Acidic Groundwater Research Articles

Chemical clogging of granular media under acidic groundwater conditions

Generation of acidic groundwater attributed to pyrite oxidation in low-lying acid sulfate soil has caused substantial damage to the soil-water environment and civil infrastructure in coastal Australia. The installation of permeable reactive barriers (PRBs) is a frontier technology in the field of acid neutralisation and removal of toxic heavy metal cations – for example, soluble iron (Fe) and aluminium (Al). This study aims to assess the potential of limestone (calcite) aggregates as the PRB’s main reactive material in low-lying pyritic land. During long-term laboratory column experiments, a significant capacity of limestone for removing contaminant chemical species was observed. Nevertheless, the formation of secondary mineral precipitates upon geochemical reactivity within the granular media in the PRB caused armouring and chemical clogging, which diminished the rate of reactivity – that is, the treatment capacity of calcite aggregates – mainly at the entrance zone of the porous media. Flow properties were altered due to blockage of pores; for instance, hydraulic conductivity was reduced by 25% at the inlet zone. Non-homogeneous clogging towards the outlet was analysed, and the time-dependent effect on the longevity of a limestone column was studied and quantified.

Hydrochemistry and isotopic studies of carbonatite groundwater systems: the alkaline–carbonatite complex of Barreiro, southeastern Brazil

In Brazil, alkaline intrusions are source rocks for several commodities (bauxite, phosphate, niobium and barite, to mention a few), including mineral water. The present study aims to understand by means of chemical and stable isotope analyses, the residence time, circulation and hydrochemical facies of the groundwater systems from the alkaline–carbonatitic complex of Barreiro (State of Minas Gerais, Brazil). This Mesozoic alkaline complex is located in the Brazilian tropical region characterized by weathered soils and fractured rocks, which play an important role in the groundwater dynamics. To assess this influence, groundwater samples from 12 points and water samples from 3 artificial lakes were collected for the determination of chemical element and natural isotope (18O, deuterium and 13C) concentrations and 14C and tritium dating. Two main groundwater categories were revealed: (a) a local, acidic and sub-modern groundwater system developed in thick, poorly mineralized weathered soil from the inner part of ACCB, and (b) a basic, hypothermal, ca. 40-ky-old fractured aquifer developed in mineralized fenitized quartzites. The younger and shallower groundwater circulation is controlled by the present intrusion relief and is prone to environmental impacts. The older, hypothermal groundwater system indicates existing geothermal residual heat provided by the Mesozoic alkaline intrusion.

Regularities in the Evolution of Groundwater Quality at Abandoned Copper Sulfide Mines at the Levikha Ore Field, Central Urals, Russia

Over the past decades, dozens of copper sulfide mines in the Urals were decommissioned, abandoned, and flooded. At many of them, acidic mine waters are still generated and poured out on the surface. Concentrations of most components in these waters are several orders of magnitude higher than the maximum permissible concentrations. The chemical composition of the acidic groundwaters is generated in a highly dynamic regime: the concentrations of all components in the water first drastically increase after flooding and then decrease during many years. At the Levikha copper sulfide deposit, Sverdlovsk oblast, the filling of the depression cone in 2007 was followed by the origin of focused groundwater discharge: an anthropogenically produced water body (pond) in a caving within the collapse and subsidence zone. The concentrations of many indicator components were higher ten years after the flooding was terminated than when the mining operations were in progress. The dominant source of the acidic waters with elevated mineralization (total dissolved solids, TDS) is the collapse zone, in which an anthropogenically generated sulfuric-acid weathering crust enriched in secondary minerals developed for tens of years in the process of mine drainage. According to results of numerical geofiltration simulations, the lateral flow makes up 60% of the hydrodynamic balance and dilutes the solution generated in the collapse zone. The movement time of this flow through the collapse zone is six–eight years, and this period of time is marked by extremely high values of practically all indicators in the anthropogenic pond. The composition of rocks whose dissolution–precipitation controls the composition of groundwater in the area of the flooded mine was determined by numerical simulations with the Visual MINTEQ ver.3.0/3.1 software. The duration of the process that forms the composition of the acidic groundwaters is estimated at tens of years.

