Groundwater Rebound Research Articles

Ground deformation monitoring via PS-InSAR time series: An industrial zone in Sacco River Valley, central Italy

Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) is an advanced technique enabling effective ground deformation monitoring. In this study, PS-InSAR time series of Sentinel-1 ascending and descending orbits for period 2015–2022 are utilized for an industrial zone in Sacco River Valley, Central Italy. The Sequential Turning Point Detection (STPD) is applied to estimate the trend turning points and their directions in PS-InSAR time series. In addition, river flow and climate time series for Sacco River near the industrial zone are analyzed using the coefficient of determination and the Least-Squares Cross-Wavelet Analysis (LSCWA) to investigate their potential impact on ground deformation. A significant land subsidence was observed prior to Fall 2016 likely as the result of drought and excessive water extraction followed by land uplift after Fall 2017 likely due to groundwater rebound. The LSCWA showed statistically significant seasonal coherency between precipitation and streamflow in 2018 when relatively much higher precipitation and streamflow were observed in this year compared to 2016 and 2017, potentially contributing to the land uplift in the study zone during 2018. These results not only highlight the capabilities of STPD for detecting trend turning points and LSCWA for analyzing streamflow and precipitation time series but also can help policymakers and stakeholders for developing a sustainable city and environment.

Hydrological and volcano-related gravity signals at Mt. Somma–Vesuvius from ∼20 yr of time-lapse gravity monitoring: implications for volcano quiescence

SUMMARY We report on about 20 yr of relative gravity measurements, acquired on Mt. Somma–Vesuvius volcano in order to investigate the hydrological and volcano-tectonic processes controlling the present-day activity of the volcano. The retrieved long-term field of time gravity change (2003–2022) shows a pattern essentially related to the subsidence, which have affected the central part of the volcano, as detected by the permanent GNSS network and InSAR data. After reducing the observations for the effect of vertical deformation, no significant residuals are found, indicating no significant mass accumulation or loss within the volcanic system. In the north-western sector of the study area, at the border of the volcano edifice, however, significant residual positive gravity changes are detected which are associated to ground-water rebound after years of intense exploitation of the aquifers. On the seasonal timescale, we find that stations within the caldera rim are affected by the seasonal hydrological effects, while the gravity stations at the base of the Vesuvius show a less clear correlation. Furthermore, within the caldera rim a multiyear gravity transient is detected with an increase phase lasting about 4 yr followed by a slower decrease phase. Analysis of rain data seem to exclude a hydrological origin, hence, we hypothesize a deeper source related to the geothermal activity, which can be present even if the volcano is in a quiescent state. We infer the depth and volume of the source by inverting the spatial pattern of the gravity field at the peak of the transient. A volume of fluids of 9.5 × 107 m3 with density of 1000 kg m−3 at 2.3 km depth is capable to fit reasonably well the observations. To explain the gravity transient, simple synthetic models are produced, that simulate the ascent of fluids from a deep reservoir up to the depth of 2.3 km and a successive diffusion within the carbonate aquifer hosting the geothermal system. The whole process appears to not significantly affect the seismicity rate and the deformation of the volcano. This study demonstrates the importance of a 4-D gravity monitoring of a volcano to understand its complex gravity signals that cover different spatial and temporal scales. Discriminating the different contributions that mix up in the observed gravity changes, in particular those due to hydrologic/anthropogenic activities form those due to the geothermal dynamics, is fundamental for a complete and reliable evaluation of the volcano state.

