Geochemical and equilibrium trends in mine pit lakes

Chemical composition and equilibrium trends in mine pit lakes were examined to provide guidance for the application of geochemical models in predicting future lake water quality at prospective open pit mines. Composition trends show that elevated solute levels generally occur only at the extremes of acidic and alkaline pH conditions. Concentrations of cationic metals (Al, Cd, Cu, Fe, Mn, Pb, and Zn) are elevated only in acidic pit lakes, whereas anionic metalloids (As and Se) are generally elevated only in alkaline pit lakes. These trends are indicative of sulfide mineral oxidation and evapoconcentration for acidic and alkaline conditions, respectively.For nearly all pit lakes, SO₄ is the dominant solute, but is limited by gypsum solubility. Fluorite, calcite, and barite are also important solubility controls. Well-defined solubility controls exist for the major metals (Al, Fe, Mn), including jurbanite and alunite for Al, ferrihydrite for Fe, and manganite, birnessite, and, possibly, rhodochrosite for Mn. Determinations of definite controls for the minor metals are less distinct, but may include otavite for Cd, brochantite and malachite for Cu, cerrusite and pyromorphite for Pb, and hydrozincite and Zn silicates for Zn. Concentrations of As and Se appear to be limited only by adsorption, but this control is sharply diminished by increased pH and SO₄ concentration. In general, the concentrations of minor metals in pit lakes are not well represented by the theoretical solubilities of pure-phase minerals contained in the thermodynamic databases. Hence, modeling efforts will generally have to rely on empirical data on the leaching characteristics of pit wall-rocks to predict the concentrations of minor metals (Cd, Cu, Pb, Zn) in mine pit lakes.Methodologies for predicting pit lake water chemistry are still evolving. Geochemical and equilibrium trends in existing pit lakes can provide valuable information for guiding the development and application of predictive models. However, mineralogical studies of pit lake sediments, suspended particles, and alteration assemblages and studies of redox transformations are still needed to validate and refine the representations of geochemical processes in water quality models of mine pit lakes.     

Geochemistry of highly acidic mine water following disposal into a natural lake with carbonate bedrock

Acid mine drainage (AMD) from the Zn–Pb(–Ag–Bi–Cu) deposit of Cerro de Pasco (Central Peru) and waste water from a Cu-extraction plant has been discharged since 1981 into Lake Yanamate, a natural lake with carbonate bedrock. The lake has developed a highly acidic pH of ∼1. Mean lake water chemistry was characterized by 16,775 mg/L acidity as CaCO₃, 4330 mg/L Fe and 29,250 mg/L SO₄. Mean trace element concentrations were 86.8 mg/L Cu, 493 mg/L Zn, 2.9 mg/L Pb and 48 mg/L As, which did not differ greatly from the discharged AMD. Most elements showed increasing concentrations from the surface to the lake bottom at a maximal depth of 41 m (e.g. from 3581 to 5433 mg/L Fe and 25,609 to 35,959 mg/L SO₄). The variations in the H and O isotope compositions and the element concentrations within the upper 10 m of the water column suggest mixing with recently discharged AMD, shallow groundwater and precipitation waters. Below 15 m a stagnant zone had developed. Gypsum (saturation index, SI ∼ 0.25) and anglesite (SI ∼ 0.1) were in equilibrium with lake water. Jarosite was oversaturated (SI ∼ 1.7) in the upper part of the water column, resulting in downward settling and re-dissolution in the lower part of the water column (SI ∼ −0.7). Accordingly, jarosite was only found in sediments from less than 7 m water depth. At the lake bottom, a layer of gel-like material (∼90 wt.% water) of pH ∼1 with a total organic C content of up to 4.40 wet wt.% originated from the kerosene discharge of the Cu-extraction plant and had contaminant element concentrations similar to the lake water. Below the organic layer followed a layer of gypsum with pH 1.5, which overlaid the dissolving carbonate sediments of pH 5.3–7. In these two layers the contaminant elements were enriched compared to lake water in the sequence As < Pb ≈ Cu < Cd < Zn = Mn with increasing depth. This sequence of enrichment was explained by the following processes: (i) adsorption of As on Fe-hydroxides coating plant roots at low pH (up to 3326 mg/kg As), (ii) adsorption at increasing pH near the gypsum/calcite boundary (up to 1812 mg/kg Pb, 2531 mg/kg Cu, and 36 mg/kg Cd), and (iii) precipitation of carbonates (up to 5177 mg/kg Zn and 810 mg/kg Mn; all data corrected to a wet base). The infiltration rate was approximately equal to the discharge rate, thus gypsum and hydroxide precipitation had not resulted in complete clogging of the lake bedrocks.     

