Long term fluctuations of groundwater mine pollution in a sulfide mining district with dry Mediterranean climate: Implications for water resources management and remediation

Analysis of self-organizing maps and explainable artificial intelligence to identify hydrochemical factors that drive drinking water quality in Haor region

Water contamination undermines human survival and economic growth. Water resource protection and management require knowledge of water hydrochemistry and drinking water quality characteristics, mechanisms, and factors. Self-organizing maps (SOM) have been developed using quantization and topographic error approaches to cluster hydrochemistry datasets. The Piper diagram, saturation index (SI), and cation exchange method were used to determine the driving mechanism of hydrochemistry in both surface and groundwater, while the Gibbs diagram was used for surface water. In addition, redundancy analysis (RDA) and a generalized linear model (GLM) were used to determine the key drinking water quality parameters in the study area. Additionally, the study aimed to utilize Explainable Artificial Intelligence (XAI) techniques to gain insights into the relative importance and impact of different parameters on the entropy water quality index (EWQI). The SOM results showed that thirty neurons generated the hydrochemical properties of water and were organized into four clusters. The Piper diagram showed that the primary hydrochemical facies were HCO3--Ca2+ (cluster 4), Cl---Na+ (all clusters), and mixed (clusters 1 and 4). Results from SI and cation exchange show that demineralization and ion exchange are the driving mechanisms of water hydrochemistry. About 45 % of the studied samples are classified as "medium quality"," that could be suitable as drinking water with further refinement. Cl- may pose increased non-carcinogenic risk to adults, with children at double risk. Cluster 4 water is low-risk, supporting EWQI findings. The RDA and GLM observations agree in that Ca2+, Mg2+, Na+, Cl- and HCO3- all have a positive and significant effect on EWQI, with the exception of K+. TDS, EC, Na+, and Ca2+ have been identified as influencing factors based on bagging-based XAI analysis at global and local levels. The analysis also addressed the importance of SO4, HCO3, Cl, Mg2+, K+, and pH at specific locations.

Spatial distribution and pollution evaluation in dry riverbeds affected by mine tailings

The objective of this study was to evaluate the level of pollution, sources and potential risk of heavy metals (Zn, Cu, Mn, Cd, Cr, Ni, Fe and Pb) and arsenic (As) in four dry riverbeds affected by mine tailing, which drain into one of the biggest coastal lagoon of Europe (Mar Menor). El Beal, La Carrasquilla, Las Matildes and Ponce dry riverbeds sediments were sampled along its course (20, 18, 13, 19 samples were collected, respectively), and total/soluble metal(loid)s, water soluble ions, nitrogen, and organic/inorganic carbon contents were analyzed. Spatial distribution, principal component analysis (PCA), hierarchical cluster analysis (HCA), contamination factor (Cf), pollution load index (PLI) and potential ecological risk index (RI) were used to identify the possible sources of metal(loid)s and to assess the sediment pollution status. The results showed that the mean total concentrations of As, Cu, Cd, Mn, Zn and Pb exceeded the natural background levels of the study area, with the highest values located close to the mining areas. Correlation and cluster analysis identified that Cd and Zn were associated mainly with anthropogenic activities for all riverbeds, while Cr and Ni come from parent. PLI graded the four riverbeds as contaminated by heavy metals, while RI manifested that 100% of samples located in El Beal, La Carrasquilla and Las Matildes had a significantly high ecological risk. Therefore, this study suggests that mine wastes are the main source of metal(loids) contamination in the dry riverbeds, which results can be used to design actions and measures to reduce the environmental impact of metal(loid)s in the Mar Menor coastal lagoon.

Comprehensive health risk analysis of heavy metal pollution using water quality indices and Monte Carlo simulation in R software

