Environmental impact and remediation of acid mine drainage: a management problem

 Work carried out at the abandoned copper (Cu) and sulphur (S) mine at Avoca (south east Ireland) has shown acid mine drainage (AMD) to be a multi-factor pollutant. It affects aquatic ecosystems by a number of direct and indirect pathways. Major impact areas are rivers, lakes, estuaries and coastal waters, although AMD affects different aquatic ecosystems in different ways. Due to its complexity, the impact of AMD is difficult to quantify and predict, especially in riverine systems. Pollutional effects of AMD are complex but can be categorized as (a) metal toxicity, (b) sedimentation processes, (c) acidity, and (d) salinization. Remediation of such impacts requires a systems management approach which is outlined. A number of working procedures which have been developed to characterise AMD sites, to produce surface water quality management plans, and to remediate mine sites and AMD are all discussed. Received: 16 January 1996 · Accepted: 5 March 1996     

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.     

Hydrochemical characterization of acute acid mine drainage at Iron Duke mine, Mazowe, Zimbabwe

 Acid mine drainage (AMD) with a minimum pH of 0.52 was recorded at Iron Duke mine near Mazowe, Zimbabwe during an investigation of the environmental geochemistry of mine waters in the Greenstone Belts of Zimbabwe. Hydrochemical data for waters emanating from the Iron Duke waste-rock pile indicate their super-saturation with respect to Fe and SO₄ ^2–. Extremely high dissolved concentrations of Al, Zn, Cu, Co, Ni, V, Cr, Cd and As also prevail. Substantial losses of metals from solution occur within 400 m of the AMD source through the precipitation of crystalline sulphates, principally melanterite. Further downstream, hydrous oxide precipitation forms the dominant mechanism of metal attenuation in waters characteristically under-saturated with respect to Fe sulphates. Speciation and saturation index data generated using the equilibrium model WATEQ4F, suggest that such codes have broad utility for generic prediction of the mineralogical contraints on metal mobility in acute AMD systems. Major discrepancies between modelled and empirical hydrochemistries are, however, evident for super-saturated waters in which the kinetics of Fe precipitation are slow, and in which total ionic strengths markedly exceed their theoretical maximum. Received: 28 August 1998 · Accepted: 7 December 1998     

Sampling riverine sediments impacted by acid mine drainage: problems and solutions

 Sampling acid mine drainage (AMD) or natural acid rock drainage (ARD)-impacted sediments is complex, requiring appropriate field sampling techniques to ensure representative samples that are both repeatable and reproducible. The important factors affecting sampling of riverine sediments are examined. These include sample site location, field observations, representative sampling, sample collection techniques, and sample preservation. A recommended sampling and processing protocol is presented for AMD- and ARD-impacted riverine sediments, which includes sediment sampling, Fe hydroxide floc sampling, chemical analysis, interstitial (pore) water collection, sediment elutriates, sediment fractionation, and physical analysis. The importance of bioassay testing is discussed, as is quality assurance and assessment approaches to define sediment quality criteria. Received: 18 September 1995 · Accepted: 23 October 1995     

A comparative mineralogical and geochemical study of sulfide mine tailings at two sites in New Mexico, USA