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 HCO3− 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 HCO3− with adsorption sites on MGNS was negligible. The inhibition of HCO3− 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 HCO3− and SiO44−), high pH, low EBCT, and low groundwater temperature decreased As removal. It was suggested that acidification and aeration of high-As groundwater and discontinuous treatment would improve the MGNS filter performance of As removal from real high-As groundwater.

Controls on iron(II) fluxes into waterways impacted by acid mine drainage: A Damköhler analysis of groundwater seepage and iron kinetics

When acidic groundwater flows into an aquatic system the sediment water interface (SWI) acts as a transition zone between the groundwater and lake water, and often exhibits strong physical and biogeochemical gradients. The fate of groundwater-borne solutes, such as Fe(II), is determined by the balance between the exposure time during transport across the SWI and the reaction time within the SWI, however the relative role of groundwater seepage rates and iron kinetics on acidity generation in lakes is unknown. Porewater seepage velocities, porewater chemical profiles, and limnological data were collected across multiple field campaigns over the last two decades, in acid Mine Lake 77, in Lusatia, Germany. This rare data set was analyzed using a Damköhler approach that compares exposure and reactions timescales, to determine that Fe(II) would typically be transported with little reaction across the SWI, spatially separating it from sediment-processes that produce alkalinity and providing a source of acidity to the lake. This Damköhler analysis further showed that remediation should be focused on reducing groundwater seepage velocities and enhancing exposure times. Strategic planting of submerged benthic macroalgae would slow groundwater inflows, as well as oxygenating overlying waters and supplying organic matter to the sediments. A similar Damköhler analysis could be used to assess the fate of any groundwater-borne reactive chemicals (e.g. phosphorus) into lakes and streams.

Research of the mineral complex and opportunities using basalts and tuffs of ivandodolinsky quarry of the Rivne region in watertreatmant

Mineral composition of basalts and red-colored tuffs of Ivanodolinsky quarry of Rivne region is investigated. Water-purifying properties of dispersed basalts have not yet been investigated. There are significant prospects in this direction. It was established that mineralization of basalts is andesine, (93%) with small part of saponite (7 %) and tuffs consist of saponite (56%) and hematite (17%). The structure of each rock includes aluminosilicates, which possess ion-exchange properties.It is known that acidic water (pH <6.5-7) without pre-treatment for drinking water needs special treatment. Transport by water supply systems is not possible as it does not meet the requirements of the State Sanitary. There are high concentrations of heavy metals ions in acidic groundwater. They are one of the most dangerous pollutants in the groundwater.The mineral composition of the basalts and tuffs of the Ivanodolinsky deposit of the Rivne region is investigated. The questions of possibilities and using dispersed basalts and tuffs of Ivanodolinsky deposit of Rivne region for treatment of water intended for consumption. Reducing the acidity of water can do tuff and basalt. The highest efficiency of the extraction of ions Mn2+ showed red-colored tuff.

Occurrence of metal-rich acidic groundwaters around the Mekong Delta (Vietnam): A phenomenon linked to well installation

Acidic groundwaters enriched with heavy metals are frequently observed in the coastal plain aquifers. The acidic pHs are observed even in the deep confined aquifers in southern Vietnam. This study geochemically explores the causes of these acidic groundwaters by investigating 41 groundwater samples, 4 soil samples and a 54 m long sediment core and the long-term monitoring data (4189 observations) obtained from 178 wells of the National Groundwater Monitoring Network for the South of Vietnam (NGMNS). The groundwater data show elevated Fe, Mn, Al, Pb, and Zn concentrations as the pH becomes acidic and suggest pyrite oxidation be the major cause for the groundwater acidification. This is further confirmed by pyrite framboids observed in the sediment or soil samples taken from the sites where strongly acidic groundwaters were observed. Results of leaching experiments using sediment and soil samples indicate that high metal concentrations in the acidic pH are associated with the increased metal solubility and mineral dissolution kinetics. The acidification of deep groundwaters is revealed to be associated with well installation, indicating the importance of proper well-installation techniques to protect water quality of deep confined aquifers.

