevent excessive seepage, you can seal it with a soil blanket. Proceed as follows.

(a) Prepare the pond bottom (and if necessary the wet sides of the dikes) by clearing it of all vegetation, sticks, stones, rock and the like. Fill all cracks, crevices and holes with well-compacted impervious soil.

(b) Build a soil layer about 15 cm thick over the entire bottom area and over the wet sides of the dikes. The soil you use should be well graded and contain at least 20 percent clay.

(c) Moisten this soil layer so as to reach optimum moisture content for compaction (see Section 10.2, Soil,6).

(d) Compact the soil layer well, preferably by four to six passes of a sheepsfoot roller (see Section 6.2, Pond construction, 20).  

(e) Bring in additional clay soil to build a second layer about 15 cm thick over the first one.

(f) Moisten and compact as before.

(g) Protect the soil blanket against rupture by:

• using layers of rocks or stones at the water inlet;
• spreading a cover of gravel 30 to 45 cm thick just below the maximum water level, if there is a danger of water freezing;
• avoiding working inside the pond over the soil blanket when the pond is drained or fish are being harvested;
• not letting the soil blanket dry too much and thus cracking;
• refilling the pond carefully as soon as possible.

Note: try to obtain the clay soil required to build the blanket from a borrow area close enough to the pond to reduce transport costs.

Laying out a subsurface soil blanket

2. For smaller ponds or if you need to work inside the pond, you may choose to use a method which provides better protection for the soil blanket. Proceed as follows.

(a) Prepare the pond bottom (and if necessary the wet sides of the dikes) by clearing it of all vegetation, sticks, stones, rocks and the like.

(b) Excavate the pond bottom deeper to a depth of about 30 cm. Store the soil close to the pond.

(c) Build a well-compacted layer of clayey soil at least 5 cm thick all over the surface.

(d) To protect this layer, cover it with the original soil and compact the soil cover.

Note: if seepage problems are suspected during planning or construction, it would be cheaper to include this subsurface blanket during pond construction. This step will make it easier to connect properly with the clay core of the dikes.

3.5 How to seal the pond with a synthetic membrane

1. Thin synthetic membranes can also be used to reduce excessive seepage, particularly in coarse-grained soils. Such a method has the advantages of:

• reducing seepage to zero in all types of soil; and
• providing a more dependable seal than the previous method.

2. However, synthetic membranes have several disadvantages which must be carefully considered.

(a) They are rather expensive.

(b) Their resistance to puncture and breakage is limited.

(c) This resistance may be weakened under the action of direct sunlight.

(d) The natural fertility of the bottom soil will be lost, and it will take some time to develop it over the membrane.

3. Three kinds of synthetic membrane material are most commonly used:

• black polyethylene;
• polyvinyl chloride (PVC); or
• butyl rubber.

4. Their relative properties and general requirements when being used to line fish ponds are summarized in Table 4.

Note: the thickness of synthetic membranes is expressed either in millimetres (mm) or in thousandths of an inch (mils). To convert one unit to the other, simply remember that 1 mil = 0.0254 mm, or approximately 40 mils = 1 mm.

5. To ensure zero seepage, the synthetic membrane must entirely cover the pond bottom and the wet sides of the dikes. Since it is relatively easy to puncture the membrane, it is especially important to clean both the bottom and the wet sides of the dikes very well.

6. To line a pond with a synthetic membrane, proceed as follows.

(a) Prepare the entire pond area (including the wet sides of the dikes) by clearing it of all vegetation, sticks, stones, rocks and the like.

(b) Fill holes and crevices with soil.

(c) If the soil material to be lined is too stony or of very coarse texture, cover it with a cushion layer of finer material about 10 cm thick.

(d) All around the pond on the top of the dikes, dig an anchor trench 25 cm deep and about 30 cm wide, set at least 30 cm back from the inner edge of the dike.

Position of synthetic membrane ending in anchor trenches

(e) Lay a first strip of the membrane along the width of the pond and along the dike. Watch for the following:

• choosing the correct slack for the membrane used (see Table 4);
• leaving 20 to 25 cm of the membrane in the bottom of the anchor trench at both ends;
• avoiding soil on top of the membrane;
• avoiding puncturing the membrane while handling it.

(f) Lay a second strip of the membrane in the same way but allowing a 15-cm overlap for joining it to the first strip.

(g) Carefully join this second strip to the first one, using the appropriate method according to the membrane used (see Table 4).

