Groundwater Rebound Research Articles

Environmental Effects on the Aquatic System and Metal Discharge to the Mediterranean Sea from a Near-Neutral Zinc-Ferrous Sulfate Mine Drainage

After mine closure in the 1980s and subsequent shutdown of the dewatering system, groundwater rebound led to drainage outflow from the Casargiu gallery (Montevecchio mine, SW Sardinia, Italy) beginning in 1997. Mine drainage had pH 6.0 and dissolved concentrations of sulfate (5000 mg/L) and metals (e.g., 1000 mg/L Zn, 230 mg/L Fe, 150 mg/L Mn) much higher than those previously measured in groundwater under dewatering conditions. As compared with the first stages of rebound at Casargiu, a very high contamination level still persists after more than 15 years of flushing. Mine drainage (20–70 L/s; pH 6.0 ± 0.2; Zn-Mg-Ca-SO4 composition) flowed into the Rio Irvi. Abundant precipitation of amorphous Fe(III)-(oxy)hydroxides occurred. Moreover, sulfate-bearing green rust was observed to flocculate in the reach of the Rio Irvi where pH was still circumneutral. Water sampling along this stream for about 6 km almost to its mouth in the Mediterranean Sea showed 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 showed small variations downstream. Lead was initially adsorbed onto Fe(III)-(oxy)hydroxides, then desorbed as pH dropped below 5. The estimated amount of dissolved metals discharged into the Mediterranean Sea is significant (e.g., 900 kg/day Zn, 1.4 kg/day Cd, 5 kg/day Ni). In particular, a conservative estimation of the amount of Zn discharged to the sea is about 330 ton/year, which would correspond to 1.4 % of the global annual flux of dissolved Zn from uncontaminated rivers to the oceans.

Development of a GRAM Model-based Program for Analyzing Groundwater Rebound in Abandoned Mines

Development of a GRAM Model-based Program for Analyzing Groundwater Rebound in Abandoned Mines

Optimizing the design of a geothermal district heating and cooling system located at a flooded mine in Canada

Flooded underground mines are attractive for groundwater heat pump systems, as the voids created during mining operations enhance the subsurface permeability and storage capacity, which allows the extraction of significant volumes of groundwater without requiring extensive drilling. Heat exchange at a flooded mine is, however, difficult to predict because of the complex geometry of the underground network of tunnels. A case study is presented here to demonstrate that numerical simulations of groundwater flow and heat transfer can help assess production temperatures required to optimize the design of a heat pump system that uses mine water. A 3D numerical model was developed for the Gaspe Mines located in Murdochville, Canada, where a district heating and cooling system is being studied. The underground mining tunnels and shafts are represented in the model with 1D elements whose flow and heat transfer contributions are superimposed to those of the 3D porous medium. The numerical model is calibrated to simultaneously reproduce the groundwater rebound that occurred when the mine closed and the drawdown measured during a pumping test conducted in a former mining shaft. Predictive simulations over a period of 50 years are subsequently performed to minimize pumping rate and determine maximum heat extraction rate.

Prediction of the Groundwater Rebound Process in a Backfilled Open Cut Mine Using an Artificial Neural Network

A neural network model was used to predict the groundwater rebound process after cessation of dewatering at a restored open cut coal site in the East Midlands area of the UK. Time (days after dewatering), water table levels in the aquifer and the backfilled site, hydraulic conductivity of the aquifer and backfilled site, and precipitation were used as input. The output of the network was the water table height, until the water table reached its equilibrium position. A feed-forward artificial neural network that uses batch gradient descent with a momentum-learning algorithm and 6-1-6-1 arrangement was found capable of predicting the groundwater rebound process. Predicted values were very close to the monitored results. The correlation coefficient values were 0.98221 for the training set, and 0.99329, 0.99499, 0.98667, 0.98289, and 0.97141 during the testing stage for the five monitoring points, showing that the model prediction was satisfactory.

