A process model of natural attenuation in drainage from a historic mining district

A process model was used to better understand the controls on the chemical evolution of drainage in a historic mining district. At the Pecos Mine Operable Unit, New Mexico, drainage near the waste rock pile is acidic (pH varies from 3.0–5.0) and carries high concentrations of Zn, Al, Cu and Pb. As drainage flows toward the Pecos River, pH increases to greater than 7 and heavy metal content decreases. A process model of natural attenuation in this drainage shows the main controls on pH are reaction with a local bedrock that contains limestone, and concurrent mixing with tributary streams. Models that account for both calcite dissolution and mixing reproduce the observed decrease in aqueous metal concentrations with increasing pH. Contaminant concentrations attenuate primarily via two distinct pathways: Al, Cu, Fe and Pb precipitate directly from solution, whereas Zn, Mg, Mn and SO₄ concentrations decrease primarily through dilution. Additionally, Pb adsorbs to precipitating hydroxide surfaces.     

Solubility products of amorphous ferric arsenate and crystalline scorodite (FeAsO4 · 2H2O) and their application to arsenic behavior in buried mine tailings

Published solubility data for amorphous ferric arsenate and scorodite have been reevaluated using the geochemical code PHREEQC with a modified thermodynamic database for the arsenic species. Solubility product calculations have emphasized measurements obtained under conditions of congruent dissolution of ferric arsenate (pH < 3), and have taken into account ion activity coefficients, and ferric hydroxide, ferric sulfate, and ferric arsenate complexes which have association constants of 10^4.04 (FeH₂AsO₄²⁺), 10^9.86 (FeHAsO₄⁺), and 10^18.9 (FeAsO₄). Derived solubility products of amorphous ferric arsenate and crystalline scorodite (as log K_sp) are −23.0 ± 0.3 and −25.83 ± 0.07, respectively, at 25 °C and 1 bar pressure. In an application of the solubility results, acid raffinate solutions (molar Fe/As = 3.6) from the JEB uranium mill at McClean Lake in northern Saskatchewan were neutralized with lime to pH 2-8. Poorly crystalline scorodite precipitated below pH 3, removing perhaps 98% of the As(V) from solution, with ferric oxyhydroxide (FO) phases precipitated starting between pH 2 and 3. Between pH 2.18 and 7.37, the apparent log K_sp of ferric arsenate decreased from −22.80 to −24.67, while that of FO (as Fe(OH)₃) increased from −39.49 to −33.5. Adsorption of As(V) by FO can also explain the decrease in the small amounts of As(V)(aq) that remain in solution above pH 2-3. The same general As(V) behavior is observed in the pore waters of neutralized tailings buried for 5 yr at depths of up to 32 m in the JEB tailings management facility (TMF), where arsenic in the pore water decreases to 1-2 mg/L with increasing age and depth. In the TMF, average apparent log K_sp values for ferric arsenate and ferric hydroxide are −25.74 ± 0.88 and −37.03 ± 0.58, respectively. In the laboratory tests and in the TMF, the increasing crystallinity of scorodite and the amorphous character of the coexisting FO phase increases the stability field of scorodite relative to that of the FO to near-neutral pH values. The kinetic inability of amorphous FO to crystallize probably results from the presence of high concentrations of sulfate and arsenate.     

Development of reaction models for ground-water systems

Methods are described for developing geochemical reaction models from the observed chemical compositions of ground water along a hydrologic flow path. The roles of thermodynamic speciation programs, mass balance calculations, and reaction-path simulations in developing and testing reaction models are contrasted. Electron transfer is included in the mass balance equations to properly account for redox reactions in ground water. The mass balance calculations determine net mass transfer models which must be checked against the thermodynamic calculations of speciation and reaction-path programs. Although reaction-path simulations of ground-water chemistry are thermodynamically valid, they must be checked against the net mass transfer defined by the mass balance calculations. An example is given testing multiple reaction hypotheses along a flow path in the Floridan aquifer where several reaction models are eliminated. Use of carbon and sulfur isotopic data with mass balance calculations indicates a net reaction of incongruent dissolution of dolomite (dolomite dissolution with calcite precipitation) driven irreversibly by gypsum dissolution, accompanied by minor sulfate reduction, ferric hydroxide dissolution, and pyrite precipitation in central Florida. Along the flow path, the aquifer appears to be open to CO₂ initially, and open to organic carbon at more distant points down gradient.     

