Dem'Eaux Roussillon: improving our knowledge of the water volumes that can be sustainably abstracted from the Roussillon coastal aquifer, around Perpignan

Dem'Eaux: understanding water issues in the Roussillon plain, around Perpignan

How will the territories adapt to climate change and the shortage of water resources? This question is at the origin of the Dem'Eaux Roussillon project, coordinated by BRGM. This large-scale project has mobilised dozens of specialists for more than 5 years to study the underground water resources of the Roussillon plain (Eastern Pyrenees), whose aquifers constitute a major resource, particularly for the drinking water of Perpignan.

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How will regions adapt to climate change and water shortages? That question was at the heart of the Dem'Eaux Roussillon project, coordinated by the BRGM, which over 5 years mobilized dozens of experts to study underground water resources in the Roussillon plain, their use and their future. The Roussillon plain, with Perpignan at its center, gets some of the least rainfall in France and has some of the highest temperatures, with an annual average of 15 degrees C. Since the 1950s, the region's population has doubled, and summer tourism has boomed on its famous beaches.

The region's water needs are substantial. More than 80 million m3 of water are pumped every year from the groundwater reservoir. Half of that water is used by people and the other half for agricultural activities. Water is pumped from this reservoir, and this intensive pumping causes a drop in water table levels. There has been a chronic shortage over the past 50 years, and it could continue in the context of climate change. It's essential to understand how these hydrogeological systems work, how groundwater is filtered and stored, how much water can be pumped from the reservoirs and how it can be best managed to face probable droughts in the future. The first step was to describe the geological formations through which the water flows. It's important to find places where you can see those formations, like here, in this quarry, where you can see the first 50m of the formations on the plain. We took the old data from the Subsurface Database. We reinterpreted the geology of onshore drilling, and to compare it to what's happening at sea, we used geophysical imagery acquired by a boat off the coast of the Roussillon plain. We compiled that data into a geological model that allows us to see the reservoirs in 3D. You can see all of the reservoirs, on land and at sea, continuously. Connecting the two had never been done before, or at least not in this area. The first part of the project consisted of centralizing all the data related to the water tables, which have been studied since the 1960s. Over time, we acquired a wealth of data on water levels, water quality, nitrates, chlorides, etc. First, we put all the data into a database, which allowed us to see the state of the water tables and how they have evolved over 60 years. Remember that 85% of people in the Pyrénées-Orientales region get drinking water from these aquifers. More than 12,000ha of land on the Roussillon plain are irrigated by waterways or groundwater, or sometimes both. Gravity-fed irrigation is where water flows from a river into a canal and then onto farms. Some of that water ends up in the water table, and it's very important for the water table. Almost half the water in the water table comes from there. More and more people are moving to the region, and more and more farmers are pumping from the water table because it's getting hotter due to climate change. We're trying to draw a red line, to determine how much water we can take from the aquifers we've studied. The main risk for aquifers in this coastal area is that they are close to the sea, which can seep into the freshwater reservoir and make the water undrinkable. When we pump from the water tables, it causes surface water to seep into them. The risk to freshwater in the deep water tables is the seeping in of surface water that could contain salt, like here by the sea. As part of this project, we set up a monitoring network in coastal areas and the rest of the region. This is one of two observatories we built as part of the project. We installed piezometers, or wells, at different depths and arranged them like panpipes to monitor the different water tables beneath our feet. When we look at what's happening in the water tables, we see that those closest to the surface are filled with salt water, but the further down you go, the fresher the water is. This means that groundwater flows deep underground, probably several kilometers under the sea.

To summarize 5 years of water resource research in the Roussillon plain, the project's partners developed a platform compiling all of the available information, most of it in real time.

