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Innovations in Fluvial Environmental Research: Core Scanning of Flooding and Contaminant Histories (INFER 300623)

1.0 Introduction and aims

Sediments deposited in marine, lake and river settings act as natural archives, containing physical and geochemical information that often records changes in climate and land-use and, in some cases, industrial activity, indicated by the presence of contaminants in the sediment source area. Accordingly, sediment cores are widely employed in scientific research to investigate aspects of past environmental change.

XRF core scanners (Fig. 1) provide a means of rapidly screening core samples for a range of environmentally significant elements at resolutions previously unachievable. The INFER (300623) project has used state-of-the-art micro-XRF core scanning and variable pressure scanning electron microscopy, both hosted at University College Dublin, Ireland, together with laser granulometry and image-processing techniques for the analysis of fine-grained floodplain sediments, with the aim of reconstructing late Holocene flood chronologies and assessing the extent of floodplain contamination, resulting from historical metal mining. The project has provided training to the Marie Curie Fellow in sampling, geochemical analysis, image capture and analysis and grain size analysis.

The project had two parallel research aims:

1. To validate statistical relationships between micro-XRF element ratios and sediment particle size using high resolution image analysis;

2. To develop and test protocols for calibrating micro-XRF core scanning results and to assess the potential for producing sufficiently accurate heavy metal concentrations in contaminated floodplain sediments, in a rapid and efficient manner.

2.0 Summary of main results

2.1 Optimisation of micro-XRF parameter settings (research aims 1 and 2)

The optimisation of micro-XRF scanning parameters (tube voltage and current, and count time) used test scans of core sections. Signal-to—noise (SNRs) were calculated using the method of Ernst et al. (2014) for each XRF spectrum, for the chemical elements rubidium (Rb) and zirconium (Zr) silicon (Si), potassium (K) and titanium (TI) in cores used in reconstructing flood histories, and for copper (Cu), zinc (Zn) and lead (Pb) for cores used in assessing floodplain contamination. Results of tests of voltage (kV) and current (mA) combinations are shown for four test data sets (Fig. 2). These show a decrease in SNRs with increasing voltage for Si and K and an increase with increasing voltage for Rb and Zr. The pattern for Ti is more complex. SNRs increase with increasing XRF count time for all elements. Appropriate voltage and current settings vary depending on the purpose of the study and the elements of interest, and may require a compromise to be made between optimal settings for elements in different parts of the XRF spectrum. XRF count times should be selected to obtain adequate signal-to-noise ratios for those elements of interest which produce the smallest peaks in the XRF spectrum (in general these are elements present at low concentrations).

2.2 Testing element log ratios as proxies for grain size using high resolution image analysis (research aim 1)

For flood reconstruction the project has focused on two sites on the River Severn, at the Roundabout (SJ 275130) and Hayes Basin (SJ 352158), two sites on the River Wye, at Letton Court (SO 334461) and the Sturts Nature Reserve (SO 339 474), and one site in Ireland at Crewbane Marsh (N 991734) on the River Boyne (Fig. 3). X-ray and micro-XRF analysis have been conducted on 51 sections of core. 179 samples from these cores were analysed for particle size using the laser granulometer facility in Trinity College Dublin (an INFER partner). Statistical analysis comparing aggregated micro-XRF data and results from particle size analysis has demonstrated high potential for the application of the Zr/Rb log-ratio on the Severn at the Roundabout and Hayes Basin and on the Wye at the Sturts, but limited or no potential for this ratio at Crewbane Marsh and Letton Court. The alternative Si/Ti log-ratio appears to be a suitable grain size proxy for the Sturts and Hayes Basin, has limited potential for the Letton Court and Crewbane Marsh sites and cannot be used as a grain size proxy at the Roundabout.

One of the primary objectives of INFER was to compare the ln(Zr/Rb) grain size proxy with direct grain size measurements from image analysis of back-scattered electron images captured from thin sections. The thin

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section samples were prepared using undisturbed sediments from the Roundabout cores. This analysis allows comparison of micro-XRF geochemical data with particle size data obtained at the scale at which the micro-XRF data were collected. Results in Fig. 4 for a thin section obtained from one of the Roundabout cores suggest a relatively good correspondence between ln(Zr/Rb) and measures of particle size derived from image analysis. Work is on-going to determine the amplitude of variation that can be confidently detected using this method.

2.3 Flood reconstruction: Britain and Ireland (research aim 1)

The project is currently waiting for results of basal radiocarbon dates for UK cores acquired at sites on the River Severn (Roundabout) and River Wye (Letton Court and Sturts), and on the River Boyne (Crewbane Marsh) in Ireland. AMS dates are based on the acquisition of charcoal that has been hand-picked under microscope. These basal radiocarbon dates will inform decisions on further dating requirements for the development of centennial to millennial-scale flood reconstructions at the study sites, expected to be mid-late Holocene in age.

