Prediction of acid mine drainage generation potential in selected mines in the Ashanti Metallogenic Belt using static geochemical methods

Acid mine/rock drainage (AMD/ARD) is the biggest environmental threat facing the mining industry. This study investigates AMD/ARD possibilities in three mines in the Ashanti Belt, using acid base accounting (ABA) and net acid generation pH (NAGpH) tests. Twenty-eight samples of rock units and mine spoil from these mines were collected for ABA and NAGpH tests. Two tailing dumps at Prestea and Nsuta were confirmed by both methods as acid generating with NAGpH of 4.5 and 4.6 and neutralization potential ratio values of 4.38 and 4.60, respectively. Six other samples are classified as potentially acid generating using a variety of established classification criteria. The rest of the samples either exhibited very low sulphur and carbonate content or had excess carbonate over sulphur. Consistency between results from ABA and NAGpH tests validates these tests as adequate tools for preliminary evaluation of AMD/ARD possibilities in any mining project in the Ashanti Belt.     

The environmental risk posed by small dumps of complex arsenic,antimony, nickel and cobalt sulphides

Mine waste dumps sited in the vicinity of mine workings typically consist of fragmented rock masses which in chemical terms may be defined as "complex systems", due to uncertainties in accurately simulating their behaviour, when water or aqueous solutions percolate through them. Remediation of the contamination problems created by abandoned mine dumps concerns many former mining areas from Europe to Africa. These dumps usually range in size from a few hundred thousand up to million cubic meters of rock defined as “waste” but which still contain significant amounts of potentially toxic elements. Although relatively small, they are often densely distributed in the territory impacting on surface and ground-water. This scenario is particularly applicable to the region of Sardinia, Italy, where several small dumps of uncommon sulphide minerals still exist on derelict mine sites. This paper reports on the results of a research project aimed at defining appropriate remedial measures of acid rock drainage. A dump of a Co–Ni–As–Sb complex sulphides mine was investigated and simulated in a laboratory leaching column. The column was irrigated with distilled water and a repetitive behaviour of the polluting elements concentrations in the effluents was observed for each run. Regardless of the campaign duration and the extent of the intervals between tests metal ions concentration peaks (250–50 ppm for Ni, 35–20 ppm for Co, 1.6–0.4 ppm for As, 20–15 ppm for Zn and 0.03–0.002 ppm for Sb) and acidic conditions (pH 2.5–3.5), on resumption of each irrigation, was noticed. As percolation continued, the ion concentrations decreased within the discharge limits after the replacement of only two pore volumes of the column material. This produces the conception that a sufficiently long rainfall period could remediate the polluting effects. Conception contradicted by field experiences: sporadic rainfall events, typical of the region, are not sufficient to completely leach out the metal ions. The repetitive nature of the leaching processes, both quantitatively and qualitatively, can be explained only by the activity of a microflora thriving in the rock mass even during long dry periods. Microbial tests and neutralisation campaigns confirmed this hypothesis and suggested that any effective remediation strategy must take into consideration the interaction between environmental factors with microbiological activities.     

Environmental geochemistry of the derelict Webbs Consols mine,New South Wales,Australia

Small-scale mining and mineral processing at the Webbs Consols polymetallic PbZnAg deposit in northern New South Wales, Australia has caused a significant environmental impact on streams, soils and vegetation. Unconfined waste rock dumps and tailings dams are the source of the problems. The partly oxidised sulphidic mine wastes contain abundant sulphides (arsenopyrite, sphalerite, galena) and oxidation products (scorodite, anglesite, smectite, Fe-oxyhydroxides), and possess extreme As and Pb (wt% levels) and elevated Ag, Cd, Cu, Sb and Zn values. Contemporary sulphide oxidation, hardpan formation, crystallisation of mineral efflorescences and acid mine drainage generation occur within the waste repositories. Acid seepages (pH 1.9–6.0) from waste dumps, tailings dams and mine workings display extreme As, Pb and Zn and elevated Cd, Cu and Sb contents. Drainage from the area is by the strongly contaminated Webbs Consols Creek and although this stream joins and is diluted by the much larger Severn River, contamination of water and stream sediments in the latter is evident for 1–5 km, and 12 km respectively, downstream of the mine site. The pronounced contamination of local and regional soils and sediments, despite the relatively small scale of the former operation, is due to the high metal tenor of abandoned waste material and the scarcity of neutralising minerals. Any rehabilitation plan of the site should include the relocation of waste materials to higher ground and capping, with only partial neutralisation of the waste to pH 4–5 in order to limit potential dissolution of scorodite and mobilisation of As into seepages and stream waters.     

