Groundwater tables in the Centre-Val de Loire region
Let's discover a little-known world hidden beneath our feet, as we explore the Centre-Val de Loire and plunge deep into its groundwater. The Loire, France's longest river, is crucial to our region. Our other important rivers are its main tributaries, the Cher, the Indre, the Vienne and the Loir. We head to the region's southeast, near Saint-Amand-Montrond. During heavy rain, water runs off, and if enough of it soaks into the ground, it will seep down into the subsoil. In this area, the soil is mainly composed of sand and sandstone which absorb rainwater. It infiltrates the first meters of the subsoil. This corresponds to the unsaturated zone. Once it reaches an impermeable layer of rock, it accumulates and forms a saturated zone, or aquifer. "Groundwater" is the water contained between the grain of the rock or in its cracks. "Aquifer" designates the water-saturated geological reservoir. The drilling bore places a pipe that lets the groundwater in via a strainer. Water level is measured with a probe inside the pipe. This probe establishes the groundwater's piezometric level. The groundwater flows horizontally. When it passes under a layer of impermeable clay, it can no longer rise. The groundwater becomes captive and drains off very slowly. Once captive, the groundwater is submitted to pressure. By drilling through the impermeable layer, water rises in the pipe, and if there's enough pressure, it can even spurt out like a spring at the surface. This is known as an artesian well. In the area between Châteauroux and Bourges, let's discover another type of aquifer. Let's go down the river Cher which crosses Champagne berrichonne. The subsoil is composed of calcareous rock. This is often fissured, so water is able to seep in and flow easily. In this zone, the groundwater is generally free and fed by rain. It feeds the river through springs or, in a more diffuse way, through the riverbed. To find out the flow direction, hydrologists take simultaneous measurements of the groundwater level in several boreholes. Then they can trace the equal-height curves and produce a piezometric map. This map shows the direction of the groundwater flow. Flow direction is decided by gravity. The groundwater is flowing towards the Cher, which is a low point. We follow the Cher until its confluence with the Loire where the city of Tours stands. In the Touraine, the hydrogeology is marked by the superposition of two very large accumulations of groundwater. The aquifer closest to the ground is composed of Seno-Turonian chalk. The rock is chalky, similar to limestone, and is often fissured. The groundwater is free and flows into the Cher and the Loire. To extract the water, a pump is inserted into the well. Water from this chalk aquifer is mostly used for farming, notably the irrigation of crops. The 2nd aquifer is composed of Cenomanian sands at a depth of about 100 meters. It's made up of sands which lie under a covering of barely permeable clay. So the groundwater is captive. The flow rate is very low, as is renewal. The age of the water is estimated at 10,000 years! This water is of very high quality as it's protected from pollutants generated by human activity. This makes the aquifer a strategic reservoir of drinking water. Sadly, due to overexploitation, the level of this groundwater dropped in the 1990s. Management measures have been deployed in recent years to reduce the amounts of water pumped out. We now go up the Loire to the city of Orléans. Here, the aquifer is composed of highly-fissured karstic limestone, meaning it's impacted by a network of underground caves. The groundwater is free and at a low depth in the Le Val area. Its piezometric level is usually between 3 and 5 meters underground. In the Val d'Orléans, large quantities of the Loire's water seep into the aquifer, notably in the Jargeau area. The river and aquifer are closely linked. The aquifer's karstic nature enables a very fast flow of groundwater, at speeds of between 200 and 300 meters per hour. The groundwater looks almost like an underground river, which is very rare. The groundwater flows towards Orléans. Its best known emergence is the Bouillon spring, the source of the river Loiret. The Beauce region lies north of Orléans. It's a vast plateau covered in wheat fields, making it a "granary of France". The subsoil is composed of fissured, sometimes karstic, limestone. Most rainwater soaks into the ground and subsoil. On contact with a clay formation, the emergence of the groundwater can create a river, like the Conie, near the village of Conie-Molitard. In summer, the groundwater level drops as it's highly exploited for crop irrigation. The flow rate of the main source feeding the river also falls. To avoid overexploitation and the drying-up of springs, measures were taken in the late 1990s. This last example shows how crucial aquifers are in guaranteeing water for rivers and for local flora and fauna. They might be invisible, but aquifers are absolutely vital!
Water, at the heart of the Beauce region
With the support of the Centre Régional Council
Water at the heart of Beauce
Directed by Philippe Claire
Beauce, near Artenay
The Beauce table is located between the Loire, Seine, Loing and Loir rivers. Here we find the Beauce limestone formations. 20 million years ago, the Beauce area was a lake, where sediments gradually accumulated and formed this limestone.
Imagine a groundwater table as being like water in a sponge. It may be because the geological formation is sand, and there's an intrinsic permeability, where rainwater seeping into the ground and reaching the subsoil occupies this porosity, the space between the grains of sand. It can also be fissures or fractures in rocks such as limestone.
