First full-scale application of barium carbonate as an effective dispersed alkaline substrate for sulfate removal from acid mine drainage.

A full-scale passive treatment plant using barium carbonate (BaCO3), the mineral witherite, as a Dispersed Alkaline Substrate (DAS) for strongly contaminated acid mine drainage was implemented for the first time globally at Mina Concepción, located in the Iberian Pyrite Belt (SW Spain). The plant was monitored over a 105-day period, covering the wettest months of the hydrological year and with a mean flow around 4 L s-1. The AMD treated in the plant exhibits a high strength, with an average sulfate concentration around 1500 mg L-1 and net acidity close to 1000 mg L-1 as CaCO3 equivalent. According to the above, the loading rate for SO4 and Fe was around 400 and 80 kg day-1, respectively. The treatment process produced an alkaline effluent with low metal content. Nearly complete removal of most metal(loid)s was achieved, with significant sulfate decrease to below 500 mg L-1 in the alkaline outflow of the barium carbonate tank. Barium carbonate demonstrated superior performance compared to magnesia, particularly in enhancing alkalinity and lowering net acidity and concentrations of sulfate and manganese. The high efficiency attained by the plant after the barium carbonate treatment is evidenced by compliance with environmental water quality standards for most contaminants.

Causes and impacts of a mine water spill from an acidic pit lake (Iberian Pyrite Belt).

In May 2017, a spill from La Zarza pit lake (SW Spain) resulted in the release of approximately 270,000 m3 of extremely acidic waters to the Odiel River. Around 780 × 103 kg of Fe, 170 × 103 kg of Al, 2.15 × 103 kg of As and high amounts of other trace metals and metalloids were spilled. The purpose of this study is to explain the causes, consequences and impacts of the mine spill on the receiving water bodies. To this end, an extensive sampling along the mine site, river and estuary as well as a hydrological model of the pit lake was performed. Around 53 km of the Odiel River's main course, which was already contaminated by acid mine drainage (AMD), were affected. The mine spill resulted in an incremental impact on the Odiel River water quality. Thus, dissolved concentrations of some elements increased in the river up to 450 times; e.g. 435 mg/L of Fe and 0.41 mg/L of As. Due to low pH values (around 2.5), most metals (e.g., Cu, Zn, Mn, Cd) were transported in the dissolved phase to the estuary, exhibiting a conservative behavior and decreasing their concentration only due to dilution. However, dissolved concentrations of Fe, Cr, Pb, Se, Sb, Ti, V and especially As decreased significantly along the river due to Fe precipitation and sorption/coprecipitation processes. At the upper zone of the estuary, a noticeable increment of metal concentrations (up to 77 times) was also recorded. The water balance illustrates the existence of groundwater inputs (at least 16% of total) to the pit lake, due probably to local infiltration of rainwater at the mining zone. The probable existence of an ancient adit connected to the pit lake indicates that potential releases could occur again if adequate prevention measures are not adopted.

Hydrogeochemical characteristics of the Tinto and Odiel Rivers (SW Spain). Factors controlling metal contents.

The Tinto and Odiel Rivers are strongly affected by acid mine drainage (AMD) due to the intense sulphide mining developed in their basins over the past 5000 years. In this study the results obtained from a weekly sampling in both rivers, before their mouth in the Ría of Huelva, over three and a half years of control are analysed. In the Tinto River, the concentrations of sulphates, Al, Cd, Co, Li and Zn are double to those of the Odiel as a consequence of lower dilution. However, the concentration of Fe in the Odiel River is 20 times lower, since the precipitation of Fe oxyhydroxysulphates caused by neutralisation processes is more intense. Lower As, Cr, Cu and Pb concentrations are also found in the Odiel River as, to a greater or lesser extent, they are sorbed and/or coprecipitated with Fe. Other elements such as Be, Mn, Ni and Mg show similar values in both systems, which is ascribed to lithological factors. The seasonal evolution of contaminants is typical of rivers affected by AMD, reaching a maximum in autumn due to the dissolution of evaporitic salts precipitated during the summer. Nevertheless, in the Tinto River, Ca, Na and Sr show a strong increase during the summer, probably due to a greater water interaction with marly materials, through which the last reach of the river flows. Barium has a different behaviour from the rest of the metals and its concentration seems to be controlled by the solubility of barite. Iron, As and Pb show different behaviours in both rivers, those for Fe and As possibly linked to the prevalence of different dissolved species of Fe. The different Pb pattern is probably due to the control of Pb solubility by anglesite or other minerals rich in Pb in the Tinto River.

