Wells should be carefully sited so that drilling only occurs where there is a high probability of successfully penetrating into water-bearing formations and the wells can be effectively used, maintained, and protected from contamination. While every borehole will not result in a good well, advanced planning with the community will maximize the number of successful wells and minimize drilling costs.
Critical factors which must be considered for siting of new water well for having high probability of getting good yield from this well are discussed below;
The history of old wells will indicate how far down the water table drops during dry seasons and will indicate how deep new wells must be. The new well should be drilled as far away as possible to ensure that both wells will produce sufficient amounts of drinking water without interfering with one another
Sub Surface Soil Types
The amount of water supplied by an aquifer (water bearing formation) is as important as its quality. The only way to know exactly how much water is available is by pumping wells. However, a rough estimate of yield can be made by identifying the soil and rock which comprise the aquifer.
Most sand and gravel deposits contain significant amounts of drinking water. However, the amount of water which can actually be pumped depends on how thick these deposits are and their permeability (how easy it is for water to flow through it). In general, the larger the grain size and the thicker the deposit, the higher the yield of the aquifer.
Wells constructed in silt or clay soils will have very low yields regardless of how they are constructed. To compensate for this, large diameter water wells should be carefully dug so that large volumes of water can slowly accumulate in the well casing over time and provide sufficient quantities when required.
Finally, limestone, sandstone or quartzitic rock may also yield adequate quantities of water. Best yields are found where there is a thick zone of weathered rock with many cracks (fractures).
During the dry season, survey for indications of groundwater by looking at the alignment of ant mounds and green vegetation in the midst of an arid landscape. Annual plants, such as grasses and ferns, are not good indicators because they come and go with the seasons. However, year-round reeds and broad leaf trees and shrubs like cedar and willow tend to grow where water is close to the surface.
The water table commonly follows the land surface. While the lowest areas (valley bottoms or depressions where water accumulates after rains) are generally the best places to drill, ensure that the site has good access, is not subject to flooding and is not close to where contaminated surface water ponds. The presence of water bearing fracture zones may be detected by surface features such as shallow linear depressions and abrupt changes in valley alignment. Often these features are difficult to see in the field but become apparent when viewed from the air.
Successful wells are often drilled near rivers; groundwater may be available even if the river is temporarily dry. Reliable wells have even been located near broad sandy riverbeds which are active once every 5-10 years. Water taken from wells located at least 15 m from a river is usually cleaner and cooler than water taken from the river. If the well water remains turbid after construction, the soils may be providing inadequate filtration and contaminated river water may be entering the well.
Look for springs since they indicate the presence of a water bearing formation (aquifer). A well can often be successfully drilled just uphill of the spring. Animal trails often lead to seeps and springs.
Finally, surface drainage patterns can be used to determine rock type :
- Trellis and rectangular drainage develops where dipping,fractured sedimentary rocks are present; these are the most favourable areas for high yield aquifers (Selby, 1985);
- Contorted drainage develops over folded rocks. Water bearing tension fractures and gaps between layers of differing hardness sometimes develop near the top of folds;
- Annular drainage typically develops over volcanic or intrusive (granitic) domes, with streams flowing along water bearing fracture zones;
- Dendritic or branching patterns with a large number of tributaries are typical of drainage in areas of impermeable crystalline rock such as gneiss. Parallel drainage patterns may develop in areas with linear water bearing structures such as faults and dikes.
Source of Contamination
Avoid drilling in areas where unsuitable quality water is known to occur and keep wells as far away as possible from potential sources of pollution.
Locate wells upgradient (uphill) of nearby potential sources of pollution (i.e., the land should NOT slope from pollution sources towards water wells). If this can not be avoided, try to locate wells as far to the side of the slope as possible (i.e., not directly downslope of possible contaminant sources).