Balance of S in a constructed wetland built to treat acid mine drainage, Idaho Springs, Colorado, U.S.A.

The wetland constructed at the Big Five Tunnel in Idaho Springs, Colorado was designed to remove, passively, heavy metals from acid mine drainage. In optimizing the design of such a wetland, an improved understanding of the chemical processes operating there was required, particularly SO₄²⁻ reduction and sulfide precipitation. For this purpose, field and laboratory data were collected to study the balance of S in the system. Field data collected included water analyses of the mine drainage and wetland effluents and measurements of H₂S gas emissions from the wetland. The concentration of sulfide in the wetland effluent ranged from 10⁻⁴ to 10⁻³ mol/l. The average rates of H₂S emission from the surface of the substrate were 150 nmol/cm²/d in the summer and 0.17 and 0.35 nmol/cm²/d in the winter. This maximum estimated loss of sulfide was not significant in reducing the amount of sulfide available for precipitation with metals. Sequential extraction experiments for S on wetland substrates showed that acid volatile sulfides (AVS) increased with time in the wetland substrate. A serum bottle experiment was conducted to study the S balance in the Big Five wetland by quantitatively measuring the amount of S in different phases as microbial SO₄²⁻ reduction progressed. The increase in AVS reasonably balanced the decrease in SO₄²⁻ concentration in the experiment, suggesting that the decrease in SO₄²⁻ concentration represented the amount of SO₄²⁻ reduced and that nearly all of the sulfide produced was precipitated as AVS. Sulfide precipitation was determined to be the primary metal removal process in the wetland system and amorphous FeS is the primary iron sulfide formed in the substrate.     

Uraniferous waters of the Arkansas River valley,Colorado, U.S.A.: a function of geology and land use

The effect of local geology and land-use practices on dissolved U was investigated by analysis of surface water and some springs in the Arkansas River valley of southeastern Colorado. Water samples were collected during a 2 week period in April, 1991. The rate of increase of U concentration with distance downriver increased markedly as the river flowed from predominantly undeveloped lands underlain by igneous and metamorphic rocks to agriculturally developed lands underlain by marine shale and limestone. An additional abrupt increase in dissolved U was observed along the section of river where discharge is often greatly reduced because of extensive diversions for irrigation and where remaining flow is largely composed of irrigation return water. Dissolved U in this last section of river and in most of its tributaries exceeded the proposed U.S. drinking water standard of 20 μ/L In water samples collected from agricultural areas dissolved U showed strong positive correlation with major dissolved constituents Na, Ca, Mg, and SO₄ that increase as a result of sulfate mineral dissolution and clay mineral ion-exchange reactions in weathered shale bedrock and shaley soils. Highly soluble minor and trace elements Cl, Li, B, Sr, and Se that are not subject to strong sorptive uptake or precipitation in this setting also correlated positively with U. These combined observations indicate that natural leaching of U-bearing shale bedrock and derivative soils, additional leaching of rock and soil by irrigation water, and evaporative concentration in a semi-arid climate can produce concentrations of dissolved U in surface water and shallow ground water that may threaten nearby drinking water supplies. Other agriculturally developed areas of the semi-arid Western U.S. with similar geology are likely to contain high concentrations of U in irrigation drain water.     

Colloid formation and metal transport through two mixing zones affected by acid mine drainage near Silverton,Colorado

