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.
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[1] Quantitative comparisons between benthic and hyporheic invertebrate communities are crucial for understanding the biological functions of the hyporheic zone, such as storage, migrations, and exchanges of invertebrates with the surface stream. Such comparisons are still hampered by the use of different techniques adapted to each habitat (benthic versus hyporheic). This work combines two different techniques for sampling the upper layers of bed sediments (0–15 cm): the semiquantitative “Bou-Rouch” pump classically used to sample the hyporheic zone (>15 cm), and the quantitative Hess sampler commonly used to sample the benthic zone (≤15 cm), in order to evaluate the quantitative efficiency of the pump in this 0–15 cm zone. First, a Bou-Rouch sample (BR) was taken within the cylinder of a Hess inserted within the streambed, then a second sample (benthic complement, BC) was collected within the Hess after removing the pump, in order to catch all invertebrates not extracted with the pump. The BR samples collected on average 14.5% of the total abundance and about 50% of the actual richness. The large range of variation indicates that the combination of the two techniques is not valid for a quantitative evaluation of benthic communities. Contrary to expectations, the pump did not collect more interstitial and groundwater invertebrates and no differences in faunal composition between upstream and downstream riffle positions were observed. Our results do not question the use of the BR technique under standard conditions i.e., when sampling the hyporheic zone, but underline how it is crucial to know its quantitative limits.
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