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GIS Spatially Weighted Averages
1. GIS and Database Action Group (GDAG) Conference 2014
Joe Luchette (JLuchette@geosyntec.com Princeton, NJ)
Tom Geiger (TGeiger@geosyntec.com Princeton, NJ)
Compliance Averaging
using Spatially Weighted Average Methodology
2. Outline
What is Compliance Averaging
When and How to Use a Spatially Weighted
Average (based on New Jersey guidance)
Case Study: Trane Trenton, NJ
Automation via python script in ArcGIS
3. What is Compliance Averaging
Compliance Averaging is…
“… statistical tests to allow for identification and elimination of data
outliers” (http://www.nj.gov/dep/srp/guidance/srra/attainment_compliance.pdf)
Methods:
Arithmetic mean for the data set where there are two or fewer distinct
sample values or nine or fewer total sample points
95 percent upper confidence limit (UCL) of the mean
Spatially weighted average (e.g., Thiessen polygons)
75 percent/10x procedure (Post Remediation)
4. When to use a spatially weighted average
Your impacts must be horizontally and vertically delineated
(to apply any compliance averaging)
Typically used when other methods don’t work
Currently only applies to soil contamination in NJ
Direct Contact Soil Remediation Standards (SRS)
Impact to Groundwater Soil Screening Levels (SSL)
5. How to use a spatially weighted average
1. Define Functional Area (this also applies to 95% UCL)
A. Shape:
Site or AOC boundary, Square (preferred) or Rectangle
B. Total Area:
Max 0.5 acres (Residential Direct contact),
Max 2.0 acres (Non-Res Direct contact),
Min 100 ft length in direction parallel to groundwater flow. (IGW)
C. Depth Zones:
Direct Contact Impact to Groundwater
(Take max in each zone) (Take max in each zone)
6. How to use a spatially weighted average
2. Create Thiessen/Voronoi polygons from point dataset
3: Add field (double) & calculate area for polygons (units are
arbitrary)
4: Add field (double) & multiply area by concentration
5: Spatially weighted mean = the sum of the weighted results
divided by the total area (same units)
6: Evaluate:
If this is less than the applicable standard, you are done!
Else, if this is still above the standard, take out the polygon with
the highest result, and start over at step 2.
Iterate until complete.
7. How to use a spatially weighted average
Example:
8. How to use a spatially weighted average
Example:
9. How to use a spatially weighted average
Example:
10. How to use a spatially weighted average
Example:
11. How to use a spatially weighted average
Example:
12. How to use a spatially weighted average
Example:
Significant reduction in “impacted” area requiring remedial action
13. Why this becomes iterative
Spatial Variation
Vertical Variation
Parameter 1 Parameter 2
Direct Contact Impact to Groundwater
14. Case Study: Trane Trenton, NJ
Operational Manufacturing Site (approx. 30 acres)
Several Historic AOCs
20+ years of soil data
Remediation standard exceedances in direct contact surface and
subsurface as well as impact to groundwater surface and
subsurface.
15. Case Study: Trane Trenton, NJ
“Holistic” data view in AOC-5 (tag map)
16. Case Study: Trane Trenton, NJ
Spatial weighted average results for AOC-5 in Direct Contact
Surface Zone
17. Case Study: Trane Trenton, NJ
Areas excavated based on Thiessen polygons
18. A Novice Python Script
Code Input files (1 per functional area)
Direct Contact
(Surface Zone)
Direct Contact
(Subsurface Zone)
Impact to
groundwater
(Surface)