GRAM 모델을 이용한 폐탄광 지역 지하수 리바운드 현상 분석

폐광된 광산에서는 채광장으로 유입되는 지하수의 펌핑 작업을 더 이상 수행하지 않기 때문에, 채광 공동이나 채광장 주변 지층에서 지하수위가 점진적으로 상승하는 지하수 리바운드 현상이 발생한다. 그동안 폐광산 지역의 지하수 리바운드 현상을 분석할 수 있는 다양한 수치모델들이 개발되었다. 본 논문에서는 폐탄광지역의 지하수 리바운드 현상 분석시 활용할 수 있는 VSS-NET, GRAM, MODFLOW 모델에 대해 조사하고, 공간적/시간적 규모에서 각 모델의 적용 범위에 대해 소개하였다. 특히, GRAM 모델에 대해서는 상세한 분석을 수행하였으며 포트란 언어를 이용하여 프로그램을 구현하였다. GRAM 모델을 이용하여 국내 동원탄광 수갱의 지하수 리바운드 현상을 분석하였으며, 현장의 지하수위 계측 결과와 GRAM 모델의 분석 결과가 유사한 것으로 나타났다. Cessation of dewatering usually results in groundwater rebound after closing an underground coal mine because the mine voids and surrounding strata flood up to the levels of decant points such as shafts and drifts. Several numerical models have been developed to predict the timing, magnitude and location of discharges resulting from groundwater rebound. This study reviews the numerical models such as VSS-NET, GRAM and MODFLOW, and compares their scopes of assessment at different spatial and time scales. In particular, the GRAM model was reviewed in details to implement it. This paper describes the implementation of GRAM model and its application to the Dongwon coal mine in Korea. The application showed that the groundwater level modeled at the shaft of Dongwon coal mine using the GRAM model is similar to the observed one in the field.

Application of the Hybrid Finite Element Mixing Cell method to an abandoned coalfield in Belgium

The Hybrid Finite Element Mixing Cell (HFEMC) method is a flexible modelling technique particularly suited to mining problems. The principle of this method is to subdivide the modelled zone into several sub-domains and to select a specific equation, ranging from the simple linear reservoir equation to the groundwater flow in porous media equation, to model groundwater flow in each sub-domain. The model can be run in transient conditions, which makes it a useful tool for managing mine closure post-issues such as groundwater rebound and water inrushes. The application of the HFEMC method to an abandoned underground coal mine near the city of Liege (Belgium) is presented. The case study zone has been discretised taking advantage of the flexibility of the method. Then, the model has been calibrated in transient conditions based on both hydraulic head and water discharge rate observation and an uncertainty analysis has been performed. Finally, the calibrated model has been used to run several scenarios in order to assess the impacts of possible future phenomena on the hydraulic heads and the water discharge rates. Among others, the simulation of an intense rainfall event shows a quick and strong increase in hydraulic heads in some zones coupled with an increase in associated water discharge rates. This could lead to stability problems in local hill slopes. These predictions will help managing and predicting mine water problems in this complex mining system.

The Hybrid Finite Element Mixing Cell Method: A New Flexible Method for Modelling Mine Ground Water Problems

Dewatering operations often stop at mine closure. The ground water rebound can have undesirable consequences, which numerical models can help one understand and manage. However, classical modelling techniques are relatively unsuitable to these contexts. While spatially distributed and physically based models suffer difficulties due to the lack of data and the complexity of geological and hydrogeological conditions, black-box models are too simple to deal with the problems effectively. A new modelling method is proposed to simulate ground water environments in which water flows through mined (exploited) and unmined (unexploited) areas. Exploited zones are simulated using a group of mixing cells possibly interconnected by pipes. Unexploited zones are simultaneously simulated using classical finite elements. This combined approach allows explicit calculation of ground water flows around the mine and mean water levels in the exploited zones. Water exchanges between exploited zones and unexploited zones are simulated in the model using specifically defined internal boundary conditions. The method is tested on synthetic cases of increasing complexity, and first results from a real case study are presented.