基于热声发射技术的陕北火烧岩烧变温度识别

开展火烧岩烧变温度的识别对于火烧岩的形成及演化具有重要的指导意义。基于热Kaiser效应,通过火烧岩在热处理过程中的声发射累计能量以及参数RA(上升时间/振幅)和AF(平均频率)值的变化,对陕北神木张家峁火烧岩的阈值温度进行了预测。结果表明:火烧岩热声发射特征能有效识别火烧岩烧变温度,与火烧岩经历温度的岩相学分析基本一致;研究区火烧岩存在显著的地层烧变温度梯度,第1—第5层岩石烧变阈值温度逐渐升高,对于第6层和第7层岩石,最高温度超过700℃,对于第8层岩石,阈值温度为245℃;当热处理温度较低时,岩石内部沿晶拉张裂纹发育,超过600℃后,穿晶剪切裂纹所占比例开始增加,声发射信号出现二次增长,同时声发射出现平静期,累计能量曲线出现多个平台。火烧岩烧变温度的识别,对于煤矿防治水及围岩稳定性研究等具有重要的现实意义。      

Spatial and temporal variability of surface water and groundwater before and after the remediation of a Portuguese uranium mine area

The old Senhora das Fontes uranium mine, in central Portugal, consists of quartz veins which penetrated along fracture shear zones at the contact between graphite schist and orthogneiss. The mine was exploited underground until a depth of 90 m and was closed down in 1971. The ores from this mine and two others were treated in the mine area by the heap-leach process which ended in 1982. Seven dumps containing a total of about 33,800 m³ of material and partially covered by natural vegetation were left in the mine area. A remediation process took place from May 2010 to January 2011. The material deposited in dumps was relocated and covered with erosion resisting covers. Surface water and groundwater were collected in the wet season just before the remediation, in the following season at the beginning of the remediation and also after the remediation in the following dry season. Before, at the beginning and after the remediation, surface water and groundwater have an acid-to-alkaline pH, which decreased with the remediation, whereas Eh increased. In general, before the remediation, uranium concentration was up to 83 μg/L in surface water and up to 116 μg/L in groundwater, whereas at the beginning of the remediation it increases up to 183 μg/L and 272 μg/L in the former and the latter, respectively, due to the remobilization of mine dumps and pyrite and chalcopyrite exposures, responsible for the pH decrease. In general, after the remediation, the U concentration decreased significantly in surface water and groundwater at the north part of the mine area, but increased in both, particularly in the latter up to 774 μg/L in the south and southwest parts of this area, attributed to the remobilization of sulphides that caused mobilization of metals and arsenic which migrated to the groundwater flow. Uranium is adsorbed in clay minerals, but also in goethite as indicated by the geochemical modelling. After the remediation, the saturation indices of oxyhydroxides decrease as pH decreases. The remediation also caused decrease in Cd, Co, Cr, Ni, Pb, Zn, Cu, As, Sr and Mn concentrations of surface water and groundwater, particularly in the north part of the mine area, which is supported by the speciation modelling that shows the decrease of most dissolved bivalent species. However, in general, after the remediation, Th, Cd, Al, Li, Pb, Sr and As concentrations increased in groundwater and surface water at south and southwest of the mine area. Before and after the remediation, surface water and groundwater are contaminated in U, Cd, Cr, Al, Mn, Ni, Pb, Cu and As. Remediation caused only some improvement at north of the mine area, because at south and southwest part, after the remediation, the groundwater is more contaminated than before the remediation.     