Stream discharges and concentrations of dissolved and colloidal metals (Al, Ca, Cu, Fe, Mg, Mn, Pb, and Zn), SO₄, and dissolved silica were measured to identify chemical transformations and determine mass transports through two mixing zones in the Animas River that receive the inflows from Cement and Mineral Creeks. The creeks were the dominant sources of Al, Cu, Fe, and Pb, whereas the upstream Animas River supplied about half of the Zn. With the exception of Fe, which was present in dissolved and colloidal forms, the metals were dissolved in the acidic, high-SO₄ waters of Cement Creek (pH 3.8). Mixing of Cement Creek with the Animas River increased pH to near-neutral values and transformed Al and some additional Fe into colloids which also contained Cu and Pb. Aluminium and Fe colloids had already formed in the mildly acidic conditions in Mineral Creek (pH 6.6) upstream of the confluence with the Animas River. Colloidal Fe continued to form downstream of both mixing zones. The Fe- and Al-rich colloids were important for transport of Cu, Pb, and Zn, which appeared to have sorbed to them. Partitioning of Zn between dissolved and colloidal phases was dependent on pH and colloid concentration. Mass balances showed conservative transports for Ca, Mg, Mn, SO₄, and dissolved silica through the two mixing zones and small losses (<10%) of colloidal Al, Fe and Zn from the water column.     

Natural or fertilizer-derived uranium in irrigation drainage: a case study in southeastern Colorado,U.S.A.

Drainage from heavily cultivated soils may be contaminated with U that is leached from the soil or added as a trace constituent of PO₄-based commercial fertilizer. The effect of decades-long application of U-rich fertilizer on the U concentration of irrigation drainage was investigated in a small (14.2 km²) drainage basin in southeastern Colorado. The basin was chosen because previous reports indicated locally anomalous concentrations of dissolved NO₃ (6–36 mg 1⁻¹) and dissolved U (61 pg 1⁻¹) at the mouth of the only stream. Results of this study indicated minimal impact of fertilizer-U compared to natural U leached from the local soils. Detailed sampling of the stream along a 6 mile (9.7 km) reach through heavily cultivated lands indicated marked decoupling of the buildup of dissolved NO₃ and U. Dissolved U increased markedly in the upstream half of the reach and correlated positively with increases in Na, Mg, SO₄, B and Li derived from leaching of surrounding shaley soils. In contrast, major increases in dissolved NO₃ occurred farther downstream where stream water was heavily impacted by ground water return from extensively fertilized fields. Nitrogen isotopic measurements confirmed that dissolved NO₃ originated from fertilizer and soil organic N (crop waste). Uranium isotopic measurements of variably uraniferous waters showed little evidence of contamination with fertilizer-derived U of isotopically distinct²³⁴U/²³⁸U alpha activity ratio (A.R. = 1.0). Leaching experiments using local alkaline soil, irrigation water and U-rich fertilizer confirmed the ready leachability of soil-bound U and the comparative immobility of U added with liquid fertilizer. Relatively insoluble precipitates containing CaPU were formed by mixing liquid fertilizer with water containing abundant dissolved Ca. In the local soils soluble Ca is provided by dissolution of abundant gypsum. Similar studies are needed elsewhere because the mobility of fertilizer-derived U is dependent on fertilizer type, porewater chemistry and soil properties (pH, moisture, mineralogy, texture).     

Selenium mobilization in a surface coal mine,Powder River Basin,Wyoming, U.S.A.

Elevated concentrations (0.6–0.9 mg/l) of selenium were detected in the groundwater of a small backfill area at a surface mine in the Powder River Basin, Wyoming. This report focuses on the source of selenium, its modes of occurrence in overburden deposits and backfill groundwater, and its fate. The immediate source of the selenium appeared to be the dissolution of preexisting soluble salts from the unsaturated zone of the overburden. The ultimate source of selenium was probably the oxidation of selenium-bearing pyrite in the geologic past. Overburden was placed partially in the saturated zone of the backfill where, upon resaturation, soluble salts dissolved in the groundwater. Water standing in the pit at the time of backfilling might have contributed to the elevated concentrations of selenium and other solutes. Selenium was found in an ash-rich coal and in clastic sediments in seven different modes of occurrence.The concentration of soluble selenium in the groundwater at this site has been decreasing since monitoring began in late 1982, and at the present rate of decrease, the concentration should drop below the State of Wyoming guideline of 0.05 mg/l for selenium in water intended for use by livestock by about mid-1992. The decrease in soluble selenium concentration may in part be due to microbially assisted reduction of selenate followed by sorption on clays and other sorbents.     