Groundwater flow in the South Wales coalfield: historical data informing 3D modelling

The South Wales Coalfield is structurally and lithologically complex. Attempts to understand the regional groundwater flow system have been largely piecemeal, focusing on single collieries or groups of collieries. Inspection of both historical data, gathered during the active mine dewatering phase of the coalfield, and of contemporary post-mining groundwater rebound data, allows a picture of the whole groundwater flow system to be derived, along with the regional water balance. The assessment is aided by application of a 3D visualization model of the geological framework, which includes the piezometry during and after the mining phase and pumping and yield data. Other valuable historical information, which can no longer be replicated, are the analyses of samples of groundwater that entered working mines from specified horizons, rather than present-day analyses of acid mine drainage. It is found that there is little deep regional groundwater flow across the basin, for example towards the sea, and most groundwater flow is of river catchment scale draining to the main rivers that traverse the coalfield. This analysis of the coalfield and the 3D model provide a foundation for future investigation into issues such as dewatering of new opencast pits, in situ gasification or anthracite mining.

The impacts of coal mining subsidence on groundwater resources management of the East Midlands Permo-Triassic Sandstone aquifer, England

There are extensive concealed coal mines beneath the East Midlands Permo-Triassic Sandstone aquifer, most of which have used longwall mining techniques. Knowledge of the pumping regimes of the concealed mines shows that there is currently little vertical leakage from the Permo-Triassic Sandstone into these mines. Long-term plans to control the groundwater rebound should prevent any large-scale pollution of the Permo-Triassic Sandstone aquifer system from ingress of mine waters. Currently, the largest impacts on the Permo-Triassic Sandstone aquifer system stem from subsidence associated with the mine workings. This paper examines the impacts on groundwater–surface water interaction, in the context of improving the surface water environment to mitigate effects of long-term over-abstraction. Integral to the assessment of the groundwater–surface water interactions are (i) the change to the hydraulic characteristics of the aquifer system related to the fracturing in the Permo-Triassic Sandstone that the subsidence induces and (ii) the reliability of the installations used to monitor groundwater levels and flows across the aquifer system. These two temporal impacts can be difficult to assess from the available data. The former tends to increase aquifer transmissivity, whereas the latter leads to the underestimation of the impacts of groundwater abstraction caused by datum subsidence of several metres. Two example case histories of impacts on groundwater–surface water interactions are considered: streambed fissuring in the catchments of the River Maun and River Meden, and flow improvements caused by subsidence in the Dover Beck catchment. These two case histories show that there are both beneficial and detrimental outcomes for groundwater resources management caused by coal mining induced subsidence.

Predicting Groundwater Rebound in the South Yorkshire Coalfield, UK

The semi-distributed model GRAM (groundwater rebound in abandoned mineworkings) has been applied to part of the South Yorkshire Coalfield, UK, to predict the pattern of groundwater rebound, in particular the timing and rates of changes in mine water flows between abandoned collieries. The model is based upon the mining hydrogeologist's concept of ‘ponds’ (discrete volumes of interconnected workings) and calculates water balances over time for all ponds in a multi-pond mined system. GRAM was successfully calibrated against observed groundwater levels over a 5 year period from 2001 to 2005 before being used to predict future rates of groundwater rebound, in accordance with different scenarios, including average, low, and high rainfall scenarios. The results reveal that it could take up to 22 years before an inflow of water occurs into the last remaining colliery in the area at Maltby, with the first surface discharge expected in approximately 15 years time from an old mine shaft. If Maltby is closed and pumping ceases across the area, then it could be 100 years before groundwater rebound is complete.

