Secrets of the Springs

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This blog is written by Heike Wanke, a hydrogeologist at the University of the West of England, Bristol. Heike researches water in drylands like Namibia.

Nature’s lifelines

Springs have been lifelines for millions of years, sustaining plant life, supplying animals with drinking water and supporting human development, particularly in drylands where surface water is scarce. Springs have influenced where humans have settled and given towns their names (such as ‘Alice Springs’ in Australia and ‘Wells’ in England).

A captured spring at Compton Abdale in the Cotswolds, England.
Hot spring at Bath, England.

We still use springs today. Some bottled water brands may be advertised as “spring water”, or “direct form spring”, but captured springs also contribute to the normal water supply from our taps. We can enjoy a visit to a spa supplied by a hot spring (like Bath in England, which has been used since Roman times). Some springs, such as the Blautopf in Germany, or Klein Gai-As in the Namib desert, are quite spectacular.

But why are springs so special to geoscientists?

A small lake fed by a spring at Klein Gai-As in the Namib Desert, forming a unique drinking place for the animals in this otherwise dry environment.

The “Blautopf” (meaning blue pot) in Blaubeurren, Germany: a spring at 18 m depth issues water from a Karst cavern system and has eroded the “pot” over the course of time. The blue appearance of the water is caused by dispersal of light on the nano-sized carbonate particles in the water (photo credit: www.blaubeurren.de)

Clues in the water

As a geologist, what fascinates me about springs is the information they carry in their water.

We can find out how deep the water has been, what rocks it has travelled through, the elevation where it first filtered into the ground, and even how long it has been in the underworld- the age of the groundwater. The location of springs can also help us to understand the geology of an area; for example, if there are several springs along a line, this could be an indication of a geological fault. We can also assess whether the spring water (that we tend to think of as being fresh, clean and healthy) is indeed fit for human consumption.

A small hot spring at Okanguati in northern Namibia. It is perceived as being healthy and clean and people often come here to fetch water. But in fact this water has a fluoride problem like many hot springs; the spring water contains of 13 mg fluoride per litre – the UK drinking water guideline allows only 1.5 mg per litre in drinking water.

Geothermal energy

The temperature of the rocks within the Earth increases with depth, at a rate known as the geothermal gradient. Within the first 3 to 5 kilometres of the Earth’s surface, temperatures increase by about 3°C per 100 m as you go down. Spring water warmer than the average water of the area (a hot spring or geothermal spring) has travelled through deeper zones within the crust, and retained that heat- and we can use the temperature of spring water to tell us how deep it has travelled.

The Ai-Ais Hot Spring in southern Namibia discharges water at a temperature of 60°C, which is 40°C warmer than the general groundwater temperature of the area (20°C). This tells us that the water must have travelled to a depth of at least 1333 m within the ground, assuming that the water rose very quickly without chance to cool down near the surface. This calculation can be verified with a ‘geo-thermometer‘, a method that uses the ratio of two elements (sodium to lithium) in the spring water. If the results from the two independent methods do not fit, it could mean that the area has an unusually high geothermal gradient. Such results can be very interesting when on the hunt for geothermal energy.

Seeking the source

If we zoom even further into a spring and look at the water molecules it is made of, we can find even more information. Each water molecule is made up of two elements, hydrogen and oxygen. Each of these elements can be made up of different isotopes (lighter and heavier variations of the same element). Water molecules containing heavier or lighter isotopes behave differently when water changes from water vapour to liquid water (when clouds change to rain drops)- and this behaviour changes with temperature and elevation.

If we examine the composition of the water molecules in a spring, and the ratio of heavy and light isotopes within them, we can estimate approximately where the rain fell that we now find in our spring- even after its long journey underground. This information can help us identify critical areas that need protection from contamination, where water is easily washed underground and could pollute the spring water.

Future challenges

If you are interested in learning more about the “secrets of the springs”, it is a great area of research under the larger umbrella of hydrogeology, geology or hydrology. Studying these subjects could get you involved in discovering more of these secrets, and involve you in a subject area that addresses one of the largest challenges of this century: providing enough water of appropriate quality for a growing world population under the influence of climate change.

Sometimes we come across false springs- this is the Okashana ‘Spring’ in Northern Namibia, which is not a spring, but an artesian borehole. In the 1970s a borehole was drilled into a deep aquifer (204 m) and as the water is under pressure, it flows out freely forming a shallow lake. Unfortunately, this water is too salty to drink.