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.     

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     

Geological controls on natural ecosystem recovery on mine waste in southern New Zealand

Slopes of an abandoned waste rock at Wangaloa coal mine, south-east New Zealand, have naturally developed variable vegetation cover over the last 40–60 years. Three distinct areas of revegetation can clearly be identified: dense cover, patchy cover, and largely unvegetated, and the differences in revegetation success are directly related to the physical properties of different rock types making up the waste rock substrate. The colonizing plants have become established in largely unweathered rock with essentially no soil development. Quartz gravel and siltstone waste rock are the two principal rock types forming substrates for revegetation. The quartz gravel has clasts up to 3 cm, and was derived from the coal-bearing sequence. Siltstone was largely derived from a Quaternary loess cap on the coal mine area. These two substrates have similar mineral contents, and this mineral material provides the low level of available nutrients. However, there is little difference in nutrient status or trace element load of the different substrates, and differences in cohesion, moisture content, and proportion of quartz pebbles control revegetation success. Finer grained matrix has been flushed from quartz gravel waste rock by rain water, leaving a dry surface armour layer of quartz pebbles. This surface layer inhibits plant establishment, so quartz gravel waste rock remains largely unvegetated. Erosion creates deep rills, and steep surfaces creep downslope. In contrast, full vegetation cover was established on the siltstone waste rock that was cohesive and did not erode. Patchy revegetation was localized by siltstone in mixed quartz gravel and siltstone substrate. Invertebrate diversity and distribution were closely linked to the spatial patterns of revegetation. The rate of revegetation and ecosystem recovery was primarily dependent on the proportion of siltstone waste rock in the last dumped truck load. A quartz pebble content <15% is optimal for plant establishment.     

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     

Atmospheric oxidation of the pyritic waste rock in Maardu, Estonia. 1 field study and modelling

 A field survey and modelling of the oxidation and carbonate buffering reactions inside the alum-shale-containing waste rock dumps located in Maardu, Estonia, was accomplished. In the slope areas, the shale has been altered at high temperatures due to the spontaneous combustion and the pyritic acidity has been eliminated through migration of SO_x gases out from the dump. In the central parts of the waste rock plateaus, low temperature pyrite oxidation fronts develop towards the dump depth and towards the centres of individual shale lumps. The main secondary phases precipitating in the weathering profile are gypsum, ferric oxyhydroxide, K-jarosite and smectite. The respective field data made it possible to calibrate the two-stage oxygen diffusion model and the characteristic pyrite oxidation rate 0.06–0.08 mol of pyrite reacted per kg of available water (pyrox/H₂O value) was estimated to describe the first tens of years of dump performance. The model is capable to compare different shale disposal strategies that are illustrated with two case scenarios. The buffering of sulphuric acid by Mg-calcite appears to be an incongruent reaction with gypsum precipitating that leads to the build-up of the high Mg/Ca ratio in the leachate. Application of the Mg/Ca method estimates the pyrox/H₂O value in the range of 0.05–0.14 mol/kg. Received: 26 January 1999 · Accepted: 23. February 1999     

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     

The Diavik Waste Rock Project: Geochemical and microbiological characterization of low sulfide content large-scale waste rock test piles

Two experimental waste-rock piles (test piles), each 15 m in height × 60 m × 50 m, were constructed at the Diavik diamond mine in Northern Canada to study the behavior of low-sulfide content waste rock, with a similarly low acid-neutralization potential, in a continuous permafrost region. One test pile with an average of 0.035 wt.% S (<50 mm fraction; referred to as Type I) and a second test pile with an average of 0.053 wt.% S (<50 mm fraction; referred to as Type III) were constructed in 2006. The average carbon content in the <50 mm fraction of waste rock in the Type I test pile was 0.031 wt.% as C and in the Type III test pile was 0.030 wt.% as C. The NP:AP ratio, based on the arithmetic mean of particle-size weighted NP and AP values, for the Type I test pile was 12.2, suggesting this test pile was non-acid generating and for the Type III test pile was 2.2, suggesting an uncertain acid-generating potential. The Type I test pile maintained near-neutral pH for the 4-year duration of the study. Sulfate and dissolved metal concentrations were low, with the exception of Ni, Zn, Cd, and Co in the fourth year following construction. The pore water in the Type III test pile contained higher concentrations of SO₄²⁻ and dissolved metals, with a decrease in pH to <4.7 and an annual depletion of alkalinity. Maximum concentrations of dissolved metals (20 mg L⁻¹ Ni, 2.3 mg L⁻¹ Cu, 3.7 mg L⁻¹ Zn, 35 μg L⁻¹ Cd, and 3.8 mg L⁻¹ Co) corresponded to decreases in flow rate, which were observed at the end of each field season when the contribution of the total outflow from the central portion of the test pile was greatest. Bacteria were present each year in spite of annual freeze/thaw cycles. The microbial community within the Type I test pile included a population of neutrophilic S-oxidizing bacteria. Each year, changes in the water quality of the Type III test-pile effluent were accompanied by changes in the microbial populations. Populations of acidophilic S-oxidizing bacteria and Fe-oxidizing bacteria became more abundant as the pH decreased and internal test pile temperatures increased. Irrespective of the cold-climate conditions and low S content of the waste rock, the geochemical and microbiological results of this study are consistent with other acid mine drainage studies; indicating that a series of mineral dissolution–precipitation reactions controls pH and metal mobility, and transport is controlled by matrix-dominated flow and internal temperatures.     