Hydrogeochemical exploration for Mississippi valley-type deposits, Arkansas, U.S.A.

Three areas located in northern Arkansas, U.S.A., representing differing host rock and mineralization, were selected to investigate the usefulness of hydrogeochemical exploration for Mississippi Valley-type Pb-Zn mineralization. Despite the geologic differences among the areas, there were no great differences in groundwater chemistry and threshold values. Anomalous Pb concentrations, and also anomalous Zn concentrations to a lesser extent, are useful in detecting the Pb-Zn mineralized areas; however, specific deposits could not be located. Because of the low threshold values (about 20 μg/1) for Pb and Zn, spring water must be utilized in order to avoid plumbing contamination.     

Heavy metal partitioning in river sediments severely polluted by acid mine drainage in the Iberian Pyrite Belt

This study provides a geochemical partitioning pattern of Fe, Mn and potentially toxic trace elements (As, Cd, Cr, Cu, Ni, Pb, Zn) in sediments historically contaminated with acid mine drainage, as determined by using a 4-step sequential extraction scheme. At the upperstream, the sediments occur as ochreous precipitates consisting of amorphous or poorly crystalline oxy-hydroxides of Fe, and locally jarosite, whereas the estuarine sediments are composed mainly of detrital quartz, illite, kaolinite, feldspars, carbonates and heavy minerals, with minor authigenic phases (gypsum, vivianite, halite, pyrite). The sediments are severely contaminated with As, Cd, Cu, Pb and Zn, especially in the vicinity of the mining pollution sources and some sites of the estuary, where the metal concentrations are several orders of magnitude above background levels. Although a significant proportion of Zn, Cd and Cu is present in a readily soluble form, the majority of heavy metals are bonded to reducible phases, suggesting that Fe oxy-hydroxides have a dominant role in the metal accumulation. In the estuary, the sediments are potentially less reactive than in the riverine environment, because relevant concentrations of heavy metals are immobilised in the crystalline structure of minerals.     

Distribution and behavior of heavy metals in a river polluted by acid mine drainage in the Dabaoshan mine area, China

Acid mine drainage （AMD） has been recognized as a major environmental pollution problem over past decades. This pollutant effluent is complex and is characterized by elevated concentrations of iron and sulfate, low pH, and high concentrations of a wide variety of metals depending on the host rock geology. Massive inadvertent discharges from acid mines have given rise to dramatic cases of ecological damage. These events indicate an improved understanding of the mechanism controlling metal transport to the river is important, since the aquatic ecology will be affected, to some degree, dependent on the phase （dissolved or particulate） in which the metal is transported. In this study, polluted water samples were collected along the Hengshi River near the Dabaoshan mine, Guangdong, China, in April 2005. The concentrations of dissolved Cu, Zn, Cd and Pb have been determined using ICP-MS and the chemical speciation of those metals in suspended particles was examined using BCR methods and SEM/EDX mineralogical analysis. Combining these two sets of data, the intention was to develop geochemical concepts, which explain the behavior of Cu, Zn, Cd and Pb in particle-water interactions of heavy metals in AMD. The results show that the dissolved heavy metals exhibited non-conservative behavior in the Hengshi River. The dissolved and particulate Cu, Zn, Cd and Pb have the similar spatial distribution, which decreased gradually along the river except in the lower reaches because of the absorption-desorption between dissolved and particulate phases. Although the metal concentrations in both phases were elevated, dissolved metals were dominant and had the maximum concentrations in the low pH region.     

Seismic hazard evaluation for dams in northern Colorado, U.S.A.

