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Case Example: Using a Stratified Sampling Design & Field XRF to Reduce the 95% UCL for Residential Soil Lead. Deana Crumbling, EPA/OSRTI/TIFSD email@example.com 703-603-0643 2009 EPA Annual Quality Conference. What things increase the interval between the sample mean & UCL?.
Deana Crumbling, EPA/OSRTI/TIFSD
2009 EPA Annual Quality Conference
What creates high data variability?
As analytical sample volumes increase, data variability decreases & distribution goes from lognormal to normal
(assumes whole sample is measured)
Physical manipulation of sample, increase volume (MIS) and/or sufficient replicate analysesReduce the UCL by addressing:
By procedures that support:
(Statistical) Stratified Sampling Design
“Dividing by 12” assumes equal weight is given to each sample (1/12th of total area)What Makes a Stratified Design Different?
To calculate average over the entire area, routine practice is that data go straight into a database, and then…
Sum(all) = 2736; then 2736 ÷ 12 = 228 ppm
* 5% of area; ave = 1070 *
20% of area
ave = 153
75% of area
ave = 20
But the CSM supports partitioning the site into 3 distinct portions based on similar populations
20(0.75) + 153(0.20) + 1070(0.05) = 99 ppm
A spatially weighted mean makes a difference!
The CSM is the basis for defining both the DU & its strata
Properties in old town near Pb battery recycling plant
XRF Pb data from bagged soil samples (~300 gram)
Plastic bag of soil
Side Yard: 5 Bagged Samples
Front Yard: 5 Samples
Back Yard: 5 Samples
Preliminary CSM of Simplified Property
Action Level (entire yard) = 500 ppm
Area fraction = 0.25
Area fx = 0.15
Area fx = 0.60
Potential release: Traffic (facility truck, Pb gasoline); Pb house paint; facility’s atmospheric deposition; combination. Expected Pb conc: Higher.
Potential release: Pb paint; atmos dep. Pb conc: Uncertain (near road, house?)
Potential release: Pb paint (near structures); atmos dep. Expected Pb conc: Lower.
NOTE: “Routine” calculation applies same weighting to data points & database loses their spatial representativeness
Note: ½ CI width = mean-to-UCL width
* Normal z-distribution used for the XRF instrument’s
counting statistics, rest of rows use the t-distribution
Deana M. Crumbling, M.S.
U.S. EPA, Office of Superfund Remediation & Technology Innovation
1200 Pennsylvania Ave., NW (5203P)
Washington, DC 20460
PH: (703) 603-0643