A comparative study of sulfide mine tailings from two sites near Silver City in southwest New Mexico has shown the need for environmental monitoring in a geological context. The Cyprus-Piños Altos and Cleveland deposits consist of Cu and Zn skarn mineralization in the Piños Altos Mountains of New Mexico. Primary ore minerals in both deposits include chalcopyrite, sphalerite, and galena. The Cyprus-Piños Altos Mine ceased operation in 1995 and the Cleveland Mill closed in 1950. The deposits have similar mineralogical characteristics; however, the tailings are different in terms of age, degree of oxidation and method of disposal. The Cyprus-Piños Altos tailings (CPAT) are stored in a lined, bermed impoundment. They are dominantly water-saturated and exhibit no secondary-phase formation. The grains are not cemented and show no evidence of primary-mineral dissolution. The geochemical data show a predominantly primary signature. The tailings pond water is neutral to slightly alkaline (pH?from 7 to 8.3), partly as a result of processing methods. The Cleveland mill tailings (CMT) were deposited in a valley at the headwaters of an ephemeral stream. They are highly oxidized and differentially cemented. They have undergone numerous wet/dry cycles resulting in extensive oxidation. Secondary minerals predominate, and consist mainly of jarosite, goethite, hematite, and Fe-oxyhydroxides and -oxyhydroxysulfates. The pH of the stream draining the CMT is as high as 2.15. Maximum metal contents in the stream immediately downstream from the tailings are 5305?ppm Zn, 454?ppm Cu, 1.16?ppm Pb, 17.5?ppm Cd, 1.4?ppm As, and 0.01?ppm Hg.     

Geochemical modeling of coal mine drainage, Summit County, Ohio

 Geochemical modeling was used to investigate downstream changes in coal mine drainage at Silver Creek Metro-park, Summit County, Ohio. A simple mixing model identified the components that are undergoing conservative transport (Cl^–, PO₄ ^3–, Ca²⁺, K⁺, Mg²⁺ and Na⁺) and those undergoing reactive transport (DO, HCO₃ ^–, SO₄ ^2–, Fe²⁺, Mn²⁺ and Si). Fe²⁺ is removed by precipitation of amorphous iron-hydroxide. Mn²⁺ are removed along with Fe²⁺ by adsorption onto surfaces of iron-hydroxides. DO increases downstream due to absorption from the atmosphere. The HCO₃ ^– concentration increases downstream as a result of oxidation of organic material. The rate of Fe²⁺ removal from the mine drainage was estimated from the linear relationship between Fe⁺² concentration and downstream distance to be 0.126 mg/s. Results of this study can be used to improve the design of aerobic wetlands used to treat acid mine drainage. Received: 4 June 1996 · Accepted: 17 September 1996     

Field assessment of acid mine drainage contamination in surface and ground water

Both sulfate and conductivity are useful indicators of acid mine drainage (AMD) contamination. Unlike pH, they are both extremely sensitive to AMD even where large dilutions have occurred. The advantage of using sulfate to trace AMD is that unlike other ions it is not removed to any great extent by sorption or precipitation processes, being unaffected by fluctuations in pH. These two parameters are also closely associated as would be expected, as conductivity is especially sensitive to sulfate ions. Therefore, as sulfate analysis is difficult in the field, conductivity can be used to predict sulfate concentration in both AMD and contaminated surface waters using regression analysis. Most accurate predictions are achieved by using equations given for specific conductivity ranges or AMD sources. There is also potential to use conductivity to predict approximate concentrations of key metals when the pH of the water is within their respective solubility ranges.     

Interaction of acid mine drainage with waters and sediments at the Corona stream, Lousal mine (Iberian Pyrite Belt, Southern Portugal)

This study investigates the geochemical characteristics of the acid mine drainage discharged from the abandoned mine adits and tailing piles in the vicinity of the Lousal mine and evaluates the extent of pollution on water and on the stream sediments of the Corona stream. Atmospheric precipitation interacting with sulphide minerals in exposed tailings produces runoff water with pH values as low as 1.9–2.9 and high concentrations of (9,249–20,700 mg l⁻¹), Fe (959–4,830 mg l⁻¹) and Al (136–624 mg l⁻¹). The acidic effluents and mixed stream water carry elevated Cu, Pb, Zn, Cd and As concentrations that exceed the water quality standards. However, the severity of contamination generally decreases 4 km downstream of the source due to mixing with fresh waters, which causes the dilution of dissolved toxic metals and neutralization of acidity. Some natural attenuation of the contaminants also occurs due to the general reduced solubility of most trace metals, which may be removed from solution, by either co-precipitation or adsorption to the iron and aluminium precipitates.     

