Using Geostatistics and Risk Assessment to Minimize Remediation

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Using Geostatistics and Risk Assessment to
Minimize Remediation

• Ronald J. Kotun, Ph.D.
• Tetra Tech

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Site Description

• 85 Acres
• Heavily forested
• Land covered with detritus material
• Ordnance disposal
• Trap and skeet range
• Lead and PAHs
• No lead pellets present
• Reuse Plan states that site is to be maintained
  as a Green Space

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Cleanup Level Development

• Protective of recreational receptors (site-wide
  exposure)
• Infrequent exposure
• Ecological receptors
• Shrew (2-acres)
• Mockingbird (site-wide exposure)
• Based on site-specific data

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Recreational Receptor (Chronic)

• Exposure assumptions
• Exposure Frequency 50 days/yr
• Exposure Duration 20 yr
• Soil Ingestion 50 mg/day
• Body Weight 70 kg
• EPA adult lead model
• Exposure concentration is average lead
  concentration
• CLEANUP LEVEL 3,281 mg/kg

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Ecological Receptors

• Least shrewsurrogate receptor to represent
  mammals that prey upon soil invertebrates
• Northern Mockingbirdsurrogate receptor to
  represent birds that prey upon soil invertebrates
• Literature data available for these species to
  develop PRGs (e.g., food consumption)

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Ecological Receptors (con't)

• CLEANUP LEVEL AVIAN 1,127 mg/kg
• Applied site-wide
• CLEANUP LEVEL SHREW 2,512 mg/kg
• Applied to 2-acre units
• Most conservative values based on a sensitivity
  analysis of variables
• Exposure concentration is average lead
  Concentration

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Site Conditions

• Pre-remediation average 990 mg/kg
• Pre-remediation Pb max 41,400 mg/kg
• Pre-remediation Pb 2 acre-max 8,875 mg/kg
• Recreational (site-wide) 3,281 mg/kg
• Avian (site-wide) 1,127 mg/kg
• Mammalian (2-acre) 2,512 mg/kg
• 2 2-acre units have average Pb gt 2,512 mg/kg
• Remediation to protect mammals would be required
  (remove soil with Pb gt 4,000 mg/kg in these two
  units)

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LeadAcute Toxicity

• Acute toxicity to a small child (single
  high-level exposure)
• Child ingests large quantity of soil (10 g)
• Conducted by University of Florida, EPA,
  Battelle Research Laboratory
• Results in a not-to-exceed Pb of 6,500 mg/kg

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Why an Acute Lead Level

• Children are more sensitive to lead than adults
• Higher soil ingestion rate
• Smaller body weight
• Greater extent of absorption
• Developing nervous system sensitive to lead

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Remediation

• Remove all soil with Pb gt 6,500 mg/kg

Post-Remediation Average 577
mg/kg Post-Remediation Pb max 6,500
mg/kg Recreational (site-wide) 3,281
mg/kg Avian (site-wide) 1,127 mg/kg Mammalian
(2-acre) 2,512 mg/kg
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Remediation Contours

• Determined using geostatistics (kriging)
• Uses the site-specific data to define an area of
  remediation that minimizes the volume (rather
  than connecting the dots)
• Use spatial representation of data to develop
  areas of remediation

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Establish the Variogram

• Structural analysis for quantifying the
  correlation between data points
• All pairs of sampling points are compared to each
  other in order to provide a consistent measure of
  their degree of spatial correlation.
• Graph difference in concentrations vs distance
• Can be done to show directional trends
  (correlation may be stronger in a certain
  direction)

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Results of Variogram Analysis

• Lead samples have a directional trend from
  northwest to southeast
• Consistent with firing range activities (trend
  would be expected)
• Strongest correlation was at 130
• Range of correlation is up to 400 feet
• Based on this information, can begin to estimate
  concentrations at points where no data are
  available

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Pb gt 6500 MG/KG
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Cleanup gt 4000 MG/KG in 2 2-Acre Units to Achieve
Mammalian Value of 2,512 MG/KG
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Remediation Areas

• Geostatistical-Based Remediation 1.8 Acres
• Pb gt3,281 mg/kg (recreational) 3.1 Acres
• Pb gt 400 mg/kg (residential) 32.6 Acres

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Conclusion

• Best use of site-specific data
• Lead concentrations
• Site reuse
• Most likely receptors
• Geostatistics
• Ultimately reduced costs associated with
  remediation with minimal impact to the site