Controls on Ra during raffinate neutralization at the Key Lake uranium mill, Saskatchewan, Canada

This study was conducted to define the geochemical controls on ²²⁶Ra during raffinate (pH = 1.2) neutralization to pH 10 at the Key Lake U mill in northern Saskatchewan, Canada. High activities (120–150 Bq/L) of aqueous phase ²²⁶Ra are present in raffinate produced during milling of U ore. The solubility control of ²²⁶Ra in the SO₄-rich, hydrometallurgical raffinate solutions often involves the addition of BaCl₂ to form a radium-barite co-precipitate (Ba(Ra)SO₄). As such, neutralization experiments were conducted with samples of mill raffinate using Ca(OH)₂ or NaOH with and without the addition of BaCl₂. Radium-226 activity decreased from 150 to <4 Bq/L for all combinations of neutralizing agents with Ca(OH)₂ + BaCl₂ being the most effective combination (final activity ∼1.0 Bq/L; ∼99.3% removal). In the absence of BaCl₂, Ca(OH)₂ more efficiently removed ²²⁶Ra than NaOH between pH 4 and 8, due to the co-precipitation of ²²⁶Ra with gypsum. Overall, neutralization with the addition of BaCl₂ reduced ²²⁶Ra activities at lower pH values (by pH 4.5), due to co-precipitation of ²²⁶Ra with BaSO₄. At varying concentrations of BaCl₂, aqueous phase activities of ²²⁶Ra converged, but did not attain steady-state values during neutralization and would continue to decrease with time. Sequential extractions indicated that ²²⁶Ra in precipitates formed during neutralization of the mill raffinate is dominated by amorphous and crystalline Fe hydroxide phases, consistent with raffinate neutralization experiments that showed that adsorption onto ferrihydrite can remove most ²²⁶Ra in the raffinate. Data generated in this study are being used to define the long-term geochemical controls on ²²⁶Ra in U mill processes and tailings.     

An experimental study of crystalline basalt dissolution from 2 ? pH ? 11 and temperatures from 5 to 75 °C

Steady-state element release rates from crystalline basalt dissolution at far-from-equilibrium were measured at pH from 2 to 11 and temperatures from 5 to 75 °C in mixed-flow reactors. Steady-state Si and Ca release rates exhibit a U-shaped variation with pH where rates decrease with increasing pH at acid condition but increase with increasing pH at alkaline conditions. Silicon release rates from crystalline basalt are comparable to Si release rates from basaltic glass of the same chemical composition at low pH and temperatures ?25 °C but slower at alkaline pH and temperatures ?50 °C. In contrast, Mg and Fe release rates decrease continuously with increasing pH at all temperatures. This behaviour is interpreted to stem from the contrasting dissolution behaviours of the three major minerals comprising the basalt: plagioclase, pyroxene, and olivine. Calcium is primarily present in plagioclase, which exhibits a U-shaped dissolution rate dependence on pH. In contrast, Mg and Fe are contained in pyroxene and olivine, minerals whose dissolution rates decrease monotonically with pH. As a result, crystalline basalt preferentially releases Mg and Fe relative to Ca at acidic conditions. The injection of acidic CO₂-charged fluids into crystalline basaltic terrain may, therefore, favour the formation of Mg and Fe carbonates rather than calcite. Element release rates estimated from the sum of the volume fraction normalized dissolution rates of plagioclase, pyroxene, and olivine are within one order of magnitude of those measured in this study.     

