Opportunities

PHOTO 8.1

Borrow pit from high-speed railway track to be converted to storage reservoir and landscaped into a local lake, Gaomi, Shandong, China

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OPPORTUNITIES

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. • The pit is located in an area of water shortage. • The pit is close to domestic, livestock, or irrigation water users.

Borrow pits may be used for water supply in three ways:

• Water retention. The borrow pit is used for direct storage of runoff water.

In such cases the pit should have a relatively impermeable bed to prevent stored water from leaking away. • 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 8.1.) • Seepage ponds. Borrow pits can serve as seepage ponds in areas with high groundwater levels, such as the floodplains of major rivers (photo 8.2). In this case, the pits will fill constantly with groundwater seeping from adjacent areas and provide an almost permanent water source.

PHOTO 8.2

Borrow pit collecting seepage water from high groundwater table in floodplain (lakes region, South Sudan)

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BOX 8.1

Converting a gravel pit into an infiltration structure in Italy

Coastal areas of southern Tuscany, Italy, face increasing water stress from construction of upstream storage dams and extensive use of wells to cultivate high-value vegetables. Because gravel quarrying was previously common and lightly regulated, gravel operations frequently closed without the gravel pits’ being refilled. These pits provided an opportunity to recharge local groundwater supplies. To achieve this goal, 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 (photo B8.1.1). The choice of pond location and the deepening of the infiltration basin were designed to tilt the groundwater recharge away from the Cornia River to reduce the risk that the new water lens underneath the gravel pit might feed back into the river instead of recharging local aquifers.

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. The pump automatically ceases if the river level falls below the minimum flow thresholds, set for environmental reasons. The pump also stops during flood events when the river water carries iron, nitrate, and excessive sediment. fully automated measurement of water quality ensures appropriate management of water quality risks. The structure recharges water at a rate of 65 liters per second. a piezometer around the

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