GIS Spatially Weighted Averages

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

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

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)

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)

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)

 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.

 Example:

 Example:
Significant reduction in “impacted” area requiring remedial action

Why this becomes iterative
Spatial Variation
Vertical Variation
Parameter 1 Parameter 2
Direct Contact Impact to Groundwater

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.

 “Holistic” data view in AOC-5 (tag map)

 Spatial weighted average results for AOC-5 in Direct Contact
Surface Zone

 Areas excavated based on Thiessen polygons

A Novice Python Script
Code Input files (1 per functional area)
Direct Contact
(Surface Zone)
Direct Contact
(Subsurface Zone)
Impact to
groundwater
(Surface)

GIS Spatially Weighted Averages

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