As part of the KARST project we investigated the groundwater flow from land to sea, the spatial distribution of SGD sites and its effects on the benthic habitats at four different study sites. We integrated a wide range of datasets and concepts from different disciplines (e.g. geomorphology, remote sensing, geophysics, biology and geostatistics) to assess the spatial variability of SGD and the movement of groundwater into the ocean. Our interdisciplinary methodology makes use of the most appropriate measurement techniques for individual water-depths and has the potential to be applied in many locations worldwide where it will not only be crucial for karstic environments but also for other geological areas where SGD occurs from point sources (e.g. volcanic islands). As freshwater becomes increasingly depleted in coastal areas and offshore freshened groundwater a more valued resource, our newly acquired data and methodologic approach provides additional monitoring options and a greater understanding of SGD to assist proper management of this potential future resource.
The four study sites investigated during the project were Veracruz (Mexico), Antalya (Türkiye), Eckernförde (Germany), and Wellington (New Zealand).
Offshore Mexico, the RV Justo Sierra undertook a voyage between the 1st and 12th of February 2023 to explore SGD in the shelf offshore Veracruz and to determine the effects of the SGD on the local environments and habitats. The data shows a previously unknown large offshore extension of a shallow coral reef system. It appeats that this reef system is closely associated to submarine springs which were monitored with deployed mini CTDs. The morphological analysis additionally shows pockmark features that suggest fluid flow from the seabed.
Offshore and onshore Antalya (Türkiye) we acquired an extensive geophysical dataset. Here we employed an innovative integration of offshore and onshore electrical resistivity and hydroacoustic techniques. This dataset allows us to investigate the dynamic interactions between fresh and salty groundwater across the land-ocean boundary. Together with our Turkish colleagues we were able to identify and characterise submarine springs in the bay, find areas where offshore groundwater is recharged from land and also identify regions where saltwater is intruding into the coastal aquifer on land.
In Eckernförde Bay we used satellite remote sensing techniques in combination with onshore electromagnetic induction data to assess the influence of SGD on local seagrass meadows. Using the electromagnetic induction data from the coast, we were able to identify regions of high and low SGD levels along the shoreline. Additionally, multibeam bathymetric data was used to identify pockmarks indicative of SGD offshore. The onshore EMI data and the offshore identification of pockmarks were remarkably good. We are therefore convinced, that we can determine areas of enhanced groundwater outflow into the Baltic Sea. The distribution of seagrass was previously assessed via satellite images and diver transects. We included detections of seagrass from existing multibeam bathymetric data. We could not find any correlation between the distribution of seagrass and the volume of SGD along the coastline.
Offshore Wellington (New Zealand), a well-defined onshore aquifer system extends beneath the harbour, where substantial amounts of freshwater seep out from the ocean floor. Here recently the first attempts worldwide have been made to use the offshore groundwater as a future source of drinking water. HWe observed several hundred seafloor depressions (pockmarks) that we attribute to continuous seepage of gas and groundwater from the seafloor. Enhanced hydroacoustic reflections in the water column within and above the pockmarks indicate suspended sediment particles, which are likely kept in suspension by discharging groundwater and density boundaries.
