Report on Flow Reversal and Saline Intrusion in Submarine Karst Springs

A Case Study of the Vise Spring, France, and its Implications for Sustainable Development Goals

Executive Summary

This report details a long-duration study of a full flow reversal event at the Vise submarine karst spring in Southern France, where saltwater intruded into a coastal freshwater aquifer. The primary driver of this regime shift is the hydraulic gradient between the aquifer and the adjacent Thau lagoon, which is influenced by changes in water density. The study identifies two critical tipping points: one that triggers the shift from normal freshwater outflow to saltwater intrusion, and a second, much higher hydraulic gradient required for the system’s recovery. This significant hysteresis is caused by the filling of the vertical karst conduit with dense saltwater, prolonging the period of degradation.

These findings have profound implications for several Sustainable Development Goals (SDGs). The contamination of freshwater aquifers by saltwater directly threatens SDG 6 (Clean Water and Sanitation). The increasing frequency and duration of these events, exacerbated by climate change-induced sea-level rise and reduced aquifer recharge, highlight a critical challenge for SDG 13 (Climate Action). Furthermore, the degradation of coastal water resources jeopardizes local economies and ecosystems, impacting SDG 14 (Life Below Water) and SDG 8 (Decent Work and Economic Growth).

1. Introduction: Freshwater Security and Coastal Ecosystems under Climate Threat

Coastal aquifers are vital freshwater resources, essential for both human consumption and the health of coastal ecosystems. Their discharge into the ocean, known as submarine groundwater discharge (SGD), plays a critical role in maintaining the ecological balance of coastal zones. However, these systems are increasingly vulnerable to saltwater intrusion, a phenomenon that threatens global progress towards key Sustainable Development Goals.

1.1. Context: The Critical Role of Coastal Aquifers (SDG 6, SDG 14)

Coastal karst aquifers represent a significant portion of global coastlines and are a crucial source of freshwater. The sustainable management of these resources is fundamental to achieving SDG 6 (Clean Water and Sanitation). The outflow from these aquifers supports coastal and marine ecosystems, directly contributing to the objectives of SDG 14 (Life Below Water). The phenomenon of flow reversal, where saltwater flows into the aquifer, represents a direct threat to both of these goals by contaminating freshwater supplies and altering the chemical balance of coastal waters.

1.2. Study Focus: The Thau Hydrosystem and its Socio-Economic Importance (SDG 8, SDG 11)

The study was conducted at the Vise submarine spring, the outlet of the Thau karst aquifer in Southern France. This hydrosystem is of immense socio-economic importance:

• It supplies drinking water to local communities.
• It supports the largest thermal bath center in France (Balaruc-les-Bains), a cornerstone of the local economy.
• The adjacent Thau Lagoon hosts extensive shellfish aquaculture and fisheries.

The integrity of this water system is therefore linked to SDG 8 (Decent Work and Economic Growth) and the resilience of local communities under SDG 11 (Sustainable Cities and Communities). Occasional saltwater intrusions create conflicts over water use and pose a significant risk to these activities.

2. Analysis of the Flow Reversal Phenomenon

A long-duration flow reversal event was monitored from November 2020 to March 2022. This provided a unique dataset to analyze the mechanisms, triggers, and impacts of saltwater intrusion in a karst system.

2.1. Hydrological Drivers and Tipping Points

The primary control parameter for the system’s state is the hydraulic gradient between the aquifer and the lagoon, corrected for water density. The flow reversal was triggered when this gradient fell to a critical threshold, or tipping point (TP1), of 6.13 m. This occurred due to a combination of factors:

1. Low water level in the aquifer following a drought period.
2. High water level in the lagoon caused by tides and a storm event.
3. High water density in the lagoon due to summer evaporation.

Once the reversal occurred, the system entered a degraded state. The return to normal freshwater outflow required the hydraulic gradient to reach a much higher second tipping point (TP2) of 10.25 m, which only occurred after a major rainfall event recharged the aquifer. During the 15-month reversal period, an estimated 6.7 million cubic meters of saltwater infiltrated the aquifer.

2.2. Physical Mechanism and System Hysteresis

The Vise spring is connected to the aquifer via a deep, sub-vertical karst conduit. The significant difference between the two tipping points (TP2 – TP1 = 4.12 m) reveals a strong hysteresis in the system. This is explained by the physical mechanism:

• Degradation: When the hydraulic gradient drops below TP1, dense saltwater from the lagoon rapidly fills the vertical conduit.
• Stabilized Intrusion: The column of dense saltwater in the conduit exerts additional pressure on the aquifer, effectively “locking” the system in a flow reversal state. This makes the degraded state highly stable.
• Recovery: A very large increase in the aquifer’s freshwater head is required to overcome the pressure from the saltwater column and flush it out, allowing the system to return to normal.

This hysteresis, a direct result of the vertical conduit geometry, is responsible for the long duration of the saltwater intrusion event.