Geochemical study of different-aged mining dump materials in the Freiberg mining district, Germany

Historical mining dumps are useful archives for the investigation of weathering processes. The objective of this study was to investigate the weathering behavior of waste-rock material derived from the 800-year-old silver ore mining in Freiberg, Germany. For identifying time-dependent weathering indices, dumped material of four dumps of different ages and corresponding rock was examined regarding the geochemical composition. The dumped material is characterized by high contents of heavy metal containing sulfidic ores, such as pyrite, arsenopyrite, sphalerite and galena. Acid mine drainage is produced by the oxidative weathering of the sulfide minerals and causes the increased dissolving of soluble metals with increasing age of dumps. As a result of these weathering processes, a clear depletion of chalcophile elements in the older dump material (800 years) compared to the youngest dump (100 years) was observed. In the soil horizons downstream the dumps, high quantities of heavy metals (e.g., up to 12,000 ppm As, 3,300 ppm Pb, 640 ppm Zn), mainly adsorbed on organic matter, were determined and indicate a time-dependent element transfer from the dumps into their surrounding soils.     

Effects of monsoonal rainfall on waste dump stability and respective geo-environmental issues: a case study

The stability of mine waste dumps is of critical importance and an issue, the mining industry of Goa, India is continually facing. The State of Goa receives high rainfall of around 3,000 mm annually. This heavy monsoonal rainfall is often believed to be the cause of dump slope instability. In light of several dump collapses encountered in the State and their damaging effects on both mine operations and local geo-environmental conditions, this paper examines their stability considering various geotechnical factors and the downstream environmental effects of a slope failure. The mechanical properties of dump waste material are reported at different compaction levels. The effect of these properties and changes in pore-water pressure are specifically examined using limit equilibrium analysis. An empirical formula is developed relating dump height, material mechanical parameters and pore-water pressure to the factor of safety of the slope. It was found that the level of compaction experienced by the material had a significant effect on the factor of safety.     

Waste rock dump investigation at Roşia Montană gold mine (Romania): a geostatistical approach

Ro?ia Montan?, the largest European gold mine, could be re-opened. Environmental problems led to severe pollution of the Ro?ia and Abrud Rivers. The two main potentially toxic element (PTE) sources in mine sites are, in general, the abandoned underground workings and the piles of waste rock. Since the composition of waste rocks is often heterogeneous, this study faces the problem of estimating their mineralogical and chemical features starting from a set of sampling point. Twenty-five samples were collected on the main waste dump of the Ro?ia Montan? mine following a virtual squared grid (knots distance about 25 m). Grain size, color, bulk chemistry, mineralogy, acid mine drainage potential and a set of selected PTE (Ag, As, Cu, Ni, Pb and Zn) concentrations were determined on each sample. In a first approximation two main waste rock groups were identified: WR1 bulk composition is dacite-dominated, while WR2 is andesite-dominated. In both of them the concentrations of PTEs are below the regulatory limits for soils, with the exception of As. In terms of acid mine drainage, WR1 has a net acid-producing potential, while WR2 has a net neutralizing potential. The anisotropy of WR properties were analysed by means of semi-variograms and displayed with contour maps. Application of positive matrix factorization for the analysis of all the data relative to waste rocks allowed defining, in a semi-quantitative way, the factors controlling pollution and their spatial distribution. The processing of score matrix G factor associated with the geostatistical elaboration promises to be a powerful tool to discern the composition of mine dumps and support the exploitation and remediation phases.     

Environmental characterisation of coal mine waste rock in the field: an example from New Zealand

Characterisation of mine waste rock with respect to acid generation potential is a necessary part of routine mine operations, so that environmentally benign waste rock stacks can be constructed for permanent storage. Standard static characterisation techniques, such as acid neutralisation capacity (ANC), maximum potential acidity, and associated acid–base accounting, require laboratory tests that can be difficult to obtain rapidly at remote mine sites. We show that a combination of paste pH and a simple portable carbonate dissolution test, both techniques that can be done in the field in a 15 min time-frame, is useful for distinguishing rocks that are potentially acid-forming from those that are acid-neutralising. Use of these techniques could allow characterisation of mine wastes at the metre scale during mine excavation operations. Our application of these techniques to pyrite-bearing (total S = 1–4 wt%) but variably calcareous coal mine overburden shows that there is a strong correlation between the portable carbonate dissolution technique and laboratory-determined ANC measurements (range of 0–10 wt% calcite equivalent). Paste pH measurements on the same rocks are bimodal, with high-sulphur, low-calcite rocks yielding pH near 3 after 10 min, whereas high-ANC rocks yield paste pH of 7–8. In our coal mine example, the field tests were most effective when used in conjunction with stratigraphy. However, the same field tests have potential for routine use in any mine in which distinction of acid-generating rocks from acid-neutralising rocks is required. Calibration of field-based acid–base accounting characteristics of the rocks with laboratory-based static and/or kinetic tests is still necessary.     