Using the GRAM Model to Reconstruct the Important Factors in Historic Groundwater Rebound in Part of the Durham Coalfield, UK

Modeling of mine water rebound in the southernmost zone of the Durham Coalfield has been undertaken in an attempt to reproduce observed water level recovery, since the 1970s, within five coal blocks south of the Butterknowle Fault. The lumped parameter model GRAM (groundwater rebound in abandoned mineworkings) was chosen to perform the simulations since it overcomes, to a large extent, a common problem found in such studies, namely a lack (especially concerning historic hydrogeological records) and superabundance of various kinds of data. The results obtained from this approach are satisfying and closely resemble the observed pattern of mine water rebound for the mining blocks studied. Such research indicates the critical dependence of predictions on factors such as the volume of water flowing into the system, the percentage runoff, and the value of storage coefficient assigned to the old workings. Model predictions are most useful as a tool for improving the conceptual understanding of abandoned mine systems and as a basis for evaluating alternative possibilities of coalfield behaviour, rather than as a strict quantitative assessment for all management purposes.

English

Following reunification of Germany in 1990 the uranium production in eastern Germany (former GDR) was abandoned. Wismut GmbH was established as a federal company and put in charge of decommissioning and rehabilitating the uranium mining liabilities. The Wismut Remediation Program comprises the full scope of mine environmental remediation activities, i.e. remediation of underground and open pit mines, waste rock dumps, tailings ponds, area cleanup, decommissioning and demolition. As a crucial element of the closure plan the Wismut project includes the flooding of five underground mines which account, after groundwater rebound has been completed, for the bulk of contaminated waters to be treated at the different mine sites over the long-term. In order to optimize long-term water treatment and to minimize annual treatment costs, a set of alternative approaches to directly influence the contaminant loads to be handled at the surface has been investigated at different scales. As a result of these activities Wismut introduced a set of different methods of direct or indirect source manipulation of water bodies in flooded underground mines into the closure plans. In the Konigstein-ISL-mine an immobilisation technology, which is based on the injection of water supersaturated in barite into former leaching blocks has been implemented as part of the closure plan in order to immobilize parts of the contaminant potential. As an alternative a reactive barrier approach had also been tested within the mine, however, this technology did not prove to be feasible under the site specific conditions of the Konigstein mine. With the intention to decrease the uranium concentration in mine waters a set of studies has been carried out where the addition of zero valent iron has been investigated as a tool to establish reducing conditions underground. At the Pohla mine site a full-scale field test had been applied to evaluate if natural attenuation processes in the mine water column can be enhanced by simple changes in the mine water management scheme. A water management approach is also the basic instrument at the Ronneburg mine site in order to limit the water treatment costs by plugging the mine which induces mine water stratification.

Using the DRASTIC System to Assess the Vulnerability of Ground Water to Pollution in Mined Areas of the Upper Silesian Coal Basin

An attempt was made to use the U.S. EPA DRASTIC ranking system to assess the vulnerability of ground water in the Upper Silesian Coal Basin. Analysis of the various system components indicate that several DRASTIC factors would have to be modified to consider the effects of mining, subsidence, and ground water rebound.

A Finite Element Model to: 2. Simulate Groundwater Rebound Problems in Backfilled Open Cut Mines

A two-dimensional finite element softwate called SEEP/W can be easily modified to simulate the groundwater rebound process within the spoil of an open cut mine taking into account saturated and unsaturated flow, hydraulic conductivity, and water content (as a function of pore-water pressure). Flexibility in the model is achieved by assigning different boundary conditions. In this paper, the results of the numerical model of the ground water rebound are presented and compared with those obtained from analytical solutions, another numerical model, and with data measured at the Horsley backfilled site in the UK. This model calculates realistic results that can be used by mine operators and environmental engineers to control the quality of mine drainage in a backfilled open cut operation.