3. Implications for Sustainable Development Goals

The observed phenomenon of hysteretic flow reversal in the Vise spring serves as a critical warning for the management of coastal resources worldwide, with direct implications for multiple SDGs.

3.1. Threats to Clean Water and Sanitation (SDG 6)

The primary impact is the large-scale contamination of a vital freshwater aquifer. The infiltration of over 200,000 tons of salt during the monitored event renders the groundwater resource unusable for drinking and other purposes without extensive and costly treatment. This directly undermines efforts to ensure the availability and sustainable management of water as mandated by SDG 6.

3.2. Climate Change Vulnerability and the Need for Climate Action (SDG 13)

The study explicitly identifies climate change as a key aggravating factor. Future scenarios predict:

• Sea-level rise: This will permanently raise the baseline water level in coastal lagoons and seas, reducing the natural hydraulic gradient.
• Decreased recharge: More frequent and intense droughts will lower aquifer water levels.

Both factors will make the conditions for flow reversal more common, pushing coastal water systems closer to their tipping points. This underscores the vulnerability of water resources to climate change and reinforces the urgency of SDG 13 (Climate Action) to mitigate these global pressures.

3.3. Risks to Marine and Coastal Economies (SDG 14, SDG 8)

The health of the Thau Lagoon’s ecosystem, which supports a significant aquaculture industry, depends on the natural discharge of fresh groundwater. The reversal of this flow alters the lagoon’s salinity and nutrient balance, threatening biodiversity and the economic viability of fisheries. This jeopardizes the sustainable use of marine resources (SDG 14) and threatens local livelihoods and economic growth (SDG 8).

3.4. Challenges for Sustainable Communities (SDG 11)

The sudden increase in aquifer pressure following a flow reversal (piezometric rebound) has been observed to cause flooding in the nearby city of Balaruc-les-Bains. This poses a direct risk to urban infrastructure and public safety, challenging the goal of creating resilient and sustainable communities (SDG 11).

4. Conclusion and Recommendations for Sustainable Management

The study of the Vise spring reveals that submarine karst systems with vertical conduits are highly susceptible to long-duration, difficult-to-reverse saltwater intrusion events. The system’s high hysteresis means that once a tipping point is crossed, recovery requires a disproportionately large environmental change, such as a major aquifer recharge event.

In the context of climate change, this phenomenon poses a severe threat to the achievement of multiple Sustainable Development Goals, particularly those related to clean water, climate action, and the health of coastal ecosystems and economies. For deeper karst systems, such flow reversals could become effectively irreversible, leading to the permanent loss of freshwater resources.

To address these challenges and build resilience, the following actions are recommended:

1. Enhanced Monitoring: Implement comprehensive monitoring of hydraulic gradients and water quality in vulnerable coastal aquifers to provide early warnings of proximity to tipping points.
2. Sustainable Water Management: Develop and enforce strategies to maintain healthy aquifer levels, including managing groundwater abstraction and promoting aquifer recharge.
3. Climate Adaptation Planning: Integrate the risk of saltwater intrusion into coastal management and climate adaptation plans, protecting both water resources and dependent communities and industries.

Further investigation is crucial to forecast and mitigate the impacts of flow reversal, ensuring the long-term sustainability of these vital freshwater resources.

Analysis of Sustainable Development Goals (SDGs) in the Article

1. Which SDGs are addressed or connected to the issues highlighted in the article?

1. SDG 6: Clean Water and Sanitation

   The core of the article revolves around the management and protection of freshwater resources. It details how coastal karst aquifers, which are significant sources of freshwater, are threatened by saltwater intrusion. The study focuses on the Vise spring, which drains the Thau karst aquifer, a resource exploited for drinking water supply. The degradation of this freshwater resource directly impacts water quality and availability, which is central to SDG 6.

2. SDG 13: Climate Action

   The article explicitly links the phenomenon of flow reversal to climate change. It states, “In a changing climate context, flow reversal at submarine karst springs could be more frequent and longer in the future due to sea level rise and a decrease in recharge.” This highlights the vulnerability of coastal water systems to climate-related hazards and underscores the need for adaptation and resilience, which are key components of SDG 13.

3. SDG 14: Life Below Water

   The article notes that submarine groundwater discharge (SGD) plays an “important role for coastal ecosystems.” The Thau Lagoon, where the studied spring is located, “supports an extensive shellfish aquaculture and fishery.” The saltwater intrusion and alteration of the freshwater outflow can negatively impact the health and productivity of these marine and coastal ecosystems, connecting the research to the goals of protecting life below water.

4. SDG 11: Sustainable Cities and Communities

   The consequences of the hydrogeological changes described in the article have a direct impact on human settlements. It is mentioned that the sudden hydraulic head changes “create catastrophic flooding of underground garages and cellars in Balaruc-les-Bains city.” This demonstrates the vulnerability of coastal communities to water-related disasters, a key concern of SDG 11.

