A reliable method for determining when appreciable volumes of groundwater are encountered is by conducting a preliminary
assessment of boreholes or water sources in the area and having a good understanding of where groundwater occurs.
It is generally good practice to inspect as many boreholes in the vicinity of interest as possible. If the inspected
boreholes encounter groundwater at approximately the same elevation and groundwater does not occur in discontinuous
lenses, groundwater should be present in the subsurface at roughly the same elevation as in the inspected boreholes.
Sometimes, however, there are no nearby boreholes to guide the drilling. In these cases it is often very difficult to
ascertain when the borehole has intercepted the water table due to the drilling mud sealing-up the borehole as the drill
bit advances.
Careful observation to the drilling sometimes reveals one or more of the following signs indicating that a good
water-bearing layer has been reached:
- The cuttings may indicate that the drill bit has hit a zone of sand and/or gravel
(formations which usually
produce abundant volumes of water if they are saturated). This is the most widely used indicator and requires
continuous, careful sampling of drill cuttings.
- There is often a significant increase in the speed with which the hole is being drilled when a permeable sand
aquifer is reached.
- When drilling into a gravel aquifer, the gravel will often cause the bit to bounce sometimes.
- The drilling fluid (drilling mud) suddenly starts to thin appreciably.
- There may be a noticeable drop in the level of drilling fluid. If a formation is permeable enough to take water,
it may also yield enough water for a borehole!
- The water temperature may drop when groundwater is encountered.
In general, boreholes should be completed as far as possible into aquifers because:
- More of the aquifer can supply the intake portion of the borehole, resulting in a higher yielding borehole
(increased specific capacity).
- Sufficient saturated thickness is available to maintain borehole yield even during periods of severe drought or heavy
pumping.
- Where clay soils are found, it is often important to drill down and slightly into underlying rock to find
significant quantities of water.
After you stop drilling, ensure that the borehole is kept full of drilling fluid until the casing
and screen have been inserted into the borehole, gravel packed and the sanitary grout seal placed.
