Site selection was based on the results of sediment pore-water salinity survey (Figure 1). The survey included salinity measurements (at two depths: 5 cm and 25 cm GSI-IX in the sediment) along parallel transects, spaced 10 m from each other, that extended seawards from the beach for some 50 m. Seawater depths ranged from 0.5 to 2 m in accordance with distance from the shore. The sediment pore water salinity surveys of the study area were performed before each of the sampling campaigns to confirm the sampling point locations. Seepage meters and groundwater

lances were installed at the selected points. Seepage rates were measured by means of seepage meters applying the end member approach (Szymczycha

et al. 2012). In short, seepage water flowing through the sediment displaces water trapped in the chamber forcing it up through the port into the PTE bag. The change in volume of water in the bag, over a measured time interval, provides the seepage water flux. The measured salinity of the samples varied from 3.7 to 6.5 (Szymczycha et al. 2012). The groundwater fraction in the samples was calculated using the end-member method (Burnett et al. 2006, Szczepańska et al. 2012), and finally the groundwater flux was calculated as the ratio of the collected groundwater volume to the device’s surface area and to time. Groundwater lances, described by Beck et al. (2007), selleck compound were used to collect pore water samples for salinity and carbon analysis. 24 h after the device had been inserted into the sediment, 35 mL of pore water were collected from several depths (0, 4, 8, 12, 16, 24, 30 cm) below the sediment-water interface (Szymczycha et al. 2012). Two groundwater lances (groundwater lance I – GLI and groundwater lance II – GLII) were used to collect samples at two groundwater seepage locations simultaneously. For comparison, a groundwater lance (groundwater lance G′) and a seepage meter were additionally deployed in an area without apparent

impact of groundwater seepage. The properties of the groundwater samples, including salinity, pH and temperature, were measured with a multimeter (WTW Multi 3400i Multi-Parameter Field Meters) with accuracies of 0.02 PSU and 0.1 °C. Several types of water samples were Sulfite dehydrogenase collected at the sampling points. These included seawater (above the seafloor; salinity ≥ 7.0) and sediment pore water (interstitial water; salinity 0.1–6.9). Sediment pore water samples of salinity ≤ 0.5 were assumed to represent groundwater, while pore water samples with salinities from 0.6 to 6.9 were assumed to be mixtures of seawater and groundwater. Since the salinities of the collected sediment pore water samples were characterised by salinity larger than these typical of groundwater, the groundwater contribution to the collected samples was calculated using the end member approach (Szymczycha et al. 2012).