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.