A physically based model of rebound in South Crofty tin mine, Cornwall

Abstract The recent closure of the South Crofty tin mine, the last working mine of this type in Europe, has raised questions over possible environmental consequences. During several centuries of operation, the mine was dewatered by a series of pumps located at different levels in the mine. Older workings near the ground surface were dewatered by a series of adits that discharge into nearby rivers and streams. The quality of water draining from these shallow workings is generally good and no treatment is required. However, because of bad experiences at the nearby Wheal Jane tin mine, the UK Environment Agency were concerned about the quality and quantity of water which was expected to discharge from the deeper workings when groundwater rebound was completed. In order to address this problem and make predictions of the timing and volume of the discharge, computer simulations using the SHETRAN/VSS-NET model have been carried out. This model has already been applied to the simulation of groundwater rebound in several UK coalfields. However, the hydrogeological characteristics of coal mines differ considerably from the South Crofty mine. In this mine, the country rocks comprise granite and metamorphic slates, strata that have negligible transmissivity and very low storativity. Most of the groundwater flow is therefore in the ‘drives’ and stopes from which tin ore was extracted. The inflows to the mine during its operation were mapped and quantified, and were found to be mainly head-dependent. These inflows usually originated from fault zones in the rock, and also from nearby disused and flooded workings, which surround the modern mine. Predictions of the water level during rebound are compared with the observed water levels in the main shaft which have been measured since the mine closed. The model was then used to predict the dates when surface discharges could be expected to commence. One of the main limitations of the predictions was a ‘blank’ depth interval in the mine plan records, which could be interpreted in two possible ways: (i) the zone was worked in the mid-nineteenth century but the plans were lost; or (ii) the zone was never worked. Local professional opinion favoured interpretation (i), and the prediction scenarios considered most probable were based on the assumption that mining-related specific yield values would be similar in this ‘blank interval’ to those applicable in better-mapped intervals below. In the event, (ii) above appears to have been the true case, resulting in a marked steepening of the rebound curve during the later stages of rebound, with surface discharge commencing in November 2000, as much as a year in advance of most other predictions. A retrospective simulation assuming very low specific yield in the ‘blank interval’ confirms that the hydrodynamics were otherwise successfully simulated using the SHETRAN/VSS-NET code.

Mine water rebound in South Nottinghamshire: risk evaluation using 3-D visualization and predictive modelling

Progressive abandonment of the South Nottinghamshire Coalfield raises concerns over the security of the Permo–Triassic Sherwood Sandstone aquifer which overlies the concealed part of the coalfield. A 3-D digital visualization package has been used to assemble and display the complex and diverse data-sets of relevance. Predictive scenarios have been run from these data using the University of Newcastle program GRAM (Groundwater Rebound in Abandoned Mineworkings). The work comprised three phases: (i) confirmation of the geological framework for the so-called ‘Pond 3’ area (southernmost part of the coalfield) and establishment of a water balance along with an outline groundwater flow path system for the Coal Measures and adjacent strata; (ii) the collation of detailed geometric information on the spatial distribution of discrete geological layers that are considered to have hydrogeological significance, the distribution of mineworkings within key horizons, and the locations of boreholes, shafts and pumping stations (both in the Coal Measures and within overlying strata). Possible flooding configurations have been assessed geometrically to identify ‘hot spots’ where mine water discharge to surface may occur, and areas where the piezometric level of the rising mine water might promote upward fluxes into the Permo–Triassic Sherwood Sandstone aquifer. In addition, critical areas where coal has been worked close to the base of the Permian and where hydraulic continuity may occur between the Sherwood Sandstone and Coal Measures have been identified; (iii) the GRAM model used data held in the 3-D visualization package VULCAN to define discrete ‘ponds’ within the coalfield. Recharge to the system allows each pond to fill until overflow pathways are reached, when the adjacent pond may start to fill. A variety of such scenarios have been completed and predictive data generated, which suggest that possible discharge to surface and into the Sherwood Sandstone might occur about 20 years after the end of dewatering.

