58. Infiltration galleries are horizontal means of groundwater abstraction. They may be subdivided into three groups:
i) Open trenches, as cut in the ground, to make the aquifer and its groundwater accessible from the ground; in emergency situations and in the right hydrogeological conditions (shallow water table) they can very quickly be developed as a source of water with the use of earth digging equipment (bulldozer). As they are easily contaminated, their use should follow the same sanitary precautions recommended for surface water sources (See 6.9); surface water should be drained away from them and access to them should be strictly limited to relevant camp staff;
ii) Buried porous conduits or drains, constructed inside the aquifer at some distance below ground level. If properly constructed, this type of infiltration gallery may provide large amounts of water when located close to, or within, medium and coarse grain (sand, gravel) river beds. Their main disadvantage is the need to construct them at the right moment, when river floods are minimal and the works may take place; sudden floods, higher than expected, have destroyed many attempts to tap groundwater for refugee camps in the past. Their construction should, however be considered as a last resort which, if successful, may provide ample water of good quality;
iii) Tunnels of large cross-sectional areas, built in consolidated (or semi-consolidated) formations by mining methods at any depth below ground level. To this type belong the Iranian qanats or the Pakistani qarrez, which are tunnels having a low gradient towards their mouth and which, by going against the slope of the mountains, are able to reach (after many kilometers) the water table of colluvial aquifers. These qanats are very ancient; they are constantly maintained by villagers and nomads who depend on them. The use of this water (several refugee camps in Iran or Pakistan have depended on these sources during initial emergency assistance) as a source of drinking water should follow the sanitary precautions recommended for surface water sources (See 6.9).