Sulfate reduction and methanogenesis at a freshwater: Seawater interface in a shallow aquifer, Skansehage Denmark

The freshwater-seawater interface was studied in a ~9-m thick anaerobic aquifer located in marine sand and gravel with thin peat lenses. Very limited amounts of iron-oxides are present. Consequently, the dominating redox processes are sulfate reduction and methanogenesis, and the groundwater is enriched in dissolved sulfide, methane and bicarbonate. Under normal conditions the seawater-freshwater interface is found at a depth of 4 m at the coastline and reaches the bottom of the aquifer 40 m inland. However, occasional flooding of the area occurs, introducing sulfate to the aquifer. Groundwater chemistry was studied in a 120 m transect perpendicular to the coast. Cores were taken for radiotracer rate measurements of sulfate reduction and methanogenesis. In the saline part of the aquifer 35 m inland, sulfate reduction was the dominant process with rates of 0.1-10 mM/year. In the freshwater part 100 m inland, the sulfate was depleted and methanogenesis consequently the dominating process with rates of 0.1-0.7 mM/year, with the bicarbonate pathway dominating over acetate fermentation. H2 concentrations were measured under natural flow conditions using a newly developed sampling device. Concentrations between 0.5 and 46 nM were found with values around 2 nM dominating. The peak values correspond to a transition state from sulfate reducing conditions towards methanogenesis. During the transition, the fermentation step continues to produce H2, which is not consumed, as sulfate is limited, and methanogenesis has not yet taken over.