Rapid urbanization, population growth, agricultural practices, and industrial activities have led to widespread groundwater contamination. This study evaluated heavy metal contamination in residential drinking water in Shiraz, Iran (2021). The analysis involved 80 groundwater samples collected across wet and dry seasons. Water quality was comprehensively assessed using several indices, including the heavy metals evaluation index (HEI), heavy metal pollution index (HPI), contamination degree (CD), and metal index (MI). Carcinogenic and non-carcinogenic risk assessments were conducted using deterministic and probabilistic approaches for exposed populations. In the non-carcinogenic risk assessment, the chronic daily intake (CDI), hazard quotient (HQ), and hazard index (HI) are employed. The precision of risk assessment was bolstered through the utilization of Monte Carlo simulation, executed using the R software platform. Based on the results, in both wet and dry seasons, Zinc (Zn) consistently demonstrates the highest mean concentration, followed by Manganese (Mn) and Chromium (Cr). During the wet and dry seasons, 25% and 40% of the regions exhibited high CD, respectively. According to non-carcinogenic risk assessment, Cr presents the highest CDI and HQ in children and adults, followed by Mn, As and HI values, indicating elevated risk for children. The highest carcinogenic risk was for Cr in adults, while the lowest was for Cd in children. The sensitivity analysis found that heavy metal concentration and ingestion rate significantly impact both carcinogenic and non-carcinogenic risks. These findings provide critical insights for shaping policy and allocating resources towards effectively managing heavy metal contamination in residential drinking water.

Wildfire effects on the hydrogeochemistry of a river severely polluted by acid mine drainage

This study evaluates for the first time the impact of a large wildfire on the hydrogeochemistry of a deeply AMD-affected river at the beginning of the wet season. To accomplish this, a high-resolution water monitoring campaign was performed within the basin coinciding with the first rainfalls after summer. Unlike similar events recorded in AMD-affected areas, where dramatic increases in most dissolved element concentrations, and decreases in pH values are observed as a result of evaporitic salts flushing and the transport of sulfide oxidation products from mine sites, a slight increase in pH values (from 2.32 to 2.88) and decrease in element concentrations (e.g.; Fe: 443 to 205 mg/L; Al: 1805 to 1059 mg/L; sulfate: 22.8 to 13.3 g/L) was observed with the first rainfalls after the fire. The washout of wildfire-ash deposited in the riverbanks and the drainage area, constituted by alkaline mineral phases, seems to have counterbalanced the usual behavior and patterns of the river hydrogeochemistry during autumn. Geochemical results indicate that a preferential dissolution occurs during ash washout (K > Ca > Na), with a quick release of K followed by an intense dissolution of Ca and Na. On the other hand, in unburnt zones parameters and concentrations vary to a lesser extent than burnt areas, being the washout of evaporitic salts the dominant process. With subsequent rainfalls ash plays a minor role on the river hydrochemistry. Elemental ratios (Fe/SO₄ and Ca/Mg) and geochemical tracers in both ash (K, Ca and Na) and AMD (S) were used to prove the importance of ash washout as the dominant geochemical process during the study period. Geochemical and mineralogical evidences point to intense schwertmannite precipitation as the main driver of reduction in metal pollution. The results of this study shed light on the response of AMD-polluted rivers to certain climate change effects, since climate models predict an increase in the number and intensity of wildfires and torrential rain events, especially in Mediterranean climates.

Hydrogeochemical characterization and quality assessment of groundwater resource in Savar - an industrialized zone of Bangladesh

Savar Upazila in the Dhaka District is a rapidly expanding city with a diverse range of industries and agricultural activities. This expansion poses environmental challenges including the threat to groundwater contamination. Based on these considerations, the objective of this research is to carry out a shallow groundwater hydrogeochemical characterization and an assessment of the suitablity of the groundwater for drinking and irrigational purposes using a geochemical approach, multivariate statistical techniques, and some indices of groundwater quality. The hydrogeochemical analyses of 42 groundwater samples from shallow depths (18 - 76 m) showed that the order of concentrations of cations, anions, and metals was Ca²⁺ > Na⁺ > Mg²⁺ > K⁺, HCO₃^- > Cl^- > SO₄^2- > NO₃^-, and Cr > As > Pb > Mn > Fe, respectively. Weathering of silicates was found to be the most significant hydrogeochemical process governing the chemistry of groundwater. Cation exchange also plays a significant role in the evolution of the groundwater chemistry. Principal component analysis and hierarchical cluster analysis suggested that anthropogenic activities are influencing groundwater quality. A drinking water quality index map showed that about 91% of the groundwater samples were in the excellent category and suitable for human consumption, with only a few samples exceeding the standards of the WHO and Bangladesh for concentrations of Ca²⁺, Mg²⁺, HCO₃^-, Fe, Mn, and As. An analysis of irrigation quality parameters found that most of the groundwater samples were either excellent or good for agricultural uses, except for one sample in the Tetuljhora Union that was unsuitable based on residual sodium carbonate. This finding may be useful to local governments in understanding the current status of groundwater quality, tracking potential threats of contamination, and initiating appropriate measures for long-term groundwater resource management.