Temporal variation and the effect of rainfall on metals flux from the historic Beatson mine,Prince William Sound,Alaska, USA

Several abandoned Cu mines are located along the shore of Prince William Sound, AK, where the effect of mining-related discharge upon shoreline ecosystems is unknown. To determine the magnitude of this effect at the former Beatson mine, the largest Cu mine in the region and a Besshi-type massive sulfide ore deposit, trace metal concentration and flux were measured in surface run-off from remnant, mineralized workings and waste. Samples were collected from seepage waters; a remnant glory hole which is now a pit lake; a braided stream draining an area of mineralized rock, underground mine workings, and waste piles; and a background location upstream of the mine workings and mineralized rock. In the background stream pH averaged ∼7.3, specific conductivity (SC) was ∼40 μS/cm, and the aqueous components indicative of sulfide mineral weathering, SO₄ and trace metals, were at detection limits or lower. In the braided stream below the mine workings and waste piles, pH usually varied from 6.7 to 7.1, SC varied from 40 to 120 μS/cm, SO₄ had maximum concentrations of 32 mg/L, and the trace metals Cu, Ni, Pb, and Zn showed maximum total acid extractable concentrations of 186, 5.9, 6.2 and 343 μg/L, respectively.     

Water quality in pit lakes in disseminated gold deposits compared to two natural, terminal lakes in Nevada

 Within the next 10–15 years, over 35 mines in Nevada will have a lake in their open pit mines after dewatering and cessation of mining. Of the ten past or existing pit lakes at eight different gold mines for which temporal data are available, most had near neutral pH, yet most had at least one constituent (e.g., As, SO₄, TDS) that exceeded drinking water standards for at least one sampling event. Most samples from pit lakes had TDS exceeding drinking water standards, but lower than that in the natural Pyramid (TDS≈5,500 mg/l) and Walker (TDS≈14,000 mg/l) Lakes. In the past century, salinity increased in both natural, terminal lakes, in part due to irrigation withdrawals and evapoconcentration. The salinity in the pit lakes may also increase through time via evapoconcentration. However, water balance models indicate that up to 132% (Walker Lake) of the total yearly inflow evaporates from the terminal lakes, whereas steady-state may be reached in the pit lakes modelled, where evaporative losses account for only ≈6% of the total pit lake volume annually and ≈100% of the net inflow (groundwater inflow minus outflow, precipitation and runoff into the lake). The effects of evapoconcentration are expected to be less significant at most pit lakes than at the natural, terminal lakes because (1) evaporation rates are lower at many pit lakes because they are located at higher elevations than the terminal lakes, and (2) the surface area to depth ratio of the pit lakes is >1000 times smaller than that of the terminal lakes. Received: 1 March 1999 · Accepted: 13 April 1999     