Hydrogeochemical characteristics of acid mine drainage and water pollution at Makum Coalfield,India

Acid mine drainage (AMD) is one of the severe environmental problems that coal mines are facing. Generation of AMD in the northeastern part of India due to the coal mining activities has long been reported. However detailed geochemical characterization of AMD and its impact on water quality of various creeks, river and groundwater in the area has never been reported. Coal and coal measure rocks in the study area show finely disseminated pyrite crystals. Secondary solid phases, resulted due to oxidation of pyrite, occur on the surface of coal, and are mainly consisting of hydrated sulphate complexes of Fe and Mg (copiapite group of minerals). The direct mine discharges are highly acidic (up to pH 2.3) to alkaline (up to pH 7.6) in nature with high concentration of SO₄²⁻. Acidic discharges are highly enriched with Fe, Al, Mn, Ni, Pb and Cd, while Cr, Cu, Zn and Co are below their maximum permissible limit in most mine discharges. Creeks that carrying the direct mine discharges are highly contaminated; whereas major rivers are not much impacted by AMD. Ground water close to the collieries and AMD affected creeks are highly contaminated by Mn, Fe and Pb. Through geochemical modeling, it is inferred that jarosite is stable at pH less than 2.5, schwertmannite at pH less than 4.5, ferrihydrite above 5.8 and goethite is stable over wide range of pH, from highly acidic to alkaline condition.     

Hydrogeochemical characteristics of acid mine drainage in the vicinity of an abandoned mine, Daduk Creek, Korea

Acid mine drainage discharged from the abandoned Daduk mine towards the Daduk creek has a pH of 3.3, and concentrations of Al, Mn, Fe, Zn and SO₄ of 18, 41, 45, 38 and 1940 mg/L, respectively. In particular, As concentration in acid mine drainage is 1000 μg/L. Removing order of metal ions normalized by SO₄ concentration downstream from discharge point is Fe > As > Al > Cu > Zn > Mn > Cd > Pb. In the Daduk creek, Fe and As are the most rapidly depleted downstream from acid mine drainage because As adsorbs, coprecipitates and forms compounds with ferric oxyhydroxide. From the results of geochemical modeling using the Phreeq C program, goethite (FeOOH) is oversaturated, and schwertmannite (Fe₈O₈(OH)_4.5(SO₄)_1.75) is the most stable solid phase at low pH in the Daduk creek. Yellowish red (orange ochre) precipitates that occurred in the study area are probably composed of goethite or schwertmannite.     

Mineralogical constraints on the determination of neutralization potential and prediction of acid mine drainage

 Acid-base accounting tests, commonly used as a screening tool in acid mine drainage (AMD) predictions, have limitations in (1) measuring with confidence the amount of neutralizers present in samples and (2) affording an interpretation of what the test results mean in terms of predicting the occurrence of acid mine drainage. Aside from the analytical difficulties inherent to the conventional methods, a potential source of error in neutralization potential (NP) measurements is the contribution from the dissolution of non-carbonate minerals. Non-carbonate alkalinity measured during static tests may or may not be available to neutralize acidity produced in the field. In order to assess the value-added of extending the NP with the knowledge of mineralogical composition and evaluate potential sources of errors in NP measurements, a suite of samples were examined and characterized in terms of their mineralogical and chemical compositions. The results indicate that although the acid-base accounting tests work well for simple compositions, the tests may result in overestimation or underestimation of NP values for field samples. Mineralogical constraint diagrams relating NP determinations to Ca, Mg and CO₂ concentrations were developed with the purpose to serve as supplementary guides to conventional static tests in identifying possible NP contributions from non-carbonate minerals and checking the quality of the chemical testing results. Mineralogical NP makes it possible to interpret the meaning of NP results and to assess the behaviour of samples over time by predicting the onset of AMD and calculating NP values for individual size fractions. Received: 1 June 1998 · Accepted: 6 October 1998     

Heavy metal geochemistry of the acid mine drainage discharged from the Hejiacun uranium mine in central Hunan,China