Hydrochemical behaviour of an underground dammed limestone aquifer in the subtropics

AbstractIn an island area with limited surface water resources, the conservation of fresh groundwater resources is important for the maintenance of the water supply and irrigation system to be used in agricultural activity. In a limestone aquifer used for irrigation on a subtropical island, the progress of the dissolution behaviour of the aquifer and the effects of CO2 storage and chemical weathering of the aquifer must be understood to ensure sustainable groundwater use. In this study, hydrochemical behaviour of the Ryukyu limestone aquifer was investigated using the residence time of slowly flowing groundwater, which is contained by a 2.3‐km‐long underground man‐made dam constructed in 2005 located at the southern edge of Okinawa Island, Japan. Groundwater dating and horizontal flow velocity measurements indicate that the groundwater horizontal flow is slow in the underground dam area, whereas the saturation index (SI) for calcite indicates that the majority of the limestone exists in a supersaturated condition. The range of SI for gypsum was indicative of subsaturation, and the SI increased with the groundwater residence time, suggesting that the dissolution of CaSO4 tends to proceed slowly through the common ion effect. The dissolution of CaCO3, however, tends to be in suspension within about a residence time of 20 years. The apparent dissolution rate of Ca in the Ryukyu limestone aquifer in this area was estimated to be approximately 2.70–4.06 nmol cm−2 year−1, a value much lower than that of the early stages of the limestone dissolution. The estimated groundwater CO2 (gas) concentrations are two order high (about 0.3–7%) relative to atmospheric CO2 (about 0.04%), and the groundwater pH shows a tendency to decrease from 7.4 to 6.9 over the last approximately 20 years, indicating that the hydration reaction of CO2 occurs in the Ryukyu limestone aquifer. These findings imply that high CO2 produced in the subtropical zone is consumed slower than CO2 production during limestone dissolution, with the exclusion of the early stage of the chemical weathering process, and causes groundwater acidification in the underground dammed limestone aquifer.

Effects of Bark Beetle Disturbance on Soil Nutrient Retention and Lake Chemistry in Glacial Catchment

Forest ecosystems worldwide are subjected to human-induced stressors, including eutrophication and acidification, and to natural disturbances (for example, insect infestation, windstorms, fires). The occurrence of the later is expected to increase due to the ongoing climate change. These multi-stressor forcings modify ecosystem biogeochemistry, including the retention of limiting nutrients, with implications for terrestrial and aquatic biodiversity. Here we present whole ecosystem nutrient (N, Ca, Mg, K) mass balances in the forested catchment of Plesne Lake, CZ, which has undergone transient changes linked to the recovery from anthropogenic acidification and to the forest disturbances caused by severe infestations by the bark beetle (Ips typographus). Measured fluxes and storage of nutrients in the lake-catchment ecosystem were used to constrain the process-oriented biogeochemical model MAGIC (Model of Acidification of Groundwater In Catchments). Simulated lake water chemistry and changes in soil nutrient pools fitted observed data and revealed that (1) the ecosystem N retention declined, thus nitrate leaching increased for 10 years following the bark beetle disturbance, with transient adverse effects on the acid–base status of lake water, (2) the kinetics of nutrient mineralisation from decaying biomass coupled with nutrient immobilisation in regrowing vegetation constrained the magnitude and duration of ecosystem losses of N, Ca and Mg, (3) the excess of mineralised base cations from decomposing biomass replenished the soil cation exchange matrix, which led to increased soil base saturation, and (4) the improvement of the catchment soil acid–base status led to an increase of lake water pH and acid neutralising capacity. Forested ecosystems underlain by nutrient-poor soils and bedrock are prone to human-induced damages caused by acidification and eutrophication, and any natural disturbance may further lead to nutrient imbalances. We demonstrated that in this natural forest ecosystem protected from human intervention, disturbances together with natural post-disturbance vegetation recovery have temporally positive effects on the nutrient stores in the soil.

Hydrological buffering during groundwater acidification in rapidly industrializing alluvial plains

