United States
Environmental Protection
Agency
Omaha Soil Mixing Study:
Redistribution of Lead in Remediated
Residential Soils Due to Excavation
or Homeowner Disturbance
Office of Research and Development
Nation Risk Management Research Laboratory
-------
Omaha Soil Mixing Study:
Redistribution of Lead in
Remediated Residential
Soils Due to Excavation or
Homeowner Disturbance.
Omaha Lead Superfund Site, Omaha,
Nebraska
Engineering Technical Support Center
Land Remediation and Pollution Control Division
National Risk Management Research Laboratory
Office of Research and Development
Todd P. Luxton1, Bradley W. Miller2, Edith Holder3,
and Jim Voit1
1U.S. EPA Office of Research and Development, Cincinnati,
OH
2U.S. EPA Office of Criminal Enforcement, Forensics, and
Training, Denver, CO
3Pegasus Technical Services, Cincinnati, OH
Project Officer: Dr. Todd Luxton
26 West Martin Luther King Ave.
Cincinnati, OH 45268
luxton.todd@epa.gov
(513)569-7210
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Notice
This document is intended for internal Agency use only. All research projects making
conclusions or recommendations based on environmental data and funded by the U.S.
Environmental Protection Agency are required to participate in the Agency Quality Assurance
Program. This project was conducted under an approved Quality Assurance Project plan (L-
16422-QP-1-3). The procedures specified in this plan were used without exception. Information
on the plan and documentation of the quality assurance activities and results are available from
Dr. Todd Luxton.
This document presents results from the fiscal years 2011-2012 field investigation at the Omaha
Lead Superfund (OLS) Site to fulfill objectives outlined in the proposal 'Redistribution of Lead
in Remediated Residential Soils Remaining at Depth Due to Excavation or Homeowner
Disturbance' (Dr. Todd Luxton, EPA/ORD and Dr. Bradley Miller, ORISE) Draft 2, March, 15
2011 for the Superfund Remedial Project Manager, Don Bahnke (EPA/Region 7) and Robert
Weber (EPA/ORD). The purpose of the study was to investigate the redistribution of Pb in
remediated residential soils after "normal" home owner excavation within the OLS.
11
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Forward
The US Environmental Protection Agency (US EPA) is charged by Congress with protecting
the Nation's land, air, and water resources. Under a mandate of national environmental laws,
the Agency strives to formulate and implement actions leading to a compatible balance
between human activities and the ability of natural systems to support and nurture life. To meet
this mandate, US EPA's research program is providing data and technical support for solving
environmental problems today and building a science knowledge base necessary to manage
our ecological resources wisely, understand how pollutants affect our health, and prevent or
reduce environmental risks in the future.
The National Risk Management Research Laboratory (NRMRL) is the Agency's center for
investigation of technological and management approaches for preventing and reducing risks
from pollution that threaten human health and the environment. The focus of the Laboratory's
research program is on methods and their cost-effectiveness for prevention and control of
pollution to air, land, water, and subsurface resources; protection of water quality in public
water systems; remediation of contaminated sites, sediments and ground water; prevention and
control of indoor air pollution; and restoration of ecosystems. NRMRL collaborates with both
public and private sector partners to foster technologies that reduce the cost of compliance and
to anticipate emerging problems. NRMRL's research provides solutions to environmental
problems by: developing and promoting technologies that protect and improve the
environment; advancing scientific and engineering information to support regulatory and
policy decisions; and providing the technical support and information transfer to ensure
implementation of environmental regulations and strategies at the national, state, and
community levels.
This publication has been produced as part of the Laboratory's strategic long-term research
plan. It is published and made available by US EPA's Office of Research and Development to
assist the user community and to link researchers with their clients.
Cynthia Sonich-Mullin, Director
National Risk Management Research Laboratory
in
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Abstract
Urban soils within the Omaha Lead Superfund (OLS) Site have been contaminated with lead (Pb)
from atmospheric deposition of particulate materials from lead smelting and recycling activities.
In May of 2009 the Final Record of Decision stated that any residential soil exceeding the
preliminary remediation goal (PRG; 400 mgpb kg'^oii) would be excavated, backfilled and re-
vegetated. The remedial action entailed excavating contaminated soil in the top 12 inches and
excavation could stop when the concentration of soil Pb was less than 400 mg kg"1 in the top 12
inches, or less than 1200 mg kg"1 at depths greater than 1 ft. After removal of the contaminated
soil, clean backfill was applied and a grass lawn was replanted. A depth of 12 inches was based
on the assumption that Pb-contaminated soil at depth greater than 1 ft would not represent a future
risk (ASTDR Health Consult, 2004). This assumption was based on the principal that mixing and
other factors encountered during normal excavation practices would not result in Pb surface
concentrations greater than the PRG.
The goal of the current study was to investigate the redistribution of Pb in remediated residential
surface soils after typical homeowner earth-disturbing activities in the OLS Site. Of specific
interest to the region for protection of human health is determining whether soil mixing associated
with normal homeowner excavation practices results in surface Pb concentrations greater than the
preliminary remediation goal (PRG) (400 mgpb kg'^oii). Results from the 18 properties
investigated indicate that when the concentration of Pb was less than 1200 mgpb kg'^oii below 12
inches, the surface concentration of Pb remained below 400 mgpb kg'^oii.
IV
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Contents
Notice ii
Forward iii
Abstract iv
Contents v
List of Tables vii
List of Figures viii
Supplemental Figures ix
Acronyms x
Acknowledgements xi
Executive Summary xii
1. Introduction 1
2. Study Site Selection 4
3. Site Locations 4
4. Methods/Materials 6
4.1. Field Methods 6
Soil Cores and Processing 8
4.2. Laboratory Analysis 9
4.3. Lead X-ray Absorption Fine Structure Spectroscopy 10
5. Results 11
5.1. Soil Samples Types 11
5.2. Pre-Excavation Soil Lead Distribution 11
5.3. Lead Spoil Concentrations 16
Post excavation Soil Lead Distribution 17
5.4. Soil Properties 18
5.5. Elemental Correlations 20
5.6. Lead Speciation 20
6. Discussion 24
6.1. Statistical Analysis of Soil Mixing 24
6.2. Frequency Analysis 33
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
6.3. Bioavailability/Bioaccessibility 38
7. Summary/Conclusions 39
References 42
Supplemental Figures 43
Appendices 46
Appendix 1. Sample Data Sets 46
Appendix 2. Correlation Figures 46
VI
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
List of Tables
Table 3.1 Property addresses, remediation quadrants, excavation depth, and Pb concentration at
depth 5
Table 5.1 Average total elemental concentrations for the fill material and underlying soil. Data
used in the calculation of the average elemental concentration and the t-test does not include soil
samples collected from Properties 2, 5, 12, and!6, a=0.05 19
Table 5.2 Linear Combination Fitting Results for the normalized and first derivative of the Pb L3
XANES spectra. Ang = Anglesite, Hy-Py = Hydroxypyromorphite, and Pb-Ferr = Plumboferrite
22
Table 6.1 Correlation coefficients for the predicted elemental concentrations in the excavated
material based on the geometric model as a function of the average concentration of specific
elements in the excavated spoil 28
Table 6.2 Post Homeowner and Standardized excavation soil total Pb concentrations in the top 1,
6, 12, 18, and 24 inches of the soil profile. Bolded numbers indicate where the soil lead
concentration exceeds 400 mg kg"1 34
Table 6.3 Frequency analysis of the number of properties that exceeded Pb concentrations of 400
mg kg"1 in either the top 1,6, 12, 18, or 24 inches. For all of the properties sampled, properties
where the soil Pb concentration exceeded 1200 mg kg"1 according to the OLS database, and
properties where the maximum soil Pb concentration exceeded 1200 mg kg"1 based on the
current study. Data corresponding to the frequency that 400 mg kg"1 was exceeded in the top 12
inches in italics. Bolded heading refer to the type of soils included in the analysis 36
vn
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
List of Figures
Figure 1.1 A) Visual barrier placed at 12 inches to indicate underlying soil exceeds 1200 mg kg"
*, B) Image showing the remediation interface and the visual distinction between the clean soil
and intact subsoil 2
Figure 3.1 Map showing the locations of the sampled residential properties (red markers) and the
former Arcos Lead recycling and smelting facility. The Numbers refer to the four grouping of
properties investigated. A list of properties within each grouping is listed in Table 1 6
Figure 4.1 Schematic representation of the Standard Excavation Technique. A. First undisturbed
soil core was collected. B. After soil core was collected a 2-person auger was used to dig a hole
in the same location as undisturbed soil core. C. Soil core collected from backfilled hole 7
Figure 4.2 Schematic representation of the Homeowner Excavation Technique. A. First
undisturbed soil core was collected. B. After soil core was collected a spade was used to dig a
hole in the same location as undisturbed soil core. C. Soil core collected from backfilled hole.... 8
Figure 4.3 Intact soil core collected pre-excavation. Visual changes in soil color or texture were
used to identify the remediation interface when the barrier was not present 9
Figure 5.1 Soil Pb concentration as a function of depth for each of the excavational units pre- and
post-excavation. Numbers refer to the property address/location. Letters refer to duplicate
samples collected from the same property. For property 1, replicate A the -A and -B refer to
samples collected within three feet of each other. Dashed line indicates the location of the
remediation interface 13
Figure 5.2 Average maximum concentration of lead and depth at which maximum concentration
occurs for the four different geographical locations 16
Figure 5.3 Soil Pb concentration in the top 18 or 24 inches of the pre-excavation soil as a
function of the Pb concentration in the spoil. The concentration in the soil was determined from
calculating the average concentration of Pb in the soil pre-excavation form the soil core data... 17
Figure 5.4 Normalized Pb L3 XANES spectra and the first derivative of the XANES spectra.
Dashed lines indicate highlight areas of where differences exist in the location, shape, and/or
presence of spectral features 21
Figure 5.5 Linear combination fit results for a sub soil sample collected prior to excavation at
residential Property 1 23
Figure 5.6 Relative abundance of hydroxypyromorphite in soil samples, from the LCF analysis,
as a function of the P:Pb molar ratio. An exponential function was used for the nonlinear
regression of the data: Adjusted R2 = 0.60, p-value = 0.016 23
Figure 6.1 Schematic representation of soil mixing. In the figure the green blocks designate the
clean soil, the red line the remediation interface, and the red blocks the un-remediated soil. The
brackets next to the Post-Disturbance column indicate different random samples taken. The
groupings of blocks contained within the brackets differ based on the redistributed soil
compartments. The red line indicates the location of the remediation interface 25
Vlll
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Figure 6.2 A Schematic Representation of the geometry of the standard and Homeowner
excavations. The image illustrates how the geometry of the hole will impact the mixing of clean
and un-remediated soil 26
Figure 6.3 Expected fraction of contaminated or "un-remediated" soil based upon geometry of
the excavation. The excavated fraction of un-remediated soil versus ratio of depth of excavation
to remediation interface 26
Figure 6.4 The predicted concentration of soil Pb, based on the geometric model, as a function of
the average concentration of Pb in the excavated spoil for the standard and Homeowner
excavation techniques. Dashed line indicates a 1:1 correlation. Correlation values are presented
in Table 3 28
Figure 6.5 Dot plots for the results predicted and actual values for the analyte concentration in
the spoil for the geometric model. Each dot represents a single sample location and a single
predicted value for a sample location. H indicates the Homeowner excavation method, S
indicates the standard method, 1 indicates the surface/clean soil analyte concentration, and 2
indicates the subsurface/un-remediated analyte concentration 29
Figure 6.6 Estimated fraction of soil mixing as a function of depth and geometry based on the Pb
soil concentrations in the post soil core data. A) Standard Excavation Technique. B)
Homeowner Excavation technique 32
Supplemental Figures
Supplemental Figure 1 Standardized excavation used a gas powered posthole digger with spoil
material brought to soil surface. Notice loss of soil structure 43
Supplemental Figure 2 The Homeowner technique used a traditional garden spade to excavate a
hole 18 inches in depth. Notice large soil clods 44
Supplemental Figure 3 JMC Environmentalist's Sub-Soil Probe. Soil core tube was driven into
the soil by hand and withdrawn using a jack 45
IX
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Acronyms
The following acronyms appear frequently in the report and are listed in alphabetical order.
Bl Initial soil surface Pb soil concentration
B2 Bottom of the excavation hole Pb soil concentration
B3 Excavated spoil Pb soil concentration
B4 Final Soil Surface Pb soil concentration
CERCLA Comprehensive Environmental Response Compensation and Liability Act
ICP-AES Inductively Coupled Plasma Atomic Emission Spectroscopy
LCF Linear Combination Fitting
LCRSH Lead Contaminated Residential Site Handbook
OLS Omaha Lead Superfund Site
Pb Lead
PRG Preliminary Remediation goal
QAPP Quality Assurance Proj ect Plan
USEPA United States Environmental Protection Agency
XRD X-ray Diffraction
XAFS X-ray Absorption Fine Structure
XANES X-ray Absorption Near Edge Structure
XRF X-ray Fluorescence Spectroscopy
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Acknowledgements
The following individuals are acknowledged for assistance in field sampling, report preparation,
and laboratory analysis: Jenifer Goetz (USEPA, ORD), Donald Bahnke (USEPA, Region 7),
Robert Weber (USEPA, ORD) Daniel Garvey (USEPA, Region 7), Nicole Bujalski (former
USEPA, Region 2), Personnel with Pegasus Technical Services, Inc. under Contract Number Ep-
C-l 1-006, and data analysis and statistical support by John Carson (CB&I/Shaw).
XI
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Executive Summary
Urban soils within the Omaha Lead Superfund Site have been contaminated with lead (Pb) from
atmospheric deposition of particulate materials from lead smelting and recycling activities. In
May of 2009 the Final Record of Decision stated that any residential soil exceeding the preliminary
remediation goal (PRG) of 400 milligrams of lead per kilogram of soil (mgpb kg'^oii) would be
excavated, backfilled and re-vegetated. The remedial action entailed excavating contaminated soil
in the top 12 inches (1 foot) and excavation could stop when the concentration of soil Pb was less
than 400 mg kg"1 in the top foot, or less than 1200 mg kg"1 at depths greater than a foot (ft). After
removal of the contaminated soil, clean backfill was applied and a grass lawn was replanted
The current study investigated the degree of soil mixing and the redistribution of lead within the
soil profile after typical Homeowner earth-disturbing or excavation activities. Two methods were
employed to evaluate soil mixing. The first method (Homeowner) entailed excavating soil using
a common garden spade to a depth of 18 inches. The second (Standard) used a two-man auger to
excavate a hole 2 ft in depth. Prior to excavation surface soil samples and an undisturbed soil core
were collected to determine the initial concentration and distribution of lead with in the soil. After
excavation a composite soil sample was collected from the spoil pile prior to backfilling the hole.
A final soil core was collected to evaluate the redistribution of within the soil profile, and a final
surface soil sample. Samples were dried and digested following EPA method 305la.
Data were collected from 18 properties, including duplicate and triplicate samples taken from
properties to evaluate soil heterogeneity within a single remediation quadrant. The results from
the duplicate and triplicate sampling were mixed. Two of the properties exhibited a large degree
of variability between the duplicate and triplicate samples. The other two properties showed a
similar distribution and concentration of within the soil profile. Results from the soil analysis of
the pre-excavation soil cores revealed a stark contrast in the soil lead concentration at the
remediation interface. Of the 18 properties sampled 17 were previously remediated. Seven of the
14 properties had maximum lead concentration at the remediation interface. The remaining 7
properties show that soil Pb concentrations increased below the remediation interface.
Post excavation cores revealed a heterogeneous distribution of lead within the soil profile
demonstrating that the soils were not well mixed. Attempts to model soil mixing based on soil Pb
concentrations and the geometry of the excavation failed due to the high variance in the data. A
future study expanding the number of properties sampled and adding composite samples to the
current study design may reduce the high levels of variance in the data. Based on the statistical
analysis, the current study should be viewed as a pilot study. The high variances within the data
make it impossible to derive any conclusions on the degree of soil mixing based on soil excavation
or disturbance.
xn
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
A frequency analysis showed that for all of the properties sampled and both excavation techniques
(n= 40), 5 or 12.5% of the samples had soil lead concentrations in excess of 400 mg kg"1 in the top
12 inches of soil after excavation and backfilling. Of the 5 properties with elevated soil lead
concentrations 3 were from the Homeowner excavation technique and two from the Standard
excavation technique. Frequency analysis limited to properties with soil lead concentration > 1200
mg kg-1at depth showed that intrusion into the contaminated soil resulted in surface soil lead
concentrations exceeding 400 mg kg"1 at three of 11 sites. However, the post surface lead
concentration did not exceed 400 mg kg"1 of lead when the maximum lead concentration at depth
was less than 1200 mg kg"1.
Lead speciation data obtained from X-ray absorption fine structure (XAFS) spectroscopy analysis
of the soils indicates that the predominant lead species present in the soils analyzed included:
anglesite, hydroxypyromorphite, plumb oferrite, and galena. Hydroxypyromorphite is not a
constituent/lead species associated with atmospheric emissions from lead smelting or recycling
activities. It is most likely that the lead phosphate mineral formed in the soil. Soil phosphorus to
lead (P:Pb) molar ratios were between 4 and 14. This would indicate there was ample P available
for the in-situ formation of phosphates. The precipitation of the lead phosphates is extremely
advantageous due to the low bioavailability of lead from pyromorphites. The precipitation of the
phase is effectively offering a secondary remediation technology by binding the lead in place in
an extremely low bioaccessible form.
Xlll
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
1. Introduction
The Omaha Lead Superfund Site (OLS; Site [CERCLIS ID # NESFN0703481]) was first
investigated by the Environmental Protection Agency (USEPA) in 1998 under the authority of the
Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) after the
Omaha City Counsel petitioned the EPA to address the frequency of elevated blood lead (Pb)
levels in children (U.S. EPA-OLS, 2009). The source of the Pb was attributed to parti culate matter
released from former Pb smelting and refining activities over the past 125 years. The subsequent
deposition of expelled particulates contaminated soils in the residential area surrounding the
former smelting facilities. EPA began sampling soils on residential properties and licensed child
care providers in March of 1999. By August, a response action was initiated under CERCLA and
EPA began excavating and replacing soil contaminated with Pb at concentrations exceeding 400
mg kg"1 at child care facilities and residences where children with elevated blood levels resided
(U.S. EPA-OLS, 2009). In 2002 a second removal action was enacted at residential properties
where the soil Pb levels exceeded 2,500 mg kg"1 in soils not adjacent to foundational structures.
The OLS was placed on the National Priorities list in 2003 and the extent of the contamination
was determined. The final area encompassed in the OLS consisted of 25.7 square miles with an
estimated 16,000 homes that could exceed 400 mg kg"1 of Pb. In May of 2009 the Final Record
of Decision Declaration for the Omaha Pb Site stated that any residential soil exceeding the
preliminary remediation goal (PRG; 400 mgpb kg'^oii) would be excavated, backfilled and re-
vegetated.
Briefly, residential properties were divided into sections. Soils in each section were sampled
according to the procedures described in the Superfund Lead-Contaminated Residential Sites
Handbook (LCRSH) (OSWER Directive 9285.7-50, 2003). A single multi-composite sample was
collected from each section and analyzed by handheld X-ray Fluorescence spectroscopy (XRF).
If one or more of the residential yard sections exceeded the PRG, the property was eligible for
EPA response and remediation. Remedial action entailed excavating soils where the Pb
concentration exceeded 400 mg kg"1 and replacing the excavated material with clean soil.
Excavation and removal of the soil continued until the concentration of Pb at the soil surface was
less than 400 mg kg"1 in the top 12 in or less than 1200 mg kg"1 at depths greater than 1 ft. When
Pb soil concentrations exceeded 1200 mg kg"1 at one foot a visual barrier was installed (Figure
1.1). After excavation, clean soil was backfilled into the excavated areas to the original grade and
a grass lawn was restored.
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Figure 1.1 A) Visual barrier placed at 12 inches to indicate underlying soil exceeds 1200 mg kg"1, B)
Image showing the remediation interface and the visual distinction between the clean soil and intact
subsoil.
In addition to the active remedial measures a Pb hazard registry was established as an institutional
control. The registry provides information to the public about conditions at specific properties, Pb
hazard information, and the current status of EPA investigations and response actions.
Excavation of contaminated soils and replacement with a soil cover is the preferred method
recommended by the LCRSH for relatively shallow contamination of Pb typically associated with
smelter sites (OSWER Directive 9285.7-50, 2003). However, when integrity of the remedial
action is compromised, exposure to soil Pb levels in excess of the PRG (400 mg kg"1) is possible.
Any excavation event in soils where the Pb concentration exceeds the PRG at depths greater than
the remediation depth may result in exposure to elevated soil Pb concentrations. Numerous
excavation activities routinely performed by residential property owners may comprise the
integrity of the of the soil cap by excavating soils below the remediation depth. Activities resulting
in the excavation of soils up to 36 inches deep and intrusion into the contaminated soils
(remediation interface at 12 in. deep) include, but are not limited to: landscaping (planting trees
and bushes), establishing a garden, fence installation, construction of an outdoor structure (deck
or garage), home addition/remodel, and installation of a driveway. A "normal" Homeowner
excavation would likely entail the use of a shovel or other hand tool for excavation while larger
projects would require the use of mechanical equipment.
Following any excavation several scenarios exist for the excavated soil:
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
1. Soil is removed and disposed of offsite
2. Soil is back filled into the excavated hole
3. Spoil pile is left at the surface
4. Soil is relocated to another location or spread over another portion of the property.
The best option would be scenario 1, this would offer the most protection against the potential
exposure of elevated soil Pb concentrations. However, scenarios 2-4 are more likely to occur. The
result of the scenarios 2-4 is the potential exposure to elevated concentrations of Pb in soils.
Exposure to elevated soil Pb concentrations will largely be dependent on how the soil is mixed
during excavation. If there is a high degree of mixing that occurs during the excavation then the
higher concentrations of Pb remaining at depth will be diluted. However, if there is little to no
mixing during excavation, then pockets of soil will exist with Pb concentrations in excess of the
PRO.
Soil mixing due to Homeowner excavation is expected to be highly variable based upon the type
of soil encountered (texture, water content, and presence of coarse fragments). Differences in
properties will result in different quantities of soil being removed by a shovel or other conventional
excavation tool. Further complicating the matter is the operator method (shoveling technique) and
operator strength (gender and/or age). Both of these variables may introduce a large degree of
variability in the natural mixing that occurs during excavation, and replacement of soil after
excavation. To help account for variability in soil composition and operator variability, the current
study employed the use of a Standardized excavation technique (gas powered auger) in addition
to the traditional Homeowner excavation (shovel). The Standardized excavation technique in this
study used a gas powered posthole digger with a 10 in diameter bit (earthen drill), while the
Homeowner excavation used a traditional garden spade. The Standardized excavation technique
will: (1) provide holes with standard dimensions (2) have minimal impact on operator variability,
(3) help reduce mixing variability from soils with different physical properties, and (4) standardize
how the excavated soil was removed from the hole and how it was deposited on the surface.
Currently, there has been no research investigating the potential hazards associated with the re-
distribution of Pb contaminated soils beneath a soil cap following intrusion into the contaminated
zone. Therefore, the goal of the study was to investigate the redistribution ofPb in remediated
residential surface soils after typical Homeowner earth-disturbing activities in the Omaha SF Site.
Of specific interest was determining whether soil mixing associated with normal Homeowner
excavation practices results in surface Pb concentrations (top inch) greater than the PRG (400
mg kg'1 ofPb).