A seismic hazard evaluation for three dams in the Rocky Mountains of northern Colorado is based on a study of the historical seismicity. To model earthquake occurrence as a random process utilizing a maximum likelihood method, the catalog must exhibit random space-time characteristics. This was achieved using a declustering procedure and correction for completeness of recording. On the basis of the resulting a- and b-values, probabilistic epicentral distances for a 2 × 10^–5 annual probability were calculated. For a random earthquake of magnitude M _L 6.0–6.5, this distance is 15 km. Suggested ground motion parameters were estimated using a probabilistic seismic hazard analysis. Critical peak horizontal accelerations at the dams are 0.22g if median values are assumed and 0.39g if variable attenuation and seismicity rates are taken into account. For structural analysis of the dams, synthetic acceleration time series were calculated to match the empirical response spectra. In addition, existing horizontal strong motion records from two Mammoth Lakes, California earthquakes were selected and scaled to fit the target horizontal acceleration response spectra.     

Assessment of metal loads in watersheds affected by acid mine drainage by using tracer injection and synoptic sampling: Cement Creek,Colorado, USA

Watersheds in mineralized zones may contain many mines, each of which can contribute to acidity and the metal load of a stream. In this study the authors delineate hydrogeologic characteristics determining the transport of metals from the watershed to the stream in the watershed of Cement Creek, Colorado. Combining the injection of a chemical tracer, to determine a discharge, with synoptic sampling, to obtain chemistry of major ions and metals, spatially detailed load profiles are quantified. Using the discharge and load profiles, the authors (1) identified sampled inflow sources which emanate from undisturbed as well as previously mined areas; (2) demonstrate, based on simple hydrologic balance, that unsampled, likely dispersed subsurface, inflows are significant; and (3) estimate attenuation. For example, along the 12-km study reach, 108 kg per day of Zn were added to Cement Creek. Almost half of this load came from 10 well-defined areas that included both mined and non-mined parts of the watershed. However, the combined effect of many smaller inflows also contributed a substantial load that could limit the effectiveness of remediation. Of the total Zn load, 58.3 kg/day came from stream segments with no visible inflow, indicating the importance of contributions from dispersed subsurface inflow. The subsurface inflow mostly occurred in areas with substantial fracturing of the bedrock or in areas downstream from tributaries with large alluvial fans. Despite a pH generally less than 4.5, there was 58.4 kg/day of Zn attenuation that occurred in mixing zones downstream from inflows with high pH. Mixing zones can have local areas of pH that are high enough for sorption and precipitation reactions to have an effect. Principal component analysis classified inflows into 7 groups with distinct chemical signatures that represent water-rock interaction with different mineral-alteration suites in the watershed. The present approach provides a detailed snapshot of metal load for the watershed to support remediation decisions, and quantifies processes affecting metal transport.     

Distribution of reduced inorganic sulfur compounds in lake sediments receiving acid mine drainage

The sediments of Lake Anna, Virginia, act as a major sink for incoming acid mine drainage (AMD) pollutants (Fe, SO₄²⁻, H⁺) due to bacterial sulfate reduction (SR). Acid-volatile sulfide (AVS), elemental S, and pyrite concentrations in the sediments of the polluted arm of the lake are significantly greater than those in unpolluted sections of the lake. Measurements of SR using ³⁵SSO₄²⁻ showed that AVS and S⁰ are the major short-term (48 h) products of SR in these sediments. Inorganic forms of S(AVS, S⁰, and FeS₂) made up from 60 to 100% of the total sediment S concentration. Pyrite concentrations in the sediment were high but decreased exponentially with distance from the AMD source, suggesting that the pyrite was deposited as stream detritus from the abandoned mines. Iron monosulfide and elemental S concentrations were highest at a station 1 km away from the AMD inflow, indicating formation in situ. There was no evidence for the formation of organic S species. The results suggest that in Fe- and S-rich locations such as those contaminated with acid mine drainage, the distribution of end products of SR may vary substantially from those reported for more moderate environments.     

Kinematics of fault-related folding in a duplex,lost river range,Idaho, U.S.A.