Temporal fluctuations in water contamination from abandoned pyrite Wieściszowice mine (Western Sudetes, Poland)

Production of acid mine drainage may occur during mining operations and may continue for many years after closure. In some cases, especially when host rock is capable of reacting with acidic drainage, metal concentrations may decrease over time and distance. Seasonal variations in water flow rate also play an important role in metals concentration both in surface and groundwater. The present study evaluates the contamination of an abandoned pyrite mining area (Wieściszowice, SW Poland) and the temporal variation of the water contamination in selected locations of 2 sampling campaigns (2000 and 2015). The mining surrounding Rudawy Janowickie Mountains range is well known for the rich mining history and is considered as one of the oldest in Europe. The Wieściszowice pyrite mine was exploited for several hundreds of years and processed Fe and Cu sulfides, and sulfur. This mine was closed in 1925 because of the high competition of pyrites from Spain. Results show clearly that water samples collected in the mining area are mainly Ca-SO₄ and acid/high metal, while spring water and surface water samples, representing the local geochemical background, are Ca-HCO₃ and neutral-low metal. The analysis of data shows an improvement in water quality from 2000 to 2015 as well as a decreasing of water risk assessment for human use. This improvement can be related to the fact that 2015 was a very dry year, with over 60% less flow than in 2000, leading to less water contamination.

Acid mine drainage and sewage impacted groundwater treatment by membrane distillation: Organic micropollutant and metal removal and membrane fouling

Groundwater is the dominant source of freshwater in many countries around the globe, and the deterioration in its quality by contaminants originating from anthropogenic sources raises serious concern. In this study, a scenario where groundwater is contaminated by acid mine drainage (AMD) from mining activities and/or sewage was envisaged, and the performance of a direct contact membrane distillation (DCMD) system was investigated comprehensively for different compositions of the AMD- and sewage-impacted groundwater. Regardless of the composition, MD membrane achieved 98-100% removal of metals and bulk organics, while the removal of the selected micropollutants ranged between 80 and 100%. Effective retention of contaminants by the MD led to their accumulation over time, which affected the hydraulic performance of the MD membrane by reducing the permeate flux by 29-76%. When persulfate (PS)-mediated oxidation process was integrated with the DCMD, degradation of bulk organics (50-71%) and micropollutants (50-100%) by PS reduced their accumulation. Characterisation of the fouling layer revealed the occurrence of membrane scaling that was mainly due to the deposition of iron oxide or oxyhydroxide precipitates. For an identical composition of the AMD- and sewage-impacted groundwater, flux decline was 10% less in PS-assisted DCMD as compared to that in the standalone DCMD. However, this did not prevent the formation of iron oxide scales on MD membrane during the operation of PS-assisted DCMD. This study demonstrates the long-term performance of a standalone and PS-assisted DCMD operated in continuous-flow mode to treat AMD- and sewage-impacted groundwater for the first time.

Sustainable water resources through harvesting rainwater and the effectiveness of a low-cost water treatment

Increases in world population and climate change are some of the pressures affecting water resources for current and future water availability. The variability in water availability can reduce agricultural yields, food supplies and potentially leads to malnutrition and spread of diseases in water-poor countries. Even some water-rich countries can experience prolonged periods of dry weather, causing a drop in water reservoirs levels, forcing more restricted water resources management. Rainwater harvesting is one key option in adapting to water shortage and future demands that may alleviate the pressure on existing water resources. This work evaluates a roof top rainwater harvesting system (RWHS) installed as part of a decentralised wastewater treatment system designed to enable a circular economy by providing a more reliable water supply system in a remote public school in rural India. The effectiveness of the RWHS in reducing the pressure on a groundwater supply was assessed along with the physical, chemical and microbial characteristics of the stored rainwater over time. Further, the application of a low-cost primary treatment to make the harvested water safe to use for multiple purposes was investigated. The results revealed that the harvested water was of acceptable quality at the start of collection, however, microbial abundance increased when the rainwater was stored for a long time without treatment. Thus, a chlorine dosing regimen for the RWHS was designed based on laboratory and field experiments. The results also demonstrated that the low-cost chlorination process was effective in the field in reducing microbial abundance in the stored water for more than 30 days. However, as the residual chlorine level was reduced with time to <0.2 mg/l in the storage vessel, the microbial abundance increased, albeit to a much lower level that meets the Indian bathing water standards. The results provide evidence that installed RWHS has reduced the pressure on existing water supply at the school by up to 25% of the water that used for washing and flushing with no treatment, and with regular chlorination, greater savings and multiple uses of the stored rainwater can be achieved.

