Global climate change is associated with significant changes to short-term weather extremes as well as
long-term weather characteristics in different regions. Whilst the magnitude of climate changes are
extremely uncertain, it is likely that summers will become warmer and drier, and there will be an increase
in the intensity of rainfall events. These intense rainfall events will lead to an increased number of flooding
events that remain for longer periods of time, and the occasional inundation of land that has rarely been
flooded in the past. There is the possibility that increased flooding intensity and frequency will influence
the soil properties, which in turn may affect the behaviour and mobilisation of potentially toxic elements
(PTEs) in floodplain soils. It likely that many floodplains downstream of urban catchments, particularly
those catchments with a history of industrial development, may harbour a legacy of contaminants that
have been deposited with floodplain sediments.
To investigate the impact of fluvial flooding on PTE mobility in floodplain soils, I used the Loddon Meadow
floodplain site; situated adjacent to the River Loddon in the Southeast of England, as a model floodplain,
typical of a lowland floodplain downstream of an urban catchment. Preliminary work characterised the
floodplain topography using geospatial techniques and compared elevation with the spatial distribution
of soil PTEs concentrations. The topography of the floodplain was found to influence the deposition of
some PTEs (e.g. Cr, Cu, Ni and Zn), providing strong evidence that the source of these PTEs to this
floodplain site originated from point source or diffuse pollution upstream in the urban catchment. The
novel combination of geospatial mapping of elevation and geochemical analyses could be adopted as a
method for determining the source of PTEs to other study sites.
Analysing soil pore water chemistry provides the more useful measurement of the mobile fraction of PTEs,
rather than the total concentration bound to the solid fraction. There are a number of methods for
extracting pore water from soil samples; we compared an example of an in-situ method (RhizonTM
sampler) with an example of an ex-situ method (centrifugation). There were no significant differences
found in the pore water chemistry, despite the centrifugation exerting a pressure on the soil sample
orders of magnitude higher than the RhizonTM sampler. We found, however, that in terms of useability
through a range of soil moistures and consistency of sample volume extracted, the centrifuge was the
preferred method for this particular study. We highlight examples where the opposite conclusion might
be reached.
Laboratory mesocosm studies have reported increased PTE mobilisation with artificial flooding events.
However, it can be difficult to extrapolate these finding due to the controlled conditions of the laboratory
set-up (e.g. room temperatures are often higher than found in the field). We found that there was a need
for on-site experiments that consider the effects of flooding using real-time field observations. We
therefore took a field-based approach; extracting soil pore waters, by centrifugation, from the Loddon
Meadow floodplain pre-flood, during a flood and post-flood. We found that the flooding event did not
influence the mobility of all of the PTEs in the same way. However, we found concentrations of Cd, Cu
and Cr significantly decreased post-flood compared to pre-flood. The dominant process identified to
explain this decrease was precipitation with sulphides, which occurred during the flood and subsequently
resulted in the significant decrease in concentrations post-flood. A slight increase in pH may have aided
adsorption processes onto organic matter and clay minerals. We also found a decrease in dissolved
organic matter in solution and this would have reduced the capability of the pore water to complex PTEs
in solution. It is possible that the decreased concentrations found were a result of dilution, due to the
increased water volume from the river and ground water. When analysed, the river and ground water had
considerably lower concentrations of PTEs than the soil pore waters.
The impact of a flooding event on PTEs mobility is the combination of multiple processes. So, while we
observed some processes increasing the concentrations of PTEs; for example, the reductive dissolution of
Mn oxides, predominantly in the lower elevation areas of the floodplain. The overall net effect of the
flooding event was a decrease in PTE concentrations, because processes like sulphide precipitation were
dominant. There were no significant increases in PTEs mobility due to the flooding event and as such, no
evidence to support the idea that floodplains become a source of PTEs. This is contrary to the evidence
from laboratory studies, that found there is mobilisation of PTEs due to flooding. This study highlights the
importance of understanding the dominant processes that drive the mobility of individual PTEs on specific
floodplains, so that site-specific predictions can be made on the impact of future flooding on the
mobilisation of legacy contaminants. Further field-based monitoring; collecting data pre-flood, during the
flood and post-flood, from varying soil types and composition (e.g. clay, sand, silt, peat and loam) is
required to support future modelling exercises. This would improve our capability to predict the impact
of increased intensity and duration of flooding on soil porewater chemistry and PTE mobility
