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Real time monitoring of groundwater flux creates insight in the complex water system of Romboutswerve Polder in Damme, Belgium

A pilot real time monitoring network is installed in the Romboutswerve polder which is a protected nature reserve of 140 hectares as a habitat of bird species and of international importance for wintering of migratory birds. The dewatering of the peaty soil caused the land to subside. More elevated zones are the result of a more sandy soil, less inclined due to the dewatering. The lower areas are used as grassland and elevated areas as farmland. To the south and the ouest of the polder are the cities of Damme and Bruges. The area is partly bounded by canals and crossed by a multitude of canals and ditches that either drain or feed the area.

The waterways are both of marine origin and man-made. The area is threatened by drought. Although after periods of intense rainfall, the water has to be discharged to protect the higher urban areas from flooding, so that there is currently little possibility of water retention and pastures are hardly flooded, even in winter. The challenge is to redevelop the area and optimize water management to tackle the consequences of climate change taking into account the different types of land use in the area and preserve or improve biodiversity. Groundwater dynamics plays a crucial role in the field of ecosystem restoration. Dynamics are often underestimated or even not taken into acount.

There is still a big gap in understanding groundwater flow dynamics although they are crucial for succesfull development and management of restoration projects. Traditional monitoring networks follow up groundwater levels which are only the result of the in’s and outs of the water system. These groundwater levels are used to callibrate complex models simulating groundwater flow, without knowlegde of the causes behind. Simulated groundwater flow is always an approximation of reality, often a simplification, which can lead to misinterpretation. Measuring groundwater flow on site helps to understand the water system and its dynamics. The installed real time groundwater monitoring network includes the unique iFLUX sensors for horizontal and vertical groundwater flux in combination with a weather station, level sensors and CTD sensor. Surface water levels provided by third partie are also added.

The combined data allow to see interaction and correlation between weather events, groundwater flow velocity and -direction and the levels of ground- and surface water. The pilot test already created new insights in the total water balance of the Romboutswerve Polder. Examples of these insights are reversing of the horizontal flow direction in case of in- of decrease of the exfiltration flow and a clear upward seapage which needs further investigation. Based on these first results we plan to expand monitoring and setup restoration pilots to quantify the infiltration and retention capacity. The monitoring network allows to identify the water balance, the effects but also the causes. Effective monitoring and quantifying the results of restoration measurements will help to convince different stakeholders and will lead to a effective restorationbased on a comprehensive water management.

Source: Joris, M. 2021. Real time monitoring of groundwater flux creates insight in the complex water system of Romboutswerve Polder in Damme, Belgium. Treatment and monitoring of Water, Reuse, Recycle & Recover. AIWW 2021

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