The acid mine drainage (AMD) discharged from the Hejiacun uranium mine in central Hunan (China) was sampled and analyzed using ICP-MS techniques. The analyzing results show that the AMD is characterized by the major ions Fe_Total, Mn, Al and Si, and is concentrated with heavy metals and metalloids including Cd, Co, Ni, Zn, U, Cu, Pb, Tl, V, Cr, Se, As and Sb. During the AMD flowing downstream, the dissolved heavy metals were removed from the AMD waters through adsorption onto and co-precipitation with metal-oxhydroxides coated on the streambed. Among these metals, Cd, Co, Ni, Zn, U, Cu, Pb and Tl are negatively correlated to pH values, and positively correlated to major ions Fe, Al, Si, Mn, Mg, Ca and K. The metals/metalloids V, Cr, Se, As and Sb are conservative in the AMD solution, and negatively-correlated to major ions Na, Ca and Mg. Due to the above different behaviors of these chemical elements, the pH-negatively related metals (PM) and the conservative metals (CM) are identified; the PM metals include Cd, Co, Ni, Zn, U, Cu, Pb and Tl, and the CM metals V, Cr, Se, As and Sb. Based on understanding the geochemistry of PM and CM metals in the AMD waters, a new equation: EXT = (Acidity + PM)/pH + CM × pH, is proposed to estimate and evaluate extent of heavy-metal pollution (EXT) of AMD. The evaluation results show that the AMD and surface waters of the mine area have high EXT values, and they could be the potential source of heavy-metal contamination of the surrounding environment. Therefore, it is suggested that both the AMD and surface waters should be treated before they are drained out of the mine district, for which the traditional dilution and neutralization methods can be applied to remove the PM metals from the AMD waters, and new techniques through reducing the pH value of the downstream AMD waters should be developed for removal of the CM metals.     

Acid mine drainage and acid rock drainage processes in the environment of Herrerías Mine (Iberian Pyrite Belt, Huelva-Spain) and impact on the Andevalo Dam

The present work describes the process of acid water discharge into the Andévalo Dam (Iberian Pyrite Belt, Huelva-Spain) starting from the interpretation of rainfall data and chemical analyses regarding pH, conductivity, metal and sulphate content in water, from a time series corresponding to the sampling of two confluent channels that discharge water into the referred dam. Statistical data treatment allows us to conclude the existence of acid mine drainage processes in the Chorrito Stream, which are translated into very low pH values and high sulphate and metal concentrations in the water coming from Herrerías Mine. On the other hand, the Higuereta Stream shows, for the same parameters, much lower values that can be interpreted as the channel response to acid rock drainage processes in its drainage basin induced by the rocky outcrops of the Iberian Pyrite Belt.     

Experimental study of the stability of metals associated with iron oxyhydroxides precipitated in acid mine drainage

Iron oxyhydroxide precipitates associated with acid mine drainage (AMD) from the Stearns Coal Zone in southeastern Kentucky were analyzed for their metal (Al, Cu, Pb, Mn, Ni, and Zn) content. The most concentrated metals within these sediments are nickel (27–32×10³μmol/kg), manganese (16–29×10³μmol/kg), and aluminum (13–22×10³μmol/kg) as determined by HCl-HNO₃ digestion. Metal concentrations associated with the organic fraction as determined by H₂O₂ digestion were generally far lower, with the exception of aluminum. "Batch" experiments (at initial pH=2.0) were used to analyze the stability of these metals associated with a contaminated soil. Aluminum was the most mobile of the metals, presumably the result of the formation of aluminum-sulfate aqueous complexes. The solubilization rates for nickel and iron were very similar, suggesting that nickel, unlike the other metals, coprecipitated with iron in these sulfatic oxyhydroxides. Received: 9 October 1997 · Accepted: 15 December 1997     

Comparison of color, chemical and mineralogical compositions of mine drainage sediments to pigment

