Marine and terrestrial influence on submarine groundwater discharge in coastal waters connected to a peatland
by Miriam Ibenthal
Date of Examination:2020-03-10
Date of issue:2020-06-03
Advisor:Prof. Dr. Thomas Ptak
Referee:Prof. Dr. Thomas Ptak
Referee:Prof. Dr. Gudrun Massmann
Persistent Address:
http://dx.doi.org/10.53846/goediss-8007
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Abstract
English
In low-lying coastal fens, groundwater flow direction, water level and input of salts with seawater determine vegetation communities and greenhouse gas emissions, while the export of nutrients and carbon with submarine groundwater discharge influences biogeochemical processes in the shallow coastal sediments. Anthropogenic interferences like drainage and diking have strong impacts on water flow. This thesis aimed at an understanding of the hydro(geo)logical system in a rewetted coastal fen typical for the southern Baltic Sea, and its changes from the pristine state over phases of moderate and intensive drainage towards rewetting with implications for biogeochemical processes. The drivers and effects of short-term (weather-induced) and long-term (hydrological interferences) processes were differentiated and their importance for flow and transport identified. Groundwater observation wells were installed both in the peat and underlying sand, and continuous readings of water level and electrical conductivity were combined with groundwater dating and analysis of the water composition (major ions, nutrients and carbon). Based on sediment cores, grain size analysis and hydraulic slug tests, a 3D numerical groundwater flow model was developed to investigate varying short-term and long-term groundwater flow patterns. Combined with analysed water composition, the consequences for transport and biogeochemistry in the peatland and at the coastal interface were estimated. On the landside, the peat thickness, depth of ditches and their resulting hydraulic connectivity to the underlying aquifer, as well as the geometry of the aquifer, defined discharge zones of upwelling groundwater from the larger catchment as well as recharge zones. Long-term moderate and intensive drainage led to enhanced upwelling from an ancient glacial river valley with possible effects for local biogeochemistry. The hydraulic state of the peatland and the confining outcropping peat defined the hydraulic gradient towards the sea with consequences for origin, location and amount of submarine groundwater discharge (SGD). SGD consisted mainly of recirculating seawater, followed by fresh SGD from a dune dike, and of nutrient-enriched water from the aquifer below the outcropping peat. A transient simulation identified short-term vertical flow reversals in the peat on land and offshore with implications for dispersive mixing of different water sources. Flow was driven by variations in evapotranspiration, recharge and surface water discharge on the landside. Offshore, hydraulic head differences between the peatland and the Baltic Sea defined the flow direction. In the long-term, the dominant flow direction, and hence the water source, depends on the hydrological state of the peatland and the mean sea level. The results suggest that both long-term and short-term drivers for groundwater flow must be considered to explain biogeochemical patterns. The knowledge of the anthropogenic history, peat degradation, and aquifer geometry – especially in heterogeneous glacial deposits – is crucial to estimate the influence of different water sources on ecosystem functioning both on land- and seaside.
Keywords: Submarine groundwater discharge; Coastal peatland; Baltic Sea; Groundwater modeling