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
2. Study Site Selection
The residential properties selected for the current study were previously remediated following the
protocol outlined in the Final Record of Decision for the Omaha Lead Site Operable Unit 2
Decision Summary (May 2009). The selection criteria for residential properties in the current study
were based on the concentration of soil Pb (mg kg"1) 12 inches (1 foot) deep as reported in the
Omaha Lead database, logistical constraints for sampling at each location, and property owner
permission. Residential properties with soil Pb concentrations greater than 800 mg kg"1 were
identified in the soil Pb database. Logistical constraints at each site included the collection of soil
samples from areas that were at least 6.5 ft (2 meters, or 2 m) away from the foundation of a
building, structure, fence, paved surface (road, sidewalk, driveway, or front walk), garden/flower
beds, and playground equipment. Soil sampling locations were also located at least 2 m away
from the drip zone of building and other structures. Finally, property owners were contacted by
either Region 7 staffer the Omaha Lead Superfund contractor in the form of a letter or phone call
to request permission for property access.
Based on the site selection process 16 properties were identified for inclusion in this study (Figure
2.1 and Table 2.1). Of the properties identified four (1, 3, 7, 9) were located on vacant lots. In
addition to the 16 properties identified, a vacant lot that had not been remediated was included.
In order to evaluate quadrant heterogeneity, four of the sites were sampled multiple times (3, 4, 7,
1).
3. Site Locations
The residential properties sampled were located within 1 mile (east or west) of U.S. Route 75 and
extended throughout the entire OLS site. The properties were grouped into 4 geographical
locations based on their proximity to each other (Figure 2.1). The groupings were used to
determine if Pb soil concentrations were related to geographical location. Only one property north
of the former smelting observations met all of the sampling criteria, and is the only property in
Group A (Figure 2.1). South of Site 4 there was a grouping of 9 residential properties all less than
a mile from each other (Group B). The third grouping of 5 properties was located further south
(Group C) and these properties were more dispersed than the second group, but still within a mile
of each other. The final two, most southern properties, encompass Group D.
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Table 3.1 Property addresses, remediation quadrants, excavation depth, and Pb concentration at depth.
Experimental
Unit
1
2
O
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Sample
ID
27966
27598
27595
36021
20953
47857
22393
27507
29827
28971
26975
27319
26807
12299
2289
18470
20319
94631
Address
2113 Grant St*§
2524PatrickAve**
2528 Patrick Ave§
2611 Browne St§
11 12 South 3 1st St
3126RSt
704 South 25th Ave*§
2033 North 20th St
25 18 Maple St*
2238 Ohio St
2094 Parker Cr
2422 Blondo St
2921 Parker St
2621 E ST
2530 Patrick Ave
19 14 South 36th St
15 16 William St
2210 South 13th St
Quad
Bl
Bl
Bl
Bl
F2
Bl
Bl
Bl
B2
B2
B2
F2
Bl
Fl
B2
Bl
Bl
F2
Group^
B
B
B
A
C
D
C
B
B
B
B
B
B
D
B
C
C
C
Excavation
Depth
(inches)^
13
N/A
13
13
6
12
13
12
13
12
13
12
13
12
12
13
12
17
Soil Pb
Concentration at
Excavation Depth
(mgkg1)"
1016
129
980
1016
118
961
938
840
981
1073
1075
920
806
1037
1530
866
1237
816
tRefers to the property grouping identified in Figure 2
^Values obtained from the Omaha Soil Lead database
*Vacant Lot
**Site 2 was not previously remediated.
locations where replicate samples were collected
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Figure 2.1 Map showing the locations of the sampled residential properties (red markers) and the
former Arcos Lead recycling and smelting facility. The Numbers refer to the four grouping of
properties investigated. A list of properties within each grouping is listed in Table 1.
4. Methods/Materials
4.1. Field Methods
Sampling Locations
Soil samples collected were obtained from OLS residential yards that have already undergone
remediation. Exclusion zones were delineated as at least 6.5 ft (2 m) from the foundation of any
building or fence and 6.5 ft from paved surfaces (road, sidewalk, driveway, or front walk). Soil
samples were collected outside the exclusion area to prevent sampling of secondary Pb sources
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
(e.g., Pb paint chips, contaminates from roads). The locations sampled within a remediated OLS
residential yard were chosen randomly outside the exclusion zone, and all locations were recorded.
Soil Mixing Techniques
Two mixing techniques were tested at each of the yards, Standardized and Homeowner. The two
different sampling locations for each technique were located at least 1.5 m (5 ft) apart to prevent
cross contamination. All excavation equipment was cleaned and dried prior to sampling to remove
any adhering soil or debris.
The Standardized excavation was performed using a gas powered 2-person auger with a 10 inch
diameter bit. The auger was used to excavate the soil to a depth of 24 inches (Figure 4.1 and
Supplemental Figure 1). Prior to excavation a 14 ft2 plastic board, with an 11 inch inner diameter
hole, was laid on the ground to catch excavated soil (Figure 4.1). Then the soil surface was cleared
of grass or debris and a soil sample was collected to a depth of 1 inch, followed by auguring to 24
inches. After excavation a subsample from each of the four quadrants of the plastic board were
collected for analyses. A grab sample from the bottom of the pit was also collected. The depth to
the remediation interface was measured and the excavated soil was returned to the hole by hand or
with a spade. The soil was lightly compacted to bring the back filled soil level with the yard. A
soil core followed by bulk surface soil samples (1 in) was then collected.
Undisturbed Soil Core
Soil Core
Samples
4 inch
Sections
Remediation
Interface
Samples
Soil
Sample
To Be
Bulked
Figure 4.1 Schematic representation of the Standard Excavation Technique. A. First undisturbed soil
core was collected. B. After soil core was collected a 2-person auger was used to dig a hole in the same
location as undisturbed soil core. C. Soil core collected from backfilled hole.
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
The Homeowner excavation was performed with a standard 5' tall garden spade. Prior to digging
a 3 ft2 plastic board was placed adjacent to the sample area (Figure 4.2 and Supplemental Figure
2). The soil surface was cleared of grass or debris and a soil sample was collected to a depth of 1
in. An 18 inch diameter hole was dug with the garden spade and all material was deposited on the
plastic board (Figure 4.2). After excavation a subsample from each of the four quadrants of the
plastic board were collected for analyses. A grab sample from the bottom of the pit was also
collected. The depth to the remediation interface was measured and the excavated soil was returned
to the hole by hand or with a spade. The soil was lightly compacted to bring the back filled soil
level with the yard. A soil core was collected followed by bulk surface soil samples (1 in).
Soil
Samples To
Be Bulked
1 in Surface
Soil Sample
Original
Remediation
Interface
Refilled
Figure 4.2 Schematic representation of the Homeowner Excavation Technique. A. First undisturbed
soil core was collected. B. After soil core was collected a spade was used to dig a hole in the same
location as undisturbed soil core. C. Soil core collected from backfilled hole.
Soil Cores and Processing
Undisturbed soil cores were also collected from each site. All soil cores were collected using a
JMC Environmentalist's Sub-Soil Probe using a "kick style" soil probe (Supplemental Figure 3).
Each core was collected using al inch (2.5 cm) diameter transparent plastic tube driven into the
ground encased within a steel tube to a depth of 36 in. The soil cores were extracted from the steel
probe, capped, sealed, and labeled (Figure 4.3). The depth to remediation interface was
immediately identified and delineated on the plastic tube from the undisturbed soil cores. The
remediation interface was identifiable based upon changes in soil color and texture. After the soil
core was removed, the depth to the bottom of the excavated soil core hole was measured as well
as length of each soil core. The measurements were conducted to account for any soil compaction
due to sample collection. Compaction was determined by dividing the length of the recovered soil
core by sampled depth. For example, if the recovered soil core is 32 inches, and the sampled depth
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
is 36 inches the sample compaction is 11% (l-(32/36)*100 = 11%). All cores were then shipped
to the EPA's Center Hill laboratory in Cincinnati, OH. Soil cores received at the Center Hill
laboratory were placed in a -80°C freezer. Soil depths, on the cores, were delineated as surface (1
in) deep, and then portioned according to depth of remediation interface and corrected for soil
compaction. The frozen cores were then cut into sections with a band saw, transferred to plastic
bags and air-dried.
Soil Cap
Remediation
Interface
Subsoil
Figure 4.3 Intact soil core collected pre-excavation. Visual changes in soil color or texture were used
to identify the remediation interface when the barrier was not present.
All soil samples, bulk samples and sections from each soil core, were force air dried at 105°C,
fractured with a mortar and pestle, and passed through an ASTM No. 10 sieve to isolate the < 2mm
fraction. The soils were homogenized and stored in plastic containers until chemical analyses.
Three replicate subsamples from each air-dried soil were collected and dried at 105°C for moisture
corrections.
4.2. Laboratory Analysis
Extractable soil Pb concentrations were quantified following the procedures of EPA Method
3051 A. Triplicate 0.5 gram subsamples of the homogenized soil were added to microwave
digestion vessels to determine elemental content. Next, 9 ml concentrated nitric acid and 3 ml of
concentrated hydrochloric acid were added to each the vessel. The samples were then subjected
to microwave assisted digestion using a MARS Express Microwave system with temperature
monitoring. The digestates were quantitatively transferred to a volumetric vessel and brought to
final volume with double distilled water (< 18.2 MOhms) for ICP-AES (Inductively Coupled
Plasma-Atomic Emission Spectroscopy) analyses. Quality control and quality assurance protocols
were followed as outlined in the project QAPP and EPA Method 3051 A.
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
The digestates were analyzed for Al, As, Ca, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, Pb, S and Zn
concentration by ICP-AES. Quality control and quality assurance protocols were followed as
outlined in the project QAPP and according to the procedures of EPA Method 6010C. If the
relative standard deviation of the concentration of Pb from a soil sample exceeded 10%, the results
were discarded and triplicate soil samples were digested and analyzed again.
4.3. Lead X-ray Absorption Fine Structure Spectroscopy
Lead speciation, in a sub set of the soils collected from residential properties, was determined
using X-ray absorption fine structure (XAFS) spectroscopy. Lead L3-edge (13, 035 eV) X-ray
absorption spectra (XAS) were collected at the Materials Research Collaborative Access Team's
(MRCAT) beamline 10-ID, Sector 10 at the Advanced Photon Source at the Argonne National
Laboratory, Argonne, IL. The electron storage ring was operated in top-up mode at 7 GeV. Spectra
were collected in fluorescence mode with either a Lytle or a solid-state silicon drift detector at
room temperature. The samples were prepared using, the < 250 |j,m soil fraction, as thin pellets
using an IR pellet press and samples were secured to sample holders using Kapton tape. For each
sample, a total of fifteen to seventeen scans were collected using the Lytle detector or 4 to 5 scans
using the solid state detector and averaged. Data were analyzed using the Athena software program
(Ravel andNewville, 2005). Sample spectra were compared with synthesized minerals and mineral
specimens acquired from the Smithsonian National Museum of Natural History (USA). All
minerals were verified with X-ray Diffraction (XRD) before use as reference materials.
Soil Pb speciation was determined by comparison of Pb standards to the field samples via Linear
Combination Fitting (LCF). Linear Combination Fitting refers to the process of selecting a
multiple component fitting function with a least-squares algorithm that minimizes the sum of the
squares of residuals. A fit range of -20 to 50 eV was utilized for the X-ray absorption near-edge
structure (XANES) portion of the XAFS spectra and up to four variables. The best fitting scenarios
are determined by the smallest residual error (/2) and the sum of all component fractions being
close to 1. Detailed descriptions of the fitting procedure are described elsewhere (Isaure et al.,
2002; Roberts et al., 2002, Scheinost et al., 2002). The reference samples used in the LCF model
were plumboferrite (PbFe/tO?), plumbonacrite (PbsO(OH)2(CO3)3), chloropyromorphite
(Pb5(PO4)3Cl), Pb-sorbed to hydroxyapatite complex (Ca5Pb5(PO4)6(OH», galena (PbS),
hydrocerussite (Pb3(CO3)2(OH)2), anglesite (PbSCU), plumbomagnetite (PbFe2O4), litharge (PbO),
lead hydroxide (Pb(OH)2), and Pb sorbed to humic and fulvic acids, goethite, gibbsite, kaolinite,
bentonite, and calcite.
10
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
5. Results
5.1. Soil Samples Types
Soil samples types were used to explore different potential end points for the excavated soil.
Previously 4 potential outcomes were identified. The soil samples collected were designed to
establish the pre-excavation conditions and to mimic different post excavation scenarios.
Two types of soil samples were collected in the field, bulk and core. Bulk soil samples were
collected as loose soil from four locations: initial soil surface (Bl), bottom of the excavation hole
(B2), excavated spoil (B3), and final soil surface (B4). The two surface bulk soil samples (Bl and
4) were collected to compare the concentration of Pb measured in the surface core sample and the
bulk sample. To ensure enough soil, free of vegetation/plant debris, was collected for analysis
purposes a bulk sample was collected from the top inch before and after excavation. The spoil
sample (B2) provided an average soil Pb concentration in the excavated material and mimicked
potential end point scenarios 3 and 4 (spoil left in place, and soil is relocated to another or spread
over another portion of the property). As discussed in the Material and Methods section, the B2
sample was thoroughly mixed in the field prior to collection. Finally the Bulk sample collected
from the bottom of the excavation was used to determine if the soil Pb concentration at the bottom
of the hole was less than the maximum soil Pb concentration in the excavated material.
Core soil samples were sections were collected from the intact soil cores at specific depth intervals
identified in the Materials and Methods. The soil core data provided detailed information
regarding the distribution and mixing of Pb throughout the soil profile pre and post excavation.
The post excavation core also mimicked scenario 2, backfilling of the soil.
5.2. Pre-Excavation Soil Lead Distribution
The total Pb concentration in the soil is presented as Pb soil concentration (mg kg"1). A profile of
extractable soil Pb concentrations by depth for the 18 properties investigated are presented in
Figure 5.1. Soil Pb concentration in Figure 5.1 was corrected for soil core compaction. As
expected, soil Pb concentrations at the surface were low pre-excavation. In general soil Pb
concentrations increased at the remediation interface and decreased with depth. The soil Pb profile
for the remediated lots were characterized by a dramatic increase in soil Pb concentration at or
near the visually identified remediation interface (Figure 5.1). The maximum soil Pb concentration
occurred either at or below the remediation interface and decreased with depth. The drastic change
was not present for properties 5,12, and 16. Following the same trend as the remediated properties,
Property 2, un-remediated, exhibited a significant decrease in soil Pb concentration at a depth
similar to the remediated soil.
11
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Property 5 differed from 2 in that the entire profile had a similar soil Pb concentration near 25 mg
kg"1. Interestingly the highest concentration of Pb was at the soil surface.
The concentration of soil Pb at Properties 5, 12 and 16 did not exhibit the same distribution as the
other remediated properties. For each of these three sites there was no apparent remediation
interface identified by the soil Pb concentrations. Visually there was no discernible remediation
interface observed for any of the three properties. The Omaha Lead data base indicates that the
quadrants sampled for Properties 5, 12 and 16 had previously been remediated and that the soil Pb
concentrations at 12 and 13 inches for Property 12 and 16 were 912 and 866 mg kg"1, respectively.
As evident from Figure 6 the maximum Pb concentration in the soil was significantly less than 60
and 225 mg kg"1 for 12 and 16 respectively, indicating that no additional contamination was left at
depth.
In general the visual appearance of the remediation interface was in good agreement with the
chemical data. The soil Pb concentration for a portion of the properties continued to increase with
depth (1, 10, 11, 13, 15, 16, 17, and 18) which encompassed geographical units (B, C, and D) The
principal source of Pb contamination in soils within the Omaha Lead superfund site is atmospheric
deposition of Pb particulate matter. The elevated concentration of Pb in the sampled soils at depth,
and the increasing soil Pb concentration with depth, would suggest that a portion of the Pb
deposited on the soil surface is mobile and is currently being leached through the soil profile. Lead
has a high affinity for organic matter, and hydrous (oxide) minerals and is generally considered
immobile in the soil. However, previous research has shown that significant quantities of Pb may
be leached through a soil profile under specific geochemical conditions and mineralogical
properties. (Kim et al. 2008).
12
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
1C
0 100200300400500600700
Pb (mg/kg)
200 400 600 800 1000
Pb (mg/kg)
0 150 300 450 600 750
Pb (mg/kg)
Pre Excavation
Post Standardized
Post Homeowner
0 800 1600 2400 3200 4000
Pb (mg/kg)
0 250 500 750 10001250
Pb (mg/kg)
Pre Excavation
Post Standardized
Post Homeowner
100 200 300 400
Pb (mg/kg)
Pb (mg/kg)
Pb (mg/kg)
0-
5
10
1 15
g
25-
Pre Excavation
Post Standardized
Post Homeowner
30:
35-
40
5-
10.
& 15-
c
0
| 20-
£ 25-
S>
"- 30
Pre Excavation
0 Post Standardized
0 Post Homeowner
40
10
20
30
40
Pb (mg/kg)
Pb (mg/kg)
300 600 900 1200 1500
Pb (mg/kg)
Pre Excavation
Post Standardized
Post Homeowner
20 40 60 80 100
Pb (mg/kg)
Figure 5.1 Soil Pb concentration as a function of depth for each of the excavation units pre- and post-
excavation. Numbers refer to the property address/location. Letters refer to duplicate samples
collected from the same property. For property 1, replicate A the -A and -B refer to samples
collected within three feet of each other. Dashed line indicates the location of the remediation
interface.
13
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
7B
Pre Excavation
Post Standardized 35 -
Post Homeowner
40
150 300 450
Pb (mg/kg)
200 400 600 800
Pb (mg/kg)
0 300 600 900 1200 1500
Pb (mg/kg)
10
vation
Post Standardized
- Post Homeowner
0 25 50 75 100 125 150 175
Pb (mg/kg)
Pb (mg/kg)
Pb (mg/kg)
150 300 450 600
Pb (mg/kg)
Pre Excavation
Post Standardized
Post Homeowner
0 25 50 75 100 125 150
Pb (mg/kg)
20^
°- 30-
Pre Excavation
•- Post Standardized 35 -
£ Post Homeowner
, 40
Pb (mg/kg)
Pb (mg/kg)
400 800 1200 1600 2000
Pb (mg/kg)
18
- Pre Excavation
Post Standardized
Post Homeowner
100 200 300 400 500
Pb (mg/kg)
Figure 5.1 (cont). Soil Pb concentration as a function of depth for each of the excavation units pre-
and post-excavation. The dashed line in each plot corresponds to the visually identified remediation
interface. In the upper right corner of the plots the numbers refer to the property address/location,
and the letters refer to duplicate samples collected from the same property. For property 1, replicate
A the -A and -B refer to samples collected within three feet of each other.
14
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
In general, the visual appearance of the remediation interface was in good agreement with the
chemical data. The soil Pb concentration for a portion of the properties continued to increase with
depth (1, 10, 11, 13, 15, 16, 17, and 18) which encompassed geographical units (B, C, and D) The
principal source of Pb contamination in soils within the Omaha Lead superfund site is atmospheric
deposition of Pb particulate matter. The elevated concentration of Pb in the sampled soils at depth,
and the increasing soil Pb concentration with depth, would suggest that a portion of the Pb
deposited on the soil surface is mobile and is currently being leached through the soil profile.
Immediately evident is the remediation interface between the clean fill and the underlying soil
(Figure 5.1). The heterogeneity shown in the distribution and concentration of Pb in the soils
between properties was expected and readily apparent in Figure 5.1. There were no discernible
patterns of maximum soil Pb at depth among the four geographical groups (Figure 5.2). Likewise,
there was large variability in soil Pb concentrations at depth among properties in close proximity
to each other within geographical groups. A prime example of soil Pb heterogeneity at depth are
properties 8, 9, and 10 located in geographical Group B. The properties are less than 0.25 miles
apart but the maximum Pb concentration in the soil profile varies from approximately 150-1500
mg kg"1. Large variation in soil Pb concentration was also present within a single property.
Property 1 included three different excavation reps (1A, B, and C) and one duplicate rep for 1A
(Figure 5.1). Visual comparison of the soil Pb distribution through the profile and the maximum
concentration of soil Pb present in any of the 4 profiles varied greatly. As previously mentioned
samples 1 A-A and 1A-B were collected within three feet of each other, and even these two samples
show significant differences. The large increase in soil Pb concentration for 1C compared to 1A
and IB is due to the presence of what appeared to be smelter slag-like material in the pre-
excavation core. The same variation in soil Pb distribution within a single property was is evident
in property 7 as well. However, the maximum concentration of soil Pb and the soil Pb profiles
were similar for Property 3 and 4. The wide variation in the distribution of soil Pb throughout the
profiles and within individual properties highlights the heterogeneity that exists over the entire
Omaha Lead site. It is unlikely that the atmospheric deposition of Pb was significantly different
within a single property given the nominal size of residential properties sampled, less than 0.25
acres. Therefore, the heterogeneity is likely related to previous anthropogenic disturbances to the
property pre-remediation. Changes in soil compaction, leveling of the property, introduction of
contaminants, and chronic drainage issues may have influenced the local distribution of Pb through
the soil profile.
15
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
-" 2000n
o>
. 1600-
c
o
O
-o
1200-
800-
X
ro
D
D)
CO
400-
.2 20
c
g
§ 15
O
_Q
Q_
E 10-
X
CO
9 6
A B C D
Geographical Group
A B C D
Geographical Group
Figure 5.2 Average maximum concentration of lead and depth at which maximum concentration
occurs for the four different geographical locations.
5.3. Lead Spoil Concentrations
The slag-like material was also present in both the Homeowner and standard excavation holes.
The slag-like material was dark grey friable channers and flagstones1 present at 18 to 20 inches.
The slag-like material was present at depths greater than 12 inches and would not have been
identified during the initial investigation. If the slag-like material was well mixed within the soil,
the concentrations of soil Pb and Pb in the slag-like material should be similar. Figure 5.3 shows
a poor relationship between the Pb concentrations in the soil and spoil. A linear regression of the
concentration of Pb in the two excavation techniques indicated there was no relationship between
the soil Pb and the spoil for the Homeowner excavation technique (p-value 0.5; R2 = -0.1) and only
a weak relationship for the standard technique (p-value 0.013; R2 = 0.37). The relationship
between the spoil-like material and the soil Pb concentration improves at lower spoil Pb
concentrations for both techniques; however the relationship is still weak. Interestingly, the slope
for the Homeowner was close to 1 (1.1) but the very low R2 value (0.159) indicating the variability
between the two soils is still very high.
Channels and flagstone are terms used to describe flat rock fragments present in soils between 2-15 mm and 15-380 mm,
respectively.
16
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
^.f"^
'CD
CD
*— ""
.2
TO
s
X
''Y
Q.
c
0
2
1
c
o
"
!
rjnnr\
oouu -
3000-
zoUU
2000-
1500-
1000-
500-
0-
5 CO
400- *
* *
300 • ",*
200.
.*•' *
100- .**'
9 -
0 100 200 300 400 5
•
_,•' *
••*/''
•5* • • *
i •
9 Homsownsr •''
• Standard
0 ,/'
•
Q
.
0 500 1000 1500 2000 2500 3000 3500
Spoil Lead Concetration (mg kg"1)
Figure 5.3 Soil Pb concentration in the top 18 or 24 inches of the pre-excavation soil as a function of
the Pb concentration in the spoil. The concentration in the soil was determined from calculating the
average concentration of Pb in the soil pre-excavation from the soil core data.
