How boreholes work
And what exactly is an aquifer?25th Apr 2020
In times of drought, it’s common practice to sink a borehole. Many estates, and individuals, do this without fully appreciating how an aquifer works, and what legal implications are associated with going off grid, even partially.
The hydrological cycle
Water is a strange substance unlike any other natural resource. The most important of its many unique properties is that, unlike most natural resources, which are a stock with a defined available quantity, water is a flux that has moved around our planet continuously since Earth was formed billions of years ago. This movement is known as the hydrological cycle. What this means is that water is an infinitely renewable resource, but only if we manage it properly. If we do, we can enjoy its benefits indefinitely.
Put simply, the hydrological cycle consists of water evaporating from oceans and rivers, from plants and even from moist soil, or wet roads after rain. This water vapour blows away in the wind where it becomes part of a planetary recycling system, falling as snow, rain or fog over distant landscapes – quite possibly literally on the other side of the world. At a planetary scale, all water is part of one continuously moving flux. This has some major implications, the biggest of which is that moisture in the ground is the same water that we find in the ocean, the ice caps, as vapour in the clouds, or flowing in mighty rivers.
So what is an aquifer?
All water obeys the laws of gravity, so when it falls onto the land, it either flows downhill over the surface to become a stream, or it percolates into the soil to become groundwater. The soil is a relatively thin layer covering base geological structures, most of which are porous or fractured. These two words open up a deeper discussion into how different aquifers work. For simplicity, we can think of two broad types of aquifer (there are more, but then it becomes complicated).
The first is the porous aquifer. Here we can think of a sponge with tiny spaces between a structure that has enough strength to not collapse under its own weight. Sand makes a good structure of this type, so think of a layer of porous material, lying on top of less porous rock. The tiny holes in between the particles of sand soak up the water and hold it in place indefinitely. Now think long geological timescales where that sand begins to consolidate into sedimentary rock, still with tiny pores containing water. This type of aquifer is quite common in defined parts of the country, often along the coastal areas. It is also a characteristic of the coal-mining areas because all coal is hosted in sedimentary geology.
The second type of aquifer is one made of hard rock. These aquifers have specific characteristics, because the rock is seldom monolithic, and is invariably broken or fractured. It’s these fractures and breaks that can cover long distances to become pathways for water from mountains down to the valley below.
There are other types of aquifer, but that’s another story.
Different host geology gives different characteristics
A porous aquifer means that a hole drilled almost anywhere will generally yield water. The flows are usually slow, so the yield can be low but reliable. A fractured hard rock aquifer means that the siting of the borehole must be done by a geophysicist who looks for fractures, breaks and volcanic intrusions such as dolerite dykes and sills. (Or a ‘diviner’ with a forked stick, but this system has not proven to be particularly reliable.) Basically what this means is that you can’t just drill a hole randomly, because intersection with water-bearing fractures is unlikely. Such aquifers tend to be higher yielding, but more costly to drill, by virtue of the precision needed in their siting. Misses result in a dry hole, but you still have to pay the driller.
The important thing to understand is that aquifers exist almost everywhere, but not all aquifers can be economically exploited. The reason is simple. Because all boreholes in a general area are probably part of the same aquifer, the abstraction of water from one can impact on the yield of another. Think of friends drinking a fruit juice from one glass, but with different straws. The same principle applies to an aquifer. In fact, we can think of a borehole as a shaft, and the process of pumping as mining. While water is infinitely renewable, the rate of flow through an aquifer can be measured in decades, centuries, or millennia. The Great Nubian Sandstone Aquifer found in the Sahara Desert was last recharged 10,000 years ago, so in effect Libya and Egypt are mining fossil water. Sandstone aquifers in the Free State can take a century or more to recharge.
Cone of depression
We like to think that borehole water is clean, unpolluted, pristine water from the depths of the Earth, but it’s not that simple. Remember, it’s not part of a huge crystal-clear underground lake kilometres under the surface – it’s the same water that hangs out in the clouds and in the rivers, just passing through.
When you pump from a borehole, it sucks water from the pores in the rock, which changes the pressure gradient in an aquifer, creating what is known as a cone of depression. This water takes time to be replaced and, because water (and almost anything, really) flows from an area of high pressure to an area of low pressure, any nearby water will flow into the cone of depression – and continued pumping will result in continued inflow from the surroundings. Now this surrounding water could come from anywhere – runoff from agriculture, septic tanks or rivers, all of which are polluted. At the coast, even seawater can flow into the aquifer, which damages it permanently. This also means that all fertilisers and pesticides used in gardens or on golf courses start to move through aquifers as these pressure gradients change. And boreholes close to any large lake will have water derived in part from that lake, even if it is pumped out some distance from the shore.
So what does that mean?
The take-home message is that, while groundwater is often a reliable source of water, it does not represent an infinite supply, and its persistent use creates a set of changing risks over time. One of the biggest risks is inflow into the aquifer of waste water from septic tanks, or contamination from open defecation on the surface but some distance away from the borehole in question. So, if you are using borehole water, you must get it tested regularly to SANS 241 standard if it ever comes into human contact. And, remember, even the water from irrigation sprinklers can come into human contact.