The Doublespring duplex, located in the Lost River Range of Idaho, is a Sevier age fault-related fold complex in massive limestones of the Upper Mississippian Scott Peak Formation. Folds within the duplex closely resemble fault-bend fold geometrics, with open interlimb angles and low-angle bed cut-offs. Narrow, widely spaced, bedding-parallel shear zones with well-developed pressure solution cleavage alternate with massive, relatively undeformed layers on fold limbs. Shear zones are developed only on the limbs of anticlines, and have similar but unique morphologies in each of three different folds. Incremental strain histories reconstructed from antitaxial fibrous overgrowths and veins within the shear zones constrain the kinematics of folding. Shear zones experienced distributed bedding-parallel simple shear (flexural flow) towards pins near axial surfaces, while adjacent massive layers experienced rotation through an externally fixed extension direction. The absence of footwall synclines and morphological differences in shear zones from adjacent folds suggest that faulting preceded folding. Kinematic histories of folds that have experienced different translational histories are identical, and are not compatible with strain histories predicted from previous kinematic models of fault-bend folding. Shear zone development and fiber growth is instead interpreted to have occurred during low amplitude fixed-hinge buckling in response to initial resistance to translation of the thrust sheet. Fault-bend folding with mobile axial surfaces occurred with translation of the thrust sheets once the initial resistance to translation was overcome and resulted in no penetrative strain.     

Sedimentology and paleoecology of middle and upper eocene carbonate shoreline sequences,Crystal River,Florida, U.S.A.

Petrographic analyses of six cores penetrating the Inglis and Avon Park Formations, from Crystal River, Florida, U.S.A., reveal a general regressive—transgressive cycle, aerial exposure and diagenesis. Three distinct lithofacies are represented: (I) biomicrite—poorly washed biosparite; (II) dolomitized biopelmicrite—biosparite and algal dololithite; and (III) pellet- and intraclast-bearing biosparite. These lithofacies were deposited under shallow-marine conditions, at times even above the strand line.Lithofacies I was deposited in a shallow basin partly covered by marine grasses and dasyclad algae. Lithofacies II was deposited in the supratidal and intertidal zones and closely resembles the sedimentary characteristics of Holocene mud-flat deposits. Lithofacies III is the shallow-marine counterpart of the supratidal dolostone of lithofacies II and can be compared with a Holocene bank environment.This study illustrates the importance of Holocene analogues in determining and interpreting environments and demonstrates the need to consider sedimentary processes, both physical and biological, which operate in these zones.     

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.     

Grain size partitioning of metals in contaminated,coarse-grained river floodplain sediment: Clark Fork River,Montana, U.S.A.

The traditional concept of the relationship between metal content and grain size assumes that the fine fraction carries most of the metals in natural sediments. This concept is supported in many cases by strong, significant linear relationships between total-sediment metal concentrations and percentages of various fine-size fractions. Such observations have led to development of methods to correct for the effects of grain size in order to accurately document geographical and temporal variations and identify trends in metal concentrations away from a particular source. Samples from the floodplain sediment of a large, coarse-grained river system indicates that these concepts do not hold for sediments contaminated by mining and milling wastes. In this particular system, the application of methods to correct for grain-size effects would lead to erroneous conclusions about trends of metals in the drainage. This indicates that the a priori application of grain-size correction factors limits interpretation of actual metal distributions and should not be used unless data indicate that correlations exist between metals and particular size fractions.     

Large-scale distribution of metal contamination in the fine-grained sediments of the Clark Fork River,Montana, U.S.A.