 Forty-three untreated and actively and passively (wetland) treated coal mine drainage sediments and five yellow-red pigments were characterized using X-ray fluorescence, fusion-inductively coupled plasma atomic emission spectroscopy, X-ray diffraction, and tristimulus colorimeter. Primary crystalline iron-bearing phases were goethite and lepidocrocite, and iron phases converted to hematite upon heating. Quartz was nearly ubiquitous except for synthetic pigments. Gypsum, bassinite, calcite, and ettringite were found in active treatment sediments. Iron concentrations from highest to lowest were synthetic pigment>wetland sediment>natural pigment>active treatment (untreated sediments varied more widely), and manganese was highest in actively treated sediments. Loss on ignition was highest for passively treated sediments. No clear trends were observed between quantified color parameters (L*, a*, b*, and Redness Index) and chemical compositions. Because sediments from passive treatment are similar in chemistry, mineralogy, and color to natural pigments, the mine drainage sediments may be an untapped resource for pigment. Received: 29 December 1997 · Accepted: 11 May 1998     

The potential formation of acid mine drainage in pyrite-bearing hard-coal tailings under water-saturated conditions: an experimental approach

 Annually, an amount of approximately 13 million cubic meters of hard-coal tailings must be disposed of in the German Ruhr Valley. Besides the waste of land in a densily populated region, the disposal of the pyrite-bearing material under atmospheric conditions may lead to the formation of acid mine drainage (AMD). Therefore, alternative disposal opportunities are of increasing importance, one of which being the use of tailings under water-saturated conditions, such as in backfilling of abandoned gravel pits or in the construction of waterways. In this case, the oxidation of pyrite, and hence the formation of AMD, is controlled by the amount of oxygen dissolved in the pore water of tailings deposited under water. In case the advective percolation of water is suppressed by sufficient compaction of the tailings, oxygen transport can be reduced to diffusive processes, which are limited by the diffusive flux of dissolved oxygen in equilibrium with the atmospheric pO₂. Calculations of the duration of pyrite oxidation based on laboratory experiments have shown that the reduction of oxygen is mainly controlled by the content of organic substance rather than the pyrite content, a fact that is supported by results from oxidation experiments with nitrate. A "worst case" study has lead to the result that the complete oxidation of a 1.5-m layer of hard-coal tailings deposited under water-saturated conditions would take as much as several hundred thousand years. Received: 6 May 1996 · Accepted: 2 August 1996     

Arsenate and arsenite adsorption onto Al-containing ferrihydrites. Implications for arsenic immobilization after neutralization of acid mine drainage

Natural ferrihydrites (Fh) often contain impurities such as aluminum, especially in acid mine drainage, and these impurities can potentially impact the chemical reactivity of Fh with respect to metal (loid) adsorption. In the present study, we have investigated the influence of aluminum on the sorption properties of ferrihydrite with respect to environmentally relevant aqueous arsenic species, arsenite and arsenate. We have conducted sorption experiments by reacting aqueous As(III) and As(V) with synthetic Al-free and Al-bearing ferrihydrite at pH 6.5. Our results reveal that, when increasing the Al:Fe molar ratio in Fh, the sorption density dramatically decreased for As(III), whereas it increased for As(V). Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy analysis at the As K-edge indicated that the As^IIIO₃ pyramid binds to FeO₆ octahedra on both Al-free Fh and Al-bearing Fh, by forming bidentate mononuclear edge-sharing (²E) and bidentate binuclear corner-sharing (²C) surface complexes characterized by As–Fe distances of 2.9 Å and 3.4 Å, respectively. The decrease in As(III) sorption density with increasing Al:Fe ratio in Fh could thus be explained by a low affinity of the As(OH)₃ molecule for Al surface sites compared to Fe ones. In contrast, on the basis of available literature on As(V) adsorption mechanisms, we suggest that, in addition to inner-sphere ²C arsenate surface complexes, outer-sphere arsenate surface complexes forming hydrogen bonds with both Al–OH and Fe–OH surface sites could explain the enhancement of As(V) sorption onto aluminous Fh relative to Al-free Fh, as observed in the present study. The presence of aluminum in Fh may thus enhance the mobility of arsenite with respect to arsenate in Acid Mine Drainage impacted systems, while mixed Al:Fe systems could present an alternative for arsenic removal from impacted waters, provided that As(III) would be oxidized to As(V).     

