Hydrogeological situation on 1 December 2024
Aquifers began to recharge very actively in October, after which the trends in November 2024 were mixed. Levels rose at only 46% of the monitored points.
The overall situation deteriorated between October and November 2024. However, the situation is still very satisfactory overall, with higher groundwater levels in 65% of aquifers than the monthly norms. The situation is worrying for the Roussillon aquifers, with very low levels, and locally in the Hérault valley (alluvial deposits and Astian sands) and on the south-eastern coast of Corsica.
The trends and developments during this winter will depend primarily on the amount of rainfall. Recharging is likely to continue in rainy areas and the situation should then remain stable or improve. If there is insufficient rainfall, depletion could become widespread and the state of water levels could deteriorate rapidly in reactive aquifers and more slowly in inertial aquifers.
With groundwater levels having fallen in reactive aquifers, the risk of flooding from them appears to be receding. However, there is a need to remain vigilant if recharging resumes actively in areas where high or very high levels were recorded in November.
Map of aquifer levels in mainland France on 1 December 2024.
© BRGM
Evolution of the trends observed on piezometers from December 2023 to November 2024.
© BRGM
Groundwater trends
The recharge period usually begins with effective rainfall at the end of August and September for reactive aquifers and between October and November for inertial aquifers. In 2024, the winter recharge began as from September in rainy areas that have reactive water tables, and became widespread during October in all aquifers.
Recharging was active in November 2024, but slowed sharply: rising levels were recorded at 46% of the monitored points (71% in October). The contrasting trends are due to the accumulated local rainfall and the reactivity of the aquifer.
Recharging continued in November in the inertial aquifers in the Paris Basin and in the Rhône-Saône corridor. The inertial chalk aquifers of Artois, the Picardy plateau and Normandy are barely being recharged, while levels are stable or falling in the Sundgau aquifer (southern Alsace). In these areas, the rainfall that infiltrated slowly over the last two months is still not enough to compensate for the outflow of water (from springs and rivers and into the sea).
Most of the reactive water tables are showing contrasting local trends, ranging from falling to stable or rising, depending on the accumulated rainfall. Levels are clearly rising in the reactive aquifers of the Armorican Massif and along the Roussillon and Languedoc coasts. In contrast, they are generally falling in the Lorraine and Alsace plains, in the Alps and in the valleys of the Saône and the upstream Rhône. Note that some piezometers are recording particularly rapid depletion. This can be explained both by a lack of effective rainfall and by a higher depletion rate when levels are very high, as was the case last month. In fact, the higher the levels and hence the higher the pressure, the faster the groundwater will flow and drain the aquifer.
Comparison between 1 December 2023 and 1 December 2024
Map of France showing the state of the aquifers on 1 December 2023 (left) and 1 December 2024 (right).
© BRGM
Evolution of the situation observed on piezometers from December 2023 to November 2024.
© BRGM
Groundwater situation
Levels at the 2024 low-water mark, which occurred between September and October, were particularly high as a result of excess recharge in 2023-2024 and of high levels from effective rainfall in the spring and occasionally in the summer. The only exceptions were the Pyrénées-Orientales, the Aude, Cap Corse and the plains of eastern Corsica, where groundwater levels remain low after insufficient recharging in 2023-2024 and there was little or no rainfall during the spring and summer of 2024.
In November 2024, groundwater levels were very satisfactory over much of the country: the levels recorded at 15% of the observation points were lower than normal for the month; 20% were comparable with monthly norms and 65% were higher (compared with 12%, 10% and 78% respectively in October 2024).
These levels were higher than those observed for the previous year, in November 2023, when only 48% of the levels recorded were higher than normal for the month. However, several aquifers are at lower levels than in 2023: the Artois, Lorraine, western Armorican Massif and western Massif Central aquifers and many aquifers in the Aquitaine Basin and in the Pyrénées-Orientales.
The state of the inertial water tables has gradually improved over the past hydrological year and was generally satisfactory in November 2024. In detail, the situation deteriorated slightly between October and November 2024 in the Artois-Picardy inertial aquifers, as recharging began to diminish. The levels there are moderately high. The condition of the inertial aquifers in the Paris Basin, Sundgau and Rhône-Saône corridor did not change between October and November. Levels are moderately high in the centre and west of the Paris Basin, and high to very high in the east (upstream) and south. The levels are comparable to the normal to moderately high levels in the Rhône-Saône corridor. Finally, the level in the Sundgau aquifer has improved very slowly over the last few months, but was still moderately low in November as a result of its high inertia.
