When borrow pits are used for surface storage (rather than as infiltration or seepage ponds—see 7.2), much of the water can disappear because of seepage. In areas with fine sediment, this may cease to be a factor over time as fine material seals the bed of the pond. In addition, some low-cost measures can be used to reduce seepage from ponds:
- Compacting the bottom of the pond (using rollers, sheep-foot rollers, or hand compaction).
- Installing clay lining or lining with soil from termite heaps, provided that these materials are available.
- Fixing cracks and seeping areas with bentonite.
- Treating the pond with dispersants: salts that change the soil structure and increase permeability (only if the soil is 15 percent fine clay); common salts include sodium carbonate, sodium chloride, and sodium polyphosphate.
The measures above may be insufficient in areas with permeable soils. Lining with geotextile may be necessary to reduce seepage and make the borrow pit suitable for storage.
Polyethylene, butyl rubber, and vinyl membranes are commonly used for pond lining. These are the recommended practices:
- After the reservoir is shaped to the desired standards, it is important to let it settle and dry before the pond is lined.
- If there are sharp rock fragments, roots, and objects at the bottom of the reservoir, it is important to lay a thin layer of fine soil to prevent piercing of the lining.
- Ideally, the lining must be covered with a 15 cm layer of fine soil to protect it from light and piercing.
- The banks of the reservoirs should preferably be shaped to a 1:1 slope (if the lining is to remain exposed) or to a 3:1 slope (if the lining is to be covered with earth).
- The side of the lining (around 30 cm) must be anchored to a 25 cm deep trench over the edge of the reservoir.
Borrow pit lined with geotextile, MozambiqueSeveral kinds of geomembranes are available on the market, with different benefits and drawbacks, as highlighted in Table 7.1 below:
Table 7.1. Geotextile materials for reservoir lining
Reinforced polyethylene (RPE)
- Thinner material compared to EPDM and PVC
- Lightest material
- Does not stretch but is rather flexible
- Requires more time to apply over complex shapes
- More puncture resistant than PVC and EPDM
- Can last up to 40 years
- Fish and plant safe
- Can be welded with heat
Ethylene polypropylene diene monomer (EPDM)
- Made of rubber
- Stretches and folds well around corners
- Requires an underlayer
- Sheets cannot be joined with simple heat guns
- Heaviest material; higher shipping cost
- Least puncture resistant
- Fish-safe material
Polyvinyl chloride (PVC)
- Heavier than RPE but lighter than EPDM
- Less puncture resistant than RPE but more resistant than EPDM
- PVC sheets are often treated with chemicals toxic to fish
- Easily degraded by direct UV exposure
- PVC sheets are easily welded together