Geochemistry and mineralogy of arsenic in (natural) anaerobic groundwaters

Here new data from field bioremediation experiments and geochemical modeling are reported to illustrate the principal geochemical behavior of As in anaerobic groundwaters. In the field bioremediation experiments, groundwater in Holocene alluvial aquifers in Bangladesh was amended with labile water-soluble organic C (molasses) and MgSO₄ to stimulate metabolism of indigenous SO₄-reducing bacteria (SRB). In the USA, the groundwater was contaminated by Zn, Cd and SO₄, and contained <10 μg/L As under oxidized conditions, and a mixture of sucrose and methanol were injected to stimulate SRB metabolism. In Bangladesh, groundwater was under moderately reducing conditions and contained ∼10 mg/L Fe and ∼100 μg/L As. In the USA experiment, groundwater rapidly became anaerobic, and dissolved Fe and As increased dramatically (As > 1000 μg/L) under geochemical conditions consistent with bacterial Fe-reducing conditions. With time, groundwater became more reducing and biogenic SO₄ reduction began, and Cd and Zn were virtually completely removed due to precipitation of sphalerite (ZnS) and other metal sulfide mineral(s). Following precipitation of chalcophile elements Zn and Cd, the concentrations of Fe and As both began to decrease in groundwater, presumably due to formation of As-bearing FeS/FeS₂. By the end of the six-month experiment, dissolved As had returned to below background levels. In the initial Bangladesh experiment, As decreased to virtually zero once biogenic SO₄ reduction commenced but increased to pre-experiment level once SO₄ reduction ended. In the ongoing experiment, both SO₄ and Fe(II) were amended to groundwater to evaluate if FeS/FeS₂ formation causes longer-lived As removal. Because As-bearing pyrite is the common product of SRB metabolism in Holocene alluvial aquifers in both the USA and Southeast Asia, it was endeavored to derive thermodynamic data for arsenian pyrite to better predict geochemical processes in naturally reducing groundwaters. Including the new data for arsenian pyrite into Geochemist’s Workbench, its stability field completely dominates in reducing Eh–pH space and “displaces” other As-sulfides (orpiment, realgar) that have been implied to be important in previous modeling exercises and reported in rare field conditions.     

Geochemistry and stable isotope composition of the Berkeley pit lake and surrounding mine waters,Butte, Montana

Samples of mine water from Butte, Montana were collected for paired geochemical and stable isotopic analysis. The samples included two sets of depth profiles from the acidic Berkeley pit lake, deep groundwater from several mine shafts in the adjacent flooded underground mine workings, and the acidic Horseshoe Bend Spring. Beginning in July-2000, the spring was a major surface water input into the Berkeley pit lake. Vertical trends in major ions and heavy metals in the pit lake show major changes across a chemocline at 10–20 m depth. The chemocline most likely represents the boundary between pre-2000 and post-2000 lake water, with lower salinity, modified Horseshoe Bend Spring water on top of higher salinity lake water below. Based on stable isotope results, the deep pit lake has lost approximately 12% of its initial water to evaporation, while the shallow lake is up to 25% evaporated. The stable isotopic composition of SO₄ in the pit lake is similar to that of Horseshoe Bend Spring, but differs markedly from SO₄ in the surrounding flooded mine shafts. The latter is heavier in both δ³⁴S and δ¹⁸O, which may be due to dissolution of hypogene SO₄ minerals (anhydrite, gypsum, barite) in the ore deposit. The isotopic and geochemical evidence suggests that much of the SO₄ and dissolved heavy metals in the deep Berkeley pit lake were generated in situ, either by leaching of soluble salts from the weathered pit walls as the lake waters rose, or by subaqueous oxidation of pyrite on the submerged mine walls by dissolved Fe(III). Laboratory experiments were performed to contrast the isotopic composition of SO₄ formed by aerobic leaching of weathered wallrock vs. SO₄ from anaerobic pyrite oxidation. The results suggest that both processes were likely important in the evolution of the Berkeley pit lake.     

Rare earth element fractionation during the precipitation and crystallisation of hydrous ferric oxides from anoxic lake water

An enrichment of light rare earth elements (LREE) is characteristic for most of the acidic, Fe- and SO₄-rich pit lakes and groundwaters in the lignite mining area of Lower Lusatia (Germany). One of these acidic lakes – the pit lake “RL 1223” – has a strong thermal and chemical stratification. The upper water layer (0–9 m) shows pH values of about 3 during all seasons. The monimolimnion (10–17 m) of the lake is anoxic and has pH values of about 7. The rare earth element (REE) patterns of the upper lake water show enriched LREE (La_N/Yb_N = 1.6) whereas the opposite patterns (depletion of LREE, La_N/Yb_N = 0.4) are found in the anoxic water of the monimolimnion. Experiments were conducted to observe the behaviour of REE during Fe oxidation in water from the monimolimnion (depth 14 m). The sampled monimolimnion water was placed in plastic bottles, and the changing water chemistry was observed for 40 weeks after sampling. Due to the initial anoxic conditions almost all Fe precipitated in the investigated water, and the pH value decreased from about 7 to 3 during the oxidation. The Fe precipitates are identified as ferrihydrite which is transformed into goethite within the oxidation process. Stable pH conditions (pH 3.0) were reached after about 10 weeks of oxidation.The original REE patterns of the investigated water are generally reflected in the Fe precipitates collected at the beginning of the experiment as well as after up to 40 weeks of oxidation. However, in the corresponding water LREE were temporally enriched with a maximum La_N/Yb_N ratio of 1.0 and a maximum La_N/Sm_N ratio of 2.3 after 6 weeks of oxidation time (pH 3.8–4.9). Although complex geochemical changes took place between the start and the end of the experiment REE patterns observed at these points in time are nearly identical. These differences of the REE pattern can be explained by the sampling procedure. The experimental findings can be transmitted to the mining dump aquifers of the study area where geochemical conditions comparable to the experimental oxidation time from 3 to 6 weeks are found and hydrous ferric oxides are precipitating. Groundwater passing through the mining dumps can preferentially desorb LREE from the Fe precipitates and display the typical LREE enrichment and carry it to the epilimnion of the acidic pit lakes in Lower Lusatia.     