Iron isotopes in acid mine waters and iron-rich solids from the Tinto–Odiel Basin (Iberian Pyrite Belt, Southwest Spain)

The isotopic composition of Fe was determined in water, Fe-oxides and sulfides from the Tinto and Odiel Basins (South West Spain). As a consequence of sulfide oxidation in mine tailings both rivers are acidic (1.45 < pH < 3.85) and display high concentrations of dissolved Fe (up to 420 mmol l^− 1) and sulphates (up to 1190 mmol l^− 1).The δ⁵⁶Fe of pyrite-rich samples from the Rio Tinto and from the Tharsis mine ranged from − 0.56 ± 0.08‰ to + 0.25 ± 0.1‰. δ⁵⁶Fe values for Fe-oxides precipitates that currently form in the riverbed varied from − 1.98 ± 0.10‰ to 1.57 ± 0.08‰. Comparatively narrower ranges of values (− 0.18 ± 0.08‰ and + 0.21 ± 0.14‰) were observed in their fossil analogues from the Pliocene–Pleistocene and in samples from the Gossan (the oxidized layer that formed through exposure to oxygen of the massive sulfide deposits) (− 0.36 ± 0.12‰ to 0.82 ± 0.07‰). In water, δ⁵⁶Fe values ranged from − 1.76 ± 0.10‰ to + 0.43 ± 0.05‰.At the source of the Tinto River, fractionation between aqueous Fe(III) and pyrite from the tailings was less than would be expected from a simple pyrite oxidation process. Similarly, the isotopic composition of Gossan oxides and that of pyrite was different from what would be expected from pyrite oxidation. In rivers, the precipitation of Fe-oxides (mainly jarosite and schwertmannite and lesser amounts of goethite) from water containing mainly (more than 99%) Fe(III) with concentrations up to 372 mmol l^− 1 causes variable fractionation between the solid and the aqueous phase (− 0.98‰ < Δ⁵⁶Fe_{solid–water} < 2.25‰). The significant magnitude of the positive fractionation factor observed in several Fe(III) dominated water may be related to the precipitation of Fe(III) sulphates containing phases.     

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.     

Speciation and solubility relationships of Al, Cu and Fe in solutions associated with sulfuric acid leached mine waste rock

 Solutions from oxidized waste rock originating from an acid-leached waste dump were studied. The dissolution data suggest that after the majority of the soluble solid phases are removed, remaining solid phases continued to buffer the solutions in the acidic pH range. Incorporating the solution data into MINTEQA2 identified controls on the solubility of Al, Cu and Fe at pH values from about 2.5 to slightly over 5. Sulfate appears to play a significant role in the formation of solubility controlling solid phases for Al and Cu. This is not the case for Fe, and is suggested that Fe and Cu solubility may be controlled by cupric ferrite at low pH values. Received: 5 April 1998 · Accepted: 27 July 1998     

Acid mine drainage at a coal mine in the eastern Transvaal, South Africa

 Several mines in the Witbank coalfield in South Africa are affected by acid mine drainage. This has led to a deterioration in the water quality in many surface streams. The Loubert Mine is one such mine. Hence, an initial investigation was carried out to determine the source of acid mine drainage pollution and the associated hydrogeological conditions. The investigation showed that most of the acid mine drainage is emanating from old opencast workings which have been backfilled. Most of the water from the backfilled area drains into control reservoirs. Unfortunately their capacity is limited, which means that water overspills and seeps from them. This water finds its way into a nearby stream, the water of which accordingly has an unacceptably low pH value and high sulphate content. The proposals advanced to control the problem basically involve inhibiting the amount of water infiltrating the backfilled opencast area on the one hand and reducing the amount of water entering the control reservoirs on the other. Received: 5 March 1997 · Accepted: 17 June 1997     