Shallow groundwater in alluvial plains is vulnerable to contamination due to infiltration of pollutants from anthropogenic activities. In the alluvial plain of the Yangtze River near Poyang Lake, a silicone monomer industrial park was found to discharge undisclosed amount of effluents containing high levels of hydrochloric acid and total dissolved solids into ponds and ditches that in turn, displayed characteristics of acidic water with high total dissolved solids. However, most shallow groundwater (n = 35, depth 5.5–22 m) collected from the alluvial aquifer downgradient of the industrial park contained low Cl− (< 59 mg/L). Only 4 groundwater samples showed high Cl− (59–790 mg/L) with two containing superlative levels (449 mg/L and 790 mg/L) located within a 50 m distance from the polluted ponds and ditches. In addition to hydrochemical data, modeling was used to explain why a highly vulnerable alluvial aquifer was not more contaminated when subjected to such intense point source pollution and to estimate the effluent discharge. Flow and solute transport modeling suggests that a hydrological buffering mechanism resulting from a topography driven groundwater outflow along the boundary between the Pleistocene (Qpw) and the Holocene (Qhl) aquifers where the polluted ponds and ditches are fortuitously located has restricted acid infiltration, with only a few percent of the estimated acid discharge in the amount of 1700–4200 tons per year entering the aquifer. Our model results show that pumping from the upland Qpw aquifer breaks this hydrological buffering much more easily than pumping from the downgradient Qhl aquifer, suggesting the vulnerability of this buffering mechanism thus compromising groundwater utilization in the future.

Effective Remediation of Groundwater Fluoride with Inexpensively Processed Indian Bauxite.

India represents one-third of the world's fluorosis burden and is the fifth global producer of bauxite ore, which has previously been identified as a potential resource for remediating fluoride-contaminated groundwater in impoverished communities. Here, we use thermal activation and/or groundwater acidification to enhance fluoride adsorption by Indian bauxite obtained from Visakhapatnam, an area proximate to endemic fluorosis regions. We compare combinatorial water treatment and bauxite-processing scenarios through batch adsorption experiments, material characterization, and detailed cost analyses. Heating Indian bauxite above 300 °C increases available surface area by > 15× (to ∼170 m2/g) through gibbsite dehydroxylation and reduces the bauxite dose for remediating 10 ppm F- to 1.5 ppm F- by ∼93% (to 21 g/L). Additionally, lowering groundwater pH to 6.0 with HCl or CO2 further reduces the average required bauxite doses by 43-73% for ores heated at 300 °C (∼12 g/L) and 100 °C (∼77 g/L). Product water in most examined treatment scenarios complies with EPA standards for drinking water (e.g., As, Cd, Pb, etc.) but potential leaching of Al, Mn, and Cr is of concern in some scenarios. Among the defluoridation options explored here, bauxite heated at 300 °C in acidified groundwater has the lowest direct costs ($6.86 per person per year) and material-intensity.

Unrefined humic substances as a potential low-cost amendment for the management of acidic groundwater contamination

The present study explores a novel application of Huma-K, a commercially available, unrefined humic substance, as a promising low-cost source of organic matter for in situ remediation of contaminated acidic groundwater plumes. This can be achieved by creating a humic-rich coating on the surface of minerals which can enhance the sorption of contaminants from groundwater. Huma-K was characterized by means of scanning electron microscopy equipped with energy dispersive spectroscopy, Fourier-transform infrared analysis, and potentiometric titrations. Batch experiments were performed to investigate the sorption-desorption behavior of Huma-K and to evaluate what conditions (pH, contact time, and initial Huma-K concentration) affect these processes upon injection into aquifer sediments. As evidenced by potentiometric titrations, Huma-K possesses functional groups that have an acidic nature, with pK values in the range of 4–6 (carboxylic) and 9–10 (phenolic). Sorption, homogeneous precipitation, and surface-induced precipitation seem to be favored in the presence of sediment at pH 4, where there is less deprotonation of acidic functional groups. As the pH is increased, functional groups become negatively charged, leading to electrostatic repulsion and dissolution of Huma-K from sediment. Kinetic experiments indicate that Huma-K sorption is a slow-rate process, most likely governed by film diffusion. The enhanced sorption of Huma-K in acidic conditions suggests that it may be used to create a subsurface treatment zone in acidic aquifers for the sequestration of contaminants such as uranium. The treatment zone will persist as long as the pH does not increase sufficiently to cause soil-bound Huma-K to be released, remobilizing aqueous contaminants.

