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SUMMARY
Assessing Soil and Groundwater Impacts of
Chemical Mixture Releases from Hazardous
Materials Transportation Incidents
Introduction
Each year, large quantities of hazardous materials are transported throughout the United
States. In the event of an incident or accident, these hazardous materials can be released
to the environment, thereby impacting soil and groundwater, leading to costly emergency
response and cleanup efforts. Many impact measurement techniques in use today concen-
trate on fatalities/injuries, property damage, and emergency impacts, but exclude environ-
mental and ecological impacts associated with releases into soil, groundwater, aquatic
features, or natural habitats. Consequently, risk management decisions are being made in
the absence of the comprehensive information necessary to mitigate long-term environ-
mental risk. The screening model developed in this research is meant to aid in addressing
this concern. As with all screening model requirements to assess the subsurface impact of
hazardous materials, chemical-specific fate and transport data, as well as site-specific
data, are necessary input parameters. While the fate and transport data are available for
pure chemicals, similar data are not usually available for mixtures of hazardous materials that
are commonly transported (e.g., herbicides, paint, cleaning compounds, motor oil, antifreeze,
gasoline, and ethanol).
The goal of this research was to develop a tool to estimate the critical fate and trans-
port parameters of chemical mixtures for use in common fate and transport models,
allowing the user to efficiently and effectively compare and predict the potential impacts
of releases from transportation incidents. Specifically, the developed tool will assess, clas-
sify, predict, and quickly communicate fate and transport characteristics of chemical
mixtures released into the soil and groundwater as a result of hazardous materials trans-
portation incidents.
The research team has completed 16 months of research on HMCRP Project 06, "Soil and
Groundwater Impacts of Chemical Mixture Releases from Hazardous Materials Transporta-
tion Incidents." A white paper submitted in June 2009 summarizes the preliminary efforts
(Phase I, Tasks 1 to 4 outlined in the contract):
· Task 1--define and categorize the environmental hazards to soil and groundwater of pure
chemicals and mixtures,
· Task 2--identify sources and collect readily available data on fate and transport properties,
· Task 3--develop a typology and identify and classify common solvents and mixtures that
are likely to be transported by the industry and significantly control or alter the hazardous
material fate and transport properties, and
· Task 4--develop a typology to estimate the key parameters for different chemical mixtures.
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In September 2009, the research team proceeded to implement Phase 2 (Tasks 5 through 8)
of the research project:
· Task 5--design a tool to characterize, predict, and communicate the impact of chemical
mixtures in soil and groundwater environments and to estimate the fate and transport pa-
rameters of chemical mixtures released to soil and groundwater as a result of hazardous
materials transportation incidents;
· Task 6--use the tool to estimate the fate and transport parameters for 5 to 10 represen-
tative mixtures commonly transported and to apply existing basic screening models to
estimate impact to soil and groundwater;
· Task 7--refine the tool to compare fate and transport characteristics of pure chemicals ver-
sus chemical mixtures in order to rank the relative impacts to soil and groundwater; and
· Task 8--prepare a final report that fully explains the tool and documents the entire
research effort, explains and justifies recommendations, provides background infor-
mation used in the development of recommendations that addresses deficiencies and
recommends further research.
Consistent with the contract scope, the research team designed the tool using the typol-
ogy table collected in Phase I as the database and refined the tool using the selected mixtures
and existing screening models. The second phase of activity incorporated the data and opin-
ions garnered in Phase I to develop the tool itself. This phase encompassed the design and
construction of the tool, and the application of the tool to provide estimates of fate and
transport values for several representative mixtures. The tool was then used to determine
the effect of changes in these fate-and-transport parameters on the impact to soil and
groundwater after a release. CRP-CD-90: Chemical Mixture Tool for HMCRP Report 2, pro-
vided with this publication, contains the chemical mixture tool, an operational manual for
the tool, and the team's final research report for HMCRP Project 06.
Findings
The summary of the literature review and expert interviews is provided for the top-ranked
transported or spilled hazardous materials from the Spill Center; Association of American
Railroads (AAR); Commodity Flow Survey (CFS); Conestoga-Rovers & Associates emer-
gency response team; and the analysis of the incident reports database search from the
Office of Hazardous Materials Safety in the U.S. DOT. Fuels and ethanol-blended fuels,
alcohols, acids and bases, paints and related materials are the dominant hazardous material
classes according to commodity transported and incidents reported. As identified in the
interviews and literature review, the most important mixture is gasoline and ethanol. No
clear second mixture was identified, although Not Otherwise Specified (NOS) mixtures
clearly make up a large volume of what is transported.