A Strategy For Modeling Ground Water Rebound In Abandoned Deep Mine Systems

Discharges of polluted water from abandoned mines are a major cause of degradation of water resources worldwide. Pollution arises after abandoned workings flood up to surface level, by the process termed ground water rebound. As flow in large, open mine voids is often turbulent, standard techniques for modeling ground water flow (which assume laminar flow) are inappropriate for predicting ground water rebound. More physically realistic models are therefore desirable, yet these are often expensive to apply to all but the smallest of systems. An overall strategy for ground water rebound modeling is proposed, with models of decreasing complexity applied as the temporal and spatial scales of the systems under analysis increase. For relatively modest systems (area < 200 km2), a physically based modeling approach has been developed, in which 3-D pipe networks (representing major mine roadways, etc.) are routed through a variably saturated, 3-D porous medium (representing the country rock). For systems extending more than 100 to 3000 km2, a semidistributed model (GRAM) has been developed, which conceptualizes extensively interconnected volumes of workings as ponds, which are connected to other ponds only at discrete overflow points, such as major inter-mine roadways, through which flow can be efficiently modeled using the Prandtl-Nikuradse pipe-flow formulation. At the very largest scales, simple water-balance calculations are probably as useful as any other approach, and a variety of proprietary codes may be used for the purpose.

Groundwater rebound in the South Yorkshire coalfield: a first approximation using the GRAM model

The closures of three deep coal mines in the Rotherham area (South Yorkshire, UK), and the cessation of associated dewatering, have given rise to concerns over possible future pollution of ground‐ and/or surface‐water resources once groundwater rebound is complete. An assessment of the legitimacy of these concerns has been made by applying the recently developed, semi‐distributed computer model GRAM (Groundwater Rebound in Abandoned Mineworkings). The extent of workings and subsurface inter‐connections relating to the three principal collieries in the Rotherham area have been identified from mine plans, augmented by testimonies of former underground workers, and assessed in terms of their hydrogeological importance. GRAM was used to predict the rate of groundwater recovery in the workings, and the timing and flow‐rates of future surface discharges. The simulations predict that major surface discharges can be expected no sooner than the year 2005, and will eventually amount to around 3.9 Ml d −1 . It is also predicted that around 0.4 Ml d −1 of water will flow from the abandoned mine‐workings into nearby workings of Maltby Colliery. By analogy with discharges elsewhere associated with seams of similar sulphur content, it is considered that the ‘first flush’ water will contain around 200 mg/l total iron (maximum estimate 600 mg/l), declining eventually to around 20 mg/l total iron.

Groundwater Rebound in the Leicestershire Coalfield

AbstractThe Leicestershire coalfield has been mined for at least eight hundred years, the last deep mine closing in 1991. The earliest shallow mines on the exposed coalfield are not recorded on plan, but were later connected to the deeper workings in many seams. In order to protect the last deep mines, pumping was maintained from the abandoned mines to the north.Water‐table rebound was observed prior to closure, and is now affecting the surface. Increased emissions of poor‐quality water into low‐discharge surface water‐courses are predicted. Renewed subsidence has implications for roads, services and property. Rising water levels will affect old opencast excavations and waste disposal sites.Fieldwork and abandoned mine plan inspections have demonstrated renewed subsidence and evidence of increased emissions in those low‐lying areas of the exposed coalfield which are believed to be most at risk.

96/03353 - ‘To pump or not to pump’ cost-benefit analysis of future environmental management options for the abandoned Durham coalfield

Deep mining activity has totally ceased in the Durham coalfield, and withdrawal of the regional dewatering scheme is being contemplated. Consequent groundwater rebound is expected to have severe and expensive environmental consequences, which could only be entirely avoided through indefinite pumping. However, continued pumping is expensive, at about $1 million/annum. The options of either abandoning or continuing pumping have differing time streams of future costs and, consequently, direct comparison of the expenditures involved is difficult. Both options are therefore analysed using economic evaluation criteria in order to determine the lowest cost alternative for the future. The overall outcome of the economic analysis, using both net present value and equivalent annual cost criteria, demonstrates that the option of continued pumping is less expensive than the option of abandoning pumping in all cases.