Modeling potential arsenic enrichment and distribution using stacking ensemble learning in the lower Yellow River Plain, China

In the high arsenic area of the northern Henan Plain in the lower Yellow River, the interaction between surface water and groundwater is frequent due to the control of groundwater over exploitation and the ecological replenishment of the rivers. It is not clear that what leads to the cause and dynamic change in the mechanism of arsenic pollution in shallow groundwater. Using machine learning algorithms to model the risk of high arsenic occurrence in the groundwater can help analyze mechanism of arsenic in groundwater. In this study, a stacking ensemble learning model was constructed to predict the risk distribution of shallow high arsenic groundwater in the lower reaches of the Yellow River in northern Henan Province using multi-variate parameters. Furthermore, we analyzed the potential areas at risk of high arsenic enrichment under different groundwater levels by control. The results show that the groundwater arsenic exceedance rate was 16.76%, and the high arsenic groundwater distributed from northeast to southwest, while that in the middle and south mainly distributed in the alluvial pre-fan depressions and the Yellow River crevasse splay. Compared with the single model, the stacking ensemble learning model has the Area Under the Curve (0.87), accuracy (0.82), and specificity (0.88) and sensitivity (0.77). With the best overall performance, the predicted risk distribution of groundwater arsenic is highly consistent with the observed results, and the potential groundwater area with high arsenic risk accounts for 19.67% of the total area. The impact of the Yellow River burst, average annual temperature, annual precipitation, ground elevation, and hydraulic gradient as the most significant indicator factors are affecting groundwater arsenic enrichment in the study area. Among these factors, the sedimentary environment accounts for 27.10% in the process of arsenic enrichment. Groundwater also plays an important role, with a relative share of 13.69%. In the simulation of groundwater rebound process, it was found that when the groundwater level raised in the range of 1 m −3 m, the high arsenic area could be reduced by up to 19.52%. The methods and findings in this work can support the management and control of arsenic enrichment and distribution in shallow groundwater systems around the world.

Understanding the Spatiotemporal Characteristics of Land Subsidence and Rebound in the Lianjiang Plain Using Time-Series InSAR with Dual-Track Sentinel-1 Data

The Lianjiang Plain, renowned for its position as ‘China’s textile hub’ and characterized by its high population density, has experienced considerable subsidence due to excessive groundwater extraction in recent years. Although some studies have investigated short-term subsidence in this plain, research on long-term subsidence and rebound remain understudied. In this paper, the characteristics of surface deformation in the Lijiang Plain during two periods (2015–2017 and 2018–2021) have been investigated using the time-series interferometric synthetic aperture radar (TS-InSAR) technique, and the correlation with the changes in groundwater level, geological factors, and urban construction are discussed. The InSAR-derived results are cross-validated with the adjacent orbit datasets. Large-scale and uneven subsidence ranging from −124 mm/year to +40 mm/year is observed from 2015 to 2017. However, a significant decrease in the subsidence rate during 2018–2021, with local rebound deformation up to +48 mm/year in three regions, is also observed. Groundwater level changes are found to be the major cause of the ground deformation, and the intercomparison between groundwater level and ground displacement time series from TS-InSAR measurements also indicates a clear relationship between them during 2018–2021. Geological factors control the range of deformation area over the study period. The impact of urban construction on surface subsidence is evident, contributing to high deformation. Our findings could improve the understanding of how deformation is affected by groundwater rebound and offer valuable insights into groundwater management, urban planning, and land subsidence mitigation.

Pointwise Modelling and Prediction for Ground Surface Uplifts in Abandoned Coal Mines from InSAR Observations

Interferometric synthetic aperture radar (InSAR) is a useful tool for monitoring surface uplifts due to groundwater rebound in abandoned coal mines. However, InSAR-based prediction for surface uplifts has rarely been focused on so far, hindering the scientifical assessment and controlling of uplift-related geohazards in a wide area. In this study, we firstly revealed that the temporal evolution of surface uplifts caused by groundwater rebound at a surface point approximately followed an exponential distribution. Following the result, a varied cumulative distribution function (CDF) of the Weibull distribution was then used to model the temporal evolution of surface uplifts on a point-by-point basis. Finally, the parameters of the varied Weibull CDF were inverted from historical InSAR observations of surface uplifts and were forward used to predict uplift trends. Two abandoned coal mines in Beipiao city, China, were chosen to test the presented method. The results suggest that the varied Weibull CDF is able to well describe the processing of time-series uplifts, and the root mean square errors of the predicted uplifts were about 1.2 mm. The presented pointwise method predicts surface uplifts based on historical uplift observations and a mathematical function (i.e., the varied Weibull CDF), without the requirement of in situ geological and hydrological information about the focused abandoned coal mines. Therefore, it offers a new tool for predicting surface uplifts in abandoned mines, especially in case they lack in situ geological and hydrological information.