Using hair and fingernails in binary logistic regression for bio-monitoring of heavy metals/metalloid in groundwater in intensively agricultural areas, Thailand

In this study, the hair and fingernails of the local people in an intensively cultivated agricultural area in Ubon Ratchathani province, Thailand, were used as biomarkers of exposure to arsenic (As) and heavy metals. The study area has shallow acidic groundwater that is contaminated with As and heavy metals. The local people often consume this shallow groundwater; thus, they are exposed to As and heavy metals. Hair and fingernail samples were collected to characterize the differences between shallow groundwater drinking (SGWD) and tap water drinking (TWD) residents. The concentrations of As and the heavy metals Cd, Pb and Hg were significantly higher in the hair samples from the SGWD group than those from the TWD group, especially for As (0.020–0.571 vs. 0.024–0.359µg/g) and Cd (0.009–0.575 vs. 0.013–0.230µg/g). Similarly, the concentrations of As and the heavy metals in the fingernail samples collected from the SGWD group were larger than those of the TWD group, especially for As (0.039–2.440µg/g vs. 0.049–0.806µg/g). The χ2 statistic and binary logistic regression were used to find the associated factors and assess the associated probabilities. The regression results show that the factors associated with the concentrations of As and the heavy metals in the hair samples were drinking water source, rate of water consumption, gender, bathing water source, education, smoking and underlying disease, whereas the factors associated with the concentrations of these species in the fingernail samples were drinking water source, gender, occupation, work hours per day, alcohol consumption, and the use of pesticides.

The distribution and origins of extremely acidic saline groundwaters in the south of Western Australia – Groundwater and digital mapping datasets provide new insights

Some of the largest extents of naturally occurring acidic waters are found across southern Australia. The origins of these systems remain poorly understood with many hypotheses for their genesis. Australian government agency groundwater datasets and mapping data (vegetation, geology, regolith and soils) for south-western Australia, unavailable to previous researchers, were statistically analysed to better understand the origins of acidic groundwater and guide additional fieldwork to study the origins of acidic saline groundwater. The groundwater data showed a distinct bimodal distribution in pH; the ‘acid’ population had a median pH of 3.5 and the larger ‘non-acid’ population had a median pH of 6.6. Acidic groundwater became progressively more common further from the coast towards the drier internally drained regions. Acidic groundwater was mostly confined to the lower slopes and valley floors with localised controls on distribution. Paradoxically, subsoil alkalinity within the internally drained inland regions had the strongest correlation with acidic groundwater (r2 = 0.85). Vegetation was also a strong predictor of acidic groundwater. Acidic groundwater had the highest occurrence under Eucalyptus woodlands and shrublands that grew on alkaline calcareous soils.Pre-clearing soil data in areas with acidic saline groundwater showed that the upper 1 m of the unsaturated zone had a pH around 8 while the pH at depths greater than 5 m decreased to <4. Based on the observations it is proposed that biogenic formation of calcareous soils occurs in the upper 1 m of the profile, calcium is sourced from the deeper profile where the root biota exchanges calcium for hydrogen ions to maintain charge balance. Iron is mobilised from the upper soil profile and concentrates lower in the profile at depths >1.5 m. There, the iron is reduced around roots and the alkalinity generated by microbial iron reduction is removed by biogenic calcification processes. The iron moves in solution further down the profile following roots where it comes in contact with the oxygenated unsaturated zone matrix and is oxidised generating acid. The resulting acidic recharging solution acidifies the unsaturated zone matrix. Saline groundwater moving through the matrix becomes acidified due to ion exchange or direct recharge. The main chemical processes were modelled in PHREEQC to test the plausibility of the hypothesis and acidic solutions with a pH of 3.8 or lower were obtained.

Tidal and seasonal variation in carbonate chemistry, pH and salinity for a mineral-acidified tropical estuarine system

Estuarine acidification and carbonate chemistry derive from multiple biogeochemical processes. Other than biogenic CO2-acidification, estuaries can be acidified allochthonously through non-carbonate sources originating in freshwater and land ecosystems. The present study considered the carbonate chemistry of a nutrified, turbid, tropical mangrove estuary, influenced by acidic groundwater discharge from pyritic soils (Acid Sulphate Soils, ASS). We studied the spatial and temporal variation of the surface water pH, salinity, total alkalinity (TA), partial pressure carbon dioxide (pCO2), dissolved inorganic carbon (DIC), and calcite (Ωcal) and aragonite (Ωara) saturation, in the Brunei Estuarine System (BES), Borneo, Southeast Asia. pH and salinity for tidal to seasonal timeframes were determined from data collected half-hourly, logged at three stations (upper, middle and lower estuary); these data were correlated with rainfall incidence and intensity. Carbonate parameters were calculated from TA using discrete samples collected from six stations. pH (6.8–7.9) and salinity (4.2–28.2) increased expectedly seawards, due to tidal forcing and freshwater dilution at opposite ends of the estuary; amplitudes within a tidal cycle became expanded landwards and during spring tides. While, the overall effect of heavy daily downpours on estuarine salinity and pH was muted, cumulative rainfall during the monsoon season distinctly lowered parameter baselines; the response was again more pronounced in the upper estuary. In the mid-to-upper estuary, we observed a remarkably low pH relative to salinity, extraordinary pCO2super-saturation (13031 ± 4412 μatm) and carbonate undersaturation (Ωcal and Ωara were 0.006–1.431 and 0.004–0.928, respectively). Although the relative contributions of heterotrophic metabolism and ASS-discharge to the estuarine pH and pCO2 were not determined, both processes are implicated in increasing both acidity and CO2 levels. This study contributes to the understanding of carbonate fluxes in mineral-acidified estuaries.