Microfluidic pore model study of precipitates induced by the pore-scale mixing of an iron sulfate solution with simulated groundwater

Precipitates induced by the pore-scale mixing of iron sulfate solutions with simulated groundwater were investigated using a microfluidic pore model to assess the environmental impacts of the infiltration of acid mine drainage into a shallow aquifer. This model was employed to visualize the formation of precipitates in a porous network and to evaluate their physicochemical influences on pore flow. Four types of groundwater (Na-HCO₃, Na-SO₄, Na-Cl, and Ca-Cl) were evaluated, and precipitation rates were calculated by processing images of precipitates in the pores captured via microscopy. The results showed that all groundwater types formed a yellow-brownish precipitate at the interface of the iron solution and simulated groundwater flow. Microscopic X-ray analyses demonstrated that precipitate morphology varied with groundwater type. Faster precipitation was observed in the following order by groundwater type: Na-HCO₃ > Na-Cl > Na-SO₄ > Ca-Cl, which was attributed to the different stability constants of the major anions in each simulated groundwater with Fe ions. Chemical equilibrium models suggested that precipitates were Fe minerals, with FeOOH as the predominant form consistent with the results of X-ray photoelectron spectrometry. The presence of FeOOH implies that precipitates may serve as an effective sorption barrier against some nutrients and heavy metals for the underlying groundwater. However, dye-flow experiments suggested that the precipitates may clog aquifer pores, thereby altering hydrogeological properties in the aquifer.

Eco-sustainable passive treatment for mine waters: Full-scale and long-term demonstration

This paper tries to analyse the technical and economic performance of a full-scale passive Disperse Alkaline Substrate (DAS) treatment plant steadily operating for 28 months (840 days) to treat extremely acidic and metal rich mine waters in the Iberian Pyrite Belt (SW Spain). For the first time, an economic evaluation of this technology and its comparison with other passive treatments is reported. During this period, around 56,000 m³ of mine waters have been treated, without significant clogging or exhaustion of the alkaline substrate. The efficiency of the system is demonstrated by a significant decrease in the average net acidity (from 2005 to -43 mg/L as CaCO₃ equivalent) and the total elimination of Al, Cu, REY, Zn, As, Cr, Mo, V, Cd, Pb, Co and other trace metals. Water quality of the treated output discharge meets the threshold values for irrigation and drinking standards, except for Fe, Mn and sulphate. The accumulation of elements of economic interest in the waste (e.g., 32 t of Fe, 6.1 t of Al, 0.8 t of Cu, 0.8 t of Zn, 39.4 kg of REE, 20 kg of Co or 1 kg of Sc), easily extractable with diluted acids, may turn a hazardous waste into a valuable resource. The benefits associated with the revalorization of this metal-rich waste could reach a total of 27478 USD, but is more reliably estimated to be around 8243 USD due to technologic limitations. This benefit would help to defray the maintenance costs (8428 €) and make DAS an economically self-sustainable treatment. The annual treatment cost for DAS was 0.27 €/m³, being the lowest value found among other reported conventional passive schemes, and from 8 to 12 times lower compared to active technologies. The results obtained prove that the DAS technology is the most technically and economically sustainable way to decontaminate acid and metal-rich mine waters in abandoned mines.