Irreversible water–rock mass transfer accompanying the generation of the neutral,Mg–HCO3 and high-pH,Ca–OH spring waters of the Genova province,Italy

In a recent survey of the spring waters of the Genova province, many neutral Mg–HCO₃ waters and some high-pH, Ca–OH waters were found in association with serpentinites. All the springs are of meteoric origin as indicated by the stable isotopes of water and dissolved N₂ and Ar. Interaction of these meteoric waters with serpentinites determines a progressive evolution in the chemistry of the aqueous phase from an immature Mg-rich, SO₄–Cl facies of low salinity to an intermediate Mg–HCO₃ facies (pH 7.0–8.5, P_CO210^−3.5–10^−2.5 bar, Eh 150–250 mV), and to a mature Ca–OH facies (pH 10–12, P_CO2 10^−9.4−10^−10.6 bar, Eh-390 to-516 mV). The irreversible water–rock mass transfer leading to these chemical changes in the aqueous phase was simulated through reaction path modeling, assuming bulk dissolution of a local serpentinite, and the precipitation of gibbsite, goethite, calcite, hydromagnesite, kaolinite, a montmorillonite solid mixture, a saponite solid mixture, sepiolite, and serpentine. The simulation was carried out in two steps, under open-system and closed-system conditions with respect to CO₂, respectively. The calculated concentrations agree with analytical data, indicating that the computed water-rock mass transfer is a realistic simulation of the natural process. Moreover, the simulation elucidates the role of calcite precipitation during closed-system serpentinite dissolution in depleting the aqueous solution of C species, allowing the concurrent increment in Ca and the acquisition of a Ca–OH composition. Calcium–OH waters, due to their high pH, tend to absorb CO₂, precipitating calcite. Therefore, these waters might be used to sequester anthropogenic CO₂, locally preventing environmental impact to the atmosphere.     

Leaching experiments on a Mn-rich slag from the recycling of alkaline batteries – Solid phase characterization and geochemical modeling

Square sections of a Mn-rich slag from an alkaline battery recycling plant were submitted to 6-month batch leaching procedures. High-Purity Water (HPW), acidic (pH 4) and alkaline (pH 12) conditions were used in order to observe the behavior of primary solid phases as well as the constituent elements (Mn, Mg, Al, Si, Ca). The experiments were coupled with both KINDIS(P) modeling and mineralogical study (SEM-EDS). Experimental results showed that the Mn-rich slag was sensitive to acidic conditions which induced the dissolution of primary phases. Moreover, pH 4 conditions did not result in the formation of newly formed solid products, leading to the greatest mobilization of metallic elements (especially Mn). Alkaline conditions favored the precipitation of secondary phases, especially rhodochrosite, calcite and Mg-saponite, inducing low mobilization of the contained elements. The KINDIS(P) modeling allowed the stability of primary phases and newly formed products to be predicted. Although the modeled results have to be considered with caution, they allow the assessment and understanding of future environmental behavior of the solid material in given conditions. In this case, the reuse of Mn-rich slag in acidic conditions has to be avoided because of the acidic dissolution of the primary phases.     

Relation between sedimentary framework and hydrogeology in the Guarani Aquifer System in São Paulo state,Brazil

This paper presents the results of a new investigation of the Guarani Aquifer System (SAG) in São Paulo state. New data were acquired about sedimentary framework, flow pattern, and hydrogeochemistry. The flow direction in the north of the state is towards the southwest and not towards the west as expected previously. This is linked to the absence of SAG outcrop in the northeast of São Paulo state. Both the underlying Pirambóia Formation and the overlying Botucatu Formation possess high porosity (18.9% and 19.5%, respectively), which was not modified significantly by diagenetic changes. Investigation of sediments confirmed a zone of chalcedony cement close to the SAG outcrop and a zone of calcite cement in the deep confined zone. The main events in the SAG post-sedimentary history were: (1) adhesion of ferrugineous coatings on grains, (2) infiltration of clays in eodiagenetic stage, (3) regeneration of coatings with formation of smectites, (4) authigenic overgrowth of quartz and K-feldspar in advanced eodiagenetic stage, (5) bitumen cementation of Pirambóia Formation in mesodiagenetic stage, (6) cementation by calcite in mesodiagenetic and telodiagenetic stages in Pirambóia Formation, (7) formation of secondary porosity by dissolution of unstable minerals after appearance of hydraulic gradient and penetration of the meteoric water caused by the uplift of the Serra do Mar coastal range in the Late Cretaceous, (8) authigenesis of kaolinite and amorphous silica in unconfined zone of the SAG and cation exchange coupled with the dissolution of calcite at the transition between unconfined and confined zone, and (9) authigenesis of analcime in the confined SAG zone. The last two processes are still under operation. The deep zone of the SAG comprises an alkaline pH, Na–HCO₃ groundwater type with old water and enriched δ¹³C values (<?3.9), which evolved from a neutral pH, Ca–HCO₃ groundwater type with young water and depleted δ¹³C values (>?18.8) close to the SAG outcrop. This is consistent with a conceptual geochemical model of the SAG, suggesting dissolution of calcite driven by cation exchange, which occurs at a relatively narrow front recently moving downgradient at much slower rate compared to groundwater flow. More depleted values of δ¹⁸O in the deep confined zone close to the Paraná River compared to values of relative recent recharged water indicate recharge occur during a period of cold climate. The SAG is a “storage-dominated” type of aquifer which has to be managed properly to avoid its overexploitation.     