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.

Water acidification trends in a reservoir of the Iberian Pyrite Belt (SW Spain).

Scarcity of waters is the main limiting factor of economic development in most arid and semi-arid regions worldwide. The construction of reservoirs may be an optimal solution to assure water availability if the drainage area shows low disturbances. This is the quandary of mining areas where economic development relies on water accessibility. Water acidification trends were investigated in the Sancho Reservoir (SW Spain) in the last 20 years. The acidity (pH3-5) and high dissolved metal concentrations (e.g., 4.4 mg/L of Al, 2.1mg/L of Mn, 1.9 mg/L of Zn) observed in the Sancho, together with the large volume stored (between 37 and 55 Mm(3)), makes this reservoir an extreme case of surface water pollution worldwide. A progressive acidification has been observed since 2003, as evidenced by decreasing pH values and increasing dissolved metal concentrations, especially noticeable after 2007. The increase in the net acidity in the reservoir originates from the higher input of metals and acidity due to the rebound effect after the mining closure in 2001. This trend was not detected in the river feeding the reservoir due to its great hydrological and hydrochemical variability, typical of the Mediterranean climate. Chemical analysis and absolute dating of sediments identified a progressive enrichment in S and metals (i.e., Fe, Zn Cu, Ni, Co and Cd) in the upper 20 cm, which reinforce the year 2002/03 as the onset of the acidification of the reservoir. The decrease of pH values from 4-5 to 3-4 occurred later than the increase in sulfate and metals due to pH-buffering by Al. The acid mine drainage (AMD) pressure has caused an increment of dissolved Fe and other metals, as well as a change in the pH buffering role, exerted now by Fe. These processes were simulated by PHREEQC, which confirms that the acidification trend will continue, causing pH values to reach 2.5 if AMD pressure persists.

Seasonal water quality variations in a river affected by acid mine drainage: the Odiel River (South West Spain).

This paper intends to analyse seasonal variations of the quality of the water of the Odiel River. This river, together with the Tinto River, drains the Iberian Pyrite Belt (IPB), a region containing an abundance of massive sulphide deposits. Because of mining activity dating back to prehistoric times, these two rivers are heavily contaminated. The Odiel and Tinto Rivers drain into a shared estuary known as the Ría of Huelva. This work studies dissolved contaminant data in water of the Odiel River collected by various organisations, between October 1980 and October 2002, close to the rivers entry into the estuary. Flow data for this location were also obtained. The most abundant metals in the water, in order of abundance, are zinc (Zn), iron (Fe), manganese (Mn) and copper (Cu). Arsenic (As), cadmium (Cd) and lead (Pb) are also present but in much lower quantities. The quality of the river water is linked to precipitation; the maximum sulphate, Fe, Zn, Mn, Cd and Pb concentrations occur during the autumn rains, which dissolve the Fe hydroxysulphates that were precipitated during the summer months. In winter, the intense rains cause an increase in the river flow, producing a dilution of the contaminants and a slight increase in the pH. During spring and summer, the sulphate and metal concentration (except Fe) recover and once again increase. The Fe concentration pattern displays a low value during summer due to increased precipitation of ferric oxyhydroxides. The arsenic concentration displays a different evolution, with maximum values in winter, and minimum in spring and summer as they are strongly adsorbed and/or coprecipitated by the ferric oxyhydroxides. Mn and sulphates are the most conservative species in the water. Relative to sulphate, Mn, Zn and Cd, copper displays greater values in winter and lower ones in summer, probably due to its coprecipitation with hydroxysulphates during the spring and summer months. Cd and Zn also appear to be affected by the same process, although to a lower degree than Cu, experiencing a slight reduction in summer with respect to Mn and sulphates.

Trace metal partitioning over a tidal cycle in an estuary affected by acid mine drainage (Tinto estuary, SW Spain).

The Tinto River estuary is highly polluted with the acid lixiviates from old sulphide mines. In this work the behaviour of dissolved and particulate trace metals under strong chemical gradients during a tidal cycle is studied. The pH values range from 4.4 with low tide to 6.9 with high tide. Precipitation of Fe and Al is intense during rising tides and As and Pb are almost exclusively found in the particulate matter (PM). Sorption processes are very important in controlling the mobility (and hence bioavailability) of some metals and particularly affect Cu below pH 6. Above pH~6 Cu is desorbed, probably by the formation of Cu(I)-chloride complexes. Although less pronounced than Cu, also Zn desorption above pH 6.5 seems to occur. Mn and Co are affected by sorption processes at pH higher than ca. 6. Cd behaves conservatively and Ni is slightly affected by sorption processes.

