Final Report Summary - SGDBALTIC (The importance of submarine groundwater discharge for the southwestern Baltic Sea)
Submarine groundwater discharge (SGD) is a common water transport route between aquifers on land and the coastal ocean. It occurs everywhere where the hydraulic gradient on land is above sea level and permeable paths allow the subsurface flow of water to the sea. As the dissolved material concentrations can be high, SGD may be an important transport mechanism for solutes impacting the coastal environment by supplying e.g. nutrients and pollutants to coastal waters.
The environmental status of the Baltic Sea is considered to be poor due to excess nutrient supply. Rivers and the atmosphere have been identified as the major sources of nutrients. However, SGD is not taken into account in the nutrient balances of the Baltic Sea. The project "Submarine groundwater discharge in the Southwestern Baltic Sea” investigates occurrences of SGD and their nutrient loads. The project concentrated its studies on the Eckernförde Bay where numerous SGD locations were identified. Based on isotopic (radon, radium isotopes, 18O, 15N), seepage meter and piezometer measurements, SGD fluxes ranging between 0.5 – 173 cm/d were determined resulting in a total SGD input to the waters of the Eckernförde Bay of ~ 0.5 – 20 x 10^6 m^3/year. SGD is mainly composed of saltwater (85% of the total SGD) with a minor contribution of terrestrial-derived freshwater SGD (~15 % of the total SGD). The nutrient concentrations of the discharging waters were determined and average concentrations of 60 μmol/L silicon (14-105 μmol/L), 1.7 μmol/L phosphate (0.1-3.8 μmol/L) and 30.4 μmol/L total nitrogen (1.5-90.6 μmol/L) were observed. The yearly SGD-born nitrogen and phosphate fluxes to the Eckernförde Bay were estimated assuming that SGD occurs along 50% of the coastline of the Eckernförde Bay; and these computed fluxes were compared to the total water-born nitrogen and phosphorous fluxes into the German Baltic Sea (nitrogen: 16,900 tons N/year; phosphorous: 425 tons P/year). SGD-born nitrogen from the Eckernförde Bay contributes ~ 17% to the total nitrogen and ~ 22% to the total phosphorous fluxes entering the German Baltic Sea. However, published total water-born loads do not explicitly include SGD as a source. Therefore, the total water-borne nutrient fluxes into the Baltic Sea are probably underestimated. Our analyses suggest nitrogen to derive from terrestrial ground waters, which are high in nutrients due to fertilizer used in agriculture. Reducing the use of fertilizer and/or removing nutrients from groundwater would be possible options to mitigate SGD-born nutrient fluxes to the western Baltic Sea. As the waters associated with SGD have tritium/helium ages between 10 and 30 years a fast response to the reduction of nutrients is unlikely.
The environmental status of the Baltic Sea is considered to be poor due to excess nutrient supply. Rivers and the atmosphere have been identified as the major sources of nutrients. However, SGD is not taken into account in the nutrient balances of the Baltic Sea. The project "Submarine groundwater discharge in the Southwestern Baltic Sea” investigates occurrences of SGD and their nutrient loads. The project concentrated its studies on the Eckernförde Bay where numerous SGD locations were identified. Based on isotopic (radon, radium isotopes, 18O, 15N), seepage meter and piezometer measurements, SGD fluxes ranging between 0.5 – 173 cm/d were determined resulting in a total SGD input to the waters of the Eckernförde Bay of ~ 0.5 – 20 x 10^6 m^3/year. SGD is mainly composed of saltwater (85% of the total SGD) with a minor contribution of terrestrial-derived freshwater SGD (~15 % of the total SGD). The nutrient concentrations of the discharging waters were determined and average concentrations of 60 μmol/L silicon (14-105 μmol/L), 1.7 μmol/L phosphate (0.1-3.8 μmol/L) and 30.4 μmol/L total nitrogen (1.5-90.6 μmol/L) were observed. The yearly SGD-born nitrogen and phosphate fluxes to the Eckernförde Bay were estimated assuming that SGD occurs along 50% of the coastline of the Eckernförde Bay; and these computed fluxes were compared to the total water-born nitrogen and phosphorous fluxes into the German Baltic Sea (nitrogen: 16,900 tons N/year; phosphorous: 425 tons P/year). SGD-born nitrogen from the Eckernförde Bay contributes ~ 17% to the total nitrogen and ~ 22% to the total phosphorous fluxes entering the German Baltic Sea. However, published total water-born loads do not explicitly include SGD as a source. Therefore, the total water-borne nutrient fluxes into the Baltic Sea are probably underestimated. Our analyses suggest nitrogen to derive from terrestrial ground waters, which are high in nutrients due to fertilizer used in agriculture. Reducing the use of fertilizer and/or removing nutrients from groundwater would be possible options to mitigate SGD-born nutrient fluxes to the western Baltic Sea. As the waters associated with SGD have tritium/helium ages between 10 and 30 years a fast response to the reduction of nutrients is unlikely.