Effect of an extreme flood event on solute transport and resilience of a mine water treatment system in a mineralised catchment

Extreme rainfall events are predicted to become more frequent with climate change and can have a major bearing on instream solute and pollutant transport in mineralised catchments. The Coledale Beck catchment in north-west England was subject to an extreme rainfall event in December 2015 that equated to a 1 in 200-year event. The catchment contains the UK's first passive metal mine water treatment system, and as such had been subject to intense monitoring of solute dynamics before and after commissioning. Due to this monitoring record, the site provides a unique opportunity to assess the effects of a major storm event on (1) catchment-scale solute transport, and (2) the resilience of the new and novel passive treatment system to extreme events. Monitoring suggests a modest decline in treatment efficiency over time that is not synchronous with the storm event and explained instead by changes in system hydraulic efficiency. There was no apparent flushing of the mine system during the event that could potentially have compromised treatment system performance. Analysis of metal transport in the catchment downstream of the mine suggests relatively subtle changes in instream chemistry with modest but statistically-significant reductions in zinc in the lower catchment irrespective of flow condition after the extreme event, but most parameters of interest show no significant change. Increased export of colloidal iron and aluminium is associated with major landslips in the mid-catchment after the storm and provide fresh sorption sites to attenuate dissolved zinc more rapidly in these locations, corroborated by laboratory experiments utilising site materials to investigate the attenuation/release of metals from stream and terrestrial sediments. The data are important as they show both the resilience of passive mine water treatment systems to extreme events and the importance of catchment-scale monitoring to ensure continued effectiveness of treatment initiatives after major perturbation.

Design and optimization of sustainable passive treatment systems for phosphogypsum leachates in an orphan disposal site

The optimization of the dispersed alkaline substrate (DAS) technology was investigated to achieve the treatment of highly acidic and polluted effluents from a phosphogypsum pile in an orphan site of SW Spain. This phosphogypsum disposal area is located on the Tinto river marsh soils, where it acts as a source of pollution for the estuarine environment, releasing high concentrations of metal(loid)s and radionuclides, which degrade the surrounding waters. The methodology consists of flowing the leachates through columns loaded with a combination of a fine-grained alkaline reagent scattered in a non-reactive matrix to raise the water pH while decreasing the solubility of dissolved contaminants. Seven columns were built, one for each of the alkaline reagent used: limestone, barium carbonate, biomass ash, fly ash, MgO, Mg(OH)₂, and Ca(OH)₂. The Ca(OH)₂-DAS and MgO-DAS treatment systems showed the highest effectiveness, reaching near-total removal for PO₄, F, Fe, Zn, Cu, Al, Cr, and U with initial reagent mass:treated volume ratios of 36.3 g/L and 7.57 g/L, respectively. Total As removal was only achieved in the Ca(OH)₂-DAS treatment. Phosphate precipitation was the main mechanism responsible for pollutants removal. Geochemical modeling using PHREEQC code and mineralogical evidence confirmed the precipitation of these minerals. This study forms the basis of an effective and environmentally sustainable treatment system for phosphogypsum leachates to reduce the impact of the fertilizer industry worldwide.

Wildfire-driven changes in hydrology mobilize arsenic and metals from legacy mine waste

Wildfires burning in watersheds that have been mined and since revegetated pose unique risks to downstream water supplies. A wildfire near Boulder, Colorado, that burned a forested watershed recovering from mining disturbance that occurred 80-160 years ago allowed us to 1) assess arsenic and metal contamination in streams draining the burned area for a five-year period after the wildfire and 2) determine the fire-affected hydrologic drivers that convey arsenic and metals to surface water. Most metal concentrations were low in the circumneutral waters draining the burned area. Water and sediment collected from streams downstream of the burned area had elevated arsenic concentrations during and after post-fire storms. Mining-related deposits were the main source of arsenic to streams. An increased proportion of overland flow relative to infiltration after the fire mobilized arsenic- and metal-rich surface deposits, along with wildfire ash and soil, into streams within and downstream of the burned area. The deposition of this material into stream channels resulted in the remobilization of arsenic for the five-year post-fire study period. It is also possible that enhanced subsurface flow after the fire increased contact of water with arsenic-bearing minerals exposed in underground mine workings. Other studies have reported that wildfire ash can be an important source of arsenic and metals to surface waters, but wildfire ash was not a major source of arsenic in this study. Predicted increases in frequency, size, and intensity of wildfires in the western U.S., a region with widely dispersed historical mines, suggest that the intersection of legacy mining and post-wildfire hydrologic response poses an increasing risk for water supplies.