Seasonal variations in sulfur isotopic composition of dissolved SO<Subscript>4</Subscript><Superscript>2−</Superscript> in the Aha Lake,Guiyang and their implications

The Aha Lake is a seasonal anoxic water system in the southwest of Guiyang City, Guizhou Province, China. Seasonal variations in SO42- concentrations and their isotopic compositions in lake water as well as in the tributaries were investigated in this study. The results showed that sulfate concentrations in river water range from 0.94 to 6.52 mmol/L and their δ34S values range from -14.9‰ and 0.9‰, while lake water has sulfate concentrations ranging from 1.91 to 2.79 mmol/L, and δ34S values from -9.8‰ to -5.9‰. It is suggested that coal mining drainage is the major source of SO42- in the Aha Lake. Rainfall, sewage discharge, sulfide oxidation and gypsum dissolution have made only limited contributions. Different depth-dependent distributions of dissolved SO42- and δ34S were de-veloped for both DB and LJK in summer and winter. Due to water overturn, δ34S values display homogenous vertical distributions in winter and spring. While in summer and autumn, significant positive shifts of δ34S were clearly ob-served in epilimnion and bottom strata as a result of water stratification. High δ34S values in epilimnion may result from the retention of rainwater during water stratification. Dissimilatory sulfate reduction by bacteria was thought to be responsible for the increase of δ34S value in hypolimnion.     

Influence of hardpan layers on arsenic mobility in historical gold mine tailings

Hardpans, or cemented layers, form by precipitation and cementation of secondary minerals in mine tailings and may act as both physical and chemical barriers. Precipitation of secondary minerals during weathering of tailings can sequester metal(loid)s, thereby limiting their release to the environment. At Montague Gold Mines in Nova Scotia, tailings are partially cemented by the Fe arsenate mineral scorodite (FeAsO₄·2H₂O). Previous studies have shown that the formation of scorodite can effectively limit aqueous As concentrations due to its relatively low solubility (<1 mg/L at pH 3–4) and high As content (43–52 wt.% As₂O₅, this study). Co-existing waters and solids were sampled at Montague Gold Mines to identify present-day field conditions influencing scorodite precipitation and dissolution, and to better understand the mineralogical and chemical relationship between hardpan and tailings. In addition to scorodite, hardpan cements were found to include amorphous Fe arsenate and Fe oxyhydroxide. Nearly all hardpan is associated with historical arsenopyrite-bearing concentrate which provides a source of acidity, As⁵⁺ and Fe³⁺ for secondary mineral precipitation. Pore waters sampled from the hardpan have pH values ranging from 2.43 to 7.06. Waters with the lowest pH values also have the highest As concentrations (up to 35.8 mg/L) and are associated with the most extensive hardpan and greatest amount of weathered sulfide. Samples from areas with discontinuous hardpan and less sulfide have near-neutral pH and lower As concentrations. Detailed petrographic observations indicate that the identity and stability of As-bearing secondary minerals depends on the continued availability of sulfide concentrate. The results of this study are being used to develop remediation strategies for highly weathered, hardpan-bearing tailings that consider the stability of both primary and secondary minerals under various cover scenarios.     

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.     

