Deep submarine groundwater discharge indicated by pore water chloride anomalies in the Gulf

INTRODUCTION Submarine groundwater discharge (SGD) is a significant pathway for material transport to the coastal zone (Burnett et al. 2006). For some elements and isotopes SGD has been thought to be the principal source (Lin et al. 2010). Therefore, the interest in coastal groundwater flow systems has increased rapidly during the last decades. Most of the studies have been focused on shallow (<20m), narrow zone (<5km) along the coastline (Lin et al. 2010). Interestingly, some deep seafloor studies (Wilson 2005; Lin et al. 2010) indicated that SGD can occur a long distance from the shoreline (~25km). In the Baltic Sea SGD has been mainly investigated in the southern part primarily at coastal zones demonstrating that groundwater seepage is comparable to river loads in case of selected chemical substances (Piekarekthis study we identified deep SGD located at acoustically turbid sediments in the Gulf of


INTRODUCTION
Submarine groundwater discharge (SGD) is a significant pathway for material transport to the coastal zone (Burnett et al. 2006). For some elements and isotopes SGD has been thought to be the principal source (Lin et al. 2010). Therefore, the interest in coastal groundwater flow systems has increased rapidly during the last decades. Most of the studies have been focused on shallow (<20m), narrow zone (<5km) along the coastline (Lin et al. 2010). Interestingly, some deep seafloor studies (Wilson 2005;Lin et al. 2010) indicated that SGD can occur a long distance from the shoreline (~25km). In the Baltic Sea SGD has been mainly investigated in the southern part primarily at coastal zones demonstrating that groundwater seepage is comparable to river loads in case of selected chemical substances (Piekarekthis study we identified deep SGD located at acoustically turbid sediments in the Gulf of sediment biogeochemical processes implications for the Baltic Sea environment, such as eutrophication, hypoxic and anoxic events, we aim to characterize the potential role of deep SGD in the Baltic Sea cycles of elements.

METHODS
water and sediments samples were collected on board the R/V Oceania during three cruises in May 2015, January 2017 and May 2017. Seawater salinity and temperature was retrieved from CTD files while sediment cores were collected by the Gemax gravity corer. Additionally acoustic observations of the sediments were made in order to detect gas distribution. In collected sediment samples water content, calcium (Ca), total organic (TOC) and inorganic carbon (IC) were analyze while in collected pore water samples nutrients (PO 4 3-, NO 3 -+NO 2 -, NH 4 + ), dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), metals (Na, K, Mg, Ca, Al, Mn, Fe, Ni, Cr, Cu, Cd, Co, Pb) and alkalinity were analyzed. Parameters such as ORP, pH and salinity were measured in situ. The SGD rate was based on numerical modelling. Figure 2. Examples of pore water depth profiles for chloride (hollow symbols) and salinity (solid symbols) in the study area.

RESULTS AND DISCUSSION
Seawater salinity, temperature and density were typical of the seasons when the samples were taken. The pore water depth profiles for both salinity and chloride significantly decreased with depth during every sampling campaign indicating freshwater source. The exemplary profiles are presented at Figure 2. The pore water concentrations of the main cations such as Na, Ca, Mg, K showed similar trend to chloride. Generally, pore water profiles for Cl, Na, Ca, Mg and, K, unaffected by freshwater, are constant or increase ture profiles The general pore water trend of DIC, DOC, trace elements, PO 4 3-, NH 4 + , TDS and alkalinity are comparable to those observed in deep sea anaerobic sediments. Interestingly, in the deepest layers of pore water increased concentrations of Mn, Fe, Al, PO 4 3-, NH 4 + , DIC, DOC and alkalinity were observed most probably due to groundwater seepage.
The calculated SGD rates ranged from 0.3 to 0.7 (L m -2 d -1 ). Comparable results were groundwater seepage, increased methane production and consequently methane release from dy area the sediments acoustic disturbance has been previously correlated with methane presence (Brodecka et al. 2013) accompanied by methane and increased P, Si and DOC fluxes (Donis et al. 2017).

CONCLUSIONS
In this study we identified deep SGD located at acoustically turbid sediments in the Gulf of efflux of chemical substances such as PO 4 3-, NH 4 + , DIC, DOC, trace elements (Mn, Fe, Al) and possibly methane. Therefore, SGD may significantly change their distribution both within the sediments and at the water-sediment interface.

ACKNOWLEDGMENTS
The results were obtained within the framework of the following projects: 2016/21/B/ST10/01213 sponsored by National Science Center and WaterPUCK financed by the National Centre for Research and Development (NCBR) within BIOSTRATEG program.