Impact of AMD on water quality in critical watershed in the Hudson River drainage basin: Phillips Mine,Hudson Highlands,New York

A sulfur and trace element enriched U–Th-laced tailings pile at the abandoned Phillips Mine in Garrison, New York, releases acid mine drainage (AMD, generally pH < 3, minimum pH 1.78) into the first-order Copper Mine Brook (CMB) that drains into the Hudson River. The pyrrhotite-rich Phillips Mine is located in the Highlands region, a critical water source for the New York metro area. A conceptual model for derivation/dissolution, sequestration, transport and dilution of contaminants is proposed. The acidic water interacts with the tailings, leaching and dissolving the trace metals. AMD evaporation during dry periods concentrates solid phase trace metals and sulfate, forming melanterite (FeSO₄·7H₂O) on sulfide-rich tailings surfaces. Wet periods dissolve these concentrates/precipitates, releasing stored acidity and trace metals into the CMB. Sediments along CMB are enriched in iron hydroxides which act as sinks for metals, indicating progressive sequestration that correlates with dilution and sharp rise in pH when mine water mixes with tributaries. Seasonal variations in metal concentrations were partly attributable to dissolution of the efflorescent salts with their sorbed metals and additional metals from surging acidic seepage induced by precipitation.     

A mass balance approach to estimate the dilution and removal of the pollutants in stream water polluted by acid mine drainage

 A few simple mass balance equations were developed to simultaneously estimate how much the pollutants from acid mine drainage (AMD) in stream water are diluted and removed during their migration. The application of the equations requires knowledge of the variations in the concentrations of the dissolved pollutants and the stoichiometry of the precipitation reaction of the pollutants when none of the pollutant shows a conservative behavior along the stream path. The calculation should be restricted to the pollutants showing much higher concentrations in the polluted main stream water than in the combining or diluting water of the same target area. The mass balance equations were applied to estimate the dilution factor and precipitation fractions of pollutants in Imgok Creek such as Fe, SO₄ and Al from the AMD of Yeongdong mine. The results show that the estimation, especially for SO₄ and Al, significantly depends on the kinds of the precipitates. When FeOHSO₄ and AlOHSO₄ are assumed to precipitate, the maximum removal fractions of SO₄ and Al by precipitation are respectively 34% and 46% of the original input, which is much higher than the values estimated when SO₄ is considered to be perfectly conservative. It indicates that the stoichiometry of precipitation reaction is very important in the interpretation of the pollutant dilution and migration and assessment of environmental impacts of AMD. The applicability of the mass balance equations may still need to be verified. However, examining the calculated dilution factor and precipitation fractions with the equations can provide invaluable information on not only the behavior but also unexpected input of the pollutants in the stream water polluted by AMD and other point sources. Received: 12 November 1997 · Accepted: 30 March 1998     

Geochemical characteristics of acid mine drainage and sediments from coal mines, Shanxi Province, China

In this study, geochemical characteristics of acid mine drainage （AMD） and its sediments from the Malan and Sitai coalmines, Shanxi Province, China, were investigated. Many analytical approaches such as IC, ICP-MS, XRD, XRF, and modeling calculation of hydrogeochemistry using PHREEQCI software were employed. The AMD is characterized by higher concentrations of iron and sulfate, a low pH, and elevated concentrations of a wide variety of heavy metals. The results of modeling calculation by PHREEQCI software demonstrate the metals in AMD are present mainly as Me^n＋ and MeSO4^n-2 species. The sediments of AMD are composed mainly of iron-beating minerals such as goethite and schwertmannite, which are controlled by pH, Fe and SO4^2- concentrations. The schwertmannite mineral has been found for the first time in China.