Some factors affecting TDS and pH values in groundwater of the Beihai coastal area in southern Guangxi,China

Groundwater of low total dissolved solids (TDS) of 10 mg/L and low pH ranging commonly from 4.0 to 7.0 occurs in the unconsolidated unconfined and confined aquifers in the coastal plain near Beihai, Guangxi, China. Minerals in the unconsolidated sediment of Quaternary are predominated by quartz (more than 50%) with small amount of clay minerals, such as kaolinite, illite and chlorite. SiO₂ accounts for most (more than 70%) of the chemical compositions of the sediments. The sediments contain low concentrations of soluble constituents (below 300 μg/g) and are predominated by SO₄. Abundant recharge from low-TDS precipitation and a long time’s dissolution of the unconsolidated sediments containing small amount of soluble compositions lead to low TDS of the groundwater in the area.     

Chemical analysis of drinking water from some communities in the Brong Ahafo region

This study consisted of the determination of the trace metals and some physiochemical properties in drinking water samples from the Brong Ahafo region of the Republic of Ghana, where drinking water samples are not treated before it is consumed. The purpose was to ascertain the quality of water from these sources. Samples were taken from fifteen sampling points and analyzed for the following parameters Fe, Cu, Mn, Zn, Al, NO₃ ^?, NO₂ ^?, SO₄ ², PO₄ ^2?, and F^? using the procedure outline in the palintest photometer method. The data showed the variation of the investigated parameters in samples as follows: pH 5.57-7.54, conductivity (EC) 35-1216 us/cm, turbidity 3.25-72.50 NTU,PO₄ ^2?1 0.32-9.30 mg/L,F 0.32-1.05 mg/L,NO₃ ^? 0.09-0.99 mg/L,NO₂ ^? 0.006-0.114 mg/L, SO₄ ^2? 3.33-8.02 mg/L, Cu 1.19-2.75 mg/L Fe 0.05-0.85mg/L, Zn 0.04-0.15 mg/L, Mn 0.003-0.011 mg/L and Al 0.05-0.15 mg/L. The concentrations of most of the investigated parameters in the drinking water samples from Brong Ahafo region were within the permissible limits of the World Health Organization drinking water quality guidelines. There were no correlations between metal concentrations in the drinking water samples.     

Vertical distribution and mobilization of arsenic in shallow alluvial aquifers of Chapai-Nawabganj district, Northwestern Bangladesh

Core sediments from two boreholes and groundwater from fifty four As-contaminated well waters were collected in the Chapai-Nawabganj area of northwestern Bangladesh for geochemical analysis. Groundwater arsenic concentrations in the uppermost aquifer (10 to 40 m of depth) range from 2.76?C315.15 mg/l (average 48.81 mg/l). Arsenic concentration in sediments ranges from 3.26?C10 mg/kg. Vertical distribution of arsenic in both groundwater and sediments shows that maximum As concentration (462 mg/l in groundwater and 10 mg/kg in sediments) occurs at a depth of 24 m. In January 2008, 2009 and 2010, maximum As concentration occurs at the same depth. Environmental scanning electron microscope (ESEM) with EDAX was used to investigate the presence of major and trace elements in the sediments. The dominant groundwater type is Ca-HCO₃ with high concentrations of As and Fe, but with low levels of NO₃ ^? and SO₃ ^?2. Statistical analysis clearly shows that As is closely associated with Fe (R² = 0.64) and Mn (R² = 0.91) in sediments while As is not correlated with Fe and Mn in groundwater samples. Comparatively low Fe and Mn concentrations in some groundwater, suggest that probably siderite and/or rhodochrosite precipitated as secondary mineral on the surface of the sediment particles. The correlations along with results of sequential leaching experiments suggest that reductive dissolution of FeOOH and MnOOH mediated by anaerobic bacteria represents mechanism for releasing arsenic into the groundwater.     