Regional Department for the Environment, Planning and Housing
The Beauce table, a major French groundwater table, in terms of size and the volume of water stored, concerns two regions, Centre and Ile-de-France, six departments: Loiret, Eure-et-Loir, Loir-et-Cher for the Centre region, Seine-et-Marne, Essonne and Yvelines for Ile-de-France. It straddles two hydrological basins: the Seine basin to the north, and the Loire basin to the south.
The BRGM is the national geological survey, a public body that deals with Earth sciences, notably groundwater. In the Centre region, we carry out research on groundwater, flow direction, quality... and also monitor groundwater, using piezometers.
I can demonstrate with this borehole. The operation consists simply of lowering the probe, on the end of a cable that measures to the nearest centimetre.
We go down to the water table. It is the table's surface that we measure. We've got to 18 metres.
We're close and usually... There we are! A light comes on when we get to... I'm rising it slightly to check the exact depth. And I'm measuring. 18.95 m. The Beauce table level, on the BRGM site.
My colleague has gone to the Ruan site to check the piezometric station. To do this, she'll take a manual measurement, by lowering an electric probe into the borehole to measure the depth of the water and compare it with the figure shown by the automatic data unit. Hello, Sylvie. Good! You're at Ruan. Have you taken the manual measurement? 17.44. I'll check that. I'll make a station call to ensure the phone line is working. In this case, the measurement taken, 17.44 m, matches the measurement taken in the field. The DREAL Centre's role is to observe fluctuations of tables in the Centre region, particularly the Beauce table.
The tools that monitor the workings of the system of tables and waterways, use a piezometric network, of 50 stations, which automatically and continuously monitor table fluctuations. This is supplemented by a network of stations that monitor the flow of the main waterways. One is on the Conie, at Conie-Molitard.
Beauce, near Viabon
Beauce has the particularity, or the luck, of having a very old station, located on the site of the Toury sugar refinery, which has provided piezometric measurements since the early 20th century. This station brings to light a cyclic phenomenon in the Beauce table's fluctuations, with periods of very low and very high water. There were some very low water levels in the early 90s. Previously, there'd been extremely low water levels in the early 1900s. The difference is that these low water levels occurred in climatic conditions much more critical than those observed in the early 90s. This reveals water withdrawal linked to the development of irrigation in Beauce from the 1960s on. This dry period and its consequences for the Beauce's rivers led to a collective realisation that we ought to better manage this resource and any withdrawals linked to it. This coincided with the setting up of a first volumetric management system, implemented in 1998. In parallel, we started in 2000, drawing up a water management plan (SAGE). The aim of SAGE for Beauce was to set management rules for water resources, i.e. the table and its waterways, for the years to come. To do so, a volumetric system was set up to manage farm irrigation withdrawals.
In the process, we monitor the flow rate of rivers. At each of the 9 reference stations, we have alert and crisis flow rate thresholds. If the thresholds are crossed, we take, in the process, constraining measures, such as bans.
Imagine a great reservoir which occupies the Beauce. The water in this reservoir is supplied, almost exclusively, by rain. The groundwater emerges in springs and feeds waterways, the surface water network.
Today, we judge a healthy groundwater resource by its ability to feed, in summer, the surface aquatic environments and, notably, the rivers. The aim is to have enough flow in the rivers to guarantee, notably, functions related to the survival of aquatic fauna.
Functions involving withdrawals from the Beauce table are firstly for drinking water, this represents 100 million m3 a year. Industrial usage withdrawals are around 30 million m3 a year. Finally, withdrawals for agricultural crop irrigation can vary from 150 million m3 a year in rainy years to up to 400 million m3 in years with a drier spring and summer, which occurred in the early 1990s. In terms of what enters the system, the table will refill over a period extending, generally, from October to April, from "effective rainfall". Effective rainfall is a part of the rain that falls in autumn and winter. This is around 130 mm per year, on average, which corresponds, in terms of the Beauce table, to a total volume of around 1.3 billion m3. The Beauce table has the problem, due to its unconfined nature, unprotected as it is by relatively impermeable geological surroundings, of being particularly vulnerable to surface pollution. Today, we're observing degraded water quality. We've taken action for some years now, mainly regulatory, but also voluntary, notably through agro-environmental measures. The prospects announced by climate specialists working on global warming are of globally drier years with warmer and shorter winters. The consequence will probably be lesser volumes of water to replenish the table and, in spring and summer, increasingly low water levels in rivers. Hence the value of implementing, in the years to come, volumetric management as employed in the SAGE today, and being able to test it and assess its efficiency if we are confronted, as in the past, by very dry periods.
If we have reliable information on climate change forecasts, in terms of temperature changes, rainfall, how rainfall will change, we could model, how the resource will evolve. On one hand, we can continue to research and advance in understanding and improving the model, which shows how the table works, and, depending on how the climate change research evolves, in France and globally, we can integrate all this and forecast on the way the resources will evolve, in the Beauce table.