Groundwater rebound and flooding in the Naples' periurban area (Italy)

AbstractGroundwater bodies in the urbanized and densely populated aquifers or coastal floodplains may be subjected to groundwater rebound (GR), often causing serious risks of groundwater flooding (GF) to underground structures and infrastructures. This paper shows the results of a study carried out in the Naples' periurban area (Italy) to: investigate the dynamics and features of GR and GF phenomena, map flooded sites during 2013–2015 period, and provide an interpretative analysis of the factors that control the GF of private buildings and agricultural lands. At the municipality scale, since 1990 and until 2015, a remarkable and widespread GR was observed, with magnitude up to +16.54 m, triggering an unexpected GF of basements of buildings and agricultural soils. Field surveys proved that there is a time delay of the GF, ranging from 17 to 20 years, compared to the start of GR. Moreover, inhomogeneous distribution of flooding episodes is controlled by anthropogenic and natural factors. The obtained results are fundamental to design mitigation measures to GF hazard, and offer new perspectives to make Naples' periurban area more robust, smart and resilient against this new hydrogeological risk, undervalued by Italian legislation and local authorities.

Utilising CoDA methods for the spatio-temporal geochemical characterisation of groundwater; a case study from Lisheen Mine, south central Ireland

Lisheen Mine in County Tipperary, Ireland exploited an underground Pb/Zn massive sulphide deposit hosted in Carboniferous (Mississippian) carbonates. During the extraction phase, the mine workings (located at an average depth of 170 m below ground level), were continuously pumped to lower the groundwater level. Following mine closure in 2015, pumping ceased and eight groundwater wells in the surrounding area were sampled monthly over an 11-month period to monitor the effects of groundwater rebound. These wells draw water from the upper 30 m of a limestone/dolostone aquifer and the monthly samples were analysed for the concentration of 31 elements and compounds (SO4, Cl, NO3, F, NH4, NO2, P, Ca, Na, K, Mg, Fe, Mn, Cu, Zn, Pb, Al, Ni, Ba, As, Hg, B, Cr, Cd, Mo, Ag, Co, Sr, Be, Sb and U). All of the water can be described as Ca–HCO3 type as expected. Standard methods for analysing groundwater geochemistry data (e.g. piper diagrams etc.) are useful, differentiating groundwaters with first-order contrasting chemical signatures, for example, distinguishing Ca–HCO3-type from Na–HCO3-type water. Samples from the 8 monitoring wells appear to be broadly similar, using this approach. However, these major ion methods fail to further distinguish between different groundwaters. The use of multivariate statistical analytical techniques has become more common in groundwater studies in recent years, allowing the interaction of all the elements and compounds to be considered simultaneously. Compositional Data Analysis (CoDA) was used on the Lisheen dataset to gain a better understanding of the spatial and temporal variation in groundwater geochemistry. Ilr-ion plotting, Hierarchical Cluster Analysis (HCA) and Principal Component Analysis (PCA) through CoDA highlights the elements and compounds that account for the majority of the variance and at Lisheen these are nitrate, manganese, ammonium, sulphate and potassium. By displaying these data visually in a CoDA bi-plot, each location can be reliably ‘geochemically fingerprinted’ despite similar concentrations of major ions within a relatively small geographical area (<30 km2). Relabeling the bi-plot observations by date of recovery reveals how one particular groundwater well (PH) subtly varies over time, most likely as a result of seasonal land-use changes (input of compounds associated with fertiliser). This type of statistical analysis has broad applications in hydrology and hydrogeology including contaminant tracing and interaction, environmental studies, land-use planning and mineral exploration.