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     

Use of sample color to estimate oxidized Fe content in mine waste rock

 Color, a readily perceived feature of natural earth materials, including mine waste, often represents compositional variation as a result of oxidative processes involving Fe. Near surface samples from excavated trenches in a mine-waste rock pile were collected to investigate the relationship between color and contents of Fe, Cu, and S. The silt+clay fraction (<0.05 mm) was isolated from recognizable colored material of the bulk sample for determination of pH, total and extractable Fe and Cu, and bulk mineralogy. Rock fragments within the pile exhibited coatings of crystalline gypsum and amorphous Fe. These coatings result from weathering (secondary products) and play important roles in surface reactions of waste rock piles, such as adsorption of anions (SO₄ ^2–) or coprecipitation of Fe with Cu. The correlation between color (Hurst method) and extractable Fe was high. Although color is influenced by site conditions such as original mineral composition, materials handling, weathering conditions etc., the results suggest that color measurements may provide an inexpensive and rapid estimation for secondary iron compounds and associated sorbed elements. Received: 5 April 1998 · Accepted: 30 June 1998     

Atmospheric oxidation of the pyritic waste rock in Maardu, Estonia, 2: an assessment of aluminosilicate buffering potential

 The assessment of the aluminosilicate buffering potential during acid weathering of the Estonian alum shale is provided. It is found that the stoichiometric interaction between dissolved pyrite oxidation products and illite of the shale best describe the buffering process and are consistent with earlier field studies. The scheme includes incongruent dissolution of illite with smectite and K-jarosite precipitating. This complex mechanism involves buffering of 8% of the acidity by K⁺ and temporary precipitation of 25% of the acidity as K-jarosite. Dissolution proceeds at a low pH (1.5–3) until all pyrite in the shale particle is oxidised. Hence, if the total amount of illite present is larger than needed for stoichiometric interactions, only part of it is involved in a buffering process, neutralising a certain percentage of acidity. The next stage in shale weathering is the incongruent dissolution of K-jarosite with the release of the precipitated acidity and the formation of ferric oxyhydroxide. Received: 3 August 1998 · Revised paper: 26 January 1999 · Accepted: 23 February 1999     

Characterization of anthropogenic and natural sources of acid rock drainage at the Cinnamon Gulch abandoned mine land inventory site,Summit County,Colorado

Colorado's Cinnamon Gulch releases acid rock drainage (ARD) from anthropogenic and natural sources. In 2001, the total discharge from Cinnamon Gulch was measured at 1.02 cfs (29 L/s) at base flow and 4.3 cfs (122 L/s) at high flow (spring runoff). At base flow, natural sources account for 98% of the discharge from the watershed, and about 96% of the chemical loading. At high flow, natural sources contribute 96% of discharge and 92 to 95% of chemical loading. The pH is acidic throughout the Cinnamon Gulch watershed, ranging from 2.9 to 5.4. At baseflow, nearly all of the trace metals analyzed in the 18 samples exceeded state hardness-dependent water quality standards for aquatic life. Maximum dissolved concentrations of selected constituents included 16 mg/L aluminum, 15 mg/L manganese, 40 mg/L iron, 2 mg/L copper, 560 g/L lead, 8.4 mg/L zinc, and 300 mg/L sulfate. Average dissolved concentrations of selected metals at baseflow were 5.5 mg/L aluminum, 5.5 mg/L manganese, 14 g/L cadmium, 260 g/L copper, 82 g/L lead, and 2.8 mg/L zinc.     

Sources of environmental sulfur in the groundwater system,southern New Zealand

Sulfide minerals commonly occur in sediments and basement rocks in southern New Zealand, as authigenic precipitates from groundwater below the oxygenated surface zone. There are two principal potential sources for sulfur in the groundwater system: weathering of sulfide minerals in the metamorphic basement and rainwater-derived marine aerosols. We present data for these two key sulfur sources: metamorphic sulfide and associated hydrothermal Au-bearing veins within the Otago Schist (average δ³⁴S = −1.8 ± 2.4‰), and an inland saline lake (S derived entirely from rainwater, δ³⁴S = 21.4 ± 0.8‰). We use these two end member δ³⁴S values to estimate the contributions of these sources of sulfur in authigenic groundwater sulfide minerals and in waters derived from oxidation of these sulfide minerals, across a range of environments. We show that authigenic groundwater pyrite along joints in the Otago schist is derived primarily from metamorphic basement sulfur. In contrast, authigenic groundwater pyrite cementing Miocene-Recent aquifers shows a substantial marine aerosol component, and represents a distinct hydrogeological system. We suggest that marine aerosols represent a significant flux to the terrestrial sulfur cycle that has been present through the groundwater system in Otago over the past 20 million years.     

