Section 6

Deciding When to Stop Drilling

A reliable method for determining when appreciable volumes of groundwater are encountered is by conducting a preliminary assessment of wells or water sources in the area and having a good understanding of where groundwater occurs. It is generally good practice to inspect as many wells in the vicinity of interest as possible. If the inspected wells 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 wells.

Sometimes, however, there are no nearby boreholes to guide the drilling. In these cases it is often very difficult to know 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 useful 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 (Brush, 197?). If a formation is permeable enough to take water, it may also yield enough water for a well! 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 well, resulting in a higher yielding well (increased specific capacity);
• sufficient saturated thickness is available to maintain well 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. To learn more about "tropical hydrogeology", see Appendix C-4.

As mentioned earlier, after you stop drilling, ensure that the borehole is kept full of drilling fluid until the casing and screen (Section 7) have been inserted into the well, gravel packed (Section 8) and the sanitary grout seal placed (Section 9).

Footnotes

Brush, R. (197?) "Wells Construction: Hand Dug and Hand Drilled", US Peace Corps, Washington DC.

Driscoll, F. (1986) Groundwater and Wells, St. Paul: Johnson Division