Post excavation Soil Lead Distribution
The redistribution of Pb through the soil after backfilling is presented in Figure 5.1 for both
excavation techniques. If the quantity of Pb in the pre-excavation core is equal to the quantity of
Pb in the post excavation core then the integration of the area along the y-axis for the pre-, post
Homeowner, and post standard soil Pb should be nearly identical. Visual inspection of the soil
profiles pre- and post-excavation indicates the total concentration of Pb in the pre and post
excavations is true for a portion of experimental units (1A-A, IB, 4A, 10, 15, 17, and 18; Figure
5.1). Conversely, the assumption does not hold for several of the properties (1C, 3B, 8, 9 and 13).
Finally, in some instances the assumption was only true for one of the specific excavation
techniques (3 A, 6, and 7B).
The lack of conservation of mass between the pre- and post-excavation soil cores is not
immediately understood. Non-sample heterogeneity of the core section prior to analysis may have
contributed to the issue. However, every attempt was made to thoroughly homogenize samples
prior to acid digestion (EPA Method 3051 A). Further, all digestions and analyses were conducted
in triplicate and average values are reported. Significant heterogeneity within the specific quadrant
may also have contributed to the issue. Sample location 1, 3, and 7 were all vacant lots. Visual
inspection of the soil cores indicated that the soils were heavily disturbed at depth below the
remediation interface. Heavily disturbed soils often exhibit a high degree of heterogeneity in the
soil over very short distances. Therefore the previous anthropogenic activities may have
contributed to the issue. As previously discussed the heterogeneity of soils within a single quadrant
17
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
was evident from the duplicate samples within a single quadrant (Figure 5.1). Based on the known
heterogeneity within the quadrant it is very possible that the centralized location of the pre-
excavation soil core Pb concentration between the shovel and auger differed from the soil Pb
concentrations at each excavation location. One potential option for determining if the reason is
due to soil heterogeneity or poor homogenization of the analytical sample would be to collect a
pre-excavation sample for each excavation technique. If the total soil Pb concentrations pre and
post exaction were similar then the differences may be attributed to soil heterogeneity. However,
if the same issue occurs the reason is related to poor sample homogenization
For the purposes of evaluating mixing of the soils based on excavation technique, the discussion
will focus on profile distribution of Pb rather than the absolute quantity of Pb present.
If a soil was well mixed then the post excavation profile should exhibit little change in the soil Pb
concentration as a function of depth as seen in experimental units 1C, 3B Standardized, and 9
Standardized (Figure 5.1). If a soil was not well mixed then the Pb soil profile will include large
variations in the soil Pb concentration as a function of depth as seen in Figure 5.1 properties IB,
3A, 4A, 6, 7, 7B, 10, 16, 17, and 18. In most instances the redistribution of Pb throughout the soil
profile post-excavation was more homogeneous (regardless of excavation technique) however,
excavation resulted is an increase of Pb at the soil surface, occasionally exceeding 400 mg Pb kg"
1 (Figure 6 properties 6, 17 and 18). Property 2 had a similar Pb profile distribution to the
properties that had undergone remediation. In this instance the Standardized technique did result
in a fairly homogeneous distribution of Pb through the soil profile
5.4. Soil Properties
For the purposes of the current study only the soil moisture content and chemical composition, and
were determined. Excel files containing all of the raw and processed data are located in Appendix
1.
Moisture Content
Moisture content is the percent mass of water present in the soil. The moisture content will
influence how the soil forms clods/aggregates during excavation and the overall strength/stability
of the clods/aggregates. The average moisture content for both the bulk and core soil samples was
19.19% with a relatively low standard deviation of 5.8%. The mass moisture content for just the
spoil was 19.5 % with a standard deviation of 3.7%. The lower moisture content, narrow variation
between all of the samples, and visual observations in the field indicate that the aggregates formed
during excavation were friable (easily breakable) and collection of bulk soil samples were not
impacted by the presence of large strong soil clods. Further, small variability in soil moisture
content ensures all of the soils had similar moisture content at the time of sampling.
Soil Chemical Composition
18
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Obtaining a background or baseline concentration of elements in the OLS for comparative
purposes was not feasible in this preliminary study. The incorporation of fill materials with
potentially high levels of contaminates and length of time since the disturbance are a just a couple
of the factors that makes it very difficult. In the current study, trends in the chemical composition
of the entire soil data set including average chemical concentration and standard deviation for the
fill/cap material and the underlying soil were calculated separately (Table 5.1). The calculated
standard deviations for the chemical analysis for both the fill and underlying soil are extremely
large with respect to the average value calculated. The large standard deviations suggest that
different materials being used as fill and the natural heterogeneity is persistent in soils.
Table 5.1 Average total elemental concentrations for the fill material and underlying soil. Data used
in the calculation of the average elemental concentration and the t-test does not include soil samples
collected from Properties 2, 5,12, and 16, a=0.05.
Element Fill Underlying Average Standard Average Standard p-
Material Soil Deviation mgkg"1 Deviation value
Al
As
Ca
Co
Cr
Cu
Fe
K
Mg
Mn
Na
Ni
P
Pb
S
Zn
12828.2
11.5
11052.9
9.2
15
26.7
18512.3
4709.8
5476.3
609.8
376.4
21.7
2265.5
51.1
868.6
130
3743
3.1
5453.2
0.9
4.1
25.9
4197.4
2849.3
2041.7
91.3
345.3
4.8
2026.3
95.1
864.9
136.9
12210.4
20.5
9855.6
10.2
17.2
40
22566.1
2790.5
3678.5
634.7
259.7
25.6
1134.1
353.3
608.2
470.2
3080.2
13.6
7965.3
1.7
5.4
39.3
6876.1
1468.6
1296.2
125.5
227.8
10.3
916.9
565.2
601
1236.4
0.232
< 0.001
0.21
< 0.001
0.0013
< 0.001
< 0.001
< 0.001
< 0.001
0.115
0.0102
< 0.001
< 0.001
< 0.001
0.023
0.0043
19
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Using an unequal variance t-test the mean elemental composition of the fill and the underlying soil
were tested to see if there was a statistical difference in the chemical composition (Table 2).
There was a statistical difference (a < 0.05) for all but three of the elements (Al, Ca, and Mn)
(Table 5.1). Aluminum and calcium are common components of nearly all major silicate minerals
present in soil, and the lack of differences in the concentration of the two elements in the fill and
underlying soil is expected. The lack of a difference in the Mn was not expected and the reason is
not apparent. Overall there is an enrichment of first row transition metals (Cr, Fe, Co, Ni, Cu, Zn),
As, and Pb in the subsoil and an enrichment of P and S in the cap material. The enrichment of the
metals in the subsoil was expected. Similar to the soil Pb data there were extremely large variations
in the elemental composition of soils as a function of depth. There were no statistical differences
in excavation techniques for any of the elements analyzed. In nearly all instances the calculated
averages had large standard deviations.
5.5. Elemental Correlations
Correlations between Pb and other elements were performed to determine if elemental correlations
could be used to predict the degree of mixing. Elemental correlations were evaluated for the entire
soil profile pre-excavation, the clean fill/cap, the underlying subsoil, the post Standardized
excavation soil cores and the post Homeowner soil cores. For all of the data sets analyzed only a
few weak elemental correlations with Pb were identified. In the capping material used for
remediation there was a weak correlation between Pb and As (R2=0.43) and Cu (R2=0.58). In the
subsoil prior to excavation Pb was again weakly correlated with Cu (R2=0.59). The correlation
between As, Cu, and Pb in the capping material is likely related to the very low concentrations of
the three elements present. The relationship between Cu and Pb in the subsoil is more likely related
to the quantity of copper present in Pb ore (galena) which may have been deposited together via
atmospheric deposition from the refinery. In the post excavation soil cores there was again a weak
correlation with Cu in both the Homeowner and Standardized soil cores (R2= 0.47 and 0.44,
respectively). Based on the presence of the weak Pb/Cu relationship in the soil pre-excavation
samples the relationship may be related.
5.6. Lead Speciation
XAFS spectroscopy was used to determine Pb speciation in a select subset of the collected soil
samples. A total of 5 samples were analyzed. Soil samples selected for XAFS analysis exhibited
high Pb concentrations present in the sub soil (below the depth of remediation) or elevated Pb
concentration in the post-excavation soil at the surface. Two soil samples were analyzed from the
same experimental unit (residential property 1), one sample from an un-remediated soil (residential
property 2), an additional sub soil sample from residential property 3, and a post excavation surface
soil sample (residential property 6). XANES spectra and the first derivative of the XANES spectra
20
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
for each of the soil samples are presented in Figure 5.4. Subtle differences exist in the shape, peak
locations, and spectral features between each of the XANES spectra and are related to the
speciation of Pb. Differences in the spectra of the 5 soils analyzed are indicated by dashed lines
in Figure 5.4.
LL
(0
0)
Q
OS
O
13025 13050 13075 13100
Energy (eV)
6 SC Post 1
3B SC Pre 6
2A SC Pre 6
1ASCPre5
13020 13040 13060
Energy (eV)
Figure 5.4 Normalized Pb L3 XANES spectra and the first derivative of the XANES spectra. Dashed
lines indicate highlight areas where differences exist in the location, shape, and/or presence of
spectral features.
Linear combination fitting was used to determine the Pb species present and their relative
abundance in the soil. Potential Pb species for inclusion in the LCF were chosen based on: 1)
previously published data on the composition and speciation of Pb atmospheric particulates
typically released during Pb smelting and recycling operations (galena, anglesite, cerussite,
plumb oferrite); 2) Pb species that are commonly found in soils with similar chemical properties
and mineralogy (plumbonacrite, Pb sorbed to different mineral surfaces, Pb organic complexes);
3) Pb dissolution products from the primary particulates previously identified (plumbonacrite,
hydrocerussite, and plumbojarosite); 4) a Pb(IV) compound typically found in paint (platternite);
and 5) several pyromorphites species (pyromorphite, chloropyromorphite, and
hydroxypyromorphite). The pyromorphite species were included in the fitting process based on
the existing P to Pb molar ratios present in the soil (between 4 and 14; Table 5.2). Similar molar
ratios have been used successfully in laboratory and field trials for immobilizing Pb through the
formation of pyromorphite. LCF modeling results indicate that four different Pb species could be
used to successfully model the 5 different soils samples: plumb of errite, Anglesite, galena, and
Hydroxyapatite. LCF was performed on both the normalized and the first derivative of the
normalized spectra. The LCF results were similar for the normalized and 1st derivative spectra,
21
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
results from the LCF analysis are presented in Table 5.2 and an example fit is presented in Figure
5.5.
Previous research has not identified Pb phosphate species present in the atmospheric particulates
released during Pb smelting and recycling operations (Ettler et al., 2005; Manceau et al., 1996;
Morin et al., 1999). This would suggest that hydroxypyromorphite formation occurs during active
weathering of the primary Pb phases initially present in the soil. In-situ formation of Pb phosphates
sequesters the free Pb present in the soil and may significantly reduce
bioavailability/bioaccessibility (Scheckel et al., 2013). The relative abundance of hydroxyapatite
in the sample was related to the total P present. A nonlinear regression of the data using an
exponential function indicates a potential maximum of Pb transformation above a specific
phosphorus to lead (P:Pb) ratio (Figure 5.6). However, additional research would be required to
more fully characterize the system and determine the influence of other Pb species present on the
formation of Pb phosphates.
