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.

Geochemical recovery of the Torna-Marcal river system after the Ajka red mud spill, Hungary

The failure of the Ajka red mud depository in October 2010 led to the largest single release of red mud into the surface water environment. This study provides a comparative assessment of stream sediment quality in the Torna-Marcal-Rába catchment between post-disaster surveys (2010) and follow up surveys at an identical suite of 21 locations in 2013. The signature of red mud apparent in initial surveys with high Al, As, Cr, Na, V was only apparent at a small number of sample stations in recent surveys. These constitute <1 km of stream, compared to the >20 km reach of affected sediments in the immediate aftermath of the spill. Concentrations of red mud-derived contaminants are predominately associated with fine fractions of the red mud (<8 μm). This enhances transport out of the system of red mud-derived contaminants and, along with extensive remedial efforts, has substantially limited the within-channel inventory of potentially ecotoxic metals and metalloids.

Mine water geochemistry and metal flux in a major historic Pb-Zn-F orefield, the Yorkshire Pennines, UK

Recent studies have shown up to 6 % of rivers in England and Wales to be impacted by discharges from abandoned metal mines. Despite the large extent of impacts, there are still many areas where mine water impact assessments are limited by data availability. This study provides an overview of water quality, trace element composition and flux arising from one such area; the Yorkshire Pennine Orefield in the UK. Mine drainage waters across the orefield are characterised by Ca-HCO3-SO4-type waters, with moderate mineralization (specific electrical conductance: 160-525 μS cm(-1)) and enrichment of dissolved Zn (≤2003 μg L(-1)), Ba (≤971 μg L(-1)), Pb (≤183 μg L(-1)) and Cd (≤12 μg L(-1)). The major ion composition of the waters reflects the Carboniferous gritstone and limestone-dominated country rock, the latter of which is heavily karstified in parts of the orefield, while sulphate and trace element enrichment is a product of the oxidation of galena, sphalerite and barite mineralization. Contaminant flux measurements at discharge sites highlight the disproportionate importance of large drainage levels across the region, which generally discharge into first-order headwater streams. Synoptic metal loading surveys undertaken in the Hebden Beck sub-catchment of the river Wharfe highlight the importance of major drainage levels to instream baseflow contamination, with diffuse sources from identifiable expanses of waste rock becoming increasingly prominent as river flows increase.

Contaminant mobility and carbon sequestration downstream of the Ajka (Hungary) red mud spill: The effects of gypsum dosing

A number of emergency pollution management measures were enacted after the accidental release of caustic bauxite processing residue that occurred in Ajka, western Hungary in October, 2010. These centred on acid and gypsum dosing to reduce pH and minimise mobility of oxyanion contaminants mobile at high pH. This study assessed the effectiveness of gypsum dosing on contaminant mobility and carbon sequestration through assessment of red mud and gypsum-affected fluvial sediments via elemental analysis and stable isotope analysis. There was a modest uptake of contaminants (notably As, Cr, and Mn) on secondary carbonate-dominated deposits in reaches subjected to gypsum dosing. C and O stable isotope ratios of carbonate precipitates formed as a result of gypsum dosing were used to quantify the importance of the neutralisation process in sequestering atmospheric carbon dioxide. This process was particularly pronounced at sites most affected by gypsum addition, where up to 36% of carbonate-C appears to be derived from atmospheric in-gassing of CO(2). The site is discussed as a large scale analogue for potential remedial approaches and carbon sequestration technologies that could be applied to red mud slurries and other hyperalkaline wastes. The results of this work have substantial implications for the aluminium production industry in which 3-4% of the direct CO(2) emissions may be offset by carbonate precipitation. Furthermore, carbonation by gypsum addition may be important for contaminant remediation, also providing a physical stabilisation strategy for the numerous historic stockpiles of red mud.

Treatment of zinc-rich acid mine water in low residence time bioreactors incorporating waste shells and methanol dosing

Bioreactors utilising bacterially mediated sulphate reduction (BSR) have been widely tested for treating metal-rich waters, but sustained treatment of mobile metals (e.g. Zn) can be difficult to achieve in short residence time systems. Data are presented providing an assessment of alkalinity generating media (shells or limestone) and modes of metal removal in bioreactors receiving a synthetic acidic metal mine discharge (pH 2.7, Zn 15 mg/L, SO(4)(2-) 200mg/L, net acidity 103 mg/L as CaCO(3)) subject to methanol dosing. In addition to alkalinity generating media (50%, v.v.), the columns comprised an organic matrix of softwood chippings (30%), manure (10%) and anaerobic digested sludge (10%). The column tests showed sustained alkalinity generation, which was significantly better in shell treatments. The first column in each treatment was effective throughout the 422 days in removing >99% of the dissolved Pb and Cu, and effective for four months in removing 99% of the dissolved Zn (residence time: 12-14 h). Methanol was added to the feedstock after Zn breakthrough and prompted almost complete removal of dissolved Zn alongside improved alkalinity generation and sulphate attenuation. While there was geochemical evidence for BSR, sequential extraction of substrates suggests that the bulk (67-80%) of removed Zn was associated with Fe-Mn oxide fractions.