Modelling groundwater rebound in recently abandoned coalfields using DInSAR

Advances in differential interferometric synthetic aperture radar (DInSAR) processing algorithms, such as the Intermittent Small Baseline Subset (ISBAS), and increased data availability from SAR systems, such as Sentinel-1, provide the opportunity to increase the spatial and temporal density of ground deformation measurements. Such measurements, when combined with modelling, have the potential to make a significant cost-effective contribution to the progressive abandonment strategy of recently closed coalfields. Applications of DInSAR over coalfields have observed heave in coal measures rocks and temporal correlations between the rise of mine water and deformation time-series. The cessation of systematic dewatering can have a variety of detrimental impacts and knowledge of the time-scales (i.e. the rate of rebound) and structure of the mine system are crucial to the remediation strategy. Although mine plans and borehole measurements provide vital information in this regard, mine plans are often incomplete or inaccurate, whereas monitoring boreholes are spatially sparse. Consequently, groundwater can flow in unanticipated directions via goaf, mine shafts and roadways, making it difficult to determine where the impacts of rebound are likely to occur. In this study, ground deformation data obtained using ISBAS DInSAR on ENVISAT (2002–2009) and Sentinel-1 (2015–2019) data are used to develop a simple method to model groundwater rebound in abandoned coalfields. A forward analytical model based upon the principle of effective stress and mine water ponds is first implemented to estimate surface heave in response to changes in groundwater levels measured in monitoring boreholes. The forward model is then calibrated and validated using the ground deformation data. The DInSAR data were subsequently inverted to map the change in groundwater levels in greater detail across the coalfield and forecast surface discharges in order to support mitigation strategies.

Satellite-Based Monitoring and Modeling of Ground Movements Caused by Water Rebound

The presented research aimed to evaluate the spatio-temporal distribution of ground movements caused by groundwater head changes induced by mining. The research was carried out in the area of one of the copper ore and anhydrite mines in Poland. To determine ground movements, classical surveying results and the persistent scatter Satellite Radar Interferometry (PSInSAR) method were applied. The mining operation triggered significant subsidence, reaching 1.4 m in the years 1944–2015. However, subsidence caused by groundwater pumping was about 0.3 m. After mine closure, an ongoing groundwater rebound was observed. Hence, land uplift occurred, reaching no more than 29 mm/y. The main part of the investigation concerned developing a novel method for uplift prediction. Therefore, an attempt was made to comparatively analyze the dynamics of ground movements correlated with the mine life and hydrogeological condition. These analyses allowed the time factor for the modeling of land uplift to be determined. The investigation also revealed that in the next six years, the uplift will reach up to 12 mm/y. The developed methodology could be applied in any post-mining area where groundwater-rebound-related uplift is observed.

Modelling the interference of underground structures with groundwater flow and remedial solutions in Milan

Underground structures and infrastructures within urban areas cause disturbance to the groundwater flow. Groundwater abstraction in Milan decreased of more than 30% since the 1970s generating a groundwater rebound which caused submergence and flooding of multiple underground structures and infrastructures. In order to evaluate the impact of underground structures on the groundwater flow in the Milan city area, a detailed analysis based on a groundwater flow model was carried out. The 3D FEM groundwater flow model was calibrated against high resolution monitoring data and used to evaluate both the disturbance induced by underground structures and their sequence of construction, and to verify the performance of mitigation strategies. Major modifications of groundwater head distribution are observed when structures transversally cut the groundwater flow, resulting in the fragmentation of the groundwater flow pathways and leading to changes in groundwater head from about ±0.05 m, at a regional scale, to about ±0.45 in relative proximity (few hundred metres) of underground structures. The effectiveness of a traditional-grey (i.e. a drainage tunnel) and a blue-green infrastructure (i.e. water extraction wells combined with heat pump systems) mitigation scenarios is discussed. Blue-green infrastructures result the most effective and efficient mitigation solutions with direct impact on sustainability and environmental conditions.

