Flooding induced changes in the mobility, bioaccessibility and solid phase distribution of potentially harmful elements

Abstract

The Intergovernmental Panel on Climate Change (IPCC) predicts that the number of extreme precipitation events will increase considerably by the end of the century for mid-latitude land masses such as the UK. Potentially harmful elements (PHEs) such as arsenic, cadmium, copper, lead and zinc can be chemically mobilised during flood events, potentially increasing their availability to receptors. The development of floodplains for residential and industrial uses increases the risk of a source - pathway - receptor linkage occurring for PHEs. This thesis aims to characterise changes in the solid phase distribution and bioaccessibility of PHEs before, during and after drying to provide new knowledge of PHE mobility in catchments. Soils were collected from a UK catchment with a history of lead and zinc mining and characterised in terms of pseudo-total PHE content, the bioaccessible content of PHEs and their solid phase distribution. Laboratory inundation studies using microcosms were conducted to understand PHE behaviour during controlled wetting and drying episodes. The results demonstrated that flooding resulted in the mobilisation of the PHEs into porewaters. However, the pattern of mobility was shown to vary for different PHEs. Bioaccessibility testing after each wet and dry cycle determined the changes in PHE availability to humans and highlighted an increase in the bioaccessible fraction of PHEs in this study in the region of 5-10 %. The solid phase distribution of PHEs during wetting and drying cycles was then determined using sequential extractions and a self-modelling mixture resolution algorithm to help explain the earlier findings on PHE availability. Broad scale changes in the solid phase distribution of PHEs varied between soils. For arsenic, generally the greatest change was observed in the iron oxide components. Other PHEs exhibited redistribution between soil components, often to those that were more labile. The spatial prediction of, and the influence of flooding on PHE bioaccessibility in the Tyne catchment was investigated. Microcosm experiments were conducted to quantity the flooding induced changes in bioaccessibility. This was followed by a combination of geospatial, regression and machine learning methods to map PHE bioaccessibility at a catchment scale. A key output was the production of maps highlighting bioaccessible content of PHEs in flood prone areas of the Tyne catchment. Furthermore, flooding induced changes in bioaccessibility were mapped at a catchment scale, which highlighted areas where there was a greater potential for flooding induced increases in bioaccessibility and consequently human exposure.