The spatial distribution of heavy metal contaminated sediment across terraced floodplains

Deposits of fine alluvial sediment contaminated by heavy metals derived from mining provide an opportunity to reconstruct the alluvial histories of mineralised catchments. Field data were collected to identify the spatial distribution of heavy metal contamination in surficial floodplain sediments and establish the relationships between terrace height, terrace age and metal concentrations. Floodplain terrace morphology was mapped and terrace heights were measured for three separate study reaches in the upper Sevem Basin, mid-Wales. Concentrations of Pb, Ba and Zn were determined in 97 samples taken from fine overbank units capping these terrace surfaces. Between the three study reaches, element concentrations showed wide ranges (Pb 11–964ppm; Ba 141–1118 ppm; Zn 73–1134 ppm). They also exhibited significant spatial variation across the floodplain at each study reach (e.g. at Llandinam, Pb 36–964ppm, Ba 141–1118 ppm and Zn 173–1134 ppm). Peak metal concentrations at Morfodion (Pb 399 ppm, Ba 474 ppm) and Welsh-pool (Pb 149 ppm, Ba 558 ppm) were found on terrace surfaces 1.5–2.5m and 2.0–3.0 m, above present river level, respectively. Lower concentrations reflect natural background levels of contaminants (higher terraces) and post-mining reworking of spoil heaps and floodplains (lower terraces). In general, the spatial distribution of floodplain contaminants is more significantly correlated with terrace height than with terrace age. Different flooding mechanisms and styles of channel instability in relation to the timing and mode of sedimentation have resulted in different patterns of floodplain contamination. Focused deposition, particularly in palaeochannels, is also an important mechanism for creating strongly contaminated sites within the floodplain environment.