How to identify a suitable site for borewell?

Vertical Electrical Sounding is one of the very common geophysical methods used for domestic borewell exploration. However, sufficient open area around the location is needed for conducting the survey. Geologists charge around Rs.1,500 to 5,000/- for identifying a borewell site. Other methods of geophysical survey are used in special cases and they are quite expensive. Local well diviners who use wooden sticks or magnetic blocks are also to some extent successful in identifying borewell sites. But this method may not be always reliable, especially in areas where the geological formation is critical.

Is there a best suited time for drilling a borewell?

As such there is no prescribed season for drilling a borewell if the location of site has adequate ground water potential. However, identifying a suitable site and drilling the borewell in summer months would give the worst possible scenario in the location at the point of time. Therefore, summer months are usually preferred by some experts as the best suited period for drilling a new borewell. Also, as most agricultural sites can be accessed by drilling rigs only during summer months, they are usually available in these periods.

What are the common methods adopted for drilling borewells?

Methods of drilling to be adopted depends on factors like suitability of a method for a particular type of geological formation (i.e. such as alluvial, bouldery and hard rock), cost factors, diameter and depth of borewell and the purpose intended. Most commonly used types of drilling methods are; Water Jetting - Shallow bores in alluvial formations.
Augur Drilling - Shallow bores in alluvial formations.
Calyx Drilling - Shallow borewells in both hard rock and alluvial formation.
Percussion Drilling - Deep bores in bouldery formation.
Rotary Drilling - Most common method used for drilling large and deep bores in alluvial formations.
Down the Hole Hammering (DTH) Drilling - Most common method for drilling large and deep borewells in hard rock formations.

What is the standard diameter of borewells drilled for domestic purposes?

For domestic purposes, 4.5 and 6 inches dia borewells are usually drilled. While 4.5 inch is most commonly drilled borewells for domestic purposes, 6 inch borewells are drilled when higher yield is required for large apartments or buildings and also for agricultural purposes. Initially larger diameter bits are used to place the casing pipes up to the hard rock zone prior to drilling specified size of the borewell.

What are the types of casing pipes used in borewells?

Currently, PVC pipes are very widely used as casing pipe. GI/Mild steel casing pipes which were used earlier are not being recommended due to the problem of corrosion. PVC casing pipes installed in borewells must confirm to IS 12818: 1992 that governs the specification for unplasticized PVC screen and casing pipes for borewell of the Bureau of Indian standard. This standard covers the requirements of ribbed screen, plain screen and plain casing pipes of nominal diameter 40 to 400 mm produced from unplasticized polyvinyl chloride for borewell for water supply. Slotted PVC casing pipes are used if aquifer yields through sandy zones to protect collapse of borewell side walls and to prevent entry of fine sand into the borewell which might clog the borewell. This requires detailed study by Geologists to decide slot size and at what depths these have to be provided along with gravel packing around the casing pipe.

What is the safe distance between a borewell and septic tank/soak pit?

Maintaining sufficient distance between a borewell and septic tank is always recommended as there are chances of contamination. If the situation warrants location of them in close proximity, it advisable to properly grout the borewell casing pipe with cement slurry till hard rock formation. In shallow aquifer zones, it is not advisable to locate borewells used for drinking purposes and the septic tank in close proximity. It is better to take professional assistance in such cases to take preventive steps around the borewell as well as the septic tank.