In places where tidal marshes were formerly embanked for agricultural land use, these marshes are nowadays increasingly restored with the aim to regain important ecosystem services. However, there is growing evidence that restored tidal marshes and their services develop slowly and differ from natural tidal marshes in many aspects. Here we focus on groundwater dynamics, because these affect several key ecosystem functions and services, such as nutrient cycling and vegetation development. We hypothesize that groundwater dynamics in restored tidal marshes are reduced as compared to natural marshes because of the difference in soil structure. In the macro-tidal Schelde estuary (Belgium), in both a natural and a restored (since 2006) freshwater tidal marsh, we measured depth profiles of soil properties (grain size distribution, LOI (loss on ignition), moisture content and bulk density) and temporal dynamics of groundwater levels along a transect with increasing distance from a tidal creek. X-ray micro CT-scanning was used to quantify soil macroporosity. The restored marsh has a twolayered soil stratigraphy with a topsoil of freshly accreted sediment (ranging in depth between 10 and 60 cm, deposited since 2006) and a subsoil of compact relict agricultural soil. We found that both the soil in the natural marsh and the topsoil of the restored marsh consist of loosely packed sediment rich in macropores and organic matter, whereas the relict agricultural soil in the restored marsh is densely packed and has few macropores. Our results show that groundwater level fluctuations in the restored marsh are restricted to the top layer of newly deposited sediment (i.e. on average 0.08 m depth). Groundwater level fluctuations in the natural marsh occur over a larger depth of the soil profile (i.e. on average 0.28 m depth). As a consequence, the reduced groundwater dynamics in restored tidal marshes are expected to alter the subsurface fluxes of water and nutrients, the sourcesink function and the development of marsh vegetation.

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<p>Groundwater dynamics play a crucial role in the spreading of a soil and groundwater contamination. However, there is still a big gap in the understanding of the groundwater flow dynamics. Heterogeneities and dynamics are often underestimated and therefore not taken into account. They are of crucial input for successful management and remediation measures. The bulk of the mass of mass often is transported through only a small layer or section within the aquifer and is in cases of seepage into surface water very dependent to rainfall and occurring tidal effects.</p><p> </p><p>This study contains the use of novel real-time iFLUX sensors to map the groundwater flow dynamics over time. The sensors provide real-time data on groundwater flow rate and flow direction. The sensor probes consist of multiple bidirectional flow sensors that are superimposed. The probes can be installed directly in the subsoil, riverbed or monitoring well. The measurement setup is unique as it can perform measurements every second, ideal to map rapid changing flow conditions. The measurement range is between 0,5 and 500 cm per day.</p><p> </p><p>We will present the measurement principles and technical aspects of the sensor, together with two case studies.</p><p> </p><p>The first case study comprises the installation of iFLUX sensors in 4 different monitoring wells in a chlorinated solvent plume to map on the one hand the flow patterns in the plume, and on the other hand the flow dynamics that are influenced by the nearby popular trees. The foreseen remediation concept here is phytoremediation. The sensors were installed for a period of in total 4 weeks. Measurement frequency was 5 minutes. The flow profiles and time series will be presented together with the determined mass fluxes.</p><p> </p><p>A second case study was performed on behalf of the remediation of a canal riverbed. Due to industrial production of tar and carbon black in the past, the soil and groundwater next to the small canal ‘De Lieve’ in Ghent, Belgium, got contaminated with aliphatic and (poly)aromatic hydrocarbons. The groundwater contaminants migrate to the canal, impact the surface water quality and cause an ecological risk. The seepage flow and mass fluxes of contaminants into the surface water were measured with the novel iFLUX streambed sensors, installed directly in the river sediment. A site conceptual model was drawn and dimensioned based on the sensor data. The remediation concept to tackle the inflowing pollution: a hydraulic conductive reactive mat on the riverbed that makes use of the natural draining function of the waterbody, the adsorption capacity of a natural or secondary adsorbent and a future habitat for micro-organisms that biodegrade contaminants. The reactive mats were successfully installed and based on the mass flux calculations a lifespan of at least 10 years is expected for the adsorption material.  </p>

