- Borrow pits rather than being backfilled can be used as future water storages
- This will benefit – as far as the sourcing of building material allows – from appropriate planning: size, location, entry
- Borrow pits can be further upgraded with proper protection, entrance control, landscaping and if necessary lined
- Converted borrow pits are a good option for water storage in semi-arid areas and in flood plains with high groundwater tables
This section provides guidance on the systematic conversion of borrow pits to water-storage structures. Borrow pits provide the source material for the construction of road embankments. Depending on the local area, the source material can be gravel/aggregates, silica sands, laterite sands, and calcite. Once no longer in use for the mining of road construction material, borrow pits can become key assets for local water supply. Rather than backfilling the pits or leaving them unattended, the borrow pits may be systematically converted into storage structures to serve as sources of irrigation or livestock water. In areas where there is no alternative, borrow pits may even become sources of domestic water. In some areas, borrow pits have even been landscaped into attractive recreation areas and urban lakes. This section discusses the opportunities for converting borrow pits and the recommended practices in planning and implementation.
In planning new borrow pits or in converting decommissioned borrow pits into water storage, preference should be given to those pits that meet the following criteria:
- The pit can be connected to a water source;
- It is located in an area of water shortage; and
- It is close to water users: domestic water, livestock, or irrigation.
There are three different uses of borrow points for water supply:
- Borrow pits may be used for water retention, i.e., direct storage of runoff water. In such cases they should have relatively impermeable beds to prevent the stored from water leaking away.
- Borrow pits can also be used as infiltration ponds. In this case, the water that is collected infiltrates and feeds the shallow groundwater. Such borrow pits should have relatively permeable beds to facilitate groundwater recharge. They may even be supplied by excess water from nearby streams diverted or pumped into the converted borrow pit. (See box 7.1.)
- Borrow pits can also serve as seepage ponds. This is the case in areas with high groundwater levels, such as the floodplains of major rivers. In this case, the pits will fill constantly with groundwater seeping from adjacent areas and provide an almost permanent water source.
Box 7.1 Converting a gravel pit into an infiltration structure in Italy
In the past, gravel mining in the coastal area of Southern Tuscany, Italy was largely unregulated. Many gravel pits were developed and many were never refilled. The industry came to an end when the exploitation of gravel no longer come automatically with the ownership of the land.
The area has become water-stressed since the construction of storage dams upstream, exacerbated by the development of wells that sustain the cultivation of high value vegetables. Under the LIFE program, a gravel pit close to Forni was deepened and transformed into an infiltration structure aimed at replenishing local groundwater resources. The excavated pit was split in two: a relatively small sediment pond and a larger infiltration basin that received the overflow of clean water from the sediment pond. The choice of pond location and the deepening of the infiltration basin were designed to tilt the groundwater recharge away from the Cornia River, as it would not make sense for the new water lens underneath the gravel pit to feed back into the river.
The infiltration structure is fed by an intake from the Cornia River, equipped with a pumping unit. The pumping unit typically operates 75 percent of the time. If the level of the river drops below the environmental flow, the pump automatically stops. The pump also stops during flood events, as the river in flood carries iron, nitrate, and too much sediment. The measurement of water quality is fully automated. The piezometer around the infiltration pond detected an 80 cm increase in groundwater level in the first month of operation.
The investment in the pond is USD 360,000, creating a storage capacity of 200,000 m3. Compared to surface storage dams this is low cost: USD 1,80 per m3. Forty percent of the cost went into instrumentation. Replicating the concept on other abandoned gravel pits may be of even lower cost. The structure recharges water at 65 litres per second. It is conceivable that in the future such structures may be used to produce and sell water, and that the operation of the structures could occur by remote control.
There is a need to systematically approach the reuse of borrow pits as water ponds, from the planning of these pits to their development, conversion, and after-care. The following elements should be considered: