Contaminant mobility as a result of sediment inundation : literature review and laboratory scale pilot study on mining contaminated sediments

This report presents a literature review of soil and sediment inundation methodologies and describes a pilot scale laboratory inundation study. Changing weather conditions, subsequent flooding events, and the increased frequency of such events both in the UK and worldwide is highlighting the need to research the area of contaminant mobility from soils and sediments under inundated conditions. The findings of such investigations impact on a wide variety of sectors, including human and ecological health, agriculture, building, transport, world economy and climate change. Standardised methodologies for the investigation of contaminant mobility resulting from soil/sediment inundation episodes are not available. Most research has been conducted in the agricultural sector for nutrient transport, as part of soil fertility and plant nutrition studies. Only recently has work been undertaken for studying potentially harmful element transport in inundated sediments/soils. A pilot scale laboratory study was undertaken using contaminated bank sediment samples collected from the Rookhope Burn catchment, Northern England, UK, with the aim to examine the extent of contaminant mobilisation from flooded sediments. The catchment has been affected by historical mining and processing of lead and zinc ore and is representative of several catchments affected by the environmental legacy related to mining in the Northern Pennine Orefield. Bank sediment Pb and Zn concentrations were found significantly above both the TEL and PEL sediment quality criteria, posing potentially a significant hazard to aquatic organisms. The source of the Pb and Zn in the sediments is related to the underlying mineralisation, mining activities and mine water discharges in the catchment. Abundances of original sulphide ore and authigenic metal-bearing phases were expected to vary through the catchment. The study design simulated rising flood water, a slow saturation of the sediment in order to induce a slow change in physico-chemical properties, followed by a 3 month (88 day) stagnation period. Natural day-night cycles were simulated by undertaking the study on the bench top during the winter of 2009/2010 (November to February). The chemical changes in the inundation water during the experiment were monitored and the sediment pore water at the end of the inundation period analysed. The inundation water pH remained alkaline to neutral, while redox measurements indicated oxic conditions in the water column throughout the inundation period. The pilot study showed that inundation of river bank sediments from the Rookhope Burn may be a significant pathway for contaminants in the catchment and that mobilisation from the sediments may pose a hazard to environmental receptors in the area, particularly with respect to Pb and Zn contamination. The different degrees and different rates of metal losses to the overlying water column observed during the flooding of the Rookhope Burn bank sediments demonstrated that the significance of metal mobilisation was dictated by the sediment composition. The inundation water composition monitored during the sediment flooding was used to indirectly infer possible processes that control contaminant fluxes from the sediments to the overlying water. Dissolved Pb concentration in the inundation water reflected the original concentration in the solid material and in sediments that had XRD-detectable galena and cerussite the dissolved Pb concentration reached a maximum value of 395 μg l-1. Cerussite, which is commonly formed as coatings on galena during the sulphide weathering, was close or supersaturated in those solutions, suggesting that the lead carbonate mineral phase provided a continuous source of Pb to these solutions. The initial dissolved Zn in the inundation waters was independent of the original concentration in the sediments. Sediments downstream a mine water discharge showed a greater availability of easily mobilised Zn, producing high initial Zn concentration in the inundation water, despite the relatively low Zn concentration in the inundated sediment. The Zn/SO4 and Cd/Zn molar ratios were both consistent with sphalerite mineral oxidation. The final inundation water solutions had the highest Zn concentrations for those sediment samples where sphalerite was detected by XRD. Redox sensitive elements such as Fe and Mn could not unequivocally indicate the presence of reducing conditions within the flooded sediments and the redox measurements were carried out only in the overlying water column (ORP above 200-350 mV). Low organic matter content and sandy texture would not have favoured the rapid formation of an anoxic layer. Yet, only extending the ORP measurements to the submerged sediment would determine the presence of flooding-induced reducing conditions. Reductive dissolution of Mn oxyhydroxides would result in release of Mn into solution, along with other trace metals, such as Pb and Zn. Mn increased in the inundation water throughout most or all the inundation period for some of the studied sediments. Their final pore water composition was significantly enriched in Mn (1300- 6500 μg l-1). Saturation indices indicated both rhodocrosite (MnCO3) and Mn oxides reached saturation. Therefore, it was not possible to preclude either the role of rhodocrosite as solubility controlling solid phase or the reductive dissolution of Mn oxides for accounting the enhanced Mn concentrations in the pore water and overlying water column without a better characterisation of the solid phase and monitoring of the sediment redox conditions. Amendments to the inundation test design have been recommended, which comprise: set-up to allow for the continuous monitoring of pore water dynamics and allow for the collection of pore water at the different times and measurement of pore water pH and Eh; inclusion of a blank test cell, to test the influence of the properties and the volume of the inundation water; inclusion of flow-cell tests to assess the influence of moving or stagnant inundation water; the inclusion of abiotic blanks to identify the role microbes play in the solubilisation of contaminants. complementary characterisation of the solid phase material and metal distribution in the sediment before and after the inundation experiment

