The effects of spatio-temporal variation in estuarine contamination

Abstract

With the coastal environment likely to be affected by climate change induced modifications to storminess (both frequency and intensity), sea level rise and increased precipitation in the near future, changes in sediment dynamics in terms of erosion, accretion and movement is expected. Given the important role coastal sediments play as sinks for environmental contaminants, understanding the fate and behaviour of these sediments is crucial in determining any potential impacts on humans and wildlife. This project investigates long-term trends in sediment movement in the Ribble estuary and how disturbance events might affect these trends. The rate of erosion of sediment within the coastal environment is a concern as the diverse array of sediment deposits in the coastal margins are vital habitat for a range of wildlife. They also act as a substantial sink of radioactive contaminants from current and past discharge practices. Remobilisation and any consequent changes in bioavailability of these contaminants is of potential concern and a key reason for this research. There is an emerging view that the reworking of sediment bound contaminants either already is or will soon become the dominant factor in the extent of inter-annual variation in estuarine contamination. Within the Ribble estuary NW, England, the physical, spatial and temporal characteristics of the exchange of contaminated sediment between different sediment deposits was investigated at the micro and macro scale. Monthly observations of changes in the sediment physical properties and contaminant concentration were conducted. These observations determined that contaminants remained strongly associated with fine grained sediments, though this association varied temporally and spatially and could become decoupled in response to a substantial siltation event. Analysis of historic data for the Ribble estuary revealed that the nature of the relationship the radiogenic contaminants 137Cs and 241Am had with the fine-grained sediments differed between the contaminants over time. For example, the activity concentration of 137Cs in the fine-grained sediments was found to decline between 1995 and 2014 and has been put down to radioactive decay and reduced inputs of 137Cs into the estuary from the Irish Sea. In contrast, 241Am activity concentrations did not show a statistically significant decline over the same time period. This was interpreted as being a function of the longer half-life of 241Am, its ingrowth from 241Pu and a reduced rate of re-dissolution. A time series statistical analysis was used to determine if storminess and high riverine discharge events could explain variation in sediment properties. The analysis showed that riverine discharge was the dominant factor. Storminess was still a statistically significant driver of change in sediments but was less so in comparison to riverine discharge. These results confirm that discreet high impact disturbance events are substantial factors in the reworking of estuarine sediments. A three-dimensional spatial analysis of the sediment movements within the Ribble estuary was conducted from 1999 – 2015 through a novel LiDAR geostatistical methodology, with the purpose of determining the nature of sediment movements within the Ribble estuary. The Ribble was confirmed to be a very dynamic estuary and exhibited substantial morphological change that was interpreted as large-scale sediment remobilisations. The geostatistical analysis of the Ribble Interestingly showed that the estuary fluctuated between erosion and accretion. The saltmarshes where observed within the 16-year study period to be more significant than the mudflats in terms of sediment erosion and accretion which is of interest given that saltmarshes are concentrated in radiogenic contaminants. Natural disturbance events are very difficult to study given the uncertainty of when they may occur, the resources and time needed to sample before and after the event. In contrast planned anthropogenic disturbance can be easier to study and can still allow conclusions to be drawn on the impacts of largescale disturbance events. To this end, the disturbance effects of a managed realignment scheme were investigated specifically the modification of saltmarsh creek hydrodynamics. The managed realignment scheme was implemented in 2009 at the Hesketh Outmarsh in the Ribble estuary. Over the following years, the scheme has been shown to promote localised erosion within the larger creeks inside the breached area. A novel geostatistical methodology was used to estimate the quantity of radiogenic contaminants (137Cs and 241Am) remobilised from the breached area. This research estimates that, from a baseline in 2007, some 52 GBq of 137Cs and 20.9 GBq of 241Am were remobilised – up to 2015. The Ribble Estuary is dynamic at the micro and macro scale and under current circumstance the contaminant enriched saltmarshes are functioning as a diffuse source of radiogenic contaminants to the wider estuary and potentially beyond. Sediment remobilisations are believed to be responsible for significant variation in contaminant sediment matrix relationships. Where climate change will see an increase in disturbance events there will also be an enhanced rate of contaminant redistribution.