Historic discharges from the mining and smelting complex at the head-waters of the Clark Fork River have resulted in elevated Ag, Cd, Cu, Pb and Zn concentrations in the <60 μm fraction of both bed and flood-plain sediments of the river. Processes affecting the trends in longitudinal distributions of these metals were investigated by repeated sampling over a 380 km river reach between August 1986 and July 1989. At the most upstream site, bed-sediment metal concentrations were enriched 18–115 times above least enriched tributaries, depending on the metal. All metals decreased exponentially with distance downstream away from mining. The exponential model predicts that elevated metal concentrations should occur over 550 km downstream, in Lake Pend Oreille. Longitudinal trends, obvious on a scale of hundreds of kilometers, were obscured by small-scale spatial variability when shorter stretches of the river were considered. Longitudinal dispersion appeared to be controlled largely by physical dilution with less-contaminated sediments.Evidence suggests that erosion of contaminated flood-plains contributes to metal contamination in the bed sediments. Tributary input appeared to have little influence on the large-scale, downstream distribution of metals; however, it did contribute to local variability in bed-sediment metal concentrations. Association of metals with specific mineral grains, as well as variability in total organic C and Fe concentration, appeared also to contribute to variability.Some year-to-year variability in bed-sediment metal concentrations was observed, however, trends in longitudinal dispersion were not significantly different between at least two of the years sampled.     

Compaction of wyodak coal,powder river basin,wyoming, U.S.A.

Precise measurements of 54 deformed tree trunks within the subbituminous Wyodak coal seem in northeastern Wyoming were made at eight locations in surface mines along the outcrop north and south of Gillette. Theory of elasticity applied to the individual measurements gives ratios of peat thickness to coal thickness which range from 1.7:1 to 31:1 and average 7.1:1. This result is similar to those previously determined by other methods in other locations. Statistical analysis establishes to a high degree of confidence, however, that the ratios for each mine site vary widely and cannot be considered to be constant or to vary uniformly within the study area.     

Manganese minerals and associated fine particulates in the streambed of Pinal Creek,Arizona, U.S.A.: a mining-related acid drainage problem

The Pinal creek drainage basin in Arizona is a good example of the principal non-coal source of mining-related acid drainage in the U.S.A., namely copper mining. Infiltration of drainage waters from mining and ore refining has created an acid groundwater plume that has reacted with calcite during passage through the alluvium, thereby becoming less acid. Where O₂ is present and the water is partially neutralized, iron oxides have precipitated and, farther downstream where the pH of the stream water is near neutral, high-Mn crusts have developed.Trace metal composition of several phases in the Pinal Creek drainage basin illustrates the changes caused by mining activities and the significant control Mn-crusts and iron oxide deposits exert on the distribution and concentration of trace metals. The phases and locales considered are the dissolved phase of Webster Lake, a former acid waste disposal pond; selected sections of cores drilled in the alluvium within the intermittent reach of Pinal Creek; and the dissolved phase, suspended sediments, and streambed deposits at specified locales along the perennial reach of Pinal creek.In the perennial reach of Pinal Creek, manganese oxides precipitate from the streamflow as non-cemented particulates and coatings of streambed material and as cemented black crusts. Chemical and X-ray diffraction analyses indicate that the non-cemented manganese oxides precipitate in the reaction sequence observed in previous laboratory experiments using simpler solution composition, Mn₃O₄ to MnOOH to an oxide of higher oxidation number usually <4.0, i.e. Na-birnessite, and that the black cemented crusts contain (Ca,Mn,Mg)CO₃ and a 7-Åphyllomanganate mixture of rancieite ((Ca,Mn)Mn₄O₉ · (3H₂O)) and takanelite ((Mn,Ca)Mn₄O₉ · (3H₂O)). In the laboratory, aerating and increasing the pH of Pinal Creek water to 9.00 precipitated (Ca,Mn,Mg)CO₃ from an anoxic groundwater that contained CO₂ HCO₃, and precipitated Mn₃O₄ and subsequently MnOOH from an oxic surface water from which most of the dissolved CO₂ had been removed.It is suggested that the black cemented crusts form by precipitation of Fe on the Mn-enriched carbonates, creating a site for the MnFe oxidation cycle and thus encouraging the conversion of the carbonates to 7-Åphysllomanganates. The non-magnetic <63-μm size-fractions of the black cemented crusts consisted mostly of the manganese-calcium oxides but also contained about 20% (Ca,Mn,Mg)CO₃, 5% Fe (calculated as FeOOH), 2–4% exchangeable cations, and trace amounts of several silicates.