(h) Repeat this process with as many membrane strips as necessary to completely cover the pond bottom and the wet sides of the dikes.

(i) Backfill the anchoring trenches with well-graded soil and bury the edges of the membrane, taking care to compact well.

(j) While laying the membrane strips down, cover the finished sections for immediate protection by:

• first carefully spreading and compacting a 7.5-cm-thick layer of soil no coarser than silty sand, and being very careful to avoid puncturing the membrane;
• then spreading and compacting a layer of available site soil at least 15 cm thick.

(k) In some cases the membrane is not covered. If so, you should use a covering sheet of plastic, butyl or felt over the upper edge that extends at least 20 cm below the pond water level to protect the main membrane from heat and sunlight. The sheet can be dug into the same trench as the main membrane.

(l) You should take special care around pipes or monks. Depending on the membrane type, it can be sealed either by taping or cementing around the pipe or monk. With monks, a finer mortar joint can also be used: ideally the membrane should be incorporated during construction of the monk. Bring the membrane at least 10 cm up the sides of the monk or around the pipe, and make sure the membrane is not split around the joint.

Note: if the synthetic membrane is to be covered with a protective soil layer, the slope of the wet side should be no steeper than 3:1; if there is no soil cover, the slope of the wet side should be no steeper than 2:1.

Slope of wet dike sides

   3.6 How to seal the pond with bentonite

1. If your pond bottom soil contains a high percentage of coarse-grained particles and not enough clay, adding bentonite to it is another method of reducing excessive seepage.

What is bentonite?

2. Bentonite is a fine-textured colloidal clay with as much as 90 percent of montmorillonite. When exposed to water, dried bentonite absorbs several times its own weight of water; at complete saturation, it swells as much as eight to twenty times its original dry volume.

3. Natural bentonite deposits exist in several places around the world, for example in the United States of America, Mexico, China and Western Europe.

Selecting this method

4. Before selecting this method to seal your pond, you should locate a satisfactory source of bentonite, as near as possible to your site to reduce cost.

5. You should also ensure that the quality of the available bentonite is good: it should contain the highest percentage of montmorillonite possible (at least 70 percent). If necessary, obtain a sample first and have it analysed in a soil laboratory. You may test the bentonite by placing it in a small jar and adding water. Good bentonite should expand to several times its original volume.

6. The water level of your pond should not fluctuate too much. Soil treated with bentonite will develop numerous cracks when it dries up. It is best if it always remains wet.

7. Whenever good bentonite is readily available, this sealing method offers several advantages:

• it is cheaper than using some synthetic membranes such as butyl rubber;
• it is relatively easy to apply;
• it can offer a long-lasting solution to excessive seepage;
• unlike plastic membranes, it is not vulnerable to vandalism, it does not stretch and break if the substrate sags under pressure, and vegetation can grow through it without affecting its porosity.

8. However, bentonite treatment has several potential disadvantages:

• cost may be excessive if the transport distance is too long;
• it is more laborious to apply than a butyl membrane, and great care should be taken to ensure complete coverage;
• it can be disrupted by cattle or eroded by running water unless well protected;
• burrowing animals such as crayfish or crabs and strong bottom-feeders such as common carp can penetrate through the bentonite-treated layer;
• bentonite treatment is not advisable in highly alkaline soils;
• it should be used with care when the water contains salts such as calcium chloride.

Taking precautions before applying the method

9. Before using this method, it is safer first to make some preliminary tests.

(a) Have a water sample analysed to discover its salt content.

(b) Have soil samples analysed for chemical composition and texture.

(c) Request specialized advice from your supplier of bentonite: which particular quality of bentonite do you require and at which rate should you apply it.

Sealing a drained pond with bentonite

10. If the results of the above analyses of water and soil confirm that conditions are good for using a particular kind of bentonite to seal your pond, proceed as follows.

(a) Prepare the pond bottom (and if necessary the wet sides of the dikes) by clearing it of all vegetation, sticks, stones, rocks and the like. Fill all cracks, crevices and holes with well-compacted impervious soil.

(b) Dry or moisten the area so that the soil moisture reaches its optimum content for compaction (see Section 10.2, Soil, 6).

(c) According to the bentonite and soil laboratory analyses, uniformly spread from 5 to 15 kg of bentonite per m² : the better the quality of bentonite and the more clay or silt your soil contains, the less bentonite you will require. Use a marked grid pattern to help you spread the bentonite on the bottom evenly.