X-ray CT and multiphase flow characterization of a ‘bio-grouted’ sandstone core: The effect of dissolution on seal longevity

Microbially induced carbonate precipitation (MICP) is a novel method for controlling permeability in the subsurface with potential for sealing or reducing leakage from subsurface engineering works such as carbon sequestration reservoirs. The purpose of this research was to measure, at core scale, the change in reservoir permeability and capillary pressure due to MICP during seal formation, then to monitor the integrity of the seal when exposed to acidic groundwater capable of causing dissolution. The experiment was carried out with a Berea sandstone core mounted in a high pressure core holder within a medical X-ray CT scanner.Multiple full volume CT scans gave spatially resolved maps of the changing porosity and saturation states throughout the experiment. Porosity and permeability decreased with MICP whilst capillary pressure was increased. Dissolution restored much of the original porosity, but not permeability nor capillary pressure. This lead to the conclusion that injection pathways were coupled with carbonate precipitation hence preferential flow paths sealed first and transport of the dissolution fluid was limited. Provided a high enough reduction in permeability can be achieved over a substantial volume, MICP may prove to be a durable bio-grout, even in acidic environments such as a carbon sequestration reservoir.

Arsenic, manganese and aluminum contamination in groundwater resources of Western Amazonia (Peru)

This paper presents a first integrated survey on the occurrence and distribution of geogenic contaminants in groundwater resources of Western Amazonia in Peru. An increasing number of groundwater wells have been constructed for drinking water purposes in the last decades; however, the chemical quality of the groundwater resources in the Amazon region is poorly studied. We collected groundwater from the regions of Iquitos and Pucallpa to analyze the hydrochemical characteristics, including trace elements. The source aquifer of each well was determined by interpretation of the available geological information, which identified four different aquifer types with distinct hydrochemical properties. The majority of the wells in two of the aquifer types tap groundwater enriched in aluminum, arsenic, or manganese at levels harmful to human health. Holocene alluvial aquifers along the main Amazon tributaries with anoxic, near pH-neutral groundwater contained high concentrations of arsenic (up to 700μg/L) and manganese (up to 4mg/L). Around Iquitos, the acidic groundwater (4.2≤pH≤5.5) from unconfined aquifers composed of pure sand had dissolved aluminum concentrations of up to 3.3mg/L. Groundwater from older or deeper aquifers generally was of good chemical quality. The high concentrations of toxic elements highlight the urgent need to assess the groundwater quality throughout Western Amazonia.

Organohalide Respiration with Chlorinated Ethenes under Low pH Conditions.

