Measurements of pore water variations and the influence of groundwater discarge on tidal flats near Sahlenburg

The tidal flat near Sahlenburg is influenced by a confined aquifer. Leakage areas in the aquitard causes an outflow of groundwater (Submarine Groundwater Discarge(SGD)). This is well detectable due to the reduced chloride concentrations in pore water. At the sediment surface the SGD creates so called "sand boils" and "pancake structures"."Sand boils" are focused groundwater outlets with diameters in the centimeter to decimeter range. During the low tide the flow rates are between 0.04 L/min and 1.8 L/min."Pancake structures" are morphologically distinctive sedimentary structures with diameters of up to several meters formed by rising ground water. They show relatively sharp horizontal boundaries in porewater chemistry.Measurements showed that the flow rates vary greatly and are influenced by the tides. Measurements of the pressure in the aquifer show variations of up to 50%. SGD can be confirmed in the whole area by using chloride as inert tracer. Advective and diffusive transport processes are visible. These processes works into and out of the sediment. The determining factors are: convection, hydraulic gradient, tidal and wave-pump processes. Based on the concentration profiles transport modeling is performed. This resulted in diffusive and advective flow through the sediment from 0.01 to 3 L/(m^2*day).The aquifer horizons are different in their properties and components. Different concentrations of chloride, silica, oxygen and nitrate and temperatures were measuered . These indicates recirulation of water and varying pathways.The methane concentrations range from values below the detection limit and 500 mikromol/L. The groundwater itself could not be recognized as a source of methane. The complex structure of the sediment formed methane traps and accumulate great concentrations.