Table 5.2 Linear Combination Fitting Results for the normalized and first derivative of the Pb L3
XANES spectra. Ang = Anglesite, Hy-Py = Hydroxypyromorphite, and Pb-Ferr = Plumboferrite
pu P/PVi
o .10 , b P/Pb Spectra
Soil Sample Modeled*
mg kg"1 mol/mol
Norm
!ASCPre5 1541 4
IstDer
Norm
1C SB 2 2692 6
IstDer
Norm
2A SC Pre 6 1202 14
IstDer
Norm
3BSCPre6 1342 7
IstDer
Norm
6SCPostl 1147 11
IstDer
Normalized LCF Fit
Ang
24
24
21
23
13
10
13
17
29
24
Hy-Py
»/
29
37
35
32
49
57
50
43
39
55
Pb-Ferr
0
48
42
45
46
38
33
37
41
0
Fit
Galena Red chi2
l.OOE-04
l.OOE-04
l.OOE-04
l.OOE-04
9.00E-05
2.00E-04
l.OOE-04
2.00E-04
32 2.00E-04
21 4.00E-04
*Norm refers to the normalized Pb L3 XANES spectra and 1st Der refers to the first derivative of the normalized Pb
L3 XANES spectra
Two samples were analyzed from Residential Property 1. One sample was from the subsoil and
the other from the spoil pile. The subsoil sample represents the Pb speciation at a specific depth
(21 inches) while the spoil sample is a bulk sample of entire excavated soil. The two samples
differed in their exact location within the quadrant and total concentration of Pb (1541 and 2692
mg kg"1 for the subsoil and spoil, respectively), however the Pb speciation and relative abundance
of each species remained similar (Table 5.2). This would suggest that the Pb speciation within
each quadrant is similar and that the vertical speciation of Pb may be similar throughout the
22
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
quadrant. A relatively homogenous distribution of the Pb species and abundances within a single
quadrant enables a more complete understanding of the potential bioaccessibility.
(D
^
CO
Fit 1A SC Pre 5
-Data
13020 13040 13060 13080 13100
Energy (eV)
Figure 5.5 Linear combination fit results for a sub soil sample collected prior to excavation at
residential Property 1.
60 -I
55-
g50
o>
2 45
9 35^
•
30
25
6 8 10 12
P:Pb Ratio
14
Figure 5.6 Relative abundance of hydroxypyromorphite in soil samples, from the LCF analysis, as a
function of the P:Pb molar ratio. An exponential function was used for the nonlinear regression of
the data: Adjusted R2 = 0.60, p-value = 0.016.
23
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
6. Discussion
6.1. Statistical Analysis of Soil Mixing
Several empirical models were developed to predict the degree of soil mixing and resulting Pb
concentrations and redistribution after soil mixing based upon the Standard and Homeowner
excavation techniques. The decision to leave soils with Pb concentrations <1200 mg kg"1 soil was
based upon the assumption that the excavated soil would be well mixed and result in an average
soil Pb concentrations <400 mg kg"1. To test this hypothesis, statistical models were created to:
1. Use the geometry of the hole to predict the ratio of un-remediated soil in a specific sample
as a function of the depth to the remediation interface and total depth excavated
2. Conduct matching means and matching means and variances to predict the distribution and
concentration of Pb in the mixed soil
3. Use Bayesian estimation analysis to estimate the mixing distribution at each location.
Modeling the degree of soil mixing and the concentration of Pb in the resulting mixture is
dependent upon the relative proportions of clean and un-remediated soil present. For example, the
soil profile can be viewed as a series of compartments above and below the remediation interface
(Figure 6.1). When the soil is disturbed, the distribution of the soil compartments changes. A
random sample from the new distribution will contain a certain number of clean and un-remediated
soil compartments. Therefore, the total concentration of any analyte of interest is based on the
relative number of clean and un-remediated soil compartments in the random sample. The
geometric model provides an estimate of the analyte of interest by predicting the amount of un-
remediated soil in a given sample.
24
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Pr<
;-Distrubance Post-Disturbance
I Random
| Sample
I Random
r Sample
Figure 6.1 Schematic representation of soil mixing. In the figure the green blocks designate the clean
soil, the red line the remediation interface, and the red blocks the un-remediated soil. The brackets
next to the Post-Disturbance column indicate different random samples taken. The groupings of
blocks contained within the brackets differ based on the redistributed soil compartments. The red
line indicates the location of the remediation interface.
For our first model, a cylinder was used to model the geometry of the hole created using the
Standard technique, and a hemisphere or hemispherical cylinder was used to model the geometry
of the Homeowner excavation technique (Figure 4.1, 4.2). For comparison of all the experimental
units, the depth of the remediation interface was normalized to 1 by evaluating the total excavation
depth as a fraction of the remediation depth (Eq.l).
Equation 1 r = —
Where r is the normalized depth, E is the excavation depth and RI is the remediation depth.
For a cylindrical (Eq. 2) and hemispherical (Eq.3) shaped exaction the expected fraction of un-
remediated soil (w) is
E-RI „ 1
Equation 2 W = - = 1 --
E T
Equation 3 W =
2r3
= 1 —
(3r2-l)
2r3
For both the cylindrical and hemispherical the equation are true for r > 1. At r values < 1 w = 0
A schematic representation of the geometric model is presented in Figure 6.2 and the normalized
ideal mixing model for the Standard and Homeowner excavation techniques, is presented in Figure
6.3. A value of 1 on the x-axis indicates the depth at which excavation has reached the remediation
interface. Therefore, at excavation depths less than 1, y = 0 because the subsoil remains
undisturbed. As the excavation depth exceeds the remediation interface the ratio will be greater
than 1. The exact value of the ratio is dependent on the geometry of the hole. For example, at an
x-axis value of 2 the excavation depth is twice that of the remediation interface. The ideal degree
25
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
of mixing (w) for the Standard excavation technique (cylinder) is 0.5 or 50%, but the degree of
mixing for the Homeowner excavation (hemispherical) is only 0.3 or 30% (Figure 6.3).
Standard Homeowner
Excavation Excavation
.
^s
1
i 3
ill i
1 \
| \
i V
i V
1 ~--J
1 /
/
/
^s_
— ""
1
1
T~r
l" tRI
1
Figure 6.2 A Schematic Representation of the geometry of the standard and Homeowner excavations.
The image illustrates how the geometry of the hole will impact the mixing of clean and un-remediated
soil.
0.7-
0.6-
1
TR °5-
(0
V 04-
5 0.3-
0 0.2-
•s
ro
"- 0.1-
0.0-
Excavaton Technique
Homeowner
- -Standard
s "
f ^ '
„ '
s
' s
/ f
' /•
/ /
I /
t '
1 '
J^/
0.0 0.5 1.0 1.5 20 25 3.0
Excavation Depth as a fraction of remedaition Depth
Figure 6.3 Expected fraction of contaminated or "un-remediated" soil based upon geometry of the
excavation. The excavated fraction of un-remediated soil versus ratio of depth of excavation to
remediation interface.
The geometric model was validated by predicting the mean concentration of a specific analyte in
the spoil pile based on the relative contribution of the given analyte above and below the
remediation interface within the spoil. The equation for the predicted concentration of an analyte
in the spoil pile is
Equation 4 Psp0jj =
Spoil mass
where w is the percentage of un-remediated soil in the sample/spoil.
26
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
As an example assume a spoil sample mass of 1 kg, 15 mg Pb kg"1 above the RI, 800 mg Pb kg"1
below the RI, and a 20% fraction of un-remediated soil in the spoil, the predicted spoil (P spoil)
concentration is
.. _x
Equation 5 Psp0ii = - - ik soil - = mg & S
k soil
Correlation coefficients from the validation of the geometric model for the analytes of interest
varied (Table 6. 1). An example of the correlation plots for Pb is presented in Figure 6.4. Plots for
other analytes are presented in Appendix 2. Dot plots were also developed to depict the range of
elemental values compared to the actual concentration (Figure 6.5). Each point on the plot
represents an individual sample. The spread of data along the x-axis provides a visual
interpretation of the distribution of analyte concentrations and the variability. Additionally, the
degree to which the predicted sample concentrations line up with the actual sample concentrations
provides an insight as to the robustness and accuracy of the geometric model predictions.
Overall, the predicted soil concentration in the spoil was better for the Standard excavation
technique as compared to the Homeowner (Table 6. 1). However, this was not the case for Pb or
nickel. The correlations were also higher for analytes whose soil concentration above and below
the remediation interface were similar, e.g. aluminum, calcium, potassium, sodium, and
phosphorus (Table 6.1; Figure 6.5). This is was expected, since a poorly mixed or well mixed soil
would have virtually the same analyte concentrations.
Less robust correlations for the model were also noted for analytes whose total concentration in
the soil was low e.g. arsenic, cobalt, copper and nickel (Table 6.1; Figure 6.5). Examining the dots
plots specifically for cobalt and arsenic the range of concentrations for the two elements in the
clean soil (1) is significantly less than the un-remediated soil (2) (Figure 6.5). The range of average
concentrations in the spoil (H and S) visually appears to more closely resemble the element
distribution in the un-remediated soil. This would suggest that the soil may not be well mixed in
the spoil. Iron, Zn, and Pb also had significantly higher concentrations in the un-remediated soil
compared to the surface soil. Unlike the previous analytes the concentrations in the un-remediated
soil were in excess of 1000 mg kg"1. However, the correlation coefficients for these elements were
also low. The smaller correlation values indicate that geometric ideal mixing model may not be
representative of the field observations. The inability of the model to accurately predict the
average spoil concentration is related to the degree of soil mixing that occurs during excavation.
Results from the geometric model are in agreement with the data for the redistribution of Pb
(Figure 5.1) indicating a heterogeneous distribution of Pb after excavation and back filling.
27
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Table 6.1 Correlation coefficients for the predicted elemental concentrations in the excavated
material based on the geometric model as a function of the average concentration of specific elements
in the excavated spoil.
Excavation Type
Element
Standard
Homeowner
Correlation Coefficient
Al
As
Ca
Co
Cr
Cu
Fe
K
Mg
Mn
Na
Ni
P
Pb
S
Zn
0.858
0.703
0.886
0.727
0.766
0.698
0.338
0.928
0.908
0.875
0.87
0.182
0.906
0.406
0.831
0.397
0.8
0.648
0.863
0.594
0.687
0.623
0.482
0.964
0.96
0.636
0.917
0.4
0.94
0.737
0.894
0.264
1000-
100-
Standard
f '
o f'
' O
O v
f *
s' O
0 x
o-o
0 *-;
/
' ® ** On
^ 0
/
/a
t O O
^ ' o
•" o
1000-
^
5*
01
£
g
ra
o 100 :
*o
0)
1
1
£
10-
Homeowner
, '
'
-/
, '
'
/
/ 00
O x
/o' oo
/ 0 o
o y
' 0 0° 0 °
^ o
f
0
^ 0
/
10 100 1000
Mean Lead Concetration (mg kg ')
10 100 1000
Mean Lead Concetration {mg kg*')
Figure 6.4 The predicted concentration of soil Pb, based on the geometric model, as a function of the
average concentration of Pb in the excavated spoil for the standard and Homeowner excavation
techniques. Dashed line indicates a 1:1 correlation. Correlation values are presented in Table 3.
28
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
H
S
pred.H
pred.S
S
Ca
• • • • •• «M ••• •»* •
« • •• • Ml* • * • ••* • •
5000 10000 1500020000
Al
• • •»•**** «M»* * •
Co
~~ .
• • «••••••«• •
7 8 9 10 11 12 13 14
As
~
• «••• •« *• *
H
S
pred.H
pred.S
S
Fe
• ••• «••• •» •* ••
..
15000 20000 25000 3000035000
Cr
. ..
• * •••»• •••»•• •
K
• • ••• • • • • • •• •
• • • MMMi • • • •• • •
2000 4000 6000 8000
Cu
.
..