Seasonal and spatial variation of diffuse (non-point) source zinc pollution in a historically metal mined river catchment, UK

Quantifying diffuse sources of pollution is becoming increasingly important when characterising river catchments in entirety - a prerequisite for environmental management. This study examines both low and high flow events, as well as spatial variability, in order to assess point and diffuse components of zinc pollution within the River West Allen catchment, which lies within the northern England lead-zinc Orefield. Zinc levels in the river are elevated under all flow regimes, and are of environmental concern. Diffuse components are of little importance at low flow, with point source mine water discharges dominating instream zinc concentration and load. During higher river flows 90% of the instream zinc load is attributed to diffuse sources, where inputs from resuspension of metal-rich sediments, and groundwater influx are likely to be more dominant. Remediating point mine water discharges should significantly improve water quality at lower flows, but contribution from diffuse sources will continue to elevate zinc flux at higher flows.

Inventory of aquatic contaminant flux arising from historical metal mining in England and Wales

The impact of discharges from abandoned metal and ironstone mines has been a much studied form of aquatic pollution in recent decades. Few attempts however, have been made to accurately determine the overall contaminant mass flux arising from abandoned mine sites at scales above catchment level. Such assessments are critical to determine the significance of former mining to national, regional and ultimately global trace metal flux. This paper presents the most comprehensive national survey to date across England and Wales of the total pollution burden discharged at source from abandoned non-coal mine sites. 338 discharges have been identified (from 4923 known abandoned metal mines) and while concurrent flow and contaminant concentration records are only available for around 30% of these, significant quantities of metals (and As) have been quantified to be discharged. A minimum of 193 tonnes of Zn, 18.5 tonnes of Pb, 0.64 tonnes Cd, 19.1 tonnes of Cu, 551 tonnes Fe, 72 tonnes Mn and 5.1 tonnes As are released in water discharges from abandoned non-coal mines to the surface water environment of England and Wales each year. Precautionary extrapolation of mass fluxes based on the frequency distribution of measured concentration and flow data, for discharges with absent data, suggests that the actual total mass flux for these contaminants could be up to 41% higher. The mass flux of Pb released from mines exceeds that of all currently permitted discharges (e.g. active industrial sites and wastewater treatment works) to surface waters across England and Wales, while those of As, Cd and Zn are of a similar magnitude. These data put into context the enduring legacy of historic mining on the water environment, highlighting its significance relative to more highly regulated polluting sites. Comparison of the figures with estimates of global trace metal flux suggests that the national total identified here is significant on a global scale.

A national strategy for identification, prioritisation and management of pollution from abandoned non-coal mine sites in England and Wales. I. Methodology development and initial results

In regions affected by historic non-coal (principally metal) mining activity, government agencies are often faced with the challenge of deploying limited remedial resources at abandoned mine sites to achieve maximum improvements in the chemical and ecological quality of impacted ground and surface waters. As such, strategies for the defensible allocation of public funds require comprehensive and systematic frameworks by which to identify and prioritise polluting sites for remediation. This paper describes the development and initial findings of such a national initiative in England and Wales which allies catchment-scale environmental impact assessments using existing public archive data, with recognition of the uncertainty in impact appraisals arising from disparities in data availability between sites and regions. The methodology identifies polluting sites and takes account not only of the chemical and ecological impacts of mine water discharges on receiving watercourses, but also of socio-economic factors such as conservation and heritage concerns, which can both impede or complement efforts to remediate mine sites. Using a Geographic Information System database and a suite of spatial analyses employing Boolean operators, both the extent of the pollution problem from abandoned non-coal mines in England and Wales (6% of 7815 surface water bodies are affected nationally) and the insight that can be gleaned from systematic analyses of existing archive data are highlighted. The results of the nationwide survey can be used as a dynamic database to inform future remedial planning, in terms of prioritising impacted river basins and abandoned non-coal mine sites themselves for either remediation or future monitoring efforts. As the assessment framework is built upon existing water quality and ecological data and mine site/geological data, there is considerable scope for the approach to be applied elsewhere where the legacy of historic mining persists through the widespread pollution of the aquatic environment.