Seasonal variability of extremely metal rich acid mine drainages from the Tharsis mines (SW Spain)

The Tharsis mine is presently abandoned, but the past intense exploitation has left large dumps and other sulphide-rich mining wastes in the area generating acid mine drainages (AMD). The main goal of this work is to study the effect of hydrogeochemical processes, hydrological regime and the waste typology on the physicochemical parameters and dissolved concentrations of pollutants in a deeply AMD-affected zone. Extreme leachates are produced in the area, reaching even negative pH and concentrations of up to 2.2 g/L of As and 194 g/L of Fe. The results of the comparison of ore grades of sulphide deposits with dissolved concentrations in waters shows that Pb is the least mobile element in dissolution probably due to the precipitation of Pb secondary minerals and/or its coprecipitation on Fe oxyhydroxysulphates. Arsenic, Cr, and V are also coprecipitated with Fe minerals. Seasonal patterns in metal contents were identified: elements coming from the host rocks, such as Al, Mn and Ni, show their maximum values in the dry period, when dilution with freshwater is lower and the interaction of water-rock processes and evaporation is higher. On the other hand, As, Cr, Fe, Pb and V show minimum concentrations in the dry period due to intense Fe oxyhydroxysulphate precipitation. In this sense, large sulphide rich waste heaps would be a temporal sink of these elements (i.e. Pb, As, Cr and V) in the dry period, and a significant source upon intense rainfalls.

Research on soil moisture prediction model based on deep learning

Soil moisture is one of the main factors in agricultural production and hydrological cycles, and its precise prediction is important for the rational use and management of water resources. However, soil moisture involves complex structural characteristics and meteorological factors, and it is difficult to establish an ideal mathematical model for soil moisture prediction. Existing prediction models have problems such as prediction accuracy, generalization, and multi-feature processing capability, and prediction performance must improve. Based on this, taking the Beijing area as the research object, the deep learning regression network (DNNR) with big data fitting capability was proposed to construct a soil moisture prediction model. By integrating the dataset, analyzing the time series of the predictive variables, and clarifying the relationship between features and predictive variables through the Taylor diagram, selected meteorological parameters can provide effective weights for moisture prediction. Test results prove that the deep learning model is feasible and effective for soil moisture prediction. Its’ good data fitting and generalization capability can enrich the input characteristics while ensuring high accuracy in predicting the trends and values of soil moisture data and provides an effective theoretical basis for water-saving irrigation and drought control.

Ecological improvement assessment of a passive remediation technology for acid mine drainage: Water quality biomonitoring using bivalves

A passive treatment plant, located in the Iberian Pyrite Belt (Huelva, Southwest Spain), was designed for acid mine drainage remediation. Since its installation, the improvement of water quality in terms of hydrochemical composition has been demonstrated successfully. However, according to the Water Framework Directive, the treated effluent must have ecological values for potential living. The freshwater clam Corbicula fluminea was chosen to carry out bioassessments (survival, biomarker responses, and metal bioaccumulation in soft tissue) with effluents from the mining site, as well as, products from the passive treatment plant in order to determine the level of quality of that water from the biological point of view in toxicity tests. Results discarded mortality as endpoint for biomonitoring purposes. Only the lipid peroxidation of the cell membrane evidenced significant responses, even in correlation with the pollution degree of each effluent. Regarding bioaccumulation, some elements displayed a strong relationship (Fe, Cu, Co, and Zn) between concentrations in the environment and in the tissue. As final conclusion, the usage of the Asian clam was validated as biomonitor tool in short term exposure to acid mine drainage, and, as early warning responses (72 h), the chosen parameters would be lipid peroxidation and bioaccumulation of a specific set of elements (Fe, Cu, Co, Zn). Ecological water quality levels reached by the passive treatment plant were in agreement with the efficiency of hydrochemical improvements.

Hydrogeochemical behavior of an anthropogenic mine aquifer: Implications for potential remediation measures

