HYDROGEOLOGY

Invisible water of life

Since world governments decided that improving the management of the planet's water reserves was a major priority, the threats hanging over groundwater have suddenly become front-page news. However, inconsistencies remain...

Main types of groundwater
Main types of groundwater

  1. Alluvial groundwater: sand and gravel, 10–50 metres thick, 100–150 l/m3 of rock, connected to a surface stream.
  2. Confined groundwater: sand, sandstone, limestone, overlain with an impermeable layer, local recharge.
  3. Unconfined groundwater: chalk, limestone, sandstone, 30–100 l/m3 of rock, no impermeable confining layer.
  4. Fractured medium: granite, shale, 5–30 l/m3 of rock, recharged over the entire surface area.
Source: Groundwater, knowledge and management, by Jean-Jacques Collin, drawing by J.F. Rieux, BRGM Editions and Hermann Science and Arts Editors
Main types of porosity
Main types of porosity

Where groundwater can be found. It fills the spaces between sand grains, in rock crevices and in limestone openings.

Source: Environment Canada Freshwater Website: www.ec.gc.ca/water/
Reproduced with the kind permission of the Minister for Public Works and Government Services, 2008.
Groundwater flow
Groundwater flow

Source: Environment Canada Freshwater Website: www.e.gc.ca/water/
Reproduced with the kind permission of the Minister for Public Works and Government Services, 2008.

Since ancient times, water diviners have doused for water armed only with a wooden stick (or divining rod). Most scientists would describe them as groping their way in the dark, as groundwater distribution is determined only by the distinctive features of local geology. The geological expertise required to exploit water was developed mainly in the 19th century and it is only in the past 20 years or so that three-dimensional land imagery has become available in some countries, making it possible to locate and manage water.

Water usually consists of a multitude of droplets that infiltrate porous subsoils, forming rivers and lakes only in impermeable karst terrain. Some soils act like vast sponges that filter some of the rainwater and store it in what is called the "saturated zone" - an area where all the cracks are filled with water.

Ins and outs of groundwater

This water forms groundwater, staying trap - ped underground for anything from a few days to a few thousand years. It forms an integral part of the hydrological cycle, with its circulation speed varying in line with the type of aquifer (the geological formation through which the groundwater travels). With the exception of fossil groundwater deposits in desert regions, reserves of which are limited by scarce precipitation, groundwater circulates and is recharged, either quickly or slowly, depending on the nature of the material through which it flows and the gradient of the impermeable layer underlying the aquifer.

While every aquifer is unique because of its own particular geological features, aquifers can be categorised according to the behaviour of the water circulating in them. In a "porous aquifer", which is composed of unconsolidated material like sand or gravel, water circulates between the individual particles. In a "fractured aquifer", water travels through the crevices criss-crossing such geological strata as granite or shale. "Karst aquifers", which are made of chalk or limestone, contain crevices, cavities and sometimes pores.

Groundwater is said to be "unconfined" when it is able to rise and fall unhindered, usually in a shallow aquifer. If it is overlain by a layer of impermeable material, the groundwater is said to be "confined". As confined groundwater is found at greater depths it is under high pressure and, in some cases, flows to the surface via an artesian well. "Alluvial groundwater" is a special type of unconfined groundwater, formed in the large expanses of sand and gravel bordering rivers and streams.

Key know-how

Most aquatic environments are primarily regulated by natural groundwater. For instance, many rivers would run dry in summer if they were not supplied by the surrounding alluvial groundwater and, conversely, would burst their banks every winter if the ground did not absorb excess precipitation. Groundwater supplies almost one third of the volume of all the Earth's water courses - i.e. around 12 000 km³ per year.

Groundwater is therefore a vital resource that plays an extremely important role in regulating surface-water flow; as such, the study of groundwater mechanics is crucial to understanding and predicting floods. Knowledge about groundwater is just as crucial for the construction sector, to prevent groundwater from rising into buildings as well as to guarantee the stability of building ground. This is because aquifers form an integral part of every region's distinctive geology. In Mexico, for example, the fall in the water table as a result of overexploiting groundwater has caused land subsidence and destroyed a lot of infrastructure. In Riyadh (Saudi Arabia), the extra drinking water that has been added from desalination plants has raised groundwater levels, damaging cellars and water distribution pipes.

As groundwater is easily accessible, tends to be of excellent quality and has a good flow rate, it is widely used by industry, mainly as a coolant. It is most important for irrigation: around 40% of the world's crops depend on groundwater to some degree.

Groundwater is also vital because it provides a tremendous reserve of drinking water. Not counting the huge quantities of unexploitable freshwater trapped in glaciers and polar icecaps, around 97% of the planet's useable reserves are hidden underground. Around 50% of Europe's drinking water is from groundwater.

A vital resource under threat

"Groundwater is usually of better quality than surface water because it has been filtered through the ‘unsaturated zone' lying above the reservoir," explains Maciej Kłonowski, hydrogeologist at EuroGeoSurveys. If the geological conditions are right, groundwater can also be pumped locally. These two advantages make groundwater relatively inexpensive to exploit.

However, groundwater's important role is too often overlooked by operators and consumers alike - an oversight for which the Mediterranean countries are now paying dearly. More than half of Spain's 100 aquifers are overexploited. In the Segura river basin, the ratio between the quantity of groundwater abstracted and the quantity of recharging precipitation shot up from less than 20% in the 1980s to 130% in 1995.

