imaging of geochemical data, by bgs ni

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Imaging of geochemical data by interpolation within geological boundaries

Don Appleton (British Geological Survey)

Presented by Claire McGinn (Geological Survey of Northern Ireland)

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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.

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Area: 14,000km2

Northern Ireland

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Tellus Project: Northern Ireland Soil data

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Who uses Tellus data?

• Environmental Agencies• Commercial Industries• Marine Scientists• Agricultural Departments• Universities• Local Councils• Regulatory Authorities

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Potentially Harmful Elements (PHE)

• Arsenic (As)• Cadmium (Cd)• Chromium (Cr)• Lead (Pb)

Nickel (Ni) in soils

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Gridding by Inverse Distance Weighting: Chromium

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Simplified Bedrock Geology Map

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Inverse Distance Weighting

versus

Soil Parent Material (i.e. within geological boundaries)

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Bedrock geology boundaries

Superficial geology boundaries

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3

2

2

34

2

44

43

4+4+4+3+2=17

17/5=3.4

4+3+3+2+2=14

14/5=2.8

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4+4+4+3+2=17

17/5=3.4

4+3+3+2+2=14

14/5=2.8

1-2.93-4.95-6.97-8.9

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4+4+4+3+2=17

17/5=3.4

4+3+3+2+2=14

14/5=2.8

1-2.93-4.95-6.97-8.9

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

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Rasterised image of the centroidsinterpolated within parent material boundaries

Sample points and centroids(Parent material method )

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Chromium in soils

New Parent Material mapping

Old method using IDW mapping.

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Parent Material soil chemistry maps: optimum number of samples for Parent Material mapping

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 5 10 15 20 25

Number of neighbouring samples

MSD

As AVCr AVNi AVPb AVAverage of As, Cr, Ni, Pb AVOverall average ALR-GALA

( )2

1

measured estimatedn1 MSD ∑

=

−=n

i

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Evaluation of Parent Material and IDW soil chemistry mapping using holdout validation

Element Spatial analyst IDW

Parent Material mapping (GM)

Parent Material mapping (IDW GM)

As 0.18 0.15 0.16Cr 0.43 0.31 0.31Pb 0.20 0.19 0.21

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

measured estimatedn1 MSD ∑

=

−=n

i

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Chromium in soils – all NI

New regional Parent Material mapping.

Old version using IDW

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

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Arsenic above Soil Guideline ValuesResidential 32 mg/kg; Allotments 43 mg/kg

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For more information

Contact GSNI: gsni@detini.gov.uk

GeoIndex:http://maps.bgs.ac.uk/gsni_geoindex/