Ground water Part One: Siting of boreholes.

There are many scientifically proven techniques to best locate a position for the drilling of a borehole. Much controversy exist in the ground water industry over what is called , as I am a scientist, I will not discuss this topic and as a scientist like to comment as follows: Not one single method currently exist that can, with 100% accuracy, indicate the exact depths and volumes of ground water occurrence.

There are however scientific methods that can, with the correct application, increase the chances for successfully locating borehole sites. The following factors are taken into consideration in ground water exploration. Firstly one has look at the geological setting of the area I.e. the types of geological formations present in an area, as well as the potential of these formations to act as aquifers. Secondly one have to look at hydrological factors such as rainfall and the percentage that contributes to groundwater recharge, as well as the water balance for a specific catchment area.

Typical questions that are asked in a groundwater investigation are: how much do we need? Can the aquifers provide in this need?, is the ground water quality suitable for the intended use? Etc. To answer such questions it is necessary to study available information such as geological and hydrogeological maps, aerial photographs, satellite images, borehole information, such as borehole depths, yields, depths at which water was intersected, water quality intersected, of all, or as many as possible of the boreholes in the area. Once this phase desktop studyll look at five of the most commonly used geophysical methods applied to identify positions for borehole drilling. The essence of these techniques is based on the understanding of the geological sequences present in the area, and none of these techniques can be used in isolation.

Magnetic method. This is probably one of the most commonly used geophysical surveys that are used to assist with the location of positions to drill boreholes. This method measures the changes in the normal magnetic field of the earth, and it is used to pick up contacts between different geological formations such as dolerite dykes.

Electrical resistivity. With this method an electrical current is passed through the underlying rock and the difference in the passing of this electrical current is through the different rock formations is measured. The differences in the resistivities of rock types can measured, which can then be correlated to water bearing zones.

Electromagnetic method. This method uses the difference in electromagnetic conducting abilities of rocks and is related to the electrical resistivity method.
Gravimetric method. This method is based on the earths gravitational field, caused as a result of the different densities and
mass of rock formations, are determined at different measurement points on the earthll probably have a 70/30 chance (depending on the area) of drilling a
successful borehole. This difference is not great, but let me explain further. Letll have another 50/50 chance on the second borehole, while
with a geohydrologist your chances will probably increase to the order of 85-90%. This is generally due to the fact that the geohydrologist can interpret the results from the first borehole and fit the information into the picture as build up during the desktop study. A Geohydrologist would also be able to assist with the correct borehole construction design for the geological formation s encountered, and ensure that all relevant information is recorded to make the borehole tests more accurate and applicable.
Next week we look at the drilling of a borehole and why it is important to have a borehole properly constructed.