Introduction to boreholes

Recognition of groundwater’s contribution to water supply, including garden irrigation, crop irrigation, stock watering, potable and town supply, is ever increasing. This is driven by inconsistent rainfall patterns, climate change and an increase in water demand. With drill rigs of all shapes and sizes running around between drill sites, it is appropriate that we spend some time outlining some important aspects regarding a successful borehole – it goes beyond just selecting the correct drill site!

Groundwater (What is groundwater?)

To understand the presence of groundwater, it is best to think of it in terms of its place in the water cycle. At some point in time, almost all groundwater was once rainfall that landed on the earth’s surface. Most of this rainfall then flowed down-gradient in small streams and rivers to eventually fill dams or reach the sea. Some of it evaporated back up into the atmosphere, while importantly, some rainfall infiltrated into the soil and rocks below the surface. The water that infiltrates below the surface accumulates becoming groundwater. Groundwater is stored and transmitted between soil, sediment or rock particles called pore spaces or interstices, and when available in meaningful quantities constitutes an aquifer.

There are three main types of aquifer, namely; a primary aquifer, where groundwater flows in pore spaces between sedimentary material (e.g. in loose sands), a secondary or fractured aquifer where groundwater occurs in fractures within the bedrock and a karst aquifer where groundwater is stored and flows through dissolution cavities (typically associated with limestone and dolomite rock). In a South African context, fractured aquifers are the predominant aquifer type. Fractures intersected during drilling are sometimes informally referred to as “veins”, which is conceptually incorrect if we consider that fracturing and faulting is typically planar and is very seldom tubular (vein like) in shape.

The sustainable yield of a borehole depends on the geological setting and rainfall in the area (with associated recharge to groundwater). The hydraulic conductivity (a property of the sub-surface that describes how transmissive it is) and storativity (analogous to the volume of water stored within an aquifer) are important geological controls on the potential yield of a borehole.

Likewise, the groundwater quality is also controlled by geology and rainfall. Host rock, with associated rock-water chemical interaction, typically determines the quality of the groundwater. Furthermore, in areas where evaporation far exceeds precipitation, groundwater tends to be more saline as the “freshening” effect of recharge is low.

Drilling a borehole

Despite public perception, drilling a borehole is not a simple task. Many questions should be asked of a drilling contractor before a borehole is drilled, and here are but a few of the more important ones: