This study assesses hydrogeological hazards in the Saint-Vaast region, Belgium, impacted by three decades of deformation post-coal mine closure. Using Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) applied to 30 years of Synthetic Aperture Radar (SAR) data along with geological investigations and 13 years of piezometric well data, we analysed subsurface dynamics leading to environmental hazards in the region. We focused on identifying key periods of significant deformation pattern changes. The soft Wealden terrains (Lower Cretaceous) are characterized by alternating sandstones, clay, and sand facies, composing the sedimentary roof of an old mine drainage adit dug at 30 m depth. The presence of this mined adit structure is contributing to the current vulnerability of the region. Especially, when the abandoned mine drainage adit was filled with water following the cessation of pumping. Due to an increase in the water level in the Wealden aquifer below the gallery, natural groundwater flow paths and pressures have increased over time. At least two significant mudflow outbursts, in 2009 and 2018, occurred. This study shows that both events were correlated with changes in water level and deformation. Moreover, results suggest a new phase of rising water levels and ground uplift due to increased pressure in the Wealden aquifer. A crucial outcome of this research is the assessment of the potential for another similar incident soon, informed by the correlation analysis of water level and ground displacement time series.

Dissolved Si (DSi) provision from land systems triggers diatom growth and CO 2 sequestration. Soils and ecosystems act as a Si “filter”, transforming DSi originated from mineral weathering into biogenic Si (BSi) after DSi uptake by plants, or into other pedogenic forms of Si (non-BSi). Land use changes the quantity of BSi and non-BSi pools along the soil profile. However, methods used to isolate Si pools include chemical extractions at high temperatures and alkaline environments and therefore are unable to provide information concerning the dissolution potential of BSi and non-BSi pools under normal conditions of temperature and pH. Here, we conducted a batch experiment where forest, pasture and cropland soil samples were mixed with water at 25 °C and pH 7. The soil samples were collected from a temperate land use gradient located in the Belgian Loess Belt. We measured dissolved Si and aluminium (Al) during 80 days. BSi and non-BSi pool contents along the soil profile were known, as they had been established previously through chemical extraction. Results show that BSi and non-BSi enriched samples present distinct Si and Al dissolution curves. While non-BSi pools contribute significantly with immediate availability of Si, BSi pools present an initial slow dissolution. Therefore, croplands that were depleted of phytoliths and had poorly organic horizons display higher concentrations of initial dissolved Si, while pastures and forests, where pedogenic pools dominate only at depths below 40 cm, have more limited initial Si release.

Complex underground flow processes can occur in flooded mine workings. As the groundwater rebounds, outbreaks, flooding, and slope stability problems can occur where hydraulic pressures build up in less drained areas. A time-series statistical analysis was conducted to understand how exploited areas in an abandoned coalfield were connected and to calculate groundwater response times to rain events by spatially and temporally correlating piezometric levels and discharge rates. Ten years of flow rate and water level data were statistically analyzed for an abandoned coalfield in Liège (Belgium). Then, the results were compared to results from physically-based simulations (a 3D groundwater flow model) based on data from the first 2 years of monitoring. The statistical approach gives qualitative indications on the interconnections between the different areas of the coalfield, as well as on the storage capacity/transmissivity of the aquifer. Improved understanding of this hydrogeological behavior can be used to prevent post-mining accidents and assess the associated risks.

Intense deforestation over large areas is an important component of global models which aim to quantify the transfer of terrestrial siliceous material from the continent to the coast. Hence, there is a need for improved understanding of land use impact on Si export. We present a model that simulates the kinetic dissolution of biogenic Si in a forest soil. A kinetic equation was calibrated based on leaching experiments of soil columns at constant water flux. The calibrated equation was then used to simulate the Si export from a forest soil under varying atmospheric conditions and provided realistic Si concentrations in soil water. The same model was used to simulate the impact of reduced evapotranspiration due to deforestation and consequently increased soil water fluxes on the Si export. Results showed that lower amounts of biogenic silicon dissolved and higher Si fluxes were released to the system once water flux increased. The use of the kinetic dissolution equation for biogenic Si improves our understanding and the modelling of the Si release from soils towards the hydrographic system.

In Belgium, numerous karstified sand-carbonate contacts are known for their association with halloysite precipitation. In the Hinnisdael caves of Vechmaal, eastern of Belgium, a similar geological setting is observed in dolines of karstified calcarenite of the Cretaceous Maastricht Formation filled with sand of the Oligocene St. Huibrechts-Hern Formation. At the sand-calcarenite interface, a discontinuous succession occurs of white clay and oxidized rust almost perfectly segregated. These lithologies are authigenic precipitations of high purity consisting of only halloysite-allophane and goethite-ferrihydrite respectively. The Al, Si and Fe necessary for this precipitation were derived from the short-range distance dissolution of glauconite and other silicate minerals present in the overlying sand unit. The mobilization of these ions was realized by the progressive oxidation of pyrite to jarosite leading to a significant acidification of the percolating water. It is furthermore observed that seemingly small variations in local conditions in the Curfs quarry of Valkenburg, the Netherlands, result in alunite, gibbsite, halloysite, allophane precipitation instead of jarosite, halloysite-allophane and goethite-ferrihydrite.D

One of the major environmental changes observed after mine closure are associated to groundwater flow and the coupled stress redistribution in the subsurface. Flooding of the mined voids starts as soon as the extensive pumping required during mining activities stops. This process called "ground water rebound" continues until mine water levels equilibrate with the regional ground water surface or reach a point of discharge. In case no such point of discharge exists (e.g. due to the collapse of drainage adits) outbreak, flooding and/or stability problems can occur in the surrounding area (Wolkersdorfer 2008). Management of risks associated to former mining activities is thus essential to ensure public safety where extraction activities have been intense for several centuries, e.g. in Wallonia (Pacyna and Salmon 2012). To assess these risks, flow rates at discharge adits and groundwater levels have been monitored for 10 years in an abandoned coalfield in the area of Liege (Eastern Belgium). A statistical study is conducted on the long term data, aiming at understanding groundwater behavior and improving the monitoring network. Annual trends calculated from time series decomposition are used to evaluate if groundwater rebound is still active or not in the study area. Spatial and temporal correlation analyses between piezometric levels and discharge rates allow identifying response time of groundwater to recharge events and to obtain indications on the storage capacity of the aquifer. This study shows how statistical analysis of a long term database can be used as an efficient low cost tool to improve a monitoring network for mining risk management purposes since it helps understanding connectivity and reactivity of the groundwater table at different locations in the mine.

The release of dissolved silicon (DSi) from A and B horizons was investigated with leaching tests on unsaturated columns. As forest A horizons have larger biogenic Si (BSi) pools than arable lands, we compared the Si release from a forest and a cropland from the same geographical region developed on a Luvisol in Belgium and a Cambisol in Sweden. The A horizons released a quickly dissolving Si fraction in contrast to the B horizons, which did contain no or only little amounts of BSi and released lower Si concentrations. Our experiments show that Si export from forest soils is high because of the presence of a large reservoir of soluble BSi as well as due to the acidity of the soil (pH<4). Leaching at two different water fluxes revealed that export in forest soils was transport controlled while cropland soils were in equilibrium.

Besides well‐known effects of climate and parent material on silicate weathering the role of land use change as a driver in the global silicon cycle is not well known. Changes in vegetation cover have altered reservoirs of silicon and carbon in plants and soils. This has potential consequences for plant‐Si availability, agricultural yields, and coastal eutrophication, as Si is a beneficial element for many crop plants and an essential nutrient for diatom growth. We here examined the role of sustained and intensive land use and human disturbance on silicon (Si) pool distribution in soils with similar climatological and bulk mineralogical characteristics. We show that land use impacts both biogenic and nonbiogenic Si pools. While biogenic Si strongly decreases along the land use change gradient (from forest to croplands), pedogenic silica fractions (e.g. pedogenic clays) increase in topsoils with a long duration of cultivation and soil disturbance. Our results suggest that nonbiogenic Si pools might compensate for the loss of reactive biogenic silicon in temperate zones.