Geochemical behavior of metals and metalloids in an estuary affected by acid mine drainage (AMD).

The Tinto and Odiel rivers in southwest Spain drain the world's largest sulfide mineral formation: the Iberian Pyrite Belt which has been worked since 2,500 BC. The Tinto and Odiel estuarine zones include both an extensive area of salt marsh and an intensively industrialized urban area. As a consequence of pyrite oxidation, the Tinto and Odiel rivers are strongly acidic (pH < 3) with unusually high and quite variable metal concentrations. In this study, seasonally varying concentrations of dissolved major and trace elements were determined in the acid mine drainage affected estuary of the Ría de Huelva. During estuarine mixing, ore-derived metal concentrations exhibit excellent correlations with pH as the main controlling parameter. As pH increases, concentrations of dissolved ore-associated elements are attenuated, and this process is enhanced during the summer months. The decrease in Fe and Al concentrations ranged from 80 to 100 % as these elements are converted from dissolved to sediment-associated forms in the estuary. Coprecipitation/adsorption processes also removed between 60 and 90 % of the originally dissolved Co, Cu, Mn, Pb, Zn, and Th; however, Cd and Ni exhibited a greater propensity to remain in solution, with an average removal of approximately 60 %. On the other hand, As and U exhibited a different behavior; it is likely that these elements remain in dissolved forms because of the formation of U carbonates and soluble As species. Concentrations of As remain at elevated levels in the outer estuary (average = 48 µg L(-1)) which exceeds concentrations present in the Tinto River. Nevertheless, the estuary has recently witnessed improvements in water quality, as compared to results of several previous studies reported in the 1990s.

Metal cycling during sediment early diagenesis in a water reservoir affected by acid mine drainage.

The discharge of acid mine drainage (AMD) into a reservoir may seriously affect the water quality. To investigate the metal transfer between the water and the sediment, three cores were collected from the Sancho Reservoir (Iberian Pyrite Belt, SW Spain) during different seasons: turnover event; oxic, stratified period; anoxic and under shallow perennially oxic conditions. The cores were sliced in an oxygen-free atmosphere, after which pore water was extracted by centrifugation and analyzed. A sequential extraction was then applied to the sediments to extract the water-soluble, monosulfide, low crystallinity Fe(III)-oxyhydroxide, crystalline Fe(III)-oxide, organic, pyrite and residual phases. The results showed that, despite the acidic chemistry of the water column (pH<4), the reservoir accumulated a high amount of autochthonous organic matter (up to 12 wt.%). Oxygen was consumed in 1mm of sediment due to organic matter and sulfide oxidation. Below the oxic layer, Fe(III) and sulfate reduction peaks developed concomitantly and the resulting Fe(II) and S(II) were removed as sulfides and probably as S linked to organic matter. During the oxic season, schwertmannite precipitated in the water column and was redissolved in the organic-rich sediment, after which iron and arsenic diffused upwards again to the water column. The flux of precipitates was found to be two orders of magnitude higher than the aqueous one, and therefore the sediment acted as a sink for As and Fe. Trace metals (Cu, Zn, Cd, Pb, Ni, Co) and Al always diffused from the reservoir water and were incorporated into the sediments as sulfides and oxyhydroxides, respectively. In spite of the fact that the benthic fluxes estimated for trace metal and Al were much higher than those reported for lake and marine sediments, they only accounted for less than 10% of their total inventory dissolved in the column water.

Assessment of the dissolved pollutant flux of the Odiel River (SW Spain) during a wet period.

The abandoned mining districts of the Iberian Pyrite Belt (IPB, SW Spain) are an extreme source of pollution by acid mine drainage (AMD) to the Tinto and Odiel rivers. The pollutant flux transported by the Odiel River during a high stage period was assessed using concentration-discharge relationships and concentration-conductivity relationships, for the hydrological year 2009/10 (which was especially wet). Both correlations were high (R(2)>0.80) for most of the elements studied. The two methods for flux calculation gave similar results with differences generally lower than 10%. The dissolved contaminant flux transported by the Odiel River just before its mouth mainly includes sulphate (257,534±13,464 t/yr), Al (13,259±1071 t/yr), Zn (4265±242 t/yr), Mn (2532±146 t/yr) and Cu (1738±136 t/yr), and minor amounts of other elements. These findings confirm that, up to our knowledge, the Odiel River can be considered to be the largest contributor of mining-related pollutants to the world's oceans.