Normal background concentrations (NBCs) of contaminants in English soils : final project report

The British Geological Survey (BGS) has been commissioned by the Department for Environment, Food and Rural Affairs (Defra) to give guidance on what are normal levels of contaminants in English soils in support of the Part 2A Contaminated Land Statutory Guidance. This has initially been done by studying the distribution of four contaminants – arsenic, lead, benzo[a]pyrene (BaP) and asbestos – in topsoils from England. This work was extended to a further four contaminants (cadmium, copper, nickel and mercury) which enabled methodologies developed to be tested on a larger range of contaminants. The first phase of the Project gathered data sets that were: nationally extensive; systematically collected so a broad range of land uses were represented; and collected and analysed to demonstrably and acceptable levels of quality. Information on the soil contaminant concentrations in urban areas was of particular importance as the normal background is considered to be a combination of both natural and diffuse anthropogenic contributions to the soil. Issues of soil quality are most important in areas where these affect most people, namely, the urban environment. The two principal data sets used in this work are the BGS Geochemical Baseline Survey of the Environment (G-BASE) rural and urban topsoils (37,269 samples) and the English NSI (National Soil Inventory) topsoils (4,864 samples) reanalysed at the BGS laboratories by X-ray fluorescence spectrometry (XRFS) so both data sets were highly compatible. These two data sets provide results for most inorganic element contaminants, though results explored for mercury and BaP are drawn from a variety of different and much less extensive data sets

