Response of a fresh-brackish groundwater system to hydrological management in and around the Naardermeer wetland, the Netherlands
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Biodiversity decline as an environmental concern has been introduced to the world relatively recently but it has quickly become one of the main environmental challenges of this millennium. In the Netherlands fen meadows, a type of wetland, represent an important rare ecosystem, but the last few centuries its unique vegetation composition has been threatened by human activities such as land reclamation and groundwater extraction. Species rich fens are frequently located at edges of river plains fed by nutrient poor, alkaline groundwater at the base of sandy glacial ridges via regional groundwater systems. Groundwater flow patterns in the Netherlands can be complex due to century-long human intervention. Adding to this complexity is the presence of brackish groundwater in Dutch aquifers. In coastal areas intrusion of seawater has been identified as a major influence on groundwater composition. In the Netherlands this influence is intensified by brackish groundwater also being present further inland. This paper aims to provide insight into the response of regional groundwater flow to hydrological management with specific focus on the behavior of fresh-brackish groundwater within the system. In this paper the Naardermeer nature reserve in the Gooi- and Vechtstreek is used as a case study. In the past few decades hydrological management there has been adjusted with the goal to restore and conserve nutrient poor, alkaline groundwater flow to the Naardermeer wetland. In order to determine the response of the regional, fresh-brackish groundwater system of the Naardermeer, seven possible hydrological management scenarios were tested using a 2-dimensional, density dependent groundwater model based on the computer code MOCDENS3D. The code combines the MODFLOW module, adjusted to density dependent groundwater flow, and the MOC3D module respectively describing groundwater and particle transport. The model shows that extraction of groundwater in the hill ridge, upstream of the nature reserve, mostly affects fresh-brackish groundwater distribution in the discharge area nearest to the hill ridge and does not have as much effect on areas further downstream in the valley. Regional hydrological management also needs to consider that adjusting controlled surface water levels in downstream polders directly adjoining as well as more distant to the polder of interest affects the brackish groundwater distribution underneath the polder of interest. Furthermore, hydrological management aimed at improving quality or quantity of groundwater supplied to vegetation in one polder might have adverse effects on groundwater conditions in adjoining polders. Overall, based on this research it can be concluded that the presence of brackish water in deep aquifers adds another dimension to the complexity of groundwater systems, and that behavior of brackish groundwater needs to be considered by authorities when deciding hydrological management strategies aimed at preserving or restoring nature.