Chemical and mineralogical forms of lead,zinc and cadmium in particle size fractions of some wastes,sediments and soils in Korea

Forms of Pb, Zn and Cd in the different size fractions (<2 μm, 2–53 μm and >53 μm) of waste dumps, stream sediments and surrounding soils from a former Au mine in Korea, were investigated chemically by sequential extraction analysis and mineralogically by XRD and analytical SEM, so as to clarify the relationships between chemical and mineralogical forms. Total concentrations for the waste dumps and the stream sediments range from 655 to 2920 mg/kg for Pb, 565 to 1191 mg/kg for Zn, and 24.4 to 71.4 mg/kg for Cd, while those for the surrounding soils do not exceed the natural background levels. Direct observations on the heavy mineral fractions of the waste dumps and the stream sediments indicates that the primary sphalerite is still the main pool of the Zn and Cd, while a large part of the primary galena has been changed into a carbonate-bound form. This is in a good agreement with the partitioning of chemical forms in the coarse fractions, in which most of the Zn (75.3 to 79.4% for the waste dumps) and Cd (54.8 to 60.1% for the waste dumps) are associated with the oxidizable form, while most of the Pb (68.8 to 71.0% for the waste dumps) is in the acid (NaOAc)-extractable form. On the other hand, the partitioning of metal forms in the clay fraction is characterised by the highest proportion of the reducible form for all metals (56.6 to 73.8% for Pb, 60.2 to 68.4% for Zn, and 27.1 to 36.8% for Cd in the waste dumps), suggesting precipitation of easily to moderately reducible oxides and hydroxides from the other forms during weathering. With the increase of pH, the dramatic changes of the acid-extractable Pb, the oxidizable Zn and Cd in the coarse fractions, and the exchangeable form, especially for Cd in the clay fraction indicate that pH is the prime factor controlling the partitioning of heavy metals.     

论烧变矿床与烧变岩研究及其意义

烧变矿床及烧变岩系煤自燃的产物。从宏观及微观上搞清其产状、分带性、物质组分及其形成机理。开拓地质科学的新领域,填补岩石矿物学及矿床学中的空白。并作为找煤的有效标志及增加耐火材料、建材、陶瓷原料的新类型和基地。     

Adverse effects of coal mine waste dumps on the environment and their management

Waste dumps generated from mining that exposes sulfur-bearing overburden can be active sources of acid generation with the potential to severely contaminate soils, surface and groundwater, and endanger both local and downstream ecosystems. A waste rock management strategy ensure that disposal of such material is inert or at least stable and contained, and minimizes the surface footprint of the wastes, and explores options for alternate uses. Reclamation of waste dumps is another or parallel alternative to decrease the potential for adverse effects. At the coal mining area of Karmozd in Iran, large volumes of wastes have been produced and disposed of without any specific care for the environment. In this paper, the impacts of waste dumps on the environment were identified and this was followed by a research program to determine the characteristics of the wastes, their acid generation potential, the availability of hazardous contaminants, and a prediction of their environmental impacts on the site. Data was collected from the target site and by comparing several reclamation alternatives using a Multi-Attribute Decision-Making technique, forestry was selected as the post-mining land use for the waste dumps. Preliminary evaluations indicated that Zelkava could be a useful tree species for this region.     

Metal mobility in alum shale from Öland,Sweden

A study was initiated to analyse metal flows from alum shale to the environment in an area in Öland, Sweden. The study was performed by elemental analysis and leaching experiments of alum shale together with analysis of groundwater and surface water samples.The metal concentrations in non-weathered alum shale were much higher than in weathered or burnt shale, especially for cadmium (Cd), nickel (Ni) and zinc (Zn), indicating a loss of metals during weathering or burning of the shale. The release of metals through weathering was clearly demonstrated by the leaching tests. A 36-week leaching period of non-weathered shale resulted in a drastic drop in pH and a significant increase in metal concentrations in the leachate. The metal concentrations in groundwater were inversely related to the pH. For surface waters, the concentrations of Cd, copper (Cu), Ni and Zn were generally increased compared to background values.In conclusion, metals are released through weathering or burning of alum shale, as well as from heaps of weathered or burnt shale. The release of metals is strongly related to low pH, especially for Cd, Ni and Zn.     