8000 10000 1200014000 18000 10
mean
20
50
15 20 25 30 10
mean
50 100
Figure 6.5 Dot plots for the results predicted and actual values for the analyte concentration in the spoil for the geometric model. Each
dot represents a single sample location and a single predicted value for a sample location. H indicates the Homeowner excavation method,
S indicates the standard method, 1 indicates the surface/clean soil analyte concentration, and 2 indicates the subsurface/un-remediated
analyte concentration
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
1
pred H
H
pred.S
1
Na
• • • •«• •••»•••••••
5 10 20 50 100 200 500 1000
Mg
•M» • • »MB • •• •
«• • • Mft • ••» • ••
• «» •• «•• • •§ ** •
Ni
• • •**«•> • • •
20 25 30 35 40 45
Mn
• ••mil • • •
• • •» • ** • • •
1
s
2
S
III III
50 100 200 500 1000 2000
P
• • ••• • ••• • * •*• •
Zn
• W«M ^ M •
II II
50 100 200 500 1000 2000
Pb
• ••••• •• «MM •• • •
• • • •«• •*»••••• • • •
4000 6000 8000 10000 400 500 600 700 800 900
mean
500 1000 2000
5000 20 50 100 200 500 1000 2000
mean
Figure 6-5 cont. Dot plots for the results predicted and actual values for the analyte concentration in the spoil for the geometric model.
Each dot represents a single sample location and a single predicted value for a sample location. H indicates the Homeowner excavation
method, S indicates the standard method, 1 indicates the surface/clean soil analyte concentration, and 2 indicates the subsurface/un-
remediated analyte concentration.
30
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
An attempt was made to model the redistribution of Pb following excavation and backfilling using
a matching means and a matching means with variances approach. Matching Means, in its simplest
form, estimates the analyte means of the 'clean', 'un-remediated' and mixed soil for each location.
For the purposes of this analysis, clean is defined as average concentration of the analyte of interest
in all of the soil samples (all properties) collected above the remediation interface, and un-
remediated is defined as the average concentration below the remediation interface. For matching
means the mixing variance is assumed to be 0. Under this assumption the mixing fractions
(clean/un-remediated) are estimated using only the means values for the specific analyte of
interest. For a given mixing fraction, w, the mean of the mixture is modeled as
Equation 6 Figure 6-5 cont. Dot plots,
with the subscripts m, c and u, designating mixed, clean and un-remediated soil, respectively, and
// being a mean.
Theoretically, this should give the best agreement between the mixing fractions estimated from
geometry and those estimated based on analytical data. Unfortunately, as seen in Figure 6.6 for
Pb, the agreement is so extremely poor as to render the estimates of little worth. The lack of any
significant agreement between the model and analytical data highlights even on a small scale
demonstrates significant heterogeneity in the distribution of Pb in the soil.
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
~ 11
nj u
Q
"ra
g
S- 0.8-
c
g
0.4 -|
CO
c
a
ro
0.2 -I
0.0 -|
Standard
Homeowner
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 08 0.9 1.0 1.1
Estmated Mixing fraction from Geometry
Figure 6.6 Estimated fraction of soil mixing as a function of depth and geometry based on the Pb soil
concentrations in the post soil core data. A) Standard Excavation Technique. B) Homeowner
Excavation technique.
Matching Means and Variances, A more sophisticated method of estimating the distributional
parameters of the mixing distribution is to match both means and their variances. This can be done
by using the formula for the variance of the product of two independent random variables.
Equation 7
~y + 1(JX + a
Then under our model, the variance of the mixed soil should be
Equation 8
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
The inability to develop a model to predict the concentration of Pb in the top 12 inches of soil
following excavation or the redistribution of Pb within the soil profile is due to sample variance.
The variances are so large that that it is impossible to provide reliable estimates for the distribution
of mixing fractions. The variances were so large because of the nature of the research objective
which was to estimate the degree of mixing following Homeowner intrusion into the un-
remediated soil. We attempted to control for the expected difficulty of this research by using a
Standardized excavation technique to represent "a best case scenario" of soil mixing which would
result in a 1:4 ratio of clean fill to un-remediated soil. Regardless of using an "idealized" mixing
technique we were not able to develop a valid predictive model. Further, based on statistical
analysis the sampling campaign and data analysis should be regarded as a pilot study and used as
the basis to design a full study of soil mixing and risk for the OLS site. The methodology, if not
the results, should be applicable to other sites.
6.2. Frequency Analysis
As discussed in the previous section, the small sample size and the variability of the data prevent
any us from developing predictive models to then make inferences the degree of soil mixing
following Homeowner intrusion into contaminated soil. A frequency analysis can be used to
evaluate the impact of soil mixing on the redistribution of Pb within the soil profile. A frequency
analysis, in this case, only allows for conclusions to be drawn from the current data set and is
observational not predictive. However, it is useful for evaluating how often surface soil Pb
concentrations exceeded 400 mg kg"1 after mixing.
In this study only properties that had been previously remediated and Property 2 were used.
Property 2 was included based on the similarity of the pre-excavation soil Pb profile and that of
the properties that were previously remediated. Further replicate samples from the same quadrant
were treated as individual experimental units. For the frequency analysis the concentration of Pb
in the top 1, 6, 12, 18, and 24 inches of the soil post excavation were calculated (Tables 6.2).
In the May 2009 Final Record of Decision for the Omaha Lead Site properties exceeding 400 mg
kg"1 Pb (equivalent to mg Pb kg"1) in the top 1 inch would trigger a remedial action. Results of our
study are broken into three categories and reported on Table 6.3.
33
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Table 6.2 Post Homeowner and Standardized excavation soil total Pb concentrations in the top 1, 6,
12, 18, and 24 inches of the soil profile. Bolded numbers indicate where the soil lead concentration
exceeds 400 mg kg"1.
Experimental
Unit
1A
IB
1C
2
3A
3B
4A
4B
5
6
7
7B
8
9
10
11
12
13
14
15
16
17
18
Soil Pb
0-1
Concentration (mg
0-6
0-12
kg"1) Homeowner
0-18
0-24
Inches
69
202
765
57
76
224
135
30
20
82
333
87
51
68
16
34
136
40
240
90
249
23
121
155
462
625
212
210
161
28
27
94
562
146
26
71
16
38
197
38
118
83
377
21
247
123
348
457
331
203
143
28
52
89
436
154
266
72
16
36
209
46
114
83
488
26
267
219
504
408
368
225
140
27
86
75
477
123
219
79
139
37
185
42
281
112
639
73
246
196
403
334
285
227
136
25
145
63
445
98
176
96
166
39
146
38
260
124
625
155
Soil Pb
0-1
Concentration (mg kg"
0-6
0-12
J) Standardized
0-18
0-24
Inches
202
80
351
407
86
159
128
166
19
1147
62
53
133
120
45
240
27
100
46
170
210
556
342
153
184
341
326
159
175
82
193
22
516
70
63
102
96
47
268
34
185
38
260
213
449
234
184
118
383
301
156
191
86
170
21
603
65
66
119
95
38
246
34
177
40
214
195
573
242
190
195
393
327
153
191
86
165
21
607
73
65
117
98
45
213
34
172
39
220
190
774
257
192
160
353
313
157
198
84
159
19
689
75
63
114
91
46
238
35
162
42
195
199
1048
237
The first is the total number of excavations conducted for each experimental unit. In this case each
experimental unit had two excavations conducted (except Property 2) the Homeowner and
Standard techniques. These results are then broken out by excavation technique, either
Homeowner or Standard. In the Table the headings indicates the conditions under which the
analysis was conducted. The category, Total Excavation, refers to the number of excavation units
used in the analysis. The category, Soils with Pb > 400 mg kg"1, refers to the number of properties
that exceed the 400 mg kg"1 concentration in a given depth interval, e.g. 0 to 1 inch or 0 to 12
inches. Finally, we report the percentage refers the percentage of properties that exceed 400 mg
kg1.
34
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
The frequency analyses were conducted for several different soil Pb conditions to analyze specific
questions. The analysis was for the total number of excavations (all experimental units; Table
6.3). This analysis is the most robust since it includes the largest number of observations. For the
entire study 10% of the excavation units resulted in a final soil Pb concentration exceeding 400
ppm in the top inch after excavation (Table 6.3). The excavation units exceeding 400 included
one property from the Homeowner excavation group and three from the Standard excavation
group. The significance of the finding is questionable due to the small sample size.
In the top 12 inches of soil, 12.5% of the excavation units resulted in a final soil Pb concentration
exceeding 400 ppm (Table 6.3). Interestingly, the 0-12 and 0-6 inch groups both included five
properties. However, the specific properties in each group were not all the same. Breaking the
information down by excavation technique shows that one more property in the Homeowner
excavation group exceeded the 400 value compared to the Standard. The significance of this
finding is questionable. The number of total samples analyzed was small. Further, the number of
properties exceeding the 400 mark was only 10 % for the 0-1 inch and 12.5% of the total samples
for 0-12 inches. Lastly, the highest concentration of Pb in each soil property investigated varied
greatly. All of these factors make it difficult to place significance on the degree of soil mixing
attributed to either excavation technique. The visual evaluation of the Pb soil profiles in Figure
5.1 provides a greater insight into the difference in the degree of mixing based on excavation
technique.
Of specific interest in the current study was potential to exceed 400 ppm of Pb in the top inch and
12 inches of the soil after mixing when the maximum concentration of Pb exceeded 1200 ppm at
the remediation interface. The second section of Table 6.3 uses data obtained from the Omaha
Lead database to answer the question. Based on the database information, only 2 the properties
investigated have soil Pb concentrations exceeding the 1200 value. Of the four excavations, one
(property 17) exceeded the 400 value in the top inch and both excavation techniques resulted in an
excess of 400 ppm in the top 12 inches.
Based on the frequency analysis using the database values, the same analysis was again performed
on soils with a maximum Pb concentration greater than 1000 ppm and less than 800 ppm using
data from the Omaha Lead database and the current study (Table 6.4). The number of properties
exceeding 400 ppm Pb in the soil surface after excavation decreases when soil Pb concentration at
depth is less than 1200 ppm. These data are observational only. They are not predictive of what
surface soil Pb concentrations may be after Homeowner intrusion into contaminated soils at other
OLS properties.
35
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Table 6.3 Frequency analysis of the number of properties that exceeded Pb concentrations of 400 mg
kg"1 in either the top 1, 6,12,18, or 24 inches. For all of the properties sampled, properties where the
soil Pb concentration exceeded 1200 mg kg"1 according to the OLS database, and properties where
the maximum soil Pb concentration exceeded 1200 mg kg"1 based on the current study. Data
corresponding to the frequency that 400 mg kg"1 was exceeded in the top 12 inches in italics. Bolded
heading refer to the type of soils included in the analysis.