Artisanal & small-scale gold mining research field work, Migori County, Kenya

Artisanal & Small-scale Gold Mining (ASGM) is a subsistence level livelihood for many rural communities across the world. In Kenya it provides work for an estimated 40,000 people and produces 5 tonnes of gold per year. The impact of ASGM is double-edged with the economic benefits offset by damage to the environment and the health of the mining communities, particularly due to the widespread use of mercury to recover gold. As a signatory to the Minamata Convention on Mercury, Kenya has agreed to eliminate the use of mercury, formalise the ASGM sector, introduce good practice and protect the health of mining communities. Migori County is a major ASGM centre in southwest Kenya where gold is produced from the quartz–carbonate reefs in the Migori greenstone belt. Recovery of gold involves extraction of the ore by mining. The deep mine shafts are unstable and dangerous places to work. There are regular reports of fatalities due to mine collapse. The gold is recovered by manual crushing, ball milling, sluice box concentration and mercury amalgamation. Residual gold in the tailings is recovered by cyanidation. The local ASGM communities are primarily concerned about the safety of the mining, the environmental impact of mercury and poor gold recovery. The extent to which pollution from the mining activities leaches into groundwater and impacts water resources is also unknown. The British Geological Survey (BGS) is working with the University of Nairobi and the Migori County Artisanal Miners Co-operative (MICA) to promote good ASGM practice, reduce mercury use and improve gold recovery using appropriate technology, alongside assessing the potential pressures ASGM poses on water resources. Samples of gold ore, crushed and milled ore, concentrates and tailings were collected from ASGM operations. On average hard rock gold is finer than 100 microns. This makes the use of a sluice box a very inefficient recovery method with expected recoveries as low as 20% for gold of 100 microns or finer. Characterisation of the ore will provide the particle-size distribution of the gold and enable the liberation size to be determined. Size analysis of the milled material is expected to show that the ore has been ‘over-milled’ with a large proportion finer than 50 microns. It is likely that some of the gold has been reduced in size to the point where simple gravity processing methods such as sluice boxes will not work. A total of 30 waters were sampled from shallow wells, boreholes, springs and mine shafts, to represent the different sources from which water is obtained by the public, during the period from the 15th to 20th November 2019, while assessment of surface water quality was carried out in a previous survey in January 2019. Mine processing waters and spoil runoff were also sampled. This work will develop good practice guidance for ASGM. It will include advice from a mining engineer to improve mine safety; the use of retorts to reduce mercury consumption; and the use of longer sluice channels (at least 3 metres), appropriate sluice box gradients, consistent sluice box feed supply, alternatives to manual crushing, modification to the milling and alternative processing methods to improve gold recovery. The analysis of the inorganic chemical status of groundwater in the ASGM areas around Migori will assess the potential pressures posed by ASGM on water resources. This BGS research project is part of the BGS Official Development Assistance (ODA) research project “From source to sink: Quantifying the local and downstream environmental impacts of ASGM”

The detection and tracking of mine-water pollution from abandoned mines using electrical tomography

Increasing emphasis is being placed on the environmental and societal impact of mining, particularly in the EU, where the environmental impacts of abandoned mine sites (spoil heaps and tailings) are now subject to the legally binding Water Framework and Mine Waste Directives. Traditional sampling to monitor the impact of mining on surface waters and groundwater is laborious, expensive and often unrepresentative. In particular, sparse and infrequent borehole sampling may fail to capture the dynamic behaviour associated with important events such as flash flooding, mine-water break-out, and subsurface acid mine drainage. Current monitoring practice is therefore failing to provide the information needed to assess the socio-economic and environmental impact of mining on vulnerable eco-systems, or to give adequate early warning to allow preventative maintenance or containment. BGS has developed a tomographic imaging system known as ALERT ( Automated time-Lapse Electrical Resistivity Tomography) which allows the near real-time measurement of geoelectric properties "on demand", thereby giving early warning of potential threats to vulnerable water systems. Permanent in-situ geoelectric measurements are used to provide surrogate indicators of hydrochemical and hydrogeological properties. The ALERT survey concept uses electrode arrays, permanently buried in shallow trenches at the surface but these arrays could equally be deployed in mine entries or shafts or underground workings. This sensor network is then interrogated from the office by wireless telemetry (e.g: GSM, low-power radio, internet, and satellite) to provide volumetric images of the subsurface at regular intervals. Once installed, no manual intervention is required; data is transmitted automatically according to a pre-programmed schedule and for specific survey parameters, both of which may be varied remotely as conditions change (i.e: an adaptive sampling approach). The entire process from data capture to visualisation on the web-portal is seamless, with no manual intervention. Examples are given where ALERT has been installed and used to remotely monitor (i) seawater intrusion in a coastal aquifer (ii) domestic landfills and contaminated land and (iii) vulnerable earth embankments. The full potential of the ALERT concept for monitoring mine-waste has yet to be demonstrated. However we have used manual electrical tomography surveys to characterise mine-waste pollution at an abandoned metalliferous mine in the Central Wales orefield in the UK. Hydrogeochemical sampling confirms that electrical tomography can provide a reliable surrogate for the mapping and long-term monitoring of mine-water pollution

Dissolution experiments in halite cores: comparisons in cavity shape and controls between brine and seawater experiments