Estimation of critical convergence and rock load in coal mine roadways — an approach based on rock mass rating

Summary Based on field instrumentation in eight different coal mines representing varying depths and strata conditions, a relation for obtaining the critical convergence value has been established. In development heading for bord and pillar workings this relation can be used successfully to control the premature collapse of the roof. An empirical relation for rock load has been established. This can be utilized for optimum design of support system. The roofs have been categorized as stable, short-term stable and unstable. Proper attention should be provided for an unstable roof and the support design is to be changed before the convergence reaches the critical value.     

Mineralogical study of base metal tailings with various sulfide contents, oxidized in laboratory columns and field lysimeters

 Oxidation of a flotation-derived, low-sulfide tailings containing approximately 0.4 wt.% S was compared with simultaneously oxidized tailings containing 1.0 wt.% S and 2.5 wt.% S to assess their acid generating characteristics. Each tailings type was exposed to oxidation for three years in laboratory columns and in lysimeter pits in the field. In these tailings the sulfide mineral of principal concern with respect to acid generation is pyrrhotite (Fe _1-x S). In past studies the alteration of pyrrhotite has been characterized by initial replacement with marcasite (FeS₂) and ferric iron sulfates, which are followed by development of ferric oxyhydroxides such as goethite and lepidocrocite. Macroscopic characterization of the tailings shows varying and progressive degrees of oxidation correlative with the three different sulfur contents. As expected, the tailings with the lowest sulfur content are the least oxidized, and those with the highest sulfur content have reacted the most. The column tests, which represent accelerated reaction conditions relative to those for the lysimeter pits, show much higher degrees of oxidation, and a markedly more distinct boundary between the oxidized and unoxidized zones; as well, differences among the three tailings types are more pronounced. Received: 31 October 1997 · Accepted: 27 May 1997     

Local partial melting of the lower crust triggered by hydration through melt–rock interaction: an example from Fiordland,New Zealand

We investigate a low‐strain outcrop of the lower crust, the Pembroke Granulite, exposed in northern Fiordland, New Zealand, which exhibits localized partial melting. Migmatite and associated tschermakite–clinozoisite (TC) gneiss form irregular, elongate bodies that cut a two‐pyroxene–pargasite (PP) gneiss. Gradational boundaries between rock types, and the progressive nature of changes in mineral assemblage, microstructure and chemistry are consistent with the TC gneiss and migmatite representing modified versions of the PP gneiss. Modification is essentially isochemical, where partial modification involves hydration of the assemblage and mineral chemistry changes, and complete modification involves additional recrystallization and in situ partial melt production. Microstructures of quartz and plagioclase, including small dihedral angles, string of beads textures and films surrounding amphibole and garnet grains are consistent with the former presence of melt in modified rock types. The documented rock modification is attributed to melt–rock interaction occurring during porous melt flow of a dominantly externally derived, hydrous silicate melt. Microstructures indicate melt flow occurred along grain boundaries and field relationships show it was focused into channels tens of metres wide, with preference for following the pre‐existing foliation. Melt–rock interaction at the grain scale resulted in hydration and modification of the host PP gneiss, which resulted in localized partial melting. These relationships indicate prograde hydration during localized melt–rock interaction drove migmatization of the lower crust.     

Potential application of oxygen-18 and deuterium in mining effluent and acid rock drainage studies

 Oxygen-18 (¹⁸O) and deuterium (D, or ²H) are routinely used in hydrologic, climatologic and geothermal studies. In hydrology, stable isotopes provide information on the type and topology (altitude and latitude) of the recharge waters and the historical effects on water, related to such physical processes as evaporation (in ponds), melting (of snow or ice), condensation, evapotranspiration and mixing. In geothermal studies, stable isotopes provide key information related to recharge and the various temperature-dependent water/rock isotope exchange reactions. The latter is assessed through the oxygen shift in the ¹⁸O/D correlation. At acid rock drainage (ARD) sites, water/rock interactions are primarily controlled by pH and oxidation potential. Using the isotopic characteristics of the rocks and the recharge waters as a basis, the relative oxygen shift of the ARD effluent can provide information on: (1) the residence time, (2) the rate of water/rock reactions, and (3) the actual pH at the rock/water interface. This paper offers a methodology for conducting oxygen and hydrogen isotope studies related to ARD and other mineral effluent problems. The methodology is based on: (1) comprehensive sampling of regional waters, ARD effluent and major contributing minerals and rocks, (2) isotopic and elemental analysis, and (3) data interpretation on the basis of a zero-dimensional (mass balance), multi-component mixing model. Received: 15 January 1999 · Accepted: 3 May 1999