High arsenic and boron concentrations in groundwaters related to mining activity in the Bigadiç borate deposits (Western Turkey)

This study documents the environmental impacts of borate mines in Bigadiç district, which are the largest colemanite and ulexite deposits in the world. Borate-bearing formations have affected the concentrations of some contaminants in groundwater. Groundwater quality is directly related to the borate zones in the mines as a result of water–rock interaction processes. Calcium is the dominant cation and waters are Ca–SO₄ and HCO₃ type in the mine (Tülü borate mine) from which colemanite is produced. However in the Simav and Acep Borate Mines, ulexite and colemanite minerals are produced and waters from these open pit mines are Na–HCO₃–SO₄ types. High SO₄ concentrations (reaching 519 mg/L) might be explained by the existence of anhydrite, gypsum and celestite minerals in the borate zone. Groundwater from tuff and borate strata showed relatively low pH values (7–8) compared to surface and mine waters (>8). EC values ranged from 270 to 2850 μS/cm. Boron and As were the two important contaminants determined in the groundwaters around the Bigadiç borate mines. Arsenic is the major pollutant and it ranged from 33 to 911 μg/L in the groundwater samples. The concentrations of B in the study area ranged from 0.05 to 391 mg/L. The highest B concentrations were detected at the mine areas. The extension of the borate zones in the aquifer systems is the essential factor in the enrichment of B and As, and some major and trace elements in groundwaters are directly related to the leaching of the host rock which are mainly composed of tuffs and limestones. According to drinking water standards, all of the samples exceed the tolerance limit for As. Copper, Mn, Zn and Li values are enriched but do not exceed the drinking water standards. Sulfate, Al and Fe concentrations are above the drinking water standard for the groundwater samples.     

Geochemical assessment of groundwater contamination with special emphasis on fluoride concentration, North Jordan

The concentrations of fluorine in groundwater of North Jordan range from 0.009 to 0.055 mg/l. Other chemical parameters, e.g. pH, EC, TDS, Cl, TH, HCO₃, PO₄, SO₄, NO₃, NH₄, K, Ca, Mg, and NO₃ have been studied and showed higher concentrations in HCO₃⁻ and NO₃⁻ of 307 and 51 mg/l, respectively. Thermodynamic considerations show that almost all the analyzed samples are undersaturated with respect to calcite and fluorite. This undersaturation is probably due to their low availability in the locations. Fluoride concentration shows a positive relation to pH and HCO₃, whereas Cl, Mg, Ca, and Na initially increase and then decrease with increasing fluoride in the water. Saturation indexes of fluorite and calcite are estimated. The chemistry of the groundwater is controlled by the fluorite and calcite solubility. The topography of the area has exerted control on the aerial extent of fluoride concentration.     

Modeling transport of arsenic through modified granular natural siderite filters for arsenic removal

Groundwater arsenic(As)contamination is a hot issue,which is severe health concern worldwide.Recently,many Fe-based adsorbents have been used for As removal from solutions.Modified granular natural siderite(MGNS),a special hybrid Fe(II)/Fe(III)system,had higher adsorption capacity for As(III)than As(V),but the feasibility of its application in treating high-As groundwater is still unclear.In combination with transport modeling,laboratory column studies and field pilot tests were performed to reveal both mechanisms and factors controlling As removal by MGNS-filled filters.Results show that weakly acid pH and discontinuous treatment enhanced As(Ⅲ)removal,with a throughput of 8700 bed volumes(BV)of 1.0 mg/L As(Ⅲ)water at breakthrough of 10 μg/L As at pH 6.Influent HCO_3~-inhibited As removal by the filters.Iron mineral species,SEM and XRD patterns of As-loading MGNS show that the important process contributing to high As(Ⅲ)removal was the mineral transformation from siderite to goethite in the filter.The homogeneous surface diffusion modeling(HSDM)shows that competition between As(III)and HCO_3~-with adsorption sites on MGNS was negligible.The inhibition of HCO_3~-on As(Ⅲ)removal was connected to inhibition of siderite dissolution and mineral transformation.Arsenic loadings were lower in field pilot tests than those in the laboratory experiments,showing that high concentrations of coexisting anions(especially HCO_3~-and SiO_4~(4-)),high pH,low EBCT,and low groundwater temperature decreased As removal.It was suggested that acidification and aeration of highAs groundwater and discontinuous treatment would improve the MGNS filter performance of As removal from real high-As groundwater.     