Carbon-14 age and chemical evolution of Ca(HCO3)2-type groundwater of age less than 8,000 years in a confined sandy and muddy Pleistocene aquifer, Japan

The Pleistocene Kimitsu aquifer was selected for examination of the relationship between groundwater age and chemical evolution of Ca(HCO₃)₂-type groundwater. For the most part, the aquifer is confined and composed mainly of quartz and feldspar with a small amount of calcite. The groundwater ages calculated by ¹⁴C were adjusted by using a carbon mass-balance method and corrected for effects of ¹⁴C diffusion. Groundwater ages in the Kimitsu aquifer vary from modern (upgradient) to approximately 2,400 years at 4.4 km from the edge of the recharge area. The ¹⁴C age was verified by groundwater velocity calculated from the hydraulic gradient and hydraulic conductivity. The confined groundwater evolved to Ca(HCO₃)₂-type around 50 years after recharge and this has been maintained for more than 8,300 years due to low chemical reactivity, derived from equilibrium with calcite, kaolinite and Ca-montmorillonite. In addition, high pH prevents the dissolution of Fe and Mn. Consequently, the rate of increase in electrical conductivity ranges from 10 to 30 μS/cm per 1,000 years. On the other hand, leakage from the deep region, which is recognized from high Cl^– levels, causes remarkable increases in CH₄ and HCO₃ ^– concentrations, resulting in an apparent sulfidic zone at 500-m depth in most downgradient regions.     

Hydrogeochemical investigation in an arid region of Iran (Tabas, Central Iran)

Groundwater is the most important source of water supply in Iran and understanding the geochemical evolution of groundwater is important for sustainable development of the water resources in Tabas area. A total of 29 samples of groundwater in Tabas area have been analyzed for ions and major elements. Groundwater of the study area is characterized by the dominance of Na–Cl water type. Groundwater was generally acidic to high alkaline with pH ranging from 5.42 to 10.75. The TDS as a function of mineralization characteristics of the groundwater ranged from 479 to 10,957 mg/l, with a mean value of 2,759 mg/l. The Ca²⁺, Mg²⁺, SO₄ ^2? and HCO₃ ^? were mainly derived from the dissolution of calcite, dolomite and gypsum. The Cu, Pb and Zn ions are not mobile in recent pH–Eh, but these conditions controlled dissolved Se, V and Mo in groundwater. The As is released in groundwater as a result of the weathering of sulfide minerals like arsenopyrite.     

Dissolution rates of earthworm-secreted calcium carbonate

The dissolution of CaCO₃ granules secreted by earthworms in soil leaching columns was governed by soil pH and exchange sites available for Ca. Results indicate that granules could last for significant periods of time in soils and that, therefore, granules could be an important source of soil calcite.     