Soil Depth (inches)
Total Excavations
Soils with Pb > 400 mg kg"1
Percentage
Homeowner Excavations
Soils with Pb > 400 mg kg"1
Percentage
Standard Excavations
Soils with Pb > 400 mg kg"1
Percentage
Soils with
Total Excavations
Soils with Pb > 400 mg kg"1
Percentage
Homeowner Excavations
Soils with Pb > 400 mg kg"1
Percentage
Standard Excavations
Soils with Pb > 400 mg kg"1
Percentage
0-1
Total Excavations
40
4
10
19
1
5.3
21
3
14.3
soil Pb > 1200 mg kg x
4
1
25
2
0
0
2
1
50
0-6
0-12
0-18
0-24
Standard + Homeowner
5
12.5
3
15.8
2
9.5
, data from
1
25
0
0
1
50
Soils with soil Pb > 1200 mg kg x
Total Excavations
Soils with Pb > 400 mg kg"1
Percentage
Homeowner Excavations
Soils with Pb > 400 mg kg-1
Percentage
Standard Excavations
Soils with Pb > 400 mg kg"1
Percentage
11
4
36.4
5
1
20
6
3
50
3
27.3
1
20
2
33.3
5
12.5
3
15.8
2
9.5
the Omaha Lead
2
50
1
50
1
50
, Current Study
3
27.3
1
20
2
33.3
6
15
4
21.1
2
9.5
Database
2
50
1
50
1
50
4
36.4
2
40
2
33.3
5
12.5
3
15.8
2
9.5
2
50
1
50
0
0
4
36.4
2
40
2
33.3
36
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Table 6.4Frequency analysis of the number of properties that exceeded Pb concentration of 400 mg
kg"1 in either the top 1, 6,12,18, or 24 inches for properties where the soil Pb concentration
exceeded 1000 mg kg"1 at the remediation interface according to the Omaha Lead database,
properties where the maximum soil Pb concentration exceeded either 1000 or 800 mg kg"1 based on
37
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
the current study. Data corresponding to the frequency that 400 mg kg"1 was exceeded in
inches in italics. Bolded heading refer to the type of soils included in the analysis.
the top 12
Soil Depth (inches)
Soils with
Total Excavations
Soils with Pb > 400 mg kg'1
Percentage
Homeowner Excavations
Soils with Pb > 400 mg kg'1
Percentage
Standard Excavations
Soils with Pb > 400 mg kg'1
Percentage
Total Excavations
Soils with Pb > 400 mg kg'1
Percentage
Homeowner Excavations
Soils with Pb > 400 mg kg'1
Percentage
Standard Excavations
Soils with Pb > 400 mg kg'1
Percentage
Total Excavations
Soils with Pb > 400 mg kg'1
Percentage
Homeowner Excavations
Soils with Pb > 400 mg kg-1
Percentage
Standard Excavations
Soils with Pb > 400 mg kg'1
Percentage
0-1
soil Pb,> 1000 mg
21
3
14
10
1
10
11
2
18
Soils with soil Pb
13
4
31
6
1
43
7
3
43
Soils with soil Pb
27
0
0
13
0
0
14
0
0
0-6
kg"1, data from
2
9.5
1
10
1
9
> 1000 mg kg"1,
3
23
1
29
2
29
< 800 mg kg"1,
1
4
1
8
0
0
0-12
the Omaha Lead
2
9.5
1
10
1
9
Current Study
3
23
1
29
2
29
Current Study
;
4
1
8
0
0
0-18
Database
3
14
2
20
1
9
4
31
2
29
2
29
1
4
1
8
0
0
0-24
3
14
2
20
1
9
4
31
2
29
2
29
1
4
1
8
0
0
6.3.Bioavailability/Bioaccessibility
38
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
XAFS data indicate that Hydroxy/Chloropyromorphite was present in all of the soils sampled at
relative abundances exceeding 30%. Based on the known speciation of Pb emitted from Pb
smelting/recycling activities and existing P:Pb molar ratios in the soils analyzed it is reasonable to
assume that phosphorus concentrations in the soil were high enough to promote precipitation of a
pyromorphite species. The addition of P amendments to Pb contaminated soils to induce
pyromorphite precipitation is a remediation technique utilized to reduce Pb bioavailability. The
in-situ formation of pyromorphite without additions of phosphorus is highly advantageous due to
the inherent reduction in potential bioavailability of soil Pb. Additional research evaluating the
geographic distribution and abundance of pyromorphite phases will aid in a better understanding
of the conditions under which phase form and determine if geographical location/soil properties
control the precipitation of Pb phosphate minerals. Currently additional soils are being
investigated by XAFS analysis to further evaluate the abundance and presence of Pb phosphates.
7. Summary/Conclusions
Urban soils within the OLS have been contaminated with Pb from atmospheric deposition of
particulate materials from Pb smelting and recycling activities. In May of 2009 the Final Record
of Decision Declaration for the Omaha Lead Site (OLS) stated that any residential soil exceeding
the preliminary remediation goal (PRG; 400 mgpb kg'^oii) would be excavated, backfilled and re-
vegetated. The remedial action entailed excavating contaminated soil in the top 12 inches and
excavation could stop when the concentration of soil Pb was less than 400 mg kg"1 in the top 12
inches or less than 1200 mg kg"1 at depths greater than 1 ft. After removal of the contaminated
soil, clean backfill was applied and a grass lawn was replanted
The initial recommendation was that soils with Pb concentration less than 1200 mg kg"1, at depths
greater than 1 ft, be left in place. The recommendation was based on the assumption that Pb-
contaminated soil at depth greater than 1 ft would not represent a future risk. This assumption was
based on the principal that mixing and other factors encountered during normal excavation
practices would not result in Pb surface concentrations greater than the PRG. The current study
was designed to evaluate the degree of soil mixing and the redistribution of Pb within the soil
profile after typical Homeowner earth-disturbing or excavation activities. Two methods were
employed to evaluate soil mixing. The first method (Homeowner) entailed excavating soil using
a common garden spade to a depth of 18 inches. The second (Standard) used a 2-person auger to
excavate a hole 2 ft in depth and would represent the best case scenario where the excavated soil
would be well mixed, and surface soil Pb concentrations would remain below the PRG. Prior to
excavation, surface soil samples and an undisturbed soil core were collected to determine the initial
concentration and distribution of Pb within the soil. After excavation, a composite soil sample
was collected from the spoil pile prior to backfilling the hole. Lastly, a final soil core and surface
soil sample were collected to evaluate the redistribution of Pb within the soil profile. Samples
were dried and digested following EPA method 305la.
39
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Data were collected from 18 properties. This includes duplicate and triplicate samples taken from
properties to evaluate soil heterogeneity within a single remediation quadrant. The results from
the duplicate and triplicate sampling were mixed. Two of the properties exhibited a large degree
of variability between the duplicate and triplicate samples. The other two properties showed a
similar distribution and concentration of Pb within the soil profile. It should be noted that large
difference in soil Pb concentration from the triplicate samples collected from property 1 is likely
caused by the presence of the slag-like material at depths below the remediation interface. Results
from the soil analysis revealed a stark contrast in the soil Pb concentration at the remediation
interface, as would be expected. Of the 18 properties sampled, 17 were previously remediated,
two did not have the expected high Pb concentration below the remediation interface. A fourth
property did have a slight increase in Pb below the remediation interface, however, the increase
was less than what was expected based on data obtained from the Omaha Lead Database. Seven
of the 14 properties had maximum Pb concentration at the remediation interface. The remaining
7 properties show that soil Pb concentrations increased below the remediation interface. Four of
the properties (Figure 6; 1C, 3B, 9, 17) had soil Pb concentrations greater than 1200 mg kg"1 below
the remediation interface.
Post excavation cores revealed a heterogeneous distribution of Pb within the soil profile
demonstrating that the soils were not well mixed. Our attempts to model soil mixing based on soil
Pb concentrations and the geometry of the excavation failed due to the high variance in the data
despite attempting to reduce the variance before beginning the study. While a conceptual model
could be developed in Excel with assumed means and variances to predict mixing, we believe this
pilot study has demonstrated the need for further investigations to develop an empirical model. We
believe a future study expanding the number of properties sampled and adding composite samples
to the current study design would reduce the high levels of variance in the data, and would therefore
validate our model. From the statistical analysis it was determined that mixing fractions for the
un-remediated and clean soil could be estimated approximately from the geometry of the
excavation and the measured depths of remediation and total excavation depth. The mixing
fraction should be viewed as a random variable with a distribution whose mean depends on the
geometry of the excavation and the relative depths of the excavation and remediation. This
conceptual model may be implemented in an Excel spread sheet with assumed means and
variances. Finally based on the statistical analysis the current study should be viewed as a pilot
study. The high variances within the data make it impossible to analyze.
A frequency analysis was conducted to evaluate soil mixing. The analysis showed that for all of
the properties sampled and both excavation techniques (n= 40), 5 or 12.5% of the samples had soil
Pb concentrations in excess of 400 mg kg"1 in the top 12 inches of soil after excavation and
backfilling. Of the 5 properties with elevated soil Pb concentrations, 3 were from the Homeowner
excavation technique and two from the Standard excavation technique. Frequency analysis limited
to properties with soil Pb concentration > 1200 mg kg-1at depth showed that intrusion into the
contaminated soil resulted in surface soil Pb concentrations exceeding 400 mg kg"1 at three of 11
40
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
sites. However, the surface Pb concentration after excavation did not exceed 400 mg kg"1 of Pb
when the maximum Pb concentration at depth was less than 1200 mg kg"1. As previously
mentioned frequency analysis are observational and are not predictive of other sites.
Lead speciation data obtained from XAFS analysis of the soils indicates that the predominant Pb
species present in the soils analyzed included: anglesite, hydroxypyromorphite, plumb oferrite, and
galena. Hydroxypyromorphite (i.e., hydroxyapatite) is not a constituent/Pb species associated with
atmospheric emissions from Pb smelting recycling activities. It is most likely the Pb phosphate
mineral formed in the soil. Soil phosphorus to lead (P:Pb) molar ratios were between 4 and 14.
This would indicate there was ample P available for the in-situ formation of Pb phosphates. The
precipitation of the Pb phosphates is extremely advantageous due to the low bioavailability of Pb
from pyromorphites. The precipitation of the phase is effectively offering a secondary remediation
technology by binding the Pb in place in an extremely low bioaccessible form.
41
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
References
Ettler, V., Johan, Z., Baronnet, A., Jankovsky, F., Gilles, C., Mihaljevic, M., Sebek, O., Strnad,
L., Bezdicka, P., 2005. Mineralogy of air-pollution-control residues from a secondary lead
smelter: Environmental implications. Environmental Science & Technology 39, 9309-9316.
Manceau, A., Boisset, M.C., Sarret, G., Hazemann, R.L., Mench, M., Cambier, P., Prost, R.,
1996. Direct determination of lead speciation in contaminated soils by EXAFS spectroscopy.
Environmental Science & Technology 30, 1540-1552.
Morin, G., Ostergren, J.D., Juillot, F., Hdefonse, P., Galas, G., Brown, G.E., 1999. XAFS
determination of the chemical form of lead in smelter-contaminated soils and mine tailings:
Importance of adsorption processes. American Mineralogist 84, 420-434.
OSWER Directive 9285.7-50, U.S. Environmental Protection Agency, Lead Sites Workgroup.,
2003. Superfund Lead-Contaminated Residential Site Handbook. U.S. Environmental Protection
Agency, Office of Emergency and Remedial Response
Ravel, B., Newville, M., 2005. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray
absorption spectroscopy using IFEFFIT. Journal of Synchrotron Radiation 12, 537-541.
Scheckel, K.G., Diamond, G.L., Burgess, M.F., Klotzbach, J.M., Maddaloni, M., Miller, B.W.,
Partridge, C.R., Serda, S.M. 2013. Amending soils with phosphate as means to mitigate soil lead
hazard: a critical review of the state of the science. Journal of Toxicology and Environmental
Health B Critical Review. 16(6):337-80.
U.S. Environmental Protection Agency, Region VII, 2009. Final Record of Decision, Omaha
Lead Site Operable Unit 2, Omaha Nebraska.
42
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Supplemental Figures
Supplemental Figure 1 Standardized excavation used a gas powered posthole digger with spoil
material brought to soil surface. Notice loss of soil structure.
43
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Supplemental Figure 2 The Homeowner technique used a traditional garden spade to excavate a
hole 18 inches in depth. Notice large soil clods.
44
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Supplemental Figure 3 JMC Environmentalist's Sub-Soil Probe. Soil core tube was driven into the
soil by hand and withdrawn using a jack.
45
-------
Omaha Soil Mixing Study - EPA/600/R-15/054
Appendices
Appendix 1. Sample Data Sets
The imbedded excel documents contained all of the analytical data collected for the current
study. The Complete Data Set file contains all of the raw and processed data. The Bulk Soil
Sample Data Set contains only the data for the bulk soil samples and the Soil Core Data Set
contains just the soil core data
Complete Data Bulk Soil Sample Data Soil Core Data
Set.xlsx Set.xlsx Set.xlsx
Appendix 2. Correlation Figures
Correlation plots for the geometric model for all of the analytes are located in the attached adobe
file.
Adobe Acrobat
Document
46
-------
r/EPA
United States
Environmental Protection
Agency
PRESORTED STANDARD
POSTAGES FEES PAID
EPA
PERMIT NO. G-35
Office of Research and
Development (8101R)
Washington, DC 20460
EPA/600/R-15/054
Offal Business
Penalty for Private Use
$300
------- |