Environmental geochemistry of the Gulf Creek copper mine area, north-eastern New South Wales, Australia

 Past mining and smelting of sulphide ore (pyrite-chalcopyrite-sphalerite) at the abandoned Gulf Creek mine has resulted in a stream highly contaminated by acid mine drainage (pH: 2.2–3.4), as well as degradation of local soil and vegetation. Physical dispersion of secondary metal-bearing minerals from abandoned ore and waste dumps into Gulf Creek and adsorption and coprecipitation of dissolved metals and metalloids in the stream bed cause elevated Ag, As, Cd, Cu, Fe, Pb and Zn values in stream sediments. The bioavailability of individual heavy metals to freshwater organisms changes downstream, however, selective bioaccumulation processes in algae reject readily bioavailable Zn and concentrate less bioavailable Cu. Polluted soils in the vicinity of the mine and smelter sites are subject to continuing soil erosion and either support no vegetation, or a depauperate flora with certain species showing bioaccumulation of metals and resistance to high metal contents. Rehabilitation of disturbed areas should involve covering and sealing sulphidic mine waste or removal of ore and waste dumps, installation of a physical and chemical plant or construction of a wetland environment (plus anoxic lime drains), and import of topsoil and planting of local, metal-tolerant plant species. Received: 17 March 1998 / Accepted: 6 October 1998     

Arsenic contamination at the Mole River mine,northern New South Wales

Mining and processing of arsenopyrite ore at the Mole River mine in the 1920–1930s resulted in abandoned mine workings, waste dumps and an arsenic oxide treatment plant. Weathering of waste material (2.6–26.6 wt% As) leads to the formation of water soluble, As‐bearing mineral salts (pharmacolite, arsenolite, krautite) and sulfates which affect surface waters after rainfall events. Highly contaminated soils, covering about 12 ha at the mine, have extreme As (mean 0.93 wt%) and elevated Fe, Ag, Cu, Pb, Sb and Zn values compared with background soils (mean 8 ppm As). Regionally contaminated soils have a mean As content of 55 ppm and the contaminated area is estimated to be 60 km². The soils have acquired their metal enrichments by hydromorphic dispersion from the dissolution of As‐rich particulates, erosion of As‐rich particulates from the dumps, and atmospheric fall‐out from processing plant emissions. Stream sediments within a radius of 2 km of the mine display metal enrichments (62 ppm to 27.5 wt% As) compared with the mean background of 23 ppm As. This enrichment has been caused by erosion and collapse of waste‐dump material into local creeks, seepages and ephemeral surface runoff, and erosion and transportation of contaminated soil into the local drainage system. Water samples from a mine shaft and waste‐dump seepages have the lowest pH (4.1) and highest As values (up to 13.9 mg/L), and contain algal blooms of Klebsormidium sp. The variable flow regime of the Mole River causes dilution of As‐rich drainage waters to background values (mean 0.0086 mg/L As) within 2.5 km downstream. Bioaccumulation of As and phytotoxicity to lower plants has been observed in the mine area, but several metal‐tolerant plant species (Angophora floribunda, Cassinia laevis, Chrysocephalum apiculatum, Cymbopogon refractus, Cynodon dactylon, Juncus subsecundus and Poa sieberiana) colonise the periphery of the contaminated site.     

Metal partitioning in sediments and mineralogical controls on the acid mine drainage in Ribeira da Água Forte (Aljustrel,Iberian Pyrite Belt,Southern Portugal)