Carbonate dissolution in the guts of benthic deposit feeders: A numerical model

We present a numerical model to quantify calcite dissolution in the guts of deposit feeding invertebrates. Deposit feeder guts were modeled as constantly stirred reactors (CSTRs) following terminology from digestion theory. Saturation state and dissolution of calcium carbonate were calculated from changes in total dissolved carbon dioxide and alkalinity resulting from sediment passage through the digestive tract, while accounting for dissolution of calcite and respiration of organic carbon. Typical dissolution rates for a gut volume of 1 ml ranged between 0.5-4 mg calcite d⁻¹. Sensitivity analysis revealed gut pH, sediment organic matter (OM) content and OM reactivity to be the critical parameters determining calcite dissolution rate. Carbonate dissolution rate was inversely related to gut pH. However, calcite dissolution was found to be possible even at alkaline gut pH due to respiration by intestinal microbes. The kinetics of calcite dissolution had only marginal influence on daily calcite dissolution rates: Varying the calcite dissolution rate constant κ by six orders of magnitude affected calcite dissolution rates by less than a factor of 10. Calcite dissolution rates were calculated for 4 different hydrographic regimes that differed in their content of sedimentary calcite and OM and furthermore in their OM reactivity. Highest dissolution rates were calculated for the shallow water setting, where relatively high OM content facilitated high microbial respiration rates depressing gut pH. However, dissolution rates for the deep sea setting were only slightly lower, due to greatly elevated ingestion rates resulting from low OM content. As a consequence of much higher faunal abundances, shallow-water benthos is likely to contribute the vast majority of gut-mediated carbonate dissolution. Nevertheless, the fraction of sedimentary calcite that dissolves during one gut passage is probably too small to be observable by simple gravimetric analysis. This may explain the notable scarcity of evidence for gut-mediated carbonate dissolution in the literature to date. Assuming depth-dependent calcite dissolution rates and deposit feeder abundances, we estimate gut-mediated carbonate dissolution to contribute approximately 5% of the annual global sedimentary carbonate dissolution rate, which corresponds to an average calcite dissolution rate of approximately 0.5 mg m⁻² d⁻¹ for the entire ocean floor.     

Rocks control the chemical composition of surface water from the high Alpine Zermatt area (Swiss Alps)

The study presents composition data of 87 surface water samples from high alpine catchments of the Zermatt area (Swiss Alps). The investigated area covers 170 km². It was found that the surface runoff acquires the dissolved solids mostly by reaction of precipitation water with the minerals of the bedrock. Total dissolved solids (TDS) vary from 6 to 268 mg L^?1. All collected water shows a clear chemical signature of the bedrock mineralogy. The contribution of atmospheric input is restricted to small amounts of ammonium nitrate and sodium chloride. NH₄ is a transient component and has not been detected in the runoff. Evaporation is not a significant mechanism for TDS increase in the Zermatt area. The chemical composition of the three main types of water can be related to the mineralogy of the dominant bedrock in the catchments. Specifically, Ca-HCO₃ (CC) waters develop from metamorphic mafic rocks and from carbonate-bearing schists. Mg-HCO₃ water originates from serpentinites and peridotites. Ca-SO₄ (CS) waters derive from continental basement rocks such as pyrite-rich granite and gneiss containing oligoclase or andesine. The collected data suggest that, together with reaction time, modal sulfide primarily controls and limits TDS of the waters by providing sulfuric acid for calcite (CC waters) and silicate (CS waters) dissolution. If calcite is present in the bedrock, its dissolution neutralizes the acid produced by sulphide weathering and buffers pH to near neutral to weakly alkaline conditions. If calcite is absent, the process produces low-pH waters in gneiss and granite catchments. The type of bedrock and its mineral assemblage can be recognized in water leaving very small catchments of some km² area. The large variety of water with a characteristic chemical signature is an impressive consequence of the richly diverse geology and the different rock inventory of the local catchments in the Zermatt area.     

Pollution of Osheepcheon Creek by abandoned coal mine drainage in Dogyae area,eastern part of Samcheok coal field,Kangwon-Do,Korea