This study characterizes the hydrogeochemical behavior of one of the most pollutant sources in the Iberian Pyrite Belt, namely, the Poderosa adit outflow. This artificial spring arises from an anthropogenic mine aquifer with a similar hydrogeological behavior to karstic systems, where the infiltration area is an endorheic zone and the aquifer shows allogenic recharge. Recent mining has markedly increased the contaminant levels. The pollutant load released from the adit to the receiving water body is very high, with average loads of 280 kg/day of Fe, 47 kg/day of Al, 17 kg/day of Cu and so on. However, a high variability is observed related to hydrological and geochemical factors, especially during intense rainy episodes. Thus, the pollutant load during these events suffers a dramatic increase, i.e., from ~100-200 kg/day of Fe during base flow to almost 2200 kg/day during the flow peak. These data highlight the importance of short but intense rainy events on metal fluxes from mining areas, which has been previously reported in surface waters but scarcely reported in mine adits, with expected lower response times to rainfall. The pollutant load released by non-point sources, i.e., spoil heaps, is lower than that released from the adit most of the year, although it increased noticeably during intense rainy events. Some remediation measures were adopted during the 1990s without a suitable hydrogeological characterization and were shown to be ineffective. On the basis of the obtained results, potential restoration measures are discussed.

Acid production potentials of massive sulfide minerals and lead-zinc mine tailings: a medium-term study

Weathering of sulfide minerals is a principal source of acid generation. To determine acid-forming potentials of sulfide-bearing materials, two basic approaches named static and kinetic tests are available. Static tests are short-term, and easily undertaken within a few days and in a laboratory. In contrast, kinetic tests are long-term procedures and mostly carried out on site. In this study, experiments were conducted over a medium-term period of 2 months, not as short as static tests and also not as long as kinetic tests. As a result, pH and electrical conductivity oscillations as a function of time, acid-forming potentials and elemental contents of synthetically prepared rainwater leachates of massive sulfides and sulfide-bearing lead-zinc tailings from abandoned and currently used deposition areas have been determined. Although the lowest final pH of 2.70 was obtained in massive pyrite leachate, massive chalcopyrite leachate showed the highest titrable acidity of 1.764 g H₂SO₄/L. On the other hand, a composite of currently deposited mine tailings showed no acidic characteristic with a final pH of 7.77. The composite abandoned mine tailing leachate had a final pH of 6.70, close to the final pH of massive galena and sphalerite leachates, and produced a slight titrable acidity of 0.130 g H₂SO₄/L.

Pyrite oxidation under simulated acid rain weathering conditions

We investigated the electrochemical corrosion behavior of pyrite in simulated acid rain with different acidities and at different temperatures. The cyclic voltammetry, polarization curve, and electrochemical impedance spectroscopy results showed that pyrite has the same electrochemical interaction mechanism under different simulated acid rain conditions, regardless of acidity or environmental temperature. Either stronger acid rain acidity or higher environmental temperature can accelerate pyrite corrosion. Compared with acid rain having a pH of 5.6 at 25 °C, the prompt efficiency of pyrite weathering reached 104.29% as the acid rain pH decreased to 3.6, and it reached 125.31% as environmental temperature increased to 45 °C. Increasing acidity dramatically decreases the charge transfer resistance, and increasing temperature dramatically decreases the passivation film resistance, when other conditions are held constant. Acid rain always causes lower acidity mine drainage, and stronger acidity or high environmental temperatures cause serious acid drainage. The natural parameters of latitude, elevation, and season have considerable influence on pyrite weathering, because temperature is an important influencing factor. These experimental results are of direct significance for the assessment and management of sulfide mineral acid drainage in regions receiving acid rain.

Potential risk assessment in stream sediments, soils and waters after remediation in an abandoned W>Sn mine (NE Portugal)

The mining complex of Murçós belongs to the Terras de Cavaleiros Geopark, located in Trás-os-Montes region, northeast Portugal. A stockwork of NW-SE-trending W>Sn quartz veins intruded Silurian metamorphic rocks and a Variscan biotite granite. The mineralized veins contain mainly quartz, cassiterite, wolframite, scheelite, arsenopyrite, pyrite, sphalerite, chalcopyrite, galena, rare pyrrhotite, stannite, native bismuth and also later bismuthinite, matildite, joseite, roosveltite, anglesite, scorodite, zavaritskite and covellite. The exploitation produced 335t of a concentrate with 70% of W and 150t of another concentrate with 70% of Sn between 1948 and 1976. The exploitation took place mainly in four open pit mines as well as underground. Three lakes were left in the area. Remediation processes of confination and control of tailings and rejected materials and phytoremediation with macrophytes from three lakes were carried out between 2005 and 2007. Stream sediments, soils and water samples were collected in 2008 and 2009, after the remediation process. Most stream sediments showed deficiency or minimum enrichment for metals. The sequential enrichment factor in stream sediments W>Bi>As>U>Cd>Sn=Ag>Cu>Sb>Pb>Be>Zn is mainly associated with the W>Sn mineralizations. Stream sediments receiving drainage of a mine dump were found to be significantly to extremely enriched with W, while stream sediments and soils were found to be contaminated with As. Two soil samples collected around mine dumps and an open pit lake were also found to be contaminated with U. The waters from the Murçós W>Sn mine area were acidic to neutral. After the remediation, the surface waters were contaminated with F(-), Al, As, Mn and Ni and must not be used for human consumption, while open pit lake waters must also not be used for agriculture because of contamination with F(-), Al, Mn and Ni. In most waters, the As occurred as As (III), which is toxic and is easily mobilized in the drainage system. The remediation promoted a decrease in metals and As concentrations of soils and waters, however the applied processes were not enough to rehabilitate the area.