2.4 Conversion of micro-XRF counts into concentrations (research aim 2)

Conversion of micro-XRF counts to concentrations in mg kg-1

is an essential step towards the mapping of heavy metal distributions in floodplains. The project has established reasonably robust calibration curves for contaminant metals based upon the discrete sub-sampling of floodplain cores at a cm-scale (Fig. 5). Quantitative geochemical analysis is based on a ‘total’ 4-acid digest of milled sub-samples. This method was followed, because the project has adopted an approach to field sampling using light weight, u-channels extracted from manual augers, rather than percussion coring. This field sampling approach facilitates rapid field reconnaissance and sampling, and significantly enhances the spatial coverage of sampling across extensive floodplain sites, but limits the volume of sediment collected.

Geochemical analysis is on-going to develop calibration models based on synthetic standards that are matrix-matched to the floodplain sediment. This approach was considered important to better account for artefacts in the XRF data associated with variable silica, metal and organic matter content (Fig. 5). The development of standards shows promise, but is strongly influenced by very high heavy metal concentrations (5-10% Zn and Pb) found in mine wastes. Additional analyses of samples within the concentration ranges (0-1000 mg kg

-1 Cu,

Zn and Pb) found in the floodplain sediments are required.

2.5 Floodplain contaminant mapping: Avoca catchment, Ireland (research aim 2)

Contaminant mapping has focussed on the Avoca catchment (650 km2), Co. Wicklow (Fig. 3), which has a

history of Pb/Zn mining in the upper catchment and Cu mining downstream of Avoca town. Field sampling took place across the catchment especially in and around the mine sites at Glendalough and Glendasan and floodplains immediately downstream. Preliminary analyses of selected cores and samples collected for standard preparation have revealed very high concentrations (c. 5% Zn and > 10% Pb) in wastes (spoil, tailings and slimes) at abandoned mine sites, and elevated heavy metal concentrations (500-10,000 mg kg

-1 Zn and Pb)

downstream of these zones. Much of this contaminated material is buried at depth and would not be detected using conventional field sampling, but still presents a hazard because of potential channel bank erosion.

3.0 References

Ernst, T., Berman, T., Buscaglia, J., Eckert-Lumsdon, T., Hanlon, C., Olsson, K., Palenik, C., Ryland, S., Trejos, T., Valadez, M and Almirall, J. R. (2014) Signal-to-noise ratios in forensic glass analysis by micro X-ray fluorescence spectrometry. X-Ray Spectrometry 43, 13-21.

4.0 Broader implications

The research being carried out under (INFER 300623) has focussed on catchments in Britain and Ireland, but has broader applications for river managers across Europe and the Globe. The methodologies that have been developed and tested in this project are transferrable to a range of environments and not limited to riverine settings.

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For further information:

Please see the project website: http://www.ucd.ie/gpep/research/researchprojects/infer/

Contacts: Dr Jonathan Turner (jonathan.turner@ucd.ie) or Dr Anna Jones (anna.jones@ucd.ie)

Acknowledgements

In addition to the generous support of the Marie Curie Intra-European Fellowship (IEF) the project team would like to thank a number of people for their time and support during the project. In particular we would like to mention Ciara Fleming, David Colgan, John Hutchings, Seamus Arnold and Harut Shahumyan in the UCD School of Geography, Planning and Environmental Policy, Eamon Hynes in the UCD School of Geological Sciences, Gwyneth MacMaster in the UCD School of Biological Sciences, and Elaine Treacy and Neil Kearney in the Department of Geography, Trinity College Dublin. We would also like to say a special thanks to the landowners of field sites we visited in Ireland and Britain.

Innovations in Fluvial Environmental Research (INFER 300623)

Funded through a Marie Curie Intra-European Fellowship (IEF) and based at the School of Geography, Planning and Environmental Policy, University College Dublin, Ireland

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Fig. 1. The UCD GPEP Itrax micro-XRF core scanner (above left) which generates ultra-high resolution XRF profiles for a range of environmental significant elements from silicon to uranium. A direct comparison of Itrax optical and X-radiographic images, with XRF profiles (above right) can reveal hidden linkages between structural and element variability. The example shown is of stratified/layered sediments in Ireland

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Fig. 2. Variation of signal-to-noise ratios (SNRs) with tube voltage and current for Si, K, Ti, Rb and Zr in XRF scans of cores from the sites in Ireland and the performed at 500 µm resolution. Negative SNRs occur where the ‘total counts’ for the peak are less than the ‘background’, resulting in a negative ‘signal’.

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Fig. 3. INFER study catchments and field sites

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Fig. 4. (A) Thin section of fine-grained floodplain sediment from River Severn, Roundabout Core 16; (B) Back-scattered electron image of 1024 micron by 768 micron area of thin section, showing silt particles (light grey and white) in a fine matrix (dark grey or black); (C) Binary image derived from (b) showing particles with areas greater than 10 microns; (D) 75th percentile of the equivalent disc diameter of particles from 19 consecutive images compared with Zr/Rb log-ratio of the corresponding interval in the core show close correspondence between micro-XRF –based grain size proxy data and direct particle size measurements.

A)

B)

C)

D)

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Fig. 5. Linear regression models for selected samples (n = 36) showing element (Cu, Zn and Pb) concentration (mg kg

-1) vs. micro-XRF counts (peak area integrals). XRF counts are based on optimised kV and mA settings

and 5 sec count times. Geochemical concentrations were determined using a 4-acid ‘total’ digest on milled samples. Outliers are linked to high organic matter in the sample matrix for selected samples.