Despite increasing recognition of the relevance of biological cycling for Si cycling in ecosystems and for Si export from soils to fluvial systems, effects of human cultivation on the Si cycle are still relatively understudied. Here we examined stable Si isotope (δ30Si) signatures in soil water samples across a temperate land use gradient. We show that – independent of geological and climatological variation – there is a depletion in light isotopes in soil water of intensive croplands and managed grasslands relative to native forests. Furthermore, our data suggest a divergence in δ30Si signatures along the land use change gradient, highlighting the imprint of vegetation cover, human cultivation and intensity of disturbance on δ30Si patterns, on top of more conventionally acknowledged drivers (i.e. mineralogy and climate).

Diffuse phosphorus (P) export from agricultural land to surface waters is a significant environmental problem. It is critical to determine the natural background P losses from diffuse sources, but their identification and quantification is difficult. In this study, three headwater catchments with differing land use (arable, pasture and forest) were monitored for 3 years to quantify exports of dissolved (<0.45 µm) reactive P and total dissolved P. Mean total P exports from the arable catchment ranged between 0.08 and 0.28 kg ha −1 year −1 . Compared with the reference condition (forest), arable land and pasture exported up to 11-fold more dissolved P. The contribution of dissolved (<0.45 µm) unreactive P was low to negligible in every catchment. Agricultural practices can exert large pressures on surface waters that are controlled by hydrological factors. Adapting policy to cope with these factors is needed for lowering these pressures in the future.

In forest ecosystems phosphorus (P) is regarded to be among the most limiting nutrients. Therefore, P is cycled in an almost closed system, so that losses of P are minimal. Litterfall and leaf decomposition are key processes that control nutrient dynamics between forest and river systems and can cause losses of P. It is important to understand the dynamics and seasonal variations of P because P input-output balances can have an effect on sustainability of forest ecosystems.

Dissolved organic C (DOC) plays an important role in the cycling and distribution of energy and nutrients. However, factors controlling the transport of DOC both within and between ecosystems are not clear. The aim of this work was to identify the contributing pathways for transport of DOC to surface water in catchments contrasting in land use and hydrogeology and during different flow regimes. Stream water was sampled to observe temporal variation of DOC concentrations and quality both seasonally and at the time scale of a rain event. Major cation and silica concentrations in stream water, groundwater, soil pore water, precipitation/throughfall, and riparian zone water samples were combined in an end‐member mixing analysis to determine the contributing end‐members for DOC delivery at the catchment outlet. Results show that the change in DOC concentrations and quality observed in the stream water during a rain event can be explained by a change in contribution of the different end‐members. In the forested catchments with deep groundwater tables, the main pathway for DOC transport from the soil to the surface water during base flow was via the groundwater. Rising stream DOC concentrations during rainfall events were attributed to additional throughfall and riparian zone transport pathways. In the grassland catchments with shallow groundwater tables, DOC in the stream mainly originated from seeps. During rain events, contributions from a surficial transport pathway and riparian zone water gained importance. The importance of contributing pathways changed seasonally and highly depended on the degree of saturation of the vadose zone.

20 This paper reviews the processes which determine the concentrations of dissolved silicon (DSi) in 21 soil water and proposes a mechanistic model for understanding the transport of Si through a typical 22 podzol soil to the river. DSi present in natural waters originates from the dissolution of mineral 23 and amorphous Si sources in the soil. However, the DSi concentration in natural waters will be 24 dependent on both dissolution and deposition/precipitation processes. The net DSi export is 25 controlled by soil composition like (mineralogy and saturated porosity) as well as water 26 composition (pH, concentrations of organic acids, CO2 and electrolytes). These state variables 27 together with production, polymerization and adsorption equations constitute a mechanistic 28 framework determining DSi concentrations. For a typical soil profile in a temperate climate, we 29 discuss how the values of these key controls differ in each soil horizon and how it influences the 30 DSi transport. Additionally, the impact of external forcings such as seasonal climatic variations 31 and land use, is evaluated. This model is a first step to better understand Si transport processes in 32 soils and should be further validated with field measurements. 33