Hydrogeochemical features of surface water and groundwater contaminated with acid mine drainage (AMD) in coal mining areas: a case study in southern Brazil

Effects of acid mine drainage (AMD) were investigated in surface waters (Laranjinha River and Ribeirão das Pedras stream) and groundwaters from a coal mining area sampled in two different seasons at Figueira city, Paraná State, Brazil. The spatial data distribution indicated that the acid effluents favor the chemical elements leaching and transport from the tailings pile into the superficial water bodies or aquifers, modifying their quality. The acid groundwaters in both sampling periods (dry: pH 2.94-6.04; rainy: pH 3.25-6.63) were probably due to the AMD generation and infiltration, after the oxidation of sulfide minerals. Such acid effluents cause an increase of the solubilization rate of metals, mainly iron and aluminum, contributing to both groundwater and surface water contamination. Sulfate in high levels is a result of waters' pollution due to AMD. In some cases, high sulfate and low iron contents, associated with less acidic pH values, could indicate that AMD, previously generated, is nowadays being neutralized. The chemistry of the waters affected by AMD is controlled by the pH, sulfide minerals' oxidation, oxygen, iron content, and microbial activity. It is also influenced by seasonal variations that allow the occurrence of dissolution processes and the concentration of some chemical elements. Under the perspective of the waters' quality evaluation, the parameters such as conductivity, dissolved sodium, and sulfate concentrations acted as AMD indicators of groundwaters and surface waters affected by acid effluents.

Metal and acidity fluxes controlled by precipitation/dissolution cycles of sulfate salts in an anthropogenic mine aquifer

Underground mine drainages are extremely difficult to study due to the lack of information about the flow path and source proximity in relation to the outflow adit. Geochemical processes controlling metals and acidity fluxes in a complex anthropogenic mine aquifer in SW Spain during the dry and rainy season were investigated by geochemical and statistical tools. High concentrations of acidity, sulfate, metals and metalloids (e.g. Fe, Cu, Zn, As, Cd, Ni, Co) were observed due to intense sulfide oxidation processes. The high residence time inside the anthropogenic aquifer, around 40days, caused the release of significant quantities of metals linked to host rocks (e.g. Al, Ca, Ge, Li, Mg, REE). The most outstanding characteristic of the acid mine drainage (AMD) outflows is the existence of higher Fe/SO4 molar ratios than those theoretical of pyrite (0.50) during most of the monitored period, due to a fire which occurred in 1949 and remained active for decades. Permanent and temporal retention mechanisms of acidity and metals were observed in the galleries. Once released from sulfide oxidation, Pb and As are sorbed on Fe oxyhydroxysulfate or precipitated as low solubility minerals (i.e. anglesite) inside the galleries. The precipitation of evaporitic sulfate salts during the dry season and the subsequent re-dissolution after rainfall control the fluxes of acidity and main metals (i.e. Fe, Mg, Al) from this anthropogenic aquifer. Some elements, such as Cd, Cu, Ni, REE and Zn, are retained in highly soluble sulfate salts while other elements, such as Ge, Pb and Sc, have a lower response to washout processes due to its incorporation in less soluble sulfate salts. In this way, metal concentration during the washout processes would be controlled by the proportion and solubility of each type of evaporitic sulfate salt stored during the dry season. The recovery of metals of economic interest contained in the AMD could help to self-finance the remediation of these waters in derelict mines worldwide.