SUBMARINE GROUNDWATER DISCHARGE DISTRIBUTION AND SALINE WEDGE LOCATION: A FIELD CASE STUDY (RINGKOBING FJORD, DENMARK)

The quantification of the submarine groundwater discharge from aquifers to coastal areas is a common objective in hydrology and coastal studies due to the implications over the water budget components of catchments as well as the contaminant and chemical processes that can trigger when fresh groundwater enters into the surficial saline water. The presence of the saltwater wedge in coastal aquifers limits the seaward extent of freshwater discharge and introduces a more complex flow pattern. In this study, the impact of the saline wedge on submarine groundwater discharge has been analyzed based on field data collected during four different seasons (March, May, August and October) in a one year period.

Submarine groundwater discharge was measured in a lagoon connected with the sea with a sluice that prevented tidal oscillations in the West coast of Denmark. More than 150 seepage meter measurements were located along 4 transects perpendicular to the shore in a 500 m shore stretch. In each transect, the distance between the seepage meters ranged from 1-5 m for characterizing the distribution pattern of discharge. The salinity distribution within the saline wedge was measured simultaneously with the seepage data collection. This required 1300 groundwater samples through hammering small diameter pipes with a 2 cm length screen along vertical profiles every 25 cm until reaching 3 m depth.

The discharge magnitudes were compared with the location of the saline wedge to assess the connection between both beyond the natural variability of the seepage meter measurements. An increase of the seepage rate can be observed in the proximity of the intersection of the saltwater-freshwater interface with the lagoon bed. Also, the movement landwards or seawards of the interface due to the seasonality displaces this pattern in the same direction. These results show the potential impact of variable density flow processes on groundwater discharge patterns in the coastal zone.