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

摘要

This report presents a literature review of soil and sediment inundation methodologies anduddescribes a pilot scale laboratory inundation study.udChanging weather conditions, subsequent flooding events, and the increased frequency of suchudevents both in the UK and worldwide is highlighting the need to research the area of contaminantudmobility from soils and sediments under inundated conditions. The findings of suchudinvestigations impact on a wide variety of sectors, including human and ecological health,udagriculture, building, transport, world economy and climate change.udStandardised methodologies for the investigation of contaminant mobility resulting fromudsoil/sediment inundation episodes are not available. Most research has been conducted in theudagricultural sector for nutrient transport, as part of soil fertility and plant nutrition studies. Onlyudrecently has work been undertaken for studying potentially harmful element transport inudinundated sediments/soils.udA pilot scale laboratory study was undertaken using contaminated bank sediment samplesudcollected from the Rookhope Burn catchment, Northern England, UK, with the aim to examineudthe extent of contaminant mobilisation from flooded sediments. The catchment has beenudaffected by historical mining and processing of lead and zinc ore and is representative of severaludcatchments affected by the environmental legacy related to mining in the Northern PennineudOrefield.udBank sediment Pb and Zn concentrations were found significantly above both the TEL and PELudsediment quality criteria, posing potentially a significant hazard to aquatic organisms. The sourceudof the Pb and Zn in the sediments is related to the underlying mineralisation, mining activitiesudand mine water discharges in the catchment. Abundances of original sulphide ore and authigenicudmetal-bearing phases were expected to vary through the catchment.udThe study design simulated rising flood water, a slow saturation of the sediment in order toudinduce a slow change in physico-chemical properties, followed by a 3 month (88 day) stagnationudperiod. Natural day-night cycles were simulated by undertaking the study on the bench topudduring the winter of 2009/2010 (November to February). The chemical changes in theudinundation water during the experiment were monitored and the sediment pore water at the endudof the inundation period analysed. The inundation water pH remained alkaline to neutral, whileudredox measurements indicated oxic conditions in the water column throughout the inundationudperiod.udThe pilot study showed that inundation of river bank sediments from the Rookhope Burn may beuda significant pathway for contaminants in the catchment and that mobilisation from theudsediments may pose a hazard to environmental receptors in the area, particularly with respect toudPb and Zn contamination. The different degrees and different rates of metal losses to theudoverlying water column observed during the flooding of the Rookhope Burn bank sedimentsuddemonstrated that the significance of metal mobilisation was dictated by the sedimentudcomposition.udThe inundation water composition monitored during the sediment flooding was used to indirectlyudinfer possible processes that control contaminant fluxes from the sediments to the overlyingudwater. Dissolved Pb concentration in the inundation water reflected the original concentration inudthe solid material and in sediments that had XRD-detectable galena and cerussite the dissolvedudPb concentration reached a maximum value of 395 μg l-1. Cerussite, which is commonly formedudas coatings on galena during the sulphide weathering, was close or supersaturated in thoseudsolutions, suggesting that the lead carbonate mineral phase provided a continuous source of Pb toudthese solutions.udThe initial dissolved Zn in the inundation waters was independent of the original concentrationudin the sediments. Sediments downstream a mine water discharge showed a greater availability ofudeasily mobilised Zn, producing high initial Zn concentration in the inundation water, despite theudrelatively low Zn concentration in the inundated sediment. The Zn/SO4 and Cd/Zn molar ratiosudwere both consistent with sphalerite mineral oxidation. The final inundation water solutions hadudthe highest Zn concentrations for those sediment samples where sphalerite was detected byudXRD.udRedox sensitive elements such as Fe and Mn could not unequivocally indicate the presence ofudreducing conditions within the flooded sediments and the redox measurements were carried outudonly in the overlying water column (ORP above 200-350 mV). Low organic matter content andudsandy texture would not have favoured the rapid formation of an anoxic layer. Yet, onlyudextending the ORP measurements to the submerged sediment would determine the presence ofudflooding-induced reducing conditions. Reductive dissolution of Mn oxyhydroxides would resultudin release of Mn into solution, along with other trace metals, such as Pb and Zn. Mn increased inudthe inundation water throughout most or all the inundation period for some of the studiedudsediments. Their final pore water composition was significantly enriched in Mn (1300-ud6500 μg l-1). Saturation indices indicated both rhodocrosite (MnCO3) and Mn oxides reachedudsaturation. Therefore, it was not possible to preclude either the role of rhodocrosite as solubilityudcontrolling solid phase or the reductive dissolution of Mn oxides for accounting the enhancedudMn concentrations in the pore water and overlying water column without a betterudcharacterisation of the solid phase and monitoring of the sediment redox conditions.udAmendments to the inundation test design have been recommended, which comprise:ud set-up to allow for the continuous monitoring of pore water dynamics and allow for theudcollection of pore water at the different times and measurement of pore water pH and Eh;ud inclusion of a blank test cell, to test the influence of the properties and the volume of theudinundation water;ud inclusion of flow-cell tests to assess the influence of moving or stagnant inundationudwater;ud the inclusion of abiotic blanks to identify the role microbes play in the solubilisation ofudcontaminants.ud complementary characterisation of the solid phase material and metal distribution in theudsediment before and after the inundation experiment.