The situation deteriorated since the previous month for the vast majority of reactive aquifers, often falling to a lower class, particularly in the Artois, the north and west of the Armorican Massif, the Aquitaine Basin and the west of the Massif Central, and even falling by two classes in the north-east quarter. Reactive water tables are sensitive to rainfall deficits and they drain at an increased rate when levels are very high, as was the case last month. However, the situation in reactive aquifers remains very satisfactory, with generally normal to moderately high levels in the east (from Provence and the east of Languedoc to Lorraine) and in Corsica, moderately high to high levels in the west (from the Aquitaine Basin to Artois and Champagne) and up to very high levels around the Paris Basin and the south of the Armorican Massif. A few levels at 20-year highs are still occasionally being recorded in the Jurassic limestone aquifers in Berry, in the basement aquifers in the south of the Armorican Massif and in the alluvial aquifers of the Garonne and the Dordogne and their main tributaries. Note that some local situations are still fragile, with low to very low levels in the Valras-Agde sands aquifer, the downstream Hérault alluvium aquifer and those on the south-east Corsican coast.
The Roussillon and Aude aquifers are the only ones to see their condition improve between October and November 2024. Alluvial groundwater levels in the Aude aquifer have returned to higher than normal levels after nearly 3 years of deficits, particularly in the upstream part of the river. However, the rain in recent weeks has not been sufficient to compensate for the accumulated lack of rainfall over the past 3 years in the Pyrénées-Orientales department. Levels are still very low on the Roussillon plain and low in the Corbières Massif.
Aquifers with surplus water
Many aquifers are showing a surplus, with high to very high levels in comparison with the levels recorded for November in previous years:
- The reactive to mixed aquifers around the eastern and southern edges of the Paris Basin benefited from effective rainfall in November and are at high to very high levels;
- The aquifers in the basement of the south-east Armorican Massif remain at very high levels due to rainfall in late November;
- Many reactive aquifers in the Aquitaine Basin and the western Massif Central are still at high levels, despite the scanty rainfall in November.
Aquifers with low water levels
The situation is unfavourable in several aquifers with moderately low to very low levels compared to those recorded for November in previous years:
- The state of the Sundgau aquifer (southern Alsace) is changing very slowly due to its significant inertia, with overall levels still moderately low;
- The local situation is still fragile, with low to very low levels in the alluvial aquifer of the Hérault (downstream section), in the Astian sands aquifer at Valras-Agde and in the aquifers of the south-eastern coast of Corsica;
- The status of the aquifers in the Roussillon multilayer formation and the karst limestone of the Corbières Massif is still poor, with low to very low levels.
How do aquifers fill up and how do they empty?
Groundwater levels vary throughout the year, from high levels in winter (when vegetation does not absorb rainwater) to low levels in summer (the traditional depletion period).
The fate of rainfall varies greatly depending on the time of year and the condition of the ground surface on which it falls. Usually, the groundwater-recharge period takes place from early autumn (September-October) to early spring (March-April), a six-month period during which vegetation is dormant (with low evapotranspiration) and rainfall is generally more abundant. If the winter is dry, groundwater recharge is very low.
From spring through summer, rising temperatures coupled with the regrowth of vegetation and thus increased evapotranspiration, limit the infiltration of rainfall into aquifers. Between May and October, unless there are exceptional rainfall episodes, aquifer depletion usually continues and levels will keep decreasing until the autumn.
Why do some aquifers recharge faster than others?
Groundwater flows at different rates depending on the porosity (percentage of gaps/cracks in the rock) and permeability (capacity to allow water to circulate, i.e. interconnectivity between these gaps/cracks) of the aquifers. The larger the gaps and the more interconnected they are, the faster the water will flow, for both refilling and depletion.
It takes a given volume of water different periods of time to travel the same distance, depending on the kind of rock formation:
- a few years in a porous formation,
- a few months in a cracked formation,
- and a few days, or even a few hours, in a karst formation.
Inertial and reactive aquifers: what are they?
The impact of the winter recharge varies according to the cyclic nature of the aquifer, i.e., its reactivity to rainfall infiltration.
We refer to aquifers that are:
- reactive (when they are composed of sand, gravel, karst limestone or weathered granite formations). These aquifers are characterised by their rapid reaction times: they can recharge during heavy summer rainfall, but are also highly sensitive to drought. Their levels can therefore vary very quickly over the course of the same season.
- inertial (when composed of chalk, non-karst limestone, sandstone formations). Their reaction times are slow. They can have multi-annual cycles, meaning that they require a long period to recharge or empty.
Cyclicity of aquifers in mainland France.
© BRGM
Forecasts
The Météo-France seasonal forecasts for December 2024 and January and February 2025 indicate higher than usual temperatures across the whole of France. No clear scenario has emerged for rainfall.