"Once groundwater is pumped, it is imperative that it be done sustainably," emphasises Maciej Kłonowski. "Overexploiting an aquifer can change the water's chemical composition. For instance, there can be a harmful increase in concentrations of iron or manganese. Another possible consequence is saltwater seeping up from underlying deep aquifers or, in coastal regions, the intrusion of seawater into groundwater. All this makes groundwater unfit for consumption. And since it is impossible, or extremely costly, to treat groundwater, in many cases such aquifers have to be abandoned for years or even permanently."

Surprisingly, groundwater is particularly poorly managed in arid and semi-arid regions, despite being such a precious resource. "In Spain, huge quantities of water are used to grow early crops like strawberries. This is compounded by reliance on unsuitable irrigation methods, where water is simply sprayed over the entire field and much is wasted by evaporation. This shows just how imperative it is to adjust above-ground human activities to the quantity of water available in order to preserve groundwater sustainably," explains Wilhem Struckmeier, Secretary General of the International Association of Hydrogeologists (IAH). "It is not only southern countries that are unaware of, or fail to acknowledge, the key role of groundwater. Although northern European countries give equally scant consideration to groundwater, in their case the consequences of poor groundwater management are simply less obvious because so much surface water is available."

Sketchy knowledge

In the 1950s, groundwater use was stepped up as a result of industrial development and the intensification of agriculture. "Unfortunately, funding for hydrogeology research was funnelled mainly into improving the expertise needed to develop civil engineering. Research efforts focused on the feasibility of extraction, rather than on studying the role of aquifers in the water cycle or the way in which groundwater functions overall. Although European countries are capable of exploiting groundwater, our hydrogeological know-how is still too sketchy to enable us to do so sustainably," bemoans Wilhem Struckmeier.

The launch of the ambitious European Water Framework Directive, in 2000, marked a turning point in EU policy on water. A host of research projects have been launched to improve the management of this vital resource. The AquaTerra project is a good example. The project aims to provide the scientific basis for improving river basin management through a better understanding of the river/sediment/ soil/groundwater system.(1) "We are studying the movement of pollutants through these various compartments, something about which we know very little. For instance, we still have no idea how the soil stores pollutants such as PAHs(2) for years or even thousands of years. Many questions remain about the microbiological processes involved in degrading certain pollutants in soils and water," explains Johannes Barth, hydrogeologist at the University of Tübingen (DE) and Scientific Coordinator of AquaTerra.

The project's greatest asset is the highly multidisciplinary group of scientists, practitioners and end-users that have come together, including geologists, socioeconomic scientists, environmental engineers, chemists, managers, policy-makers, regional and urban land planners and others. All are pooling their efforts to build the scientific bases for developing digital models to support river basin management. The theories devised in the laboratory will be tested on the drainage basins of the Ebro, Danube, Meuse and Elba rivers and the Brévilles springs, all very different hydrological systems that have been selected with a view to extra - polating AquaTerra results to other basins.

The project has also improved our understanding of the importance of the alluvial plains bordering rivers and streams. "Not only do they help to prevent flooding, their pH dynamics and special oxidation-reduction properties give them a decisive role in the sorption cycle of pollutants," explains Johannes Barth.

From word to deed

As all the ingredients for a "water crisis" scenario are present (a demographic explosion, uncertainty about the consequences of global warming and unequal access to water for the world's nations), this calls for the sustainable global management of groundwater. However, there remain enormous gaps in scientific knowledge about groundwater. Even though they are buried deep underground, these freshwater reservoirs are connected with the soil and with the rest of the water cycle. Their sensitivity to above-ground human activities could also force us to re-examine our modes of consumption and production, especially in agriculture. All these factors mean that only a multi-disciplinary approach will enable us to find appropriate means for ensuring sustainable groundwater management.

In 2004, European governments underlined the importance of the Earth's hidden water in a groundwater directive. Nonetheless, all their good intentions have resulted in very little action. "In the Seventh Framework Programme, only one call for proposals directly concerned groundwater," says Johannes Barth with regret. "It is absurd to expect to tackle such momentous environmental issues if only meagre funding is provided for scientific research on water."

This is a view shared by Wilhem Struckmeier. "A lot more basic science is needed before an effective system for safeguarding groundwater can be established. There is not even a clear definition of what a body of groundwater (the term used in the 1980 European Groundwater Directive) actually means. The procedures for collecting data on groundwater quality are also too diverse, and in many European countries they are not even relevant. EU funding for groundwater initiatives focuses excessively on pollution reduction and groundwater overexploitation and not enough on projects to improve groundwater management. The way I see it, rather than tackling the problems at source, Europe is concentrating mainly on the effects."

Julie Van Rossom

  1. A river basin is an expanse of land where water from rain or snow melt drains downhill into a body of water, such as a river, lake, reservoir, estuary, wetland, sea or ocean. Topographically a river basin is separated from adjacent basins by a geographical barrier such as a ridge, hill or mountain.

  2. Polycyclic aromatic hydrocarbons, a type of persistent organic pollutant (POP).

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  • Aquaterra

    46 partners - 15 countries (AT- BE- CH- CZ- DE- DK- ES- FR- IT- NL- PL- RO- RS- SK- UK) www.eu-aqua
    terra.de

  • International Association of Hydrogeology (IAH)

    More than 3 500 members from some 135 countries.
    www.iah.org