Sediment and associated radionuclide dynamics within the Ribble Estuary, North West England.

Abstract

Intertidal environments represent a spatially complex and dynamic system, rendering point sampling geographically and temporally isolated within the context of the entire estuarine system. Airborne remote sensing has the potential to place these spatially isolated sampling points into a quantitative spatial context. Furthermore it provides a valuable data source for quantifying processes within estuarine zones and can supply calibration and validation information for hydrodynamic models. This study focuses on the Ribble Estuary, Lancashire, England which is accumulating elevated radionuclide concentrations derived from authorized industrial discharges from BNFL Sellafield and Westinghouse Springfields. An image mixture modeling approach was used on Airborne Thematic Mapper (ATM) data to derive accurate estimates of intertidal clay, in comparison to concurrent field sampling (r2=0.828) and radionuclide concentrations (r2= 0.822). Data processed for 2003 was compared with similar data from May 1997 (Rainey 1999; Rainey et aL., 2000; 2003) to investigate spatial changes in intertidal clay and 137Cs contamination. These results compared with field sampling data demonstrates considerable reduction (c.52%) in the activity concentrations, which is primarily attributed to processes of sediment dilution. Calibrated Compact Airborne Spectrographic Imager (CASI) imagery combined with concomitant ground reference data, was used to characterize the suspended sediment concentrations and the total suspended load over each flight line. Two sets of time series image data were compared to assess the spatial and temporal changes in suspended sediment and associated radionuclide transportation within the estuarine environment. In conjunction with total volumetric estimates generated from a two-dimensional vertically resolving hydrodynamic model, this data then allowed estimation of the total flux of suspended sediment and radionuclide over the flood and ebb tidal cycle to a reasonable precision (40%). To establish whether these flux estimates are realistic, the results are compared with time series field based observations collected from monthly observations over a two year cycle. The results provide a unique quantitative insight into the understanding of contaminant and sediment transport within this estuarine environment and the environmental processes controlling them. The contribution of field data with the intertidal and flood-ebb tide imagery has provided an enhanced understanding of the interactions of tides and fluvial flow on the spatial distribution of sediments within the Ribble Estuary. It could also be possible to apply the calibrated clay intertidal maps to other heavy metal pollutants that have a high affinity with fine-grained clay particles i.e. Pb, Zn, Cu, Al in estuarine sediments.