Bioremediation at chlorinated solvent sites often leads to groundwater acidification due to electron donor fermentation and enhanced dechlorination activity. The microbial reductive dechlorination process is robust at circumneutral pH, but activity declines at groundwater pH values below 6.0. Consistent with this observation, the activity of tetrachloroethene (PCE) dechlorinating cultures declined at pH 6.0 and was not sustained in pH 5.5 medium, with one notable exception. Sulfurospirillum multivorans dechlorinated PCE to cis-1,2-dichloroethene (cDCE) in pH 5.5 medium and maintained this activity upon repeated transfers. Microcosms established with soil and aquifer materials from five distinct locations dechlorinated PCE-to-ethene at pH 5.5 and pH 7.2. Dechlorination to ethene was maintained following repeated transfers at pH 7.2, but no ethene was produced at pH 5.5, and only the transfer cultures derived from the Axton Cross Superfund (ACS) microcosms sustained PCE dechlorination to cDCE as a final product. 16S rRNA gene amplicon sequencing of pH 7.2 and pH 5.5 ACS enrichments revealed distinct microbial communities, with the dominant dechlorinator being Dehalococcoides in pH 7.2 and Sulfurospirillum in pH 5.5 cultures. PCE-to-trichloroethene- (TCE-) and PCE-to-cDCE-dechlorinating isolates obtained from the ACS pH 5.5 enrichment shared 98.6%, and 98.5% 16S rRNA gene sequence similarities to Sulfurospirillum multivorans. These findings imply that sustained Dehalococcoides activity cannot be expected in low pH (i.e., ≤ 5.5) groundwater, and organohalide-respiring Sulfurospirillum spp. are key contributors to in situ PCE reductive dechlorination under low pH conditions.

Metavolcanic host rocks, mineralization, and gossans of the Shaib al Tair and Rabathan volcanogenic massive sulphide deposits of the Wadi Bidah Mineral District, Saudi Arabia

ABSTRACTThe Wadi Bidah Mineral District of Saudi Arabia contains more than 16 small outcropping stratabound volcanogenic Cu–Zn–(Pb) ± Au-bearing massive sulphide deposits and associated zones of hydrothermal alteration. Here, we use major and trace element analyses of massive sulphides, gossans, and hydrothermally altered and least altered metamorphosed host rock (schist) from two of the deposits (Shaib al Tair and Rabathan) to interpret the geochemical and petrological evolution of the host rocks and gossanization of the mineralization. Tectonic interpretations utilize high-field-strength elements, including the rare earth elements (REE), because they are relatively immobile during hydrothermal alteration, low-grade metamorphism, and supergene weathering and therefore are useful in constraining the source, composition, and physicochemical parameters of the primary igneous rocks, the mineralizing hydrothermal fluid and subsequent supergene weathering processes. Positive Eu anomalies in some of the massive sulphide samples are consistent with a high temperature (>250°C) hydrothermal origin, consistent with the Cu contents (up to 2 wt.%) of the massive sulphides. The REE profiles of the gossans are topologically similar to nearby hydrothermally altered felsic schists (light REE (LREE)-enriched to concave-up REE profiles, with or without positive Eu anomalies) suggesting that the REE experienced little fractionation during metamorphism or supergene weathering. Hydrothermally altered rocks (now schists) close to the massive sulphide deposits have high base metals and Ba contents and have concave-up REE patterns, in contrast to the least altered host rocks, consistent with greater mobility of the middle REE compared to the light and heavy REE during hydrothermal alteration. The gossans are interpreted to represent relict massive sulphides that have undergone supergene weathering; ‘chert’ beds within these massive sulphide deposits may be leached wall-rock gossans that experienced silicification and Pb–Ba–Fe enrichment from acidic groundwaters generated during gossan formation.

Sodium silicate treatment for the attenuation of U(VI) by iron‐bearing sediments in acidic groundwater plumes

AbstractBACKGROUNDAnthropogenic activities, such as uranium mining and the nuclear industry, have resulted in groundwater contamination and the creation of uranium‐affected acidic plumes. In situ immobilization through base injection is a favorable way of uranium attenuation. The present study explores the use of sodium silicate for the restoration of neutral pH of the affected zone and consequently, uranium immobilization under circumneutral conditions.RESULTS70 mg L−1 sodium silicate restored the pH of uranium bearing, acidic groundwater to neutral in batch experiments consisting of Savannah River Site (SRS) soil and the aqueous phase. SRS soil main components are quartz, kaolinite and goethite and the U(VI) removal was ∼60%. Identical experiments consisting only of quartz and kaolinite showed only 19% U(VI) removal. Binding of uranium may be improved by inner‐sphere complexation and is not affected by the presence of competitive cations, such as Ca2+ and Mg2+. Recovery of uranium under acidic conditions (pH 3.5) was ∼60%, whereas sorption is not reversible under circumneutral conditions.CONCLUSIONSodium silicate restores the pH of acidic groundwater systems to circumneutral conditions, where uranium retention by iron bearing sediments is favored. Goethite is the soil's most reactive phase and contributes to stronger binding of uranium through inner‐sphere complexation. © 2017 Society of Chemical Industry