The platform gives local stakeholders one place to access all existing water resource data, whether for surface water, groundwater, meteorology, dams or canals. There's a lot of interaction between all of these divisions in the region, and we thought it would be good to allow them to visualize all the available data in real time. As in many other regions, drought committees meet in the summer to assess the situation. We believe this platform could be a very good decision-making tool for that kind of committee, since they can see, in real time, the situation in waterways and water tables and rainfall compared to seasonal averages, in order to decide on possible water restrictions based on that information. One strength of this project is that we put together a multidisciplinary team with a wide range of researchers, engineers, technicians, municipal representatives and user representatives, and they all worked together to better understand how the water table works and is used. In addition, all of the data acquired during the project, which we will continue to acquire from our two observatories, is freely and openly accessible to allow scientists to pursue their research. As managers, it's up to us to manage and process these data to improve water resource management.

Located between the sea and the mountains in an area where water needs are growing and affecting the available resources, this groundwater resource was studied through to the end of 2021 in order to better understand its properties and constraints in the context of climate change.

The ultimate objective of this project, entitled Dem'Eaux Roussillon, was to gain a better understanding of the volumes of water that can be sustainably abstracted from the different layers of the aquifer. The project produced a large amount of knowledge on various topics, presented in the project outputs.

Strategic framework

Groundwater provides 90% of the drinking water supply in the former Languedoc-Roussillon region. It represents a major challenge, particularly with regard to climate change.

The karst and multi-layered sedimentary systems from which this resource is pumped are representative of the main aquifers exploited around the Mediterranean and in other regions of the world. Accordingly, the development of tools to optimise the use of these aquifers has the potential to generate international activities for the region's laboratories and companies.

The Dem'Eaux Roussillon project demonstrates solutions for the characterisation and concerted management of the water resources of the Roussillon multi-layer Pliocene-Quaternary sedimentary system.

The development of tools and methods for characterising and managing the Roussillon aquifer on the basis of the skills and know-how of the region's public research establishments and companies has contributed to their mutual technological and scientific development and has improved their visibility.

The Roussillon plain is bordered by the Pyrenees to the west, the Corbières massif to the north and the Mediterranean to the east.

Background and objectives

Located on the coastal margin of the Pyrénées-Orientales département, the Roussillon plain is one of the driest regions in France with annual rainfall of 570 mm (average over the 1980-2010 period). Nevertheless, this area has a high level of urban development (the Perpignan agglomeration and coastal resorts) and agricultural development (approximately 15,000 ha of fruit and vegetable crops). Much of this development was made possible by the underground water resources contained in the Pliocene-Quaternary formations of the Roussillon plain.

Situation of sedimentary aquifers and interactions with their environment. The question marks illustrate the main issues and interactions involved in the Pliocene-Quaternary aquifer.

Abstraction from the water resource contained in the Pliocene-Quaternary aquifers currently amounts to 80 million m³/year (SAGE Roussillon, 2012). This abstraction has steadily depleted the aquifer over the entire plain. In order to deal with this, the local water management stakeholders created a Water Development and Management Scheme (SAGE "Nappes Plio-Quaternaires de la plaine du Roussillon") in 2008 to achieve a satisfactory groundwater status (this objective was set by the Water Framework Directive (WFD) for 2015) and rational water resource management.

Despite the technical and scientific work carried out to date (including a research project financed by the ANR – VULCAIN – and a study carried out in 2012 to determine the volumes that can be abstracted), the complexity of this aquifer (in terms of the geometry of the deposits, the organisation of water flows and the relationship with the interfaces) and the multiple issues at stake (numerous uncontrolled abstraction points, intense summer abstraction and problems with localised and diffuse pollution) mean that the conditions for the protection and the optimum, rational use of this aquifer are not well known at present.

Although the deep water in this aquifer (located largely under an impermeable cover) is not considered particularly vulnerable to human and agricultural activities, several catchments are affected by nitrate and pesticide pollution. This contamination is often attributed mainly to defective boreholes which connect shallow aquifers with the deep Pliocene aquifers, but this has not been documented with certainty.

In addition to this anthropogenic pollution, there is also the risk of seawater intrusion into the horizons exploited along the coast. Coupled with a chronic drop in piezometric levels throughout the plain, the scale of abstraction during the summer periods creates a risk of saline intrusion which could affect the entire water resource. Finally, existing climate projections (decreased recharge and rise in sea level) and the demand for water due to sustained population growth will probably exacerbate the existing pressure on this resource in the future.