This study investigates the effect of fine-scale clay drapes on tracer transport. A tracer test was performed in a sandbar deposit consisting of cross-bedded sandy units intercalated with many fine-scale clay drapes. The heterogeneous spatial distribution of the clay drapes causes a spatially variable hydraulic conductivity and sorption coefficient. A fluorescent tracer (sodium naphthionate) was injected in two injection wells and ground water was sampled and analyzed from five pumping wells. To determine (1) whether the fine-scale clay drapes have a significant effect on the measured concentrations and (2) whether application of multiple-point geostatistics can improve interpretation of tracer tests in media with complex geological heterogeneity, this tracer test is analyzed with a local three-dimensional ground-water flow and transport model in which fine-scale sedimentary heterogeneity is modeled using multiple-point geostatistics. To reduce memory needs and calculation time for the multiple-point geostatistical simulation step, this study uses the technique of direct multiple-point geostatistical simulation of edge properties. Instead of simulating pixel values, model cell edge properties indicating the presence of irregularly shaped surfaces are simulated using multiple-point geostatistical simulations. Results of a sensitivity analysis show under which conditions clay drapes have a significant effect on the concentration distribution. Calibration of the model against measured concentrations from the tracer tests reduces the uncertainty on the clay-drape parameters. The calibrated model shows which features of the breakthrough curves can be attributed to the geological heterogeneity of the aquifer and which features are caused by other processes.

Silicon (Si) is a key element in global biogeochemical cycling and recent research has shown that changes in the biological component of the Si cycle may lead to more rapid variations in the land–ocean Si transfer than previously thought. The objective of this paper is to better understand the controls on temporal Si dynamics in terrestrial ecosystems, by studying Si fluxes from a small forested catchment in central Belgium. An intensive monitoring program (2008–2010) of dissolved and amorphous silica (DSi and ASi) concentrations and load patterns show that DSi concentrations are significantly lower during winter–spring periods than during summer–autumn periods. In contrast to what was found in other studies, seasonal dynamics in Meerdaal forest are not controlled by variations in biological uptake or temperature, but mainly by the more important supply of pore-water to the groundwater table in winter–spring periods. Analysis of seasonal and event fluctuations in stream water DSi concentrations showed that final stream water is a mixture of old, DSi rich water pushed out of the soil, and new, DSi poor water delivered by quick flow. The mixing of old and new water finally resulted in streamwater DSi concentrations responding only moderately to variations in discharge (near-chemostatic behaviour). We estimated the total DSi export from the system to be ca. 65.1 × 103 mol km−2 year−1. Because Si delivery is biologically regulated through an important Si cycle in the vegetation-soil continuum, an anthropogenic (e.g. agricultural expansion) or climatic disturbance of terrestrial ecosystems may alter both water residence times through shifts in hydrological regimes and the DSi chemical equilibrium concentration in soils. In turn, these perturbations will potentially alter long-term DSi and ASi inputs to aquatic systems.

This paper reviews the processes which determine the concentrations of dissolved silicon (DSi) in soil water and proposes a conceptual mechanistic model for understanding the transport of Si through soils to rivers. The net DSi present in natural waters originates from the dissolution of mineral and amorphous Si sources in the soil, as well as precipitation processes. Important controlling factors are soil composition (mineralogy and saturated porosity) and soil water chemistry (pH, concentrations of organic acids, CO2 and electrolytes). Together with production, polymerization and adsorption equations they constitute a mechanistic framework determining DSi concentrations. We discuss how key controls differ across soil horizons and how this can influence the DSi transport. A typical podzol soil profile in a temperate climate is used as an example, but the proposed model is transferrable to other soil types. Additionally, the impact of external forcing factors such as seasonal climatic variations and land use is evaluated. This blueprint for an integrated model is a first step to mechanistic modelling of Si transport processes in soils. Future implementation with numerical methods should validate the model with field measurements.