Qualitative and quantitative monitoring of water catchments subject to saline intrusion in western France

Our client is a water operator for a local authority in western France. As part of its public service delegation, it operates and manages a catchment area comprising two boreholes extracting water from an aquifer located three kilometres from the Atlantic coast. This resource is periodically subject to quality problems due to the presence of chlorides, linked to saline intrusion (seawater entering the aquifer in coastal areas).
To overcome this historical problem, the pumped water is diluted with water purchased from the neighbouring municipality to lower the chloride content and make the water drinkable.

We were tasked with monitoring the intrusion of seawater into the aquifer and understanding how saltwater intrusion works as a function of seasonality and pumping on the catchment field.

Three systems for continuously monitoring the conductivity of the water in the aquifer over its entire height have been deployed in piezometers from the coast to the wellfield. This high-frequency vertical and temporal data will make it possible to detect the arrival of chloride plumes and serve as a warning and management aid for the wellfield, by validating or identifying management practices.

Innovative real-time continuous salinity measurement tools, combined with analysis by our engineering department, have enabled us to draw up operating guidelines (particularly in summer) to limit the impact of salt water intrusion.

The results of adopting these management and operating guidelines are as follows:

• Reduction in the summer period during which the pumped water was subject to chloride problems (140 days in 2016 – 0 in 2020);
• 80% savings in the volumes purchased to dilute raw water to make it drinkable (110,000 m3 in 2017 vs. 12,000 m3 in 2020);
• Ensuring the long-term future of the catchment field and maintaining its DUP.

For this high-frequency temporal and spatial monitoring, three multi-probe monitoring systems (SMD) for water conductivity have been installed in piezometers between the coast and the catchment field. These autonomous, robust systems measure the conductivity of the water in the aquifer at different depths in three different locations. Thanks to their high-frequency measurement over time, conductivity can be monitored almost continuously.

Continuous conductivity measurement system from the coast to the catchment camp and diagram of how the salt-water wedge works

Continuously measured vertical conductivity profiles make it possible to monitor the dynamics of the salt wedge and changes in its depth as a function of seasonality as close as possible to the coast. Its propagation towards the wellfield is observed by the intermediate SMDs.

A relationship is then establishedbetween this advance and the chloride content of the raw water extracted from the wellfield.

Changes in aquifer conductivity in terms of depth (ordinate) and time (abscissa) between 2015 and 2017 on the SMD close to the coast and visualisation of the dynamics of the salt wedge

Statistical cross-analysis of SMD and wellfield data (conductivity, groundwater level, volumes abstracted) from 2015 onwards, in particular via EMI, makes it possible to establish relationships between these variables.

Based on these analyses, operating guidelines have been drawn up:

1. In a deficit year (recharge limited by rainfall), the operating limit is 2,000 m3/day for more than 20 consecutive days in July.
2. The maximum volume that can be extracted in summer depends on recharge up to May, and can be adjusted according to rainfall in June, July and August (exploitation chart).
3. The sustainable exploitation of this resource must be based on the groundwater level, by adapting the exploitation rate so as never to lower the groundwater level below 0 m NFG for more than 20 days.
4. A statistical analysis of previous years was used to refine this rule on the volumes that can be extracted during the summer months (June-August), and the abacus used to estimate the volumes that can be extracted during the summer is based on the pre-summer water table level and the basic conductivity of the water table.

Abacus used to estimate the volumes that can be extracted in summer without exceeding the chloride limits on the raw water extracted.

The data obtained from these continuous measurement systems and their analysis will enable :

• The local authority to implement steering and management actions to :
  • • Sustainable use of the resource associated with the risk of saltwater intrusion;
    • Maintain operating volumes by managing this risk (alerts), and thus renew the DUP (renewal in 2019);
    • Avoid the cost of creating a new wellfield or network interconnection.
• The operator to implement operational actions on pumping volumes and rhythms in order to limit the purchase of water for the dilution of chlorides in summer.
• 80% reduction in water purchases between 2017 and 2020, i.e. 90,000 m3 saved.