This work focuses on the geochemical processes taking place in the acid drainage in the Ribeira da Água Forte, located in the Aljustrel mining area in the Iberian Pyrite Belt. The approach involved water and stream sediment geochemical analyses, as well as other techniques such as sequential extraction, Mössbauer spectroscopy, and X-ray diffraction. Ribeira da Água Forte is a stream that drains the area of the old mine dumps of the Aljustrel mine, which have for decades been a source of acid waters. This stream flows to the north for a little over than 10 km, but mixes with a reduced, organic-rich, high pH waste water from the municipal waste water pools of the village. This water input produces two different results in the chemistry of the stream depending upon the season: (i) in the winter season, effective water mixing takes place, and the flux of acid water from the mine dumps is continuous, resulting in the immediate precipitation of the Fe from the acid waters; (ii) during the summer season, acid drainage is interrupted and only the waste water feeds the stream, resulting in the reductive dissolution of Fe hydroxides and hydroxysulfates in the stream sediments, releasing significant quantities of metals into solution. Throughout the year, water pH stays invariably within 4.0–4.5 for several meters downstream of this mixing zone even when the source waters come from the waste water pools, which have a pH around 8.4. The coupled interplay of dissolution and precipitation of the secondary minerals (hydroxides and sulfates), keeps the system pH between 3.9 and 4.5 all along the stream. In particular, evidence suggests that schwertmannite may be precipitating and later decomposing into Fe hydroxides to sustain the stream water pH at those levels. While Fe content decreases by 50% from solution, the most important trace metals are only slightly attenuated before the solution mixes with the Ribeira do Rôxo stream waters. Concentrations of As are the only ones effectively reduced along the flow path. Partitioning of Cu, Zn and Pb in the contaminated sediments also showed different behavior. Specific/non-specific adsorption is relevant for Cu and Zn in the upstream branch of Ribeira da Água Forte with acid drainage conditions, whereas the mixture with the waste water causes that the association of these metals with oxyhydroxide to be more important. Metals bound to oxyhydroxides are on the order of 60–70% for Pb, 50% for Cu and 30–60% for Zn. Organic matter is only marginally important around the waste water input area showing 2–8% Cu bound to this phase. These results also show that, although the mixing process of both acid and organic-rich waters can suppress and briefly mitigate some adverse effects of acid drainage, the continuing discharge of these waste waters into a dry stream promotes the remobilization of metals fixed in the secondary solid phases in the stream bed back into solution, a situation that can hardly be amended back to its original state.     

Assessment of degradation of agricultural soils arising from brick burning in selected soil profiles

The study was conducted with the selected soil profile of burnt (soil around brick kilns) and unburnt (agricultural land) soils in the Dinajpur, Rangpur, Rajshahi, Khulna and Patuakhali districts at the western part of Bangladesh to evaluate the effects of brick kilns on soil degradation and environmental pollution. The pH values of the unburnt soils increased as a function of the soil depth for Rangpur, Khulna and Patuakhali, while decreased for the soil profiles in Dinajpur. Burning of soils significantly (p<0.05) decreased the average pH values of soils by 0.4 pH units (7 % increased over average content = IOAC), but strikingly increased the average EC values from 0.26 to 1.77 mS/cm (592 % IOAC) and the effect was pronounced with the depth function. The average sand content of the soil profiles increased by 330%, while the silt and clay contents decreased by 49 and 40 %, respectively. The average losses arising from the burning of agricultural soils were amounted to 63% for organic matter, 56 to 86 % and 23 to 88 % for available and total N, P, K and S, respectively. This huge loss through the burning of 1 m deep soil profile, i.e. almost 3/4^th of the deterioration of soil fertility is not only reducing the crop production but also polluting the associated environment and atmosphere. The burning of enormous C, N and S not only degrade the agricultural soils but also contributing to the changes in the global climate.     

Rate controls on the chemical weathering of natural polymineralic material. II. Rate-controlling mechanisms and mineral sources and sinks for element release from four UK mine sites,and implications for comparison of laboratory and field scale weathering studies

Predictions of mine-related water pollution are often based on laboratory assays of mine-site material. However, many of the factors that control the rate of element release from a site, such as pH, water–rock ratio, the presence of secondary minerals, particle size, and the relative roles of surface-kinetic and mineral equilibria processes can exhibit considerable variation between small-scale laboratory experiments and large-scale field sites.Monthly monitoring of mine effluent and analysis of natural geological material from four very different mine sites have been used to determine the factors that control the rate of element release and mineral sources and sinks for major elements and for the contaminant metals Zn, Pb, and Cu. The sites are: a coal spoil tip; a limestone-hosted Pb mine, abandoned for the last 200 a; a coal mine; and a slate-hosted Cu mine that was abandoned 150 a ago. Hydrogeological analysis of these sites has been performed to allow field fluxes of elements suitable for comparison with laboratory results to be calculated. Hydrogeological and mineral equilibrium control of element fluxes are common at the field sites, far more so than in laboratory studies. This is attributed to long residence times and low water–rock ratios at the field sites. The high water storativity at many mine sites, and the formation of soluble secondary minerals that can efficiently adsorb metals onto their surfaces provides a large potential source of pollution. This can be released rapidly if conditions change significantly, as in, for example, the case of flooding or disturbance.     