(d) Thoroughly mix the bentonite with the bottom soil to a depth of about 15 cm. A rototiller is best for this operation but an agricultural disc harrow can also be used. (e) Compact the area well, preferably with four to six passes of a sheepsfoot roller. You could also use other types of compacting equipment (see Section 6.2, Pond construction, 20).

(f) If you cannot fill the pond with water immediately, you should protect the treated area against drying and cracking. Use a good mulch* of straw and hay on top of the surface and keep it moist if necessary.

(g) Protect areas where erosion could damage the layer containing bentonite such as under the water inlet and around the pond where wave action is thought to be the strongest (see Section 4.2). Use gravel, stones, rocks and other available materials.

Note: when you plan to use bentonite to seal a pond and its wet dike sides, the dike slope ratio should be no steeper than 3:1.

Sealing a full pond with bentonite

11. If you discover that seepage losses from a pond in use are excessive and bentonite is easily available, you can seal the pond without draining it. First check on this possibility by having water and soil samples tested.

12. At the pond inlet, throw bentonite in the inflowing water at the rate of 5 to 15 kg/m² of water surface, according to the results of the bentonite and bottom soil analyses. You may also sprinkle the bentonite on the entire water surface. The bentonite particles will settle down on the pond bottom and penetrate the soil pores as the water seeps through. As they swell, they will block these pores and reduce seepage losses.

Note: it is best to treat your pond with bentonite in the absence of fish, because it is potentially harmful to their gills.

3.7 How to maintain earthen ponds

1. An earthen fish pond needs to be given most attention during the first years of operation, because it is then that the unforeseen, hidden faults come to the fore. Maintenance should therefore be carried out continuously from the beginning (see Sections 16.1 and 16.2).

2. With earthen ponds and in particular with dikes, you should expect several kinds of problems, which you should take care of as soon as possible.

(a) Surface erosion of the dikes is usually caused by rain trickling down the side slopes. Protect these slopes with a healthy cover of grass (see Section 4.4).

(b) Erosion of the wet side of the dikes at the water surface level mainly attributable to strong wave action may justify additional protection (see Section 4.3).

(c) Part of the soil may slip down from the slope in some places, most frequently resulting from bad compaction, drenching of the dike or quick water discharge. Replace the slipped soil with well-compacted horizontal layers of soil.

(d) Part of the dike may slump, resulting either from bad compaction or from incomplete removal of organic matter from the dike's site. Loosen the surface soil of the dike top, moisten to optimum moisture content for compaction, add a layer of good quality soil and moisten again if necessary and compact well.

Protect dikes with grass cover

Replace dike areas which slip or slump

3. It a dike breaks at one point and a gap develops, the water level must be lowered immediately to a level below the gap.

Lower water level and repair

4. Then carefully repair the dike as follows.

(a) Widen and deepen the gap slightly to expose fresh and clean dike material. Eliminate all organic matter that might be present, such as roots. Make irregular cuts to give the repair a good foundation.

(b) Moisten the old dike material, and pack in layer after layer of good quality new soil. As you build up the new dike section, give particular attention to the junction of the old soil with the new soil.

(c) If a clay core is present, rebuild it also as you progress upwards, layer by layer (see Pond construction, 20).

5. Leaks may develop locally in dikes for various reasons, such as accidental damage, the presence of a rotting piece of root or the burrow of an animal. To repair local leaks, you can proceed in the following way.

(a) Lower the water level, slightly widen the leak and fill it with well-compacted clayey soil. You can also use a mixture of clay and hydrated lime (see Section 4.6), for example in the proportion of 4 to 6:1, which will set more firmly. In some areas, hydraulic limes, made from clayey limestone* (10 to 30 percent clay) can also be applied directly.

(b) If the leak is not too large, throw a certain amount of clayey soil into the water directly above the leak. You can also use sifted cinders (less than 2 mm in diameter) or a 5:1 mixture of finely grained soil with bentonite (see Section 3.6).

(c) For emergency use, you may also apply a section of polythene sheet, well weighted down with soil.

6. In tropical countries, particularly during the dry season, termites may establish themselves in the dike of a pond. The dike becomes weakened by underground galleries, which may result in severe water leaks and even breakages. To eliminate termites, proceed as follows.

(a) Partly open the anthill at the top.

(b) Pour used motor oil into the galleries. If available, use xylophene, because it has a stronger effect.

(c) Repeat this treatment each day for one week.

(d) Close the galleries with clayey soil, moistening as necessary and compacting well.