The main goal of this study is to understand and estimate the amount of submarine groundwater discharge into Ringkøbing Fjord from shallow and deep aquifer systems at the Eastern shoreline from Ringkøbing catchment in Western Denmark. In order to accomplish this objective, the study was initiated using an existing large-scale airborne geophysical survey and hydrogeological data from the boreholes in the study area. This data helped in locating zones of groundwater discharge as well estimating complex salinity distribution under the sediment bed along with information about geology under lagoon sediment bed up to Miocene age. However, in order to understand what caused such a complex distribution of salinity so that a better estimate of groundwater discharge can be obtained, it was necessary to model the salinity distribution. Two large-scale hydrogeological conceptual models along with historical evolution of salinity in the lagoon aided in setting up a hydrogeological model to locate and quantify submarine groundwater discharge along with salinity distribution under the sediment bed to locate saltwater – freshwater interface. The results from the simulations showed that most of the groundwater discharge occurred near the shoreline of the lagoon, but also off-shore discharge from deep confined aquifers system occurred at places where confining clay layers are eroded by buried valleys. The simulated fresh groundwater discharge was a non-negligible component, 59 % of recharge on the lagoon and 6 % of river input into the lagoon.
This large-scale study was the motivation to conduct field investigation techniques in order to understand the dynamic processes in the near-shore environment. Field campaigns were conducted every two months in order to understand the seasonal groundwater discharge pattern and brackish water – freshwater interface movement on the same transects. Groundwater discharge distribution showed a non-exponential pattern from shoreline to offshore with a small peak around the shoreline and two larger peaks farther offshore, contrary to existing literature. The salinity distribution
indicated no significant interface movement seasonally but the groundwater discharge showed more temporal changes. The conceptual model constructed from the observed data gave a range from 66 - 388 ld-1 per meter of shore of freshwater discharge in a 20 meters wide fringe. In order to closely observe the dynamics and factors that affect the temporal and spatial distribution of groundwater discharge and brackish water – freshwater interface, small-scale numerical modeling was carried out using the new hydrogeological data obtained from these field campaigns. The salinity data from the lagoon water and freshwater head in the unconfined aquifer were used as transient boundary conditions used in the small scale hydrogeological model, model 1 along with some data on viii hydraulic conductivity. The transient model 1 results showed no significant changes in the brackish water – freshwater interface between the seasons but the groundwater discharge varied considerably being highest during winter and lowest during summer, which was also observed in field investigations. Surficial mixing zone in the discharge zone also showed seasonal changes. However the spatial distribution of simulated groundwater discharge showed an exponentially falling trend from shoreline to offshore in contrast to irregular trend in the observed data. The field investigations
– based conceptual model required a vegetation zone around the shoreline along with wavepumping effect in order to better reason the irregular pattern of the groundwater discharge pattern.
So, in model 1 we tested that vegetation layer as a low permeable zone around the shoreline, now modified to model 2. The location of brackish water – freshwater interface was slightly farther offshore in model 1 compared to the observed data, while model 2 simulated poorer than model 1. However, the irregular groundwater discharge trend in model 2 from shoreline to offshore matched very well with the observed, one small peak around the shoreline and one large peak farther offshore. Eventually, model 2 proved to be a better choice for this study since the range of total groundwater discharge along the shoreline matched very well with the observed data, 25 – 591 ld-1 in contrast to 75 – 7521 ld-1 from model 1. Eventually the study suggests that groundwater discharge is an important component in our study area and thus cannot be ignored.