An integrated approach to environmental risk assessment of cumulatively impacted drainage basin from mining activities in southwestern Ghana

Acid rock/mine drainage and metal leaching constitute major environmental management risks in the mining industry. This paper assesses the environmental risks due to acid rock/mine drainage, and the metal leaching potential of multiple mines of gold and manganese on the Ankobra River Drainage Basin in Southwestern Ghana. The basin is a hub of mining activity in Ghana, hosting several mines. A combination of mineralogical, and static geochemical acid drainage predictive investigation of overburden of varied geological units, complimented with hydrochemical drainage quality analysis was used to assess potential environmental risks posed by acid-generating lithologies and mine spoil. Mineralogical investigations revealed sulphide-bearing lithological units with profound compositional variations due to the incorporation of potentially toxic heavy metals and metalloids, in association with carbonates and silicates. Accounting Base Accounting (ABA) and Net acid generation potential pH (NAGpH) tests delimited two tailing sites as potentially acid generating with NAGpH of 3.5 and 4.8, respectively. Five other samples, representing specific lithological units in the stratigraphic sequence, with net acid neutralization potential ratio (NNPR) less than 5.0, were classified as being potentially acid generating according to the categorization requirement of the US Forestry Service. The rest of the samples exhibited moderate to very strong buffering capabilities. The assessment also evaluated drainage quality of the network of streams and rivers constituting the basin and identified sources of drainage contaminants. Acidic waters emanate from identified acid generating sources, while high metal load regimes were identified with both low pH waters and high pH regimes, coincident with high sulphide and carbonate alteration sites, respectively. The study results show that Zn, Cu, Ni, As, Co, Sb, SO₄ ²⁻, pH, alkalinity and conductivity are essential and adequate parameters in routine environmental risk monitoring programmes of mines in the area. Sites characterized by low pH (<5.5) with high sulphate and metal ions are suggestive of acid mine drainage, while sites with high pH (>7.5), metal ions and sulphate are suggestive of net acid neutralizing.     

Stream water geochemistry from mine wastes in Peña de Hierro,Riotinto area,SW Spain: a case of extreme acid mine drainage

In this work we have studied the geochemistry of stream waters arising from waste dumps at the Peña de Hierro mine (Iberian Pyrite Belt, SW Spain), and we have correlated them with the mineralogical and geochemical characteristics of the wastes to asses the source and factors affecting the release of trace elements. The mineralogical composition and geochemistry of 58 borehole samples of waste dumps were studied in the <2 mm fraction. Twenty-eight water samples collected in winter and summer from streams emerging from the waste dumps were analysed for pH, Eh, conductivity, temperature, sulphates and major and trace elements. The leachates from pyrite-rich volcanic tuffs produced very acidic waters, usually with pHs below 2 and reaching values as low as 0.7. The partial dissolution of gossan, which is mainly composed of Fe oxy-hydroxides and is rich in trace elements, released high concentrations of Fe_tot (up to 33 g/L), As (up to 72), Mo (up to 11 mg/L). On the other hand Cd, Zn and Pb reached up to 0.85, 142 and 0.42 mg/L, respectively, in the stream arising from roasted pyrite ashes and other pyritic wastes. Several elements such as Al, Fe, As, Co, Cu and Mo were strongly correlated with the pH, but Cd and Zn were not correlated under such acidic conditions. The precipitation of jarosite seems to be an important factor in the retention of Pb. The mobility sequence of trace elements shows that Co, Zn and Cd were among the most mobile elements; Cu, As and Mo had intermediate mobility, and Pb was the most immobile. This work shows that uncontrolled waste dumping increases the pollution potential, and a selective management could reduce the release of trace elements into stream waters and mitigate the contamination.