We investigated submarine groundwater discharge in the Eckernfoerde Bay, located in the western Baltic Sea (Schlüter et al., 2004).

The Bay has a length of approximately 19 km and a width of 9 km at its mouth. The maximum water depths are 26 to 28 m. In the eastern part of the bay, the basin is divided into two elongated troughs by the Mittelgrund, a moraine remnant, where water depths are less than 10 m (Jensen et al., 2002; Whiticar, 2002).

Pockmarks, elongated depressions approximately 1 to 3 m below adjacent seafloor levels, are located in water depths of 24 to 26 m around the Mittelgrund and along the southern shore.

High organic carbon contents of 4% and occurrences of free gas are reported for the muddy sediments (Wever et al., 1998; Whiticar, 2002). In this part of the Baltic Sea, the tides are a few centimeters at the most and changes in water level are generally wind driven.

During several cruises by research vessels as Littorina, Alkor or Alexander von Humboldt the water column and sediments of the Eckerfoerde Bay were sampled.

By the Rumohr Lot surface sediments were recovered for pore water and sediment analysis. A vibrocorer system (IOW, Warnemünde) was applied to recover sediment cores of up to 6 m length. A temporary well was mounted within the vibro-corer system  (Sauter et al., 200*), which allowed sampling of groundwater from the sub-seafloor aquifer.

By shallow seismic investigations (GEUSS, DK) sub-seafloor strata were identified. In combination with sediment analysis this allowed to characterize the sub-seafloor aquifer.

The pore water concentration of Cl –an inert tracer suitable for computation of fluid flow- were analyzed by Mohr–Knudsen titration (Grasshoff et al., 1983). Major elements, such as Na, K, Mg, Ca, were determined by ICP-AES (inductively coupled plasma atomic emission spectroscopy; Jobin-Yvon JY 170) for several pore water profiles.

A standard headspace technique (Albert et al., 1998) was used to determine methane, which was measured on a Shimatzu gas chromatograph equipped with a flame ionization detector.

²²²Radon (half-life 3.8 d) and ²²⁶Radium (half-life 1,600 yr) were analyzed in water column and sediment samples. ²²²Rn is the product of 226Ra decay in sediments and seawater.

In seawater ²²⁶Ra activity is generally low, whereas considerably higher levels of ²²⁶Ra activity are found in the mineral matrix of soils and sediments. Groundwater samples and fluids escaping from the seafloor are therefore enriched in ²²²Rn as compared to seawater. These natural radio nuclides are applied to trace fluid discharge from the seafloor.

Detailed information concerning the shape and spatial alignment of pockmark locations was obtained during dives with the autonomous underwater vehicle (AUV) M600 (A/ S Maridan). The M600 was equipped with a dual channel Klein 2000 side-scan sonar system (100 kHz and 500 kHz), which provides a horizontal resolution of 15 cm, a Geo-Acoustics Chirp for subbottom profiling, an (Sonar Research & Development Ltd. (SRD) seabed visualization system, and a Seabird conductivity, temperature, depth (CTD) recorder.

During eight dives, each lasting as long as 7 h, the M600 operated at 3 or 12 m above seafloor and surveyed regions formerly identified by pore water studies as discharge areas (including pockmarks and nonpockmark regions).

Pore Water
Sediment sampling provided site-specific information concerning fluid discharge and its effect on benthic methane and nutrient cycles. Pore water profiles for Cl provided information about fluid transport in sediments. The measured profiles could be grouped into three categories with respect to shape and asymptotic concentration at depth. Sites unaffected by freshwater were characterized by nearly constant Cl concentrations from the bottom water to sediment depths of more than 40 cm. The second category was characterized by a nearly linear decrease in Cl concentrations with depth. As supported by our pore water modeling, this suggests admixture by freshwater due to molecular diffusion with negligible fluid advection. The third category had low Cl concentrations at depth with profile curvature close to the sediment–water interface indicating active flow of freshwater.

At several sites, the intense admixture of freshwater resulted in Cl concentrations of <30 mmol/L (10% of bottom-water concentration) at sediment depths of 10–20 cm. In addition to chloride concentrations, major ions, such as K, Mg, and Na, were also affected by freshwater admixture.