<p>Interactions between groundwater and surface water are of crucial importance for the ecological functioning of wetland systems since they control groundwater levels in the wetlands, water temperature in the river and exchange of solutes. Anthropogenic impacts such as the construction of drainage systems in the vicinity of wetlands can completely change the magnitude and direction of groundwater – surface water exchange, often negatively affecting the ecological functioning of the wetlands.</p><p>Management practices aiming to conserve the ecological status strongly depend on estimates of groundwater – surface water exchange. However, currently established methods to estimate groundwater flow (i) rely on point measurements, missing the effect of crucial short term events (e.g. precipitation), (ii) rely on differences in physical characteristics between the groundwater and surface water (e.g. temperature and/or conductivity), which are not always present or (iii) require extensive modelling.</p><p>In this presentation, we present a newly developed sensor, the iFLUX sensor. Two versions of this sensor exist, for measuring horizontal and vertical flow, respectively. The sensor probe for horizontal flow consists of two bidirectional flow sensors that are superimposed and is installed in a monitoring well with dedicated pre-pack filter, allowing for measurement of both groundwater flux magnitude and direction. The probe measuring vertical flow can be installed directly in the soil, in the riverbed or in a monitoring well. With a broad measuring range of groundwater fluxes from 0.5 cm/day to 2000 cm/day and measurements every second, this setup can map rapidly changing flow conditions.</p><p>Here, we show a selection of results from a case study in North-East Poland. In the Biebrza National Park, high groundwater levels resulting from subsurface runoff from the uplands protect the highly valuable peatland system. During most of the year, the river is gaining, with a sharp increase in upward groundwater flux in the hyporheic zone during summer months. In the valley surrounding the river, groundwater flows towards the river, as expected. However, the data show a remarkable diurnal pattern of both flow magnitude and direction, with the highest flow velocity occurring in the late afternoon, suggesting a relation with evapotranspiration. After large precipitation events, the flow direction reverses, suggesting infiltration of surface water into the aquifer.</p><p>Since these events occur on a small temporal scale, they were never measured before in the area with traditional methods. As such, our sensors provide new insights in groundwater – surface water interactions and will become an invaluable tool in ecohydrological studies worldwide, ultimately leading to more integrated management strategies to protect our remaining wetlands.</p>

<p>Along estuaries and coasts, tidal marsh restoration projects are increasingly being executed on formerly embanked agricultural land to regain the ecosystem services provided by tidal wetlands. There are, however, more and more indications that restored tidal marshes do not deliver these ecosystem services to the same extent as natural tidal marshes. In particular, we found that marsh restoration on a compacted agricultural soil (which has a very low porosity and hydraulic conductivity) leads to reduced groundwater fluxes and soil aeration, which may imply decreased soil-water interactions, reduced biogeochemical cycling and impaired vegetation development.</p><p>We studied the subsurface hydrology in the restored marsh Lippenbroek (Scheldt estuary, Belgium). To investigate spatial and temporal variation of groundwater fluxes in the restored tidal marsh, we developed a real-time groundwater flux sensor (iFLUX sensor) that enables us to measure both the groundwater flow velocity and flow direction in real-time. With these instruments installed at multiple locations and depths in the marsh soil, we were able to capture the effects of the tidal regime and soil stratigraphy on groundwater flow in high detail.</p><p>Furthermore, we set up a 2D vertical model in HYDRUS with a domain representing a creek and marsh cross-section. The model enables variably saturated flow calculations for groundwater flow and solute transport in dual porosity soils. Input parameters for the model were obtained by soil sampling and laboratory measurements of saturated hydraulic conductivity and soil water retention curves. Simulated results are in good agreement with in situ measured groundwater levels in monitoring wells.</p><p>With a scenario analysis, we showed that in a scenario in which the compact subsoil is absent, 6 times more water passes through the marsh soil during a spring tide – neap tide cycle  compared to the reference scenario in which the compact soil starts at a depth of 60 cm. In the compact layer, which is always saturated, flow rates are so low that this layer is expected not to contribute to nutrient cycling.</p><p>We then simulated the effect of (i) creek excavation (by varying the number of creeks in the domain) and (ii) soil amendments (by varying the depth to the compact layer) on groundwater flow in newly restored tidal marshes.  We found that increasing the creek density from 1 creek to 2 creeks per 50 m marsh, or changing the depth to the compact layer from 20 cm to 40 cm, both more than doubles the volume of water processed by the marsh soil. As such , our study demonstrates that groundwater modelling is a useful tool in support of designing marsh restoration measures aiming to optimize groundwater fluxes and related ecosystem services.   </p>