Approaches of Applying Non-Darcian Flow Equations to Predicting Groundwater Rebound in Response to Coal Mine Closure

Approaches of Applying Non-Darcian Flow Equations to Predicting Groundwater Rebound in Response to Coal Mine Closure

Conceptualization and finite element groundwater flow modeling of a flooded underground mine reservoir in the Asturian Coal Basin, Spain

Secure workings in underground coal mines usually require the lowering of water levels via pumping. After mine closure, pump systems are stopped and the water level recovers in a process known as groundwater rebound. In the Asturian Coal Basin, Spain, the closure and flooding of underground coal mines carries associated environmental impacts that ought to be assessed. Among them, those related with groundwater flow and pollutant transport are of main interest. To evaluate the environmental risks during and after mine closure the construction of numerical flow models is suggested, which rely on an appropriate conceptualization of the mine-hydrogeological system. Underground mines of this region are characterized by very steep coal seams due to a folded and faulted geological structure. The resulting geometry translates into a complex network of hundreds of kilometers of mine tunnels and galleries. This is a challenge for the construction of numerical models in issues like geometrical problems, assignation of hydraulic parameters, coupling of physical laws and high computational cost. This paper presents the conceptual and numerical model of two linked mines of the region, Mosquitera and Pumarabule, that recently ended the flooding stage, to be used in assessment of post-closure environmental risks. To implement the geometry of the underground mine, a methodological approach to translate the mining information to the numerical simulator has been developed, that retains the hydraulic behavior of the key underground mine elements. A methodological approach for coupled simulation of mine conduits and porous media flow has been explicitly adapted. The results, in terms of the velocity distribution field inside the mine and possible outflow sites, are useful to provide a basis for later extension of transport models and assessment of hydro chemical impacts on the area.

Environmental effects of the groundwater rebound in the eastern plain of Naples (Italy)

Since 1990, in the eastern plain of Naples a generalized groundwater rebound phenomenon has occurred, caused by the sharp reduction of the withdrawals which had overexploited the plain aquifer until 1989.The present study aims to investigate, for 1989-2013 period, the groundwater flooding and ground uplift related to the groundwater rebound, by a coupled analysis of multi-temporal hydrogeological and DInSAR data of this area.Results show a generalized rise of groundwater levels from 1990, varying from 6.0 to 16.0 m, whereas localized impacts of groundwater flooding on buildings and agricultural soils have started since 2006.An uplift trend is observed in ground deformations (up to +40 mm), caused by the increase of pore water pressure in the aquifer system, following to groundwater rebound.This result opens new perspectives in assessment, mitigation and management of geohazards not considered by the Italian legislation.

Uplift Evidences Related to the Recession of Groundwater Abstraction in a Pyroclastic-Alluvial Aquifer of Southern Italy

Aquifer mismanagement is a common anthropogenic cause of subsidence and uplift phenomena in alluvial plains, representing one of the main natural hazards in urban areas due to related damage to urban structures and infrastructures. In this work, the groundwater rebound phenomenon that occurred in the last decades of the 20th century in the Lufrano area (Metropolitan area of Naples, Southern Italy) has been studied by integrating geological data, hydrogeological continuous monitoring and spaceborne SAR information derived from ERS-1/2 and ENVISAT satellites. In the period of 1989–2006, the Lufrano area, which hosts an important well field made up of 180 wells extracting groundwater for drinking use, suffered an initial over-exploitation of the aquifer which was followed by a sudden and severe decrease of the volume abstraction, resulting this last in a rapid ground uplift. The coupled analysis of hydrogeological and DInSAR data have shown a correspondence between piezometric level rise (up to 15 m) and ground uplift (up to 50 mm) trends in the period 1989–2006. In order to examine the spatio-temporal evolution of the phenomena and the cause-effect relationships, showing the link between the two phenomena and their rates, longitudinal cross-sections were carried out and comparisons between piezometric level rise and time-series of displacements were reconstructed. The obtained results represent an initial contribution to the definition of ground deformation related to groundwater level rise phenomena, providing a basis for future studies focused on the modelling of the hydro-mechanical properties of the aquifer.