The Levenberg–Marquardt method was applied to derive an optimal fit of calculated to measured data for modeling Cl pore water profiles to compute fluid flow through the sediment. High flow rates of  >5 L/m2 d were observed only at a few sites. The average flow rate was 0.54 L/m2 d. For Cl profiles that had an essentially linear decrease with depth, modeling showed that molecular diffusion was the dominant transport process, whereas fluid advection was negligible.

Methane concentrations were measured in water column and sediment samples. In the water column, CH₄ concentrations were generally between 13 and 700 nmol/L. At some sites high concentrations (e.g., up to 1,900 nmol/L CH₄) were measured close to the seafloor. This suggests the release of CH₄ associated with seepage of fluids from the seafloor.

High CH₄ concentrations were observed at several sites in the organic rich sediments of the bay. Pore water

concentrations of methane are often close to CH₄ saturation. Free gas was detected in sediments (Wever et al., 1998; Whiticar, 2002), and ebullition of gas bubbles from the seafloor was occasionally observed by our towed video systems. Nevertheless, low CH₄ concentration was measured at some sites, although high organic carbon contents were favoring formation of CH₄. Flushing of CH₄ from sediments by fluid flow was suggested as a mechanism causing low concentrations (Albert et al., 1998; Bussmann et al., 1998; Whiticar, 2002). Even though some of our pore water profiles support this assumption, fluid flow of freshwater from below did not always cause low CH₄ concentrations. For example, at pockmark sites, very high as well as low concentrations of Cl and CH₄ were observed in close proximity, suggesting strong horizontal and lateral gradients.

Within the entire bay (~70 km²), pore water modeling suggests a submarine fluid discharge of 4 x 10⁶ to 57 x 10⁶ m3/yr. The budget based on ²²²Rn inventories of the water column gave an estimate of 37x 10⁶ m3/yr to 337 x 10⁶ m3/y. Although both attempts rely on differing techniques and constraints (e.g., the pore water profiles assumed steady state conditions and the ²²²Rn estimates ignore molecular diffusion of radon from the seafloor) the budgets were within similar ranges. As a conservative estimate for fluid flow from the seafloor we suggest 4 x 10⁶ to 57x 10⁶ m3/yr, equivalent to 0.3–4.1% of the entire water volume of the bay.

The highest discharge rates were derived for pockmark sites, suggesting a coupling of pockmark formation and fluid flow. One mechanism for the formation of circular depressions in lake and river deposits is the fluidization of sediments caused by fluid flow from below, forming ‘‘sand boils’’ (Guhman and Pederson, 1992). For muddy sediments in Eckernfoerde Bay, the interaction of fluid flow and bottom currents is suggested to be a mechanism for the formation and preservation of pockmarks. Fluid flow through sediments decreases the shear strength of the sediment matrix, and periodic strong bottom currents, including episodic storm surges, cause preferential erosion of these areas when compared to adjacent seafloor, which in turn preserves sharp edges and the considerable slope within the pockmarks observed by AUV and towed video systems.

Fluid flow was also obvious for sites at which no distinct morphological features were observed at the seafloor. The considerable small-scale variability of Cl and CH₄ at pockmarks suggests an intensive interaction of transport processes with methane production and consumption kinetics. Changes in the forcing factors of fluid flow such as hydraulic head (here, e.g., the water level height in Eckernfoerde Bay) cause lower/higher flow velocities and different flow paths. This permits diffusion of methane from sites of higher content to regions formerly depleted due to fluid flow, underlining the need for improved sampling techniques to record small-scale variation at discharge sites.

Ecological aspects associated with fluid discharge, although not at the center of this study, seem to be likely. The large area affected by freshwater (~22% of the entire bay) might affect benthic habitats, and nutrient release from sediments to the water column will be enhanced due to advective transport. The geological setting of glacial coastal sediments covered by organic rich mud deposits observed for Eckernfoerde Bay may be representative for other coastal zones of the Baltic Sea where submarine groundwater discharge from subseafloor aquifers could be of similar relevance.