The trends and changes over the next few weeks will depend exclusively on the amount of rainwater infiltration, and hence the total rainfall, and the reaction time of the aquifers (reactivity/inertia). In the event of normal to surplus effective rainfall, active recharging is likely to resume. The state of the aquifers should then stabilise or gradually improve. If there is insufficient precipitation, rainfall infiltration will not compensate for the output volumes (natural outlets and abstraction). Depletion could then initially spread to all the reactive aquifers, then to the inertial aquifers if the rainfall deficit continues for several weeks. The state of the aquifers would then deteriorate, slowly for inertial aquifers and rapidly for reactive aquifers.
Longer-term forecasts clearly indicate that areas where groundwater levels are currently higher than normal for the month will not experience a winter drought practically anywhere in the country.
As far as reactive aquifers are concerned, the start of winter recharge means that we can expect satisfactory levels at the end of the winter. However, reactive aquifer levels in early spring 2025 are still uncertain. The situation may also deteriorate rapidly if there is insufficient rainfall. The groundwater levels in the summer of 2025 will depend on whether there is an abundant winter recharge continuing into the spring. This would push back the start of the depletion period.
As for the inertial aquifers of Artois, the Paris Basin, Sundgau (southern Alsace) and the Rhône-Saône corridor, given the satisfactory low-water level and an early start to the recharge period, levels in spring 2025 will probably be satisfactory, close to or higher than normal.
It will be more difficult for aquifers in the Roussillon plain, which had very low levels in November, to reach normal levels in Spring 2025. It appears unlikely that there will be a sustainable replenishment of these aquifers in the coming months.
The short-term risk of flooding from rising groundwater levels appears to be receding in the case of reactive aquifers. After widespread high to very high levels in many aquifers in October, the water tables in November 2024 fell back to lower levels. However, a particularly abundant recharge of reactive aquifers over the next few weeks could once again result in exceptionally high levels and have a significant impact on surface environments.
The risk of flooding by rising groundwater in more inertial aquifers is still largely uncertain overall, as it depends on the accumulated rainfall received by the aquifer over the entire 2024-2025 recharge period. Changes in inertial aquifer levels will therefore need to be monitored particularly closely during the winter, with a greater risk in the spring of 2025. However, high to very high levels were recorded in less inertial water tables in the Paris Basin in November and could reach historically high levels during the winter. Enhanced monitoring is recommended in the following areas over the coming weeks:
- Semi-inertial Turonian chalk marl aquifer in the Champagne region;
- Semi-inertial Tertiary limestone and sand aquifer in the Brie Tardenois region.
Next groundwater tables status report
Our groundwater report is now published every month, in the middle of the month.
The next issue will be published in mid-January 2025.
State of groundwater: monitoring by BRGM
Groundwater is a widely used resource: in metropolitan France, it accounts for nearly two-thirds of drinking water consumption and more than one-third of agricultural water consumption. It is also widely used in the industrial sector. Groundwater tables depend on cyclical recharges.
BRGM monitors groundwater levels and quality in mainland France. Discover the actions carried out by the French geological survey and the resources and databases available on groundwater in France.
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Map drawn up by BRGM on 10 December 2024, based on data from the ADES database, acquired up to 30 November 2024. Data source: ADES database (ades.eaufrance.fr) / Hydroportail (hydro.eaufrance.fr) / Background map © IGN. Data producers and contributors: APRONA, BRGM, Conseil Départemental de la Vendée, Conseil Départemental des Landes, Conseil Départemental du Lot, EPTB Vistre Vistrenque, Parc Naturel Régional des Grandes Causses, Syndicat Mixte d'Etudes et de Travaux de l'Astien (SMETA), Syndicat Mixte pour la protection et la gestion des nappes souterraines de la plaine du Roussillon (SMNPR).
This map shows the global indicators reflecting the average fluctuations of the aquifers. They are based on point indicators collected at groundwater monitoring points (by means of piezometers).
The "Aquifer levels" indicator compares the current month’s figures with those of the same months in the entire record, i.e. at least 15 years of data and sometimes up to as much as 100 years of data. It is divided into 7 classes, from the lowest level (in red) to the highest (in dark blue).
The grey areas correspond to areas without unconfined aquifers, i.e. with an impermeable or semi-permeable layer above the aquifer, and/or sectors with a very low density of measuring points. This last case primarily concerns mountainous areas with small, heterogeneous aquifers.
The "Evolution of levels" indicator reflects the variation of the water level of the past month compared to the two previous months (stable, increasing or decreasing).
These global indicators reflect general situations and trends and do not take into account possible local disparities.