Coupled ground uplift and groundwater rebound in the metropolitan city of Naples (southern Italy)

Subsidence and ground uplift are phenomena that may have natural or anthropogenic origin and cause strong socioeconomic and environmental impacts, especially in urban areas. In this paper, the coupling between ground uplift and groundwater rebound in a complex multi-layered aquifer system in the metropolitan city of Naples (southern Italy) has been investigated. By an integrated use of hydrogeological and DInSAR monitoring data covering the periods 1989–2013 and 1993–2010, respectively, the spatio-temporal evolution of natural ground deformation following the strong reduction and interruption of groundwater pumping has been analysed. Hydrogeological monitoring data have shown large-scale groundwater rebound, with the highest amplitude of approximately 16 m and a groundwater rising velocity up to approximately 1.0 m/yr. DInSAR data show a general ground uplift with a magnitude of approximately 40 mm on average, except for the southern sector of the study area where some local natural and human-induced subsidence has been identified. A comparison between trends of groundwater level and ground displacement indicates that the ground uplift is linked to a poro-elastic rebound mechanism in the porous aquifer system, which is triggered by the increase of pore pressure, while the rate of ground displacement is controlled by the hydrostratigraphic characteristics of the multi-layered aquifer system. The results improve the knowledge of man-induced deformation in complex urban aquifers affected by groundwater rebound and highlight that the implementation of an appropriate hydro-mechanical deformation model is a necessary step to correctly model and manage a new geohazard in a highly urbanized environment.

Land deformation associated with exploitation of groundwater in Changzhou City measured by COSMO-SkyMed and Sentinel-1A SAR data

Abstract The Urban Agglomeration in Yangtze River Delta is one of the most important economic and industrial regions in China. The City of Changzhou is one of the most important industrial citys in Yangtze River Delta Urban Agglomeration. Activities here include groundwater exploration. Groundwater overexploitation has contributed to the major land deformation in this city. The severity and magnitude of land deformation over time were investigated in Changzhou City. A Small Baseline Subset Interferometric Synthetic Aperture Radar (SBAS-InSAR) technology, provides a useful tool in measuring urban land deformation. In this study, a time series of COSMO-SkyMed and Sentinel-1A SAR images covering Changzhou City were acquired. SBAS-InSAR imaging technique was used to survey the extent and severity of land deformation associated with the exploitation of groundwater in Changzhou City. Leveling data was used to validate the SBAR-InSAR productions, the error of SBAR-InSAR annual subsidence results was within 2 mm. The results showed that three main land subsidence zones were detected at Xinbei, Tianning and Wujin District. Four subsidence points were selected to analyze the temporal and spatial evolution characteristics of land subsidence. The subsidence rate of P1 to P4 was −2.48 mm/year, −12.78 mm/year, −18.09 mm/year, and −12.69 mm/year respectively. Land subsidence over Changzhou showed a trend of slowing down from 2011 to 2017, especially in Wujin District. SBAR-InSAR derived land deformation that correlates with the water level change in six groundwater stations. Indicated that with groundwater rebound, the land rebound obviously, and the maximum rebound vale reached 9.13 mm.

SIMPL: A Simplified Model-Based Program for the Analysis and Visualization of Groundwater Rebound in Abandoned Mines to Prevent Contamination of Water and Soils by Acid Mine Drainage.

Cessation of dewatering following underground mine closure typically results in groundwater rebound, because mine voids and surrounding strata undergo flooding up to the levels of the decant points, such as shafts and drifts. SIMPL (Simplified groundwater program In Mine workings using the Pipe equation and Lumped parameter model), a simplified lumped parameter model-based program for predicting groundwater levels in abandoned mines, is presented herein. The program comprises a simulation engine module, 3D visualization module, and graphical user interface, which aids data processing, analysis, and visualization of results. The 3D viewer facilitates effective visualization of the predicted groundwater level rebound phenomenon together with a topographic map, mine drift, goaf, and geological properties from borehole data. SIMPL is applied to data from the Dongwon coal mine and Dalsung copper mine in Korea, with strong similarities in simulated and observed results. By considering mine workings and interpond connections, SIMPL can thus be used to effectively analyze and visualize groundwater rebound. In addition, the predictions by SIMPL can be utilized to prevent the surrounding environment (water and soil) from being polluted by acid mine drainage.