Geochemical patterns of arsenic-enriched ground water in fractured,crystalline bedrock,Northport, Maine,USA

High mean As concentrations of up to 26.6 μmol/L (1990 μg/L) occur in ground water collected from a fractured-bedrock system composed of sulfidic schist with granitic to dioritic intrusions. Sulfides in the bedrock are the primary source of the As in the ground water, but the presence of arsenopyrite in rock core retrieved from a borehole with As concentrations in the ground water barely above the detection limit of 2.0 μmol/L, shows that there are complicating factors. Chemical analyses of water from 35 bedrock wells throughout a small watershed reveal spatial clustering of wells with high As concentrations. Stiff diagrams and box plots distinguish three distinct types; calcium-bicarbonate-dominated water with low As concentrations (CaHCO₃ type), sodium-bicarbonate-dominated water with moderately high As concentrations (NaHCO₃ type), and calcium-bicarbonate-dominated water with very high As concentrations (High-As type). It is proposed that differences in recharge area and ground-water evolution, and possible bedrock composition difference are responsible for the chemical distinctions within the watershed. Lack of correlation of As concentrations with pH indicates that desorption of As is an insignificant control on As concentration. Correlations of As concentrations with Fe and redox parameters indicates that reductive dissolution of Fe(III) oxyhydroxides may play a role in the occurrence of high As concentrations in the NaHCO₃ and High-As type water. The oxidation of sulfide minerals occurs within the ground-water system and is ultimately responsible for the existence of As in the ground water, but there is no correlation between As and SO₄ concentrations, probably due to precipitation of Fe(III) oxyhydroxides and adsorption of As under oxidizing conditions.     

Pollution of a water course impacted by acid mine drainage in the Imgok creek of the Gangreung coal field,Korea

The purposes of this study are to (i) determine the geochemical characteristics of Imgok creek impacted by acid mine drainage (AMD) generated from abandoned coal mines, (ii) to assess the pollution of heavy metals in the stream sediments and soils, and (iii) to identify the chemical form of Fe precipitates collected in the study area where there are 4 abandoned coal mines, which belong to the Grangreung coal field at the eastern part of Korea. AMD generated from mine adits and coal refuse piles shows low pH, and high concentrations of Fe, Al and SO₄, especially in the Youngdong coal mine. In Imgok creek, pH values increased, and total dissolved solids (TDS) values decreased with distance. The concentrations of toxic heavy metals and major cations except Fe decreased by dilution, but the concentration of Fe decreased rapidly due to the formation of precipitates. The quality of groundwater samples did not exceed the Korean drinking-water standard. In the stream sediments, the concentrations of Fe are relatively high in the Youngdong tributary and Imgok creek, but the concentrations of heavy metals are similar to those of unpolluted sediments. Pollution indices of agricultural soils range from 0.28 to 0.47. Yellowish red Fe precipitates collected in the study area turned out to be amorphous or poorly crystallized minerals (determined by X-ray diffraction patterns and Fe_ox/Fe_tot ratios) and to contain chemically bonded SO₄ and OH [determined by infra-red (IR) spectral analysis]. With these, the mol ratios of Fe/S ranging from 4.6 to 6.1 determined by electron probe micro-analysis (EPMA) in precipitates strongly support the existence of schwertmannite.     

Chloride imbalance in a catchment undergoing hydrological change: Upper Barwon River,southeast Australia

Documenting whether surface water catchments are in net chemical mass balance is important to understanding hydrological systems. Catchments that export significantly greater volumes of solutes than are delivered via rainfall are not in hydrologic equilibrium and indicate a changing hydrological system. Here an assessment is made of whether a saline catchment in southeast Australia is in chemical mass balance based on Cl. The upper reaches of the Barwon River, southeast Australia, has total dissolved solids, TDS, concentrations of up to 5860 mg/L and Cl concentrations of up to 3370 mg/L. The high river TDS concentrations are due to the influxes of groundwater with TDS concentrations of up to 68,000 mg/L. Between 1989 and 2011, the median annual Cl flux from the upper Barwon catchment was 17.8 × 10⁶ kg (∼140 kg/a/ha). This represents 340–2230% of the annual Cl input by rainfall to the catchment. Major ion and stable isotope geochemistry indicate that the dominant source of solutes in the catchment is evapotranspiration of rainfall, precluding mineral dissolution as a source of excess Cl. The upper Barwon catchment is not in chemical mass balance and is a net exporter of solutes. The chemical imbalance may reflect the transition within the last 100 ka from an endorheic lake system where solutes were recycled producing shallow groundwater with high TDS concentrations to a better drained catchment. Alternatively, a rise in the regional water table following land clearing may have increased the input of groundwater with high TDS concentrations to the river system.