There is an increasing need for underground storage of natural gas (and potentially hydrogen) to meet the UK’s energy demands and ensure its energy security. In addition, the growth of renewable energy technologies, such as wind power, will be facilitated by the development of grid-scale energy storage facilities to balance grid demand. One solution lies in creating large-scale compressed-air energy storage (CAES) facilities underground. Whilst a number of lithologies offer storage potential, only three operational CAES facilities exist in the UK. They are constructed in specifically designed solution-mined salt (halite) caverns, similar to those currently used for natural gas storage. The influences exerted on salt dissolution by petrology, structure and fabric during cavern construction are not fully understood, with some occurences of caverns with noncircular cross-sections being less than optimum for gas storage and especially CAES

Investigation of sulphate sulphur isotope variations in the Skerne Magnesian Limestone water body

This report presents the results of a sulphur isotope investigation undertaken in the Skerne catchment, located in County Durham, north of Darlington, to investigate the source of groundwater sulphate in the Magnesian Limestone Aquifer. Groundwater and surface waters in the catchment are at risk from a number of current and historic anthropogenic activities. Sulphate is the biggest risk to the public water supplies; as there is currently no cost-effective treatment available and it could render supplies unusable. The elevated sulphate could be both naturally occurring, due to the presence of gypsum or anhydrite bands in the Magnesian Limestone, or it could be due to abandoned coal mine water, or even saline intrusion pollution. Because of the large difference in the sulphate sulphur isotope composition expected between “marine sulphate”, including sulphate derived from marine evaporites, and “non-marine sulphate” derived from the oxidation of sulphide in the coal seams and mine workings, sulphur isotopes were considered promising tracers to discern mine water sources from natural Permian evaporite sources of sulphate. A survey was carried out at 28 sites where groundwater was sampled in July 2018 from boreholes in the Magnesian Limestone Aquifer and in the Coal Measures, following a pilot study comprising 7 boreholes in July 2017. A small number of surface waters, hyporheic zone waters, springs, and soil leachates, sampled during 2017-2018, were also analysed for sulphur isotopes to complement the borehole data. This has allowed the characterisation of the sulphur isotope composition of potential sources of dissolved sulphate. Most of the Magnesian Limestone aquifer groundwaters cluster close to the Global Meteoric Water Line (GMWL) on the dual water δ 18O and δ2H graph with no evidence of mixing with Narich coal mine water, the latter being more depleted in 18O and 2H; there is a small number of boreholes immediately in proximity of the coal seam boreholes, clearly showing signs of water mixing. With higher δ18O and δ2H than the main Magnesian Limestone group, and slightly offset from the GMWL, is also a small group of Magnesian Limestone boreholes. Repeated sampling would better discern the different recharge paths suggested by this single sampling event in July 2018. Groundwaters associated with the worked and unworked coal seam boreholes in this study are of two water types: sodium sulphate (Na–SO4) and sodium bicarbonate (Na–HCO3) waters, variably enriched in dissolved sulphate. Two δ 34S measurements of the dissolved sulphate in the Na–SO4 coal seam boreholes are +13.1‰ and +23.4‰. The lack of the more typical 34S-depleted sulphate derived from the oxidation of pyrite is hence apparent. A similar range of high sulphate δ 34S values has been described in recent studies, and attributed to deep coal mine systems. From a review of published δ34S values for marine evaporites, groundwaters containing sulphate solely derived from the dissolution of Permian marine evaporites are characterised by 34Senriched sulphate (δ34S values range from +8.2 to +11.1‰). There is, therefore, less of a contrasting isotope signature between potential “evaporite” and “coal mine water” end-members. For example, one sample of coal mine water with δ 34S values of +13.1‰ is not too dissimilar to the average Permian evaporite sulphate with δ 34S value of around +10‰. This makes