A Methodology to Detect and Characterize Uplift Phenomena in Urban Areas Using Sentinel-1 Data

This paper presents a methodology to exploit the Persistent Scatterer Interferometry (PSI) time series acquired by Sentinel-1 sensors for the detection and characterization of uplift phenomena in urban areas. The methodology has been applied to the Tower Hamlets Council area of London (United Kingdom) using Sentinel-1 data covering the period 2015–2017. The test area is a representative high-urbanized site affected by geohazards due to natural processes such as compaction of recent deposits, and also anthropogenic causes due to groundwater management and engineering works. The methodology has allowed the detection and characterization of a 5 km2 area recording average uplift rates of 7 mm/year and a maximum rate of 18 mm/year in the period May 2015–March 2017. Furthermore, the analysis of the Sentinel-1 time series highlights that starting from August 2016 uplift rates began to decrease. A comparison between the uplift rates and urban developments as well as geological, geotechnical, and hydrogeological factors suggests that the ground displacements occur in a particular geological context and are mainly attributed to the swelling of clayey soils. The detected uplift could be attributed to a transient effect of the groundwater rebound after completion of dewatering works for the recent underground constructions.

Impact on Streams and Sea Water of a Near-neutral Drainage from a Flooded Mine in Sardinia, Italy

After mine closure and subsequent shutdown of the dewatering system, groundwater rebound led to drainage outflow from the Casargiu gallery (Montevecchio mine, SW Sardinia, Italy) since 1997. As compared with the first discharge, a very high contamination level still persists after almost 20 years of flushing. Mine drainage (20-70 L s-1; pH 6.0±0.2; Zn-Mg-Ca-SO4 composition) flows into the Rio Irvi. Abundant precipitation of amorphous Fe(III)-(oxy)hydroxides occurs. Moreover, sulfate-bearing green rust is observed to flocculate in the reach of the Rio Irvi where pH is still circumneutral. Water sampling along this stream shows a pH decrease from 6.0 to 4.0 and a significant removal of Fe (46%) and As (96%), while sulfate, Zn, Mn, Co, Ni and Cd show small variations downstream. Lead is initially adsorbed onto Fe(III)-(oxy)hydroxides, then desorbed as pH drops below 5.4. A conservative estimation of dissolved metals discharged into the Mediterranean Sea is significant (e.g. 900 kg day-1 Zn, 1.4 kg day-1 Cd, 5 kg day-1 Ni).

Spatial monitoring of groundwater drawdown and rebound associated with quarry dewatering using automated time-lapse electrical resistivity tomography and distribution guided clustering

Dewatering systems used for mining and quarrying operations often result in highly artificial and complex groundwater conditions, which can be difficult to characterise and monitor using borehole point sampling approaches. Here automated time-lapse electrical resistivity tomography (ALERT) is considered as a means of monitoring subsurface groundwater dynamics associated with changes in the dewatering regime in an operational sand and gravel quarry. We considered two scenarios: the first was unplanned interruption to dewatering due to a pump failure for a period of several days, which involved comparing ALERT monitoring results before and after groundwater rebound; the second involved a planned interruption to pumping over a period of 6h, for which near-continuous ALERT monitoring of groundwater rebound and drawdown was undertaken. The results of the second test were analysed using distribution guided clustering (DGC) to provide a more quantitative and objective assessment of changes in the subsurface over time.ALERT successfully identified groundwater level changes during both monitoring scenarios. It provided a more useful indication of the rate of water level rise and maximum water levels than piezometer monitoring results. This was due to the piezometers rapidly responding to pressure changes at depth, whilst ALERT/DGC provided information of slower changes associated with the storage and delayed drainage of water within the sediment. By applying DGC we were able to automatically and quantitatively define changes in the resistivity sections, which correlated well with the direct observations of groundwater at site. For ERT monitoring applications that generate numerous time series, the use of DGC could significantly enhance the efficiency of data interpretation, and provide a means of automating groundwater monitoring through assigning alarm thresholds associated with rapid changes in groundwater conditions.