Osheepcheon Creek running through the Dogyae area is being polluted by the influx of the abandoned coal mine drainage. Generally, the more polluted water has lower pH and Eh and higher conductivity values. The concentrations of Mg, Ca, Fe, SO₄, and some trace elements, such as Cd, Co, Cr, Mo, Ni, Pb, Rb, Sr, U and Zn, are tens to hundreds of times more concentrated in the abandoned coal mine drainage than in the unpolluted streamwater. However, most immobile toxic pollutants from the mine drainage are quickly removed from the streamwater by the precipitation of amorphous Fe hydroxide and sorption on the precipitated Fe hydroxide. The fast removal of the pollutants from the streamwater maintains the water quality of the creek as acceptable at most places along the stream path, except where the abandoned coal mine drainage flows in. However, the creek has the potential of deteriorating quickly if the mine drainage is allowed to be continuously combined with the streams. A function of pH, Eh, and conductivity has been developed with discriminant function analysis for the purpose of easy, fast, and inexpensive measurement of the degrees of pollution of the streams. The estimated pollution of the streams with the discriminant function are consistent with what the chemical compositions of the water samples indicate. The pollution map of the study area was constructed from the calculated scores with the discriminant function. The pollution map suggests that the pollutants mainly come from the west side of Osheepcheon Creek. Thus, the abandoned coal mine drainage from the west side has to be appropriately treated as soon as possible to prevent Osheepcheon Creek from being further polluted. Considering the topography, climate, and the amount of the mine drainage, an active treatment method is recommended.     

Crude oil in a shallow sand and gravel aquifer—III. Biogeochemical reactions and mass balance modeling in anoxic groundwater

Crude oil floating on the water table in a sand and gravel aquifer provides a constant source of hydrocarbons to the groundwater at a site near Bemidji, Minnesota. The degradation of hydrocarbons affects the concentrations of oxidized and reduced aqueous species in the anoxic part of the contaminant plume that developed downgradient from the oil body. The concentrations of Fe²⁺, Mn²⁺ and CH₄, Eh measurements, and the δ¹³C ratios of the total inorganic C indicate that the plume became more reducing ver a 5-a period. However, the size of the contaminant plume remained stable during this time. Field data coupled with laboratory microcosm experiments indicate that benzene and the alkylbenzenes are degraded in an anoxic environment. In anaerobic microcosm experiments conducted under field conditions, almost complete degradation (98%) was observed for benzene in 125 d and for toluene in 45 d. Concentrations of aqueous Fe²⁺ and Mn²⁺ increased in these experiments, indicating that the primary reactions were hydrocarbon degradation coupled with Fe and Mn reduction.Mass transfer calculations on a 40-m flowpath in the anoxic zone, downgradient from the oil body, indicated that the primary reactions in the anoxic zone are oxidation of organic compounds, precipitation of siderite and a ferroan calcite, dissolution of iron oxide and outgassing of CH₄ and CO₂. The major difference in the two models presented is the ratio of CO₂ and CH₄ that outgasses. Both models indicate quantitatively that large amounts of Fe are dissolved and reprecipitated as ferrous iron in the anoxic zone of the contaminant plume.     

Chemical evolution of the Mt. Hekla,Iceland, groundwaters: A natural analogue for CO2 sequestration in basaltic rocks

A detailed study of the chemical composition of the groundwater surrounding the Mt. Hekla volcano in south Iceland was performed to assess fluid evolution and toxic metal mobility during CO₂-rich fluid basalt interaction. These fluids provide a natural analogue for evaluating the consequences of CO₂ sequestration in basalt. The concentration of dissolved inorganic C in these groundwaters decreases from 3.88 to 0.746 mmol/kg with increasing basalt dissolution while the pH increases from 6.9 to 9.2. This observation provides direct evidence of the potential for basalt dissolution to sequester CO₂. Reaction path calculations suggest that dolomite and calcite precipitation is largely responsible for this drop in groundwater dissolved C concentration. The concentrations of toxic metal(loid)s in the waters are low, for example the maximum measured concentrations of Cd, As and Pb were 0.09, 22.8 and 0.06 nmol/kg, respectively. Reaction path modelling indicates that although many toxic metals may be initially liberated by the dissolution of basalt by acidic CO₂-rich solutions, these metals are reincorporated into solid phases as the groundwaters are neutralized by continued basalt dissolution. The identity of the secondary toxic metal bearing phases depends on the metal. For example, calculations suggest that Sr and Ba are incorporated into carbonates, while Pb, Zn and Cd are incorporated into Fe (oxy)hydroxide phases.     

