This document discusses mapping of geochemical data in soils in Northern Ireland using two different interpolation methods: inverse distance weighting (IDW) and interpolation within geological boundaries based on soil parent material. It finds that the parent material method produces more accurate maps for potentially harmful elements like chromium, nickel, and arsenic when evaluated using a holdout validation method. Maps of Northern Ireland are shown comparing the two methods and highlighting areas where levels of chromium, nickel, and arsenic exceed regulatory guidelines.

1. Talking
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2010
Imaging of geochemical data
by interpolation within
geological boundaries
Don Appleton
(British Geological Survey)
Presented by Claire McGinn
(Geological Survey of Northern Ireland)

2. Talking
With Maps Acknowledgements
2010
This work is based on research using a
VB.Net tool compiled for ArcGIS 9.3 by Keith
Adlam (BGS)
Data for this work were provided by the Tellus
Project, funded by the Department of
Enterprise, Trade and Investment of Northern
Ireland.
The contributions of GSNI and BGS staff who
collected, prepared and analysed the Tellus
soil samples is gratefully acknowledged.

3. Talking Northern Ireland
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2010
Area: 14,000km2

4. Tellus Project: Northern Ireland
Talking
With Maps Soil data
2010

5. Talking Who uses Tellus data?
With Maps
2010
• Environmental Agencies
• Commercial Industries
• Marine Scientists
• Agricultural Departments
• Universities
• Local Councils
• Regulatory Authorities

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Potentially Harmful Elements (PHE)
With Maps Nickel (Ni) in soils
2010
• Arsenic (As)
• Cadmium (Cd)
• Chromium (Cr)
• Lead (Pb)

7. Gridding by Inverse Distance
Talking
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Weighting: Chromium
2010

8. Talking Simplified Bedrock Geology Map
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2010

9. Talking
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2010 Methodologies
Inverse Distance Weighting
versus
Soil Parent Material
(i.e. within geological boundaries)

10. Talking Bedrock geology
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2010
boundaries
Superficial geology
boundaries

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14. Talking
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16. 4+4+4+3+2=17
Talking 17/5=3.4
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2010
3 4
4 4
2 3 2
4 2 3
4+3+3+2+2=14
14/5=2.8

17. 1-2.9
4+4+4+3+2=17
3-4.9
Talking 17/5=3.4 5-6.9
7-8.9
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2010
4+3+3+2+2=14
14/5=2.8

18. 1-2.9
4+4+4+3+2=17
3-4.9
Talking 17/5=3.4 5-6.9
7-8.9
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2010
4+3+3+2+2=14
14/5=2.8

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Step 1: Geology polygons are intersected with the 1km Grid
Step 2: Centroids are created for each geological polygon within
each 1km polygon
Step 3: VB.Net Tool identifies the nearest 5 soil sites to the centroid
for the Parent material and superficial geology

20. Talking Sample points
With Maps and centroids
2010
(Parent material
method )
Rasterised image
of the centroids
interpolated
within parent
material
boundaries

21. Chromium in soils
Talking
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2010 New Parent
Material mapping
Old method using
IDW mapping.

22. Parent Material soil chemistry maps:
Talking optimum number of samples for Parent
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2010
Material mapping
2
1 n
MSD = ∑ (estimated − measured )
n i =1
1.8
As AV
1.6 Cr AV
Ni AV
1.4 Pb AV
Average of As, Cr, Ni, Pb AV
Overall average ALR-GALA
1.2
1
MSD
0.8
0.6
0.4
0.2
0
0 5 10 15 20 25
Number of neighbouring samples

23. Evaluation of Parent Material and IDW
Talking
soil chemistry mapping using holdout
With Maps
2010 validation
Mean squared deviation (MSD) for As, Cr and Pb for the
three mapping methods based on combining the results
for two 10% holdout subsets. Lowest MSD values for
each element indicated in bold type. (Lne data).
2
1 n
MSD = ∑ (estimated − measured )
n i =1
Element Spatial Parent Parent
analyst Material Material
IDW mapping mapping
(GM) (IDW GM)
As 0.18 0.15 0.16
Cr 0.43 0.31 0.31
Pb 0.20 0.19 0.21

24. Chromium in
Talking
soils – all NI
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2010
New regional
Parent
Material
mapping.
Old version
using IDW

25. Nickel above Soil Guideline Values (SGV)
Talking Residential 130 mg/kg; Allotments 230 mg/kg
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2010

26. Arsenic above Soil Guideline Values
Talking Residential 32 mg/kg; Allotments 43 mg/kg
With Maps
2010

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2010
For more information
Contact GSNI: gsni@detini.gov.uk
GeoIndex:
http://maps.bgs.ac.uk/gsni_geoindex/