5. SDG 8: Decent Work and Economic Growth

   The article points out that the water resources of the Thau hydrosystem are crucial for significant economic activities. It mentions that “Balaruc-les-Bains is the largest thermal bath centre in France, with 55,000 visitors annually” and that the Thau Lagoon supports “extensive shellfish aquaculture and fishery.” The threat of saltwater intrusion jeopardizes these industries, potentially leading to economic losses and impacting livelihoods, which connects the issue to SDG 8.

2. What specific targets under those SDGs can be identified based on the article’s content?

1. SDG 6: Clean Water and Sanitation

   • Target 6.3: By 2030, improve water quality by reducing pollution. The article directly addresses the degradation of water quality through “saltwater intrusion,” where “6.7 Mm3 of saltwater and more than 200,000 tons of salts infiltrated into the aquifer.”
   • Target 6.4: By 2030, substantially increase water-use efficiency and ensure sustainable withdrawals and supply of freshwater. The article highlights periods of “drought” and “low water level in the aquifer,” leading to “water use conflicts… between stakeholders,” which points to issues of water scarcity and the need for sustainable management.
   • Target 6.5: By 2030, implement integrated water resources management. The study of the complex interactions between the aquifer, lagoon, climate, and human activities (drinking water supply, thermal baths) is a foundational step for integrated management of the Thau hydrosystem.
   • Target 6.6: By 2020, protect and restore water-related ecosystems. The focus on the health of the karst aquifer, a critical water-related ecosystem, and the negative impacts of saltwater intrusion aligns with this target.

2. SDG 13: Climate Action

   • Target 13.1: Strengthen resilience and adaptive capacity to climate-related hazards. The article’s conclusion that flow reversals could be “more frequent and longer in the future due to climate change” and its aim to “forecast and mitigate the impacts” directly relates to building resilience against climate-exacerbated hazards.

3. SDG 14: Life Below Water

   • Target 14.2: By 2020, sustainably manage and protect marine and coastal ecosystems. The article’s mention of the Thau Lagoon’s “extensive shellfish aquaculture and fishery” and the importance of SGD for “coastal ecosystems” connects the health of the aquifer to the health of the marine environment.

4. SDG 11: Sustainable Cities and Communities

   • Target 11.5: By 2030, significantly reduce the number of people affected and… direct economic losses… from disasters, including water-related disasters. The article describes “catastrophic flooding of underground garages and cellars in Balaruc-les-Bains city” as a direct consequence of the flow reversal, identifying a specific water-related disaster impacting a community.

5. SDG 8: Decent Work and Economic Growth

   • Target 8.4: Improve resource efficiency and decouple economic growth from environmental degradation. The conflict over water resources for drinking, thermal activities (55,000 visitors), and aquaculture highlights the dependence of local economies on the health of the environment and the need for sustainable resource management to maintain economic activity.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

1. For SDG 6 (Clean Water and Sanitation)

   • Water Quality: The article provides direct measurements of water quality degradation. The Specific Electrical Conductivity (SEC) is used as a proxy for salinity, rising from “lower than 5 mS/cm” to “up to 60 mS/cm” during intrusion. The total infiltrated salt mass (“more than 200,000 tons of salts”) is another clear indicator.
   • Water Stress/Scarcity: The “hydraulic gradient between the aquifer and the lagoon” is a key quantitative indicator of the balance between freshwater availability and pressure from the sea. A low gradient (down to 6.13 m) indicates high stress and triggers the flow reversal. Aquifer water level (hP) is also monitored as a direct measure of freshwater storage.
   • Ecosystem Health: The flow rate (Q) at the spring, which changes from positive (outflow) to negative (inflow), and the total volume of saltwater intrusion (“6.7 Mm3”) are indicators of the health and functioning of the aquifer ecosystem.

2. For SDG 13 (Climate Action)

   • Hazard Characterization: The identification of two specific “tipping points” (TP1 = 6.13 m and TP2 ≈ 10.25 m) and the calculation of the system’s “hysteresis” (HY = 4.12 m) are crucial indicators for understanding and forecasting this climate-related hazard. The duration of the flow reversal event (“15-month-long period”) also serves as an indicator of the severity of the hazard.

3. For SDG 11 (Sustainable Cities and Communities)

   • Disaster Impact: The “hydraulic head rebound” measured in boreholes, with “water level rises ranging from 0.30 to 2.3 m,” is a direct physical indicator of the pressure change that leads to the “catastrophic flooding” in the city of Balaruc-les-Bains.

4. For SDG 14 (Life Below Water) and SDG 8 (Decent Work and Economic Growth)

   • Ecosystem Service Provision: While not directly measured, the article implies that the stability of the freshwater outflow (positive flow rate Q) is an indicator of the aquifer’s ability to support coastal ecosystems and the economic activities (aquaculture, thermal baths) that depend on it. The duration of the flow reversal (“15-month-long period”) is an indicator of the disruption to these services.

4. Summary Table of SDGs, Targets, and Indicators

Source: nature.com