discrimination of the dissolved sulphate sources based on sulphur isotope less certain, especially at low sulphate concentrations. To help the data interpretation, we have modelled the sulphate and sulphur isotope compositions of mixtures of hypothetical end-members and used the evidence from these simulations to constrain possible groundwater contributions and mixing. In particular we simulate how the HARDWICK HALL borehole, representing the Magnesian Limestone aquifer background, with a sulphate concentration of 89 mg/l, and a δ34S value of +1.0‰, evolves during mixing with the following end-members: i) the coal mine waters in this study, ii) a Permian evaporite source, iii) seawater and iv) acid mine drainage. A summary of the data interpretation based on the above modelling is as follows. Over the mine plume area, inputs of coal mine water-derived sulphate are significant in at least one Magnesian Limestone borehole, and detectable in others, supported by the water isotope δ 18O and δ2H data, indicating for these samples water mixing between the coal mine water and the Magnesian Limestone aquifer. Among the Magnesian Limestone boreholes, where gypsum or anhydrite were noted in the borehole logs, only DALTON PIERCY NO 3 and NO 6 boreholes have high sulphate concentrations and display constant δ 34S values of +10.2‰. Given how close this value is to the Permian evaporites’ δ34S values, it could be plausibly explained by a gypsum dissolution source, although a “coal mine water” contribution with a δ34S signature of +13‰ cannot be totally excluded, as shown by the mixing curves. Many of the Magnesian Limestone boreholes with a sulphate concentration around 100 mg/l (range 85–130 mg/l) are characterised instead by a low δ 34S range (-0.7 to +7.2‰). For most of these low sulphate Magnesian Limestone boreholes, uncertainties in discriminating the source of sulphate are higher. The contribution of sulphate from seawater is difficult to discern in the present data for the saline waters of HART RESERVOIR and HARTLEPOOL IND ESTATE REPLACEMENT boreholes, with similar δ34S values of +21.1‰ and +27‰, as they fall far away from the Seawater–Magnesian Limestone mixing line. Many samples fall far outside of these mixing envelopes, suggesting non-conservative behaviour of the sulphate. The very high δ34S and low sulphate concentrations can be interpreted as a possible sign of reduction of sulphates and enrichment in the heavier 34S isotope of the residual (low concentration) sulphate. Additional samples obtained during this study include: i) A spring in the Ford Formation from AYCLIFFE QUARRY to the south east of Aycliffe Village which provides an additional background sample characterised for sulphur isotopes. The water has a SO4 of 69 mg/l and a δ34S value of +2.3‰ and well resembles the composition of HARDWICK HALL borehole. ii) A Mg–SO4 spring, sampled in Woodham Burn and described in previous studies for its impact on the surface water quality because of its high sulphate concentrations of ~800 mg/l. It has a stable δ34S value of ~ +5.5‰. iii) a surface water impacted by mine water inflow with a Mg–SO4 composition, and a δ34S value of +6.9‰. The δ34S value of +5.5‰ of the Mg-SO4 spring at Woodham Burn points to a contribution of low δ 34S-sulphate, as expected from the oxidation of pyrite. These data support the mechanism, hypothesised in Palumbo-Roe et al. (2020) to account for the spring composition, of dissolution of dolomite in the presence of acidic water, where the source of acidity comes from coal mine water due to the oxidation of pyrite. There is a much narrower and lower range of δ 34S in surface water compared to the groundwater samples. With most δ 34S values less than +7‰, none of the high values measured in the boreholes were noted in the surface water, hyporheic zone or soil leachate samples, except for two samples in the hyporheic zone of Woodham Burn with δ 34S +36.3‰ and +13.4‰, values taken as further evidence of the sulphate reduction during the 2018 summer indicated by the hydrochemistry. Recommendations for future work, building upon these findings, are suggested

Understanding chromium speciation and mobility in urban-industrial environments

This project has characterised the distribution of Cr in the Polmadie Burn system using a range of analytical techniques and aims to predict how changing environmental conditions affect how Cr behaves in the soils, sediments and water