Coastal Groundwater Discharge with Electrical Resistivity Mapping

Unraveling the Coastal Groundwater Puzzle

Coastal regions are complex and dynamic environments where groundwater and seawater interact in intricate ways. One key process in this land-ocean interface is submarine groundwater discharge (SGD) – the flow of fresh or brackish groundwater into the ocean. SGD is a vital pathway for the transport of nutrients, metals, and other dissolved compounds from land to the coastal marine environment, with significant implications for coastal ecosystems and ecosystem services.

Accurately assessing SGD, however, is a challenging task due to the inaccessibility of coastal aquifers, dynamic environmental conditions, and the need for long-term monitoring to capture spatial and temporal variations. Traditional methods, such as salinity measurements and radium isotope tracing, provide valuable insights, but they are often limited in their ability to delineate the full extent and dynamics of SGD.

Mapping Coastal Groundwater Discharge with Electrical Resistivity

In this study, researchers employed a cutting-edge geophysical technique called marine continuous resistivity profiling (MCRP) to overcome these challenges and gain a more comprehensive understanding of SGD in the coastal region of Maresme, Spain. MCRP involves towing a network of electrodes behind a boat, which allows the researchers to measure the electrical resistivity of the seafloor and subsurface sediments.

The key principle behind this approach is that the electrical resistivity of marine sediments is closely linked to their salinity content. Sediments saturated with freshwater have higher resistivity values compared to those saturated with seawater, which has a much lower resistivity. By mapping the spatial and temporal variations in seafloor resistivity, the researchers were able to identify areas of increased freshwater discharge and track seasonal changes in SGD patterns.

Fig. 2

Revealing Seasonal and Spatial Variations in Groundwater Discharge

The researchers conducted two field campaigns in the Maresme region – one during the dry season (June 2020) and another during the wet season (October 2020) – to capture the seasonal dynamics of SGD. The MCRP data revealed several key findings:

1. Spatial Variations in SGD: The electrical resistivity models showed higher resistivity values closer to the coastline, indicating the presence of freshwater discharge. These higher resistivity zones were more prominent in the northern part of the study area, which corresponds to the location of a larger ephemeral stream (the Argentona stream). This suggests that the geological and hydrogeological heterogeneity of the coastal aquifer system plays a significant role in the spatial distribution of SGD.

2. Seasonal Changes in SGD: Comparing the resistivity models from the dry and wet seasons, the researchers observed a notable increase in the extent and magnitude of higher resistivity zones during the wet season. This is consistent with the observed rise in piezometric levels (groundwater levels) in the coastal aquifer, indicating that increased rainfall and recharge lead to a higher discharge of freshwater into the ocean.

Fig. 3

Integrating Multiple Lines of Evidence

To validate and complement the MCRP findings, the researchers also collected additional data, including:

– Salinity Profiles: Measurements of seawater salinity along the seafloor showed a decrease in salinity closer to the coast, consistent with the presence of freshwater discharge.
– Radium Isotopes: Analyses of radium isotopes in seawater samples also indicated higher SGD rates during the wet season, corroborating the MCRP results.
– Piezometric Levels: Monitoring of groundwater levels in a nearby piezometer (well) confirmed the seasonal fluctuations in aquifer recharge and the resulting impact on SGD.

This multi-faceted approach, combining geophysical, geochemical, and hydrogeological data, provides a comprehensive understanding of the complex coastal groundwater dynamics and the factors driving seasonal variations in SGD.

Fig. 4

Implications and Future Directions

The findings of this study have several important implications:

1. Coastal Hydrogeology and Ecosystem Dynamics: By mapping the spatial and temporal patterns of SGD, this research enhances our understanding of coastal hydrogeology and the complex interactions between groundwater and the marine environment. This knowledge is crucial for assessing the impacts of SGD on coastal ecosystems and their ecosystem services.

2. Nutrient and Pollutant Cycling: SGD can be a significant pathway for the transport of nutrients, metals, and other dissolved compounds from land to the ocean, with implications for marine ecology and water quality. The seasonal variations in SGD revealed by this study highlight the need to consider temporal dynamics when studying these processes.

3. Groundwater Resource Management: The identification of preferential SGD zones can inform the sustainable management of coastal groundwater resources, particularly in areas where surface water resources are limited.

Going forward, the researchers suggest that the integration of MCRP with other techniques, such as seismic refraction and electromagnetic induction, could provide even more detailed insights into the complex hydrogeological structure and dynamics of coastal aquifers. Additionally, long-term monitoring of SGD patterns could help researchers better understand the impacts of climate change and human activities on these critical land-ocean interfaces.

Author credit: This article is based on research by Jose Tur-Piedra, Juanjo Ledo, Marc Diego-Feliu, Pilar Queralt, Alex Marcuello, Valentí Rodellas, Albert Folch.

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