The Dem'Eaux Roussillon project has combined expert regional knowledge in geology, geophysics, geochemistry, hydraulics, economics, electronics, spatial imagery and computer science to overcome the barriers to understanding the behaviour of the Roussillon Pliocene-Quaternary aquifer by taking into account all of the natural and anthropic issues associated with it and their future trends.

Work programme

The Dem'Eaux Roussillon project has proposed a multidisciplinary approach involving numerous areas of expert scientific knowledge needed to study the selected reservoirs, available in the Occitania region. It has substantially improved our understanding of the way the Pliocene-Quaternary aquifers work and of their hydraulic relationships with the natural systems (surrounding mountain ranges, rivers, sea) or anthropic systems (irrigation channels, water uses) with which they interact. This project has thus made an important contribution to optimising the protection and use of the groundwater resource of the Roussillon plain.

The work was subdivided into several approaches:

Geological approach

Establishing a 3D geological model, using existing data acquired in particular by oil companies and academic teams (maps, seismic profiles, drilling databases, etc.) and acquiring additional data (two core drillings, field surveys, geophysics, exploratory boreholes, etc.). Land-sea geological correlations will be one of the strengths of this approach. This will follow on from the PhD work of C. Labaune, J. Lofi, E. Husson, C. Duvail, M. Rabineau and G. Jouet.

Prospective socio-economic approach

Using high-resolution spatial and temporal characterisation of water use and its future trends and through socio-economic forecasting scenarios based on the existing situation (ANR VULCAIN, ANR EAU&3E and ERA-NET AQUIMED projects).

Coastal approach

Interactions of water resources and use with the sea, including coastal erosion and sea level rise. The coastal sedimentary compartment cannot be properly understood without an overview of the water sources (river inflows) and offshore losses. This approach will follow on from the PhD work of P. Durand, C. Brunel, P. Ferrer and M. Gervais and from the Life Response, ANR Miseeva and Med Coastance projects.

Geochemical and hydrogeological approach

Combined with a study to understand the fluid circulation, in response to the issues of deep karst contributions and saline intrusions for Roussillon. The knowledge acquired will then be used to test and/or improve the conceptual model by interpreting the data acquired over many years and by carrying out new acquisitions (pumping tests, multi-sensor acquisition, conductivity, thermal and flow profiles, etc.). The project will focus on issues of quantifying the resource and its vulnerability to saline intrusions. High-resolution spatial and temporal characterisation of the quantitative and qualitative state (with respect to saline intrusions, but also diffuse pollution) of the groundwater resource in the aquifer and hydrodynamic modelling of its future development will be carried out. In particular, two hydro-geophysical observatories will be set up for acquiring water pressure, conductivity and temperature data with high spatial and temporal resolution over a complete profile of the aquifer. These observatories will observe the vertical and lateral flow dynamics (recharge processes and saline intrusions respectively). This work follows on from the PhD work by B. Aunay and the ANR VULCAIN and GRAIND'SEL projects.

Calculation and modelling approach

To establish a conceptual model for the functioning of the hydrosystem by integrating it with the 3D geological model. All the results will then be made available on an interactive online platform for managers and the public.

Collaborative R&D project

Attempts were made in the Dem'Eaux Roussillon project to develop tools and methods for aquifer characterisation and management by pooling the skills and know-how of public research establishments and companies in the Occitania region. The outputs of the project are public and available on this website. The data acquired or produced are accessible on the Follow Roussillon platform set up under the project.

Project outputs

The results of the various tasks carried out by the project partners are summarised in the form of reports or publications, which can be accessed below (links are activated as and when they are published).

Internship reports

Funding

With a budget of 5.8 million euros, 20% of the project is financed by the French Ministry of Higher Education and Research and the Occitanie/Pyrénées-Méditerranée Region (under the 2015-2020 State-Regional Plan Contract), 15% by the European ERDF fund and the Rhône-Méditerranée-Corse Water Agency, 5% by Perpignan Méditerranée Métropole and 3% by the Pyrénées Orientales Departmental Council. The remaining 57% of the project funding is provided through the financial participation of most of the partners.