Wetland treatment at extremes of pH: a review

Constructed wetlands are an established treatment technology for a diverse range of polluted effluents. There is a long history of using wetlands as a unit process in treating acid mine drainage, while recent research has highlighted the potential for wetlands to buffer highly alkaline (pH>12) drainage. This paper reviews recent evidence on this topic, looking at wetlands treating acidic mine drainage, and highly alkaline leachates associated with drainage from lime-rich industrial by-products or where such residues are used as filter media in constructed wetlands for wastewater treatment. The limiting factors to the success of wetlands treating highly acidic waters are discussed with regard to design practice for the emerging application of wetlands to treat highly alkaline industrial discharges. While empirically derived guidelines (with area-adjusted contaminant removal rates typically quoted at 10 g Fe m(2)/day for influent waters pH>5.5; and 3.5-7 g acidity/m(2)/day for pH>4 to <5.5) for informing sizing of mine drainage treatment wetlands have generally been proved robust (probably due to conservatism), such data exhibit large variability within and between sites. Key areas highlighted for future research efforts include: (1) wider collation of mine drainage wetland performance data in regionalised datasets to improve empirically-derived design guidelines and (2) obtaining an improved understanding of nature of the extremophile microbial communities, microbially-mediated pollutant attenuation and rhizospheral processes in wetlands at extremes of pH. An enhanced knowledge of these (through multi-scale laboratory and field studies), will inform engineering design of treatment wetlands and assist in the move from the empirically-derived conservative sizing estimates that currently prevail to process-based optimal design guidance that could reduce costs and enhance the performance and longevity of wetlands for treating acidic and highly alkaline drainage waters.

Preliminary evaluation of a constructed wetland for treating extremely alkaline (pH 12) steel slag drainage

High pH (> 12) leachates are an environmental problem associated with drainage from lime (CaO)-rich industrial residues such as steel slags, lime spoil and coal combustion residues. Recent research has highlighted the potential for natural ('volunteer') wetlands to buffer extremely alkaline influent waters. This appears ascribable to high CO(2) partial pressures in the wetland waters from microbial respiration, which accelerates precipitation of calcium carbonate (CaCO(3)), and the high specific surface area for mineral precipitation offered by macrophytes. The research presented here builds on this and provides preliminary evaluation of a constructed wetland built in March 2008 to buffer drainage from steel slag heaps in north-east England. The drainage water from the slag mounds is characterised by a mean pH of 11.9, high concentrations of Ca (up to 700 mg/L), total alkalinity (up to 800 mg/L as CaCO(3)) and are slightly brackish (Na = 300 mg/L; Cl = 400 mg/L) reflecting native groundwaters at this coastal setting. Documented calcite precipitation rates (mean of 5 g CaCO(3)/m(2)/day) from nearby volunteer sites receiving steel slag drainage were used to scale the constructed wetland planted with Phragmites australis; a species found to spontaneously grow in the vicinity of the discharge. Improved performance of the wetland during summer months may at least in part be due to biological activity which enhances rates of calcite precipitation and thus lowering of pH. Secondary Ca-rich precipitates also serve as a sink for some trace elements present at low concentrations in the slag leachate such as Ni and V. The implications for scaling and applying constructed wetlands for highly alkaline drainage are discussed.

Quantifying the importance of diffuse minewater pollution in a historically heavily coal mined catchment

There has been considerable progress in developing treatment systems for point sources of minewater pollution in recent years; however, there remains a knowledge gap in the characterisation and remediation of diffuse minewater sources. Data are presented from the River Gaunless catchment, a historically heavily coal mined catchment in the northeast of England. Instream iron (Fe) loadings were monitored alongside loadings arising from point minewater discharges over a 12-month period to assess the dynamic importance of diffuse sources of minewater pollution. In low flow, diffuse sources account for around 50% of instream loading, a proportion which increases to 98% in high flow conditions. The low flow sources appear to be dominated by direct discharge of contaminated groundwater to surface waters in lower reaches of the catchment. In high flow, resuspended Fe-rich sediments, which are both naturally occurring and derived from historic mining, become the dominant diffuse source of Fe in the water column.

The impact of pumped water from a de-watered Magnesian limestone quarry on an adjacent wetland: Thrislington, County Durham, UK

Although quarrying is often cited as a potential threat to wetland systems, there is a lack of relevant, quantitative case studies in the literature. The impact of pumped groundwater discharged from a quarry into a wetland area was assessed relative to reference conditions in an adjacent fen wetland that receives only natural runoff. Analysis of vegetation patterns at the quarry wetland site, using Detrended Correspondence Analysis and the species indicator values of Ellenberg, revealed a clear disparity between community transitions in the quarry wetland and the reference site. Limited establishment of moisture-sensitive taxa, the preferential proliferation of robust wetland species and an overall shift towards lower species diversity in the quarry wetland were explicable primarily by the physico-chemical environment created by quarry dewatering. This encompassed high pH (up to 12.8), sediment-rich effluent creating a nutrient-poor substrate with poor moisture retention in the quarry wetland, and large fluctuations in water levels.