Geochemistry of arsenic in uranium mine mill tailings,Saskatchewan, Canada

The Rabbit Lake U mine in-pit tailings management facility (TMF) (425 m long×300 m wide×91 m deep) is located in northern Saskatchewan, Canada. The objectives of this study were to quantify the distribution of As phases in the tailings and evaluate the present-day geochemical controls on dissolved As. These objectives were met by analyzing pore fluid samples collected from the tailings body for dissolved constituents, measuring Eh, pH, and temperature of tailings core and pore fluid samples, conducting sequential extractions on solid samples, conducting geochemical modeling of pore fluid chemistry using available thermodynamic data, and by reviewing historical chemical mill process records. Dissolved As concentrations in 5 monitoring wells installed within the tailings body ranged from 9.6 to 71 mg/l. Pore fluid in the wells had a pH between 9.3 and 10.3 and Eh between +58 and +213 mV. Sequential extraction analyses of tailings samples showed that the composition of the solid phase As changed at a depth of 34 m. The As above 34 m was primarily associated with amorphous Fe and metal hydroxides while the As below 34 m was associated with Ca, likely as amorphous poorly ordered calcium arsenate precipitates. The change in the dominant As solid phases at this depth was attributed to the differences in the molar ratio of Fe to As in the mill tailings. Below 34 m it was <2 whereas above 34 m it was >4. The high Ca/As ratio during tailings neutralization would likely precipitate Ca₄(OH)₂(AsO₄)₂:4H₂O type Ca arsenate minerals. Geochemical modeling suggested that if the pore fluids were brought to equilibrium with this Ca-arsenate, the long-term dissolved As concentrations would range between 13 and 126 mg/l.     

A hydrogeochemical transport model for an oxidation experiment with pyrite/calcite/exchangers/organic matter containing sand

We report the hydrogeochemical modeling of a complicated suite of reactions that take place during the oxidation of pyrite in a marine sediment. The sediment was equilibrated in a column with MgCl₂ solution and subsequently oxidized with H₂O₂. The oxidation of pyrite triggers dissolution of calcite, cation and proton exchange, and CO₂ sorption. The composition of the column effluent was modeled with PHREEQC, a hydrogeochemical transport model. The model was extended with a formal ID transport module which includes dispersion and diffusion. The algorithm solves the advection-reaction-dispersion equation with explicit finite differences in a split-operator scheme. Also, kinetic reactions for pyrite oxidation, calcite dissolution and precipitation, and organic C oxidation were included. Kinetic relations for pyrite oxidation and calcite dissolution were taken from the literature, and a coefficient equivalent to the ratio A/V (surface over volume), was adjusted to fit the experimental data. The comparison of model and experiment shows that ion exchange and sorption are dominant chemical processes in regulating and buffering water quality changes upon the oxidation of pyrite. Cation exchange was assigned to the colloidal fraction ( < 2 μm) and deprotonated organic matter, proton buffering to organic matter, and CO₂ sorption to amorphous Fe-oxyhydroxide. These processes have been neglected in earlier modeling studies of pyrite oxidation in natural sediments.     

Geochemistry of an acidic chromium sulfate plume

The historical disposal of acidic chromium sulfate solutions into unlined lagoons between 1953 and 1970 at an industrial site resulted in formation of a dense aqueous phase liquid (DAPL) plume [specific gravity 1.11 g/cm³, pH 3, up to 4700 mg/L Cr(III), and up to 90,000 mg/L SO₄]. The DAPL sank through the shallow glacial till aquifer to an underlying impermeable gneissic bedrock from where it migrated downgradient along buried channels incised in the bedrock. Because of its high density, the plume chemistry is sharply stratified vertically. Chromium(III) predominates in the DAPL because excess Cr(VI) not reduced in the original process has been reduced by Fe(II) derived from silicates, while Cr(OH)₃(am) occurs as surface coatings on silicate minerals and as discrete particles mixed with Fe(OH)₃(am) and Al(OH)₃(am). The solubility of Cr(OH)₃(am) accurately describes Cr(III) concentrations in the plume and nearby groundwater, while Al and Fe in solution are also consistent with solubility-controlling oxyhydroxides. Because of these solubility controls, metal cations are attenuated relative to more mobile Cl and SO₄, resulting in a chromatographic separation of solutes downgradient from the plume origin. The good agreement between predicted and observed solution concentrations illustrates the utility of equilibrium modeling when interpreting metal transport characteristics and in determining the efficacy of natural attenuation in subsurface systems.