The research team has reviewed and assimilated numerous scientific articles and agency
reports regarding chemical fate and transport and the methods for estimating the properties
of mixtures, including Raoult's Law, Universal Functional Activity Coefficient (UNIFAC),
Cosolvency Effect, and Linear Solvation Energy Relationship (LSER). The approach to esti-
mate the properties of chemical mixtures and the design of a tool have been provided for
both ideal and non-ideal chemical mixtures. Considering that UNIFAC has been more ver-
satile over other methods since it works for various solution systems, including those with
high nonideality, the research team built a Microsoft Excel UNIFAC (xlUNIFAC) to func-
tion as the basis of the chemical mixture tool. This feature is a major strength of UNIFAC and
is extremely valuable in estimating solubility of hydrophobic environmental contaminants
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in multiple-component systems, which are very difficult to characterize experimentally. In
the scenarios where xlUNIFAC does not function for a chemical mixture due to the lack of
the molecular volume and surface area (i.e., Rk and Qk) or the group interaction parameters
(i.e., anm), the cosolvent effects were incorporated into the tool as a second module to esti-
mate the solubilities of chemicals in case of the presence of major cosolvents.
A chemical mixture tool was developed to estimate the fate and transport properties of
chemical mixtures using the xUNIFAC model, Raoult's Law, and the Cosolvency--Log Kow
Model. The tool is capable of modeling a mixture containing up to 29 components. Approx-
imately 530 chemicals have UNIFAC group assignments, and the linear free energy relation-
ships (LFERs) between the cosolvency power and log Kow are included for 15 completely
water-miscible solvents, which are often used in industrial and environmental activities. The
output table of the tool consists of the following: (1) chemical identification (name, CAS#,
molecular weight), hazardous information (U.S. DOT Hazardous Class and UN/NA#);
(2) mixture characteristics (mass percent, volume percent, mole fraction); and (3) physical
chemical properties for the input mixture and its components (water solubility, vapor pres-
sure, surface tension, viscosity, partitioning among mixture/water/air, partitioning between
water and organic carbon/octanol, diffusion coefficients in air/water/mixture, and half-life
time), which can be used to simulate the characteristics of non-aqueous phase liquid (NAPL)
(where applicable) in soil, water, and air.
In tests with 11 representative mixtures--including gasoline, methyl tertiary butyl ether
(MTBE)-blended gasoline, ethanol-blended gasoline, coal tar, paint, ink, lacquer thinner,
and drycleaner solvent--the tool has been versatile at estimating the fate and transport prop-
erties of hazardous mixtures. Hydrocarbon Spill Screening Model (HSSM), Multiphase
Flow and Multicomponent Transport Model (MOFAT), and BIOSCREEN-AT are used as
screening models to simulate the fate and transport of selected mixtures in subsurface. Ben-
zene, toluene, ethylbenzene and xylene (BTEX), particularly benzene, was selected as the
target compound to analyze the impact of ethanol and MTBE on gasoline based on the
equivalent spill scenarios of oxygenate-free gasoline because benzene is the most mobile
gasoline-derived contaminant that possesses significant toxicity and groundwater impact.
Results indicated that the presence of 20% ethanol may cause a benzene plume in ground-
water to be 30% longer than that in equivalent gasoline under anaerobic conditions, while
there were no significant changes in benzene transport under aerobic biodegradation. The
MTBE addition to gasoline does not significantly affect the gasoline component transport.
However, the effect of MTBE itself on the environment is a concern due to MTBE's high
water solubility and low biodegradation under both aerobic and anaerobic conditions.
Conclusions
The chemical mixture tool was developed with a wide range of users in mind. For highly
technical users, the property output table generates the fate and transport properties of an
input mixture. For emergency response teams, the emergency response guide provides a
quick review of the emergency response to a spill. For non-technical users, a color-coding
function is included in the tool to compare the critical fate and transport properties to their
pure chemical counterparts and highlight the key parameters affecting the mixture transport
in the unsaturated (i.e., vadose) zone. A simplified version of Domenico's model, designed
by the research team, is included to simulate chemical fate and transport in groundwater.
Without the availability of external screening models, the research team screening model can
be applied directly to simulate the transport of a hazardous mixture in groundwater.
Tool comparison, calibration, sensitivity analysis, and uncertainty analysis showed that
the tool estimates mixture properties (e.g., interfacial tension and viscosity) within a mean
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error of 30% and the NAPL-water interface properties (e.g., solubility and partition coeffi-
cient) within a maximum factor of 5.0, which is relatively small compared with the imprecise
knowledge of subsurface gasoline release volumes and scenarios. When data are not avail-
able, this tool can be utilized to estimate the properties of a mixture.
The tool results can be used to determine whether shipping certain chemicals separately
or in mixtures will have significantly higher costs if an incident occurs, aiding in the emer-
gency planning costs. The tool results can be used to estimate the relative costs and time-
frames of cleanup after an incident occurs. The tool can also be used by remediation engi-
neers to provide better remediation alternatives, given the availability of different properties
of mixtures versus pure chemicals. For example, regulators and scientists could use the tool
to estimate the properties of novel additives in future fuel formulations and thereby provide
key inputs for determining the transportation facility upgrade and assessing environmental
transport of these compounds using external screening models.
Tool Limitations and Future Research Recommendations
The main limitation of the tool is that it cannot be used for all chemical substances con-
tained in the database of approximately 740 components derived from literary research and
interviews with professional personnel based on hazardous material classification and com-
modity flow survey and incident reports. That said, data for new chemicals can be added as
pure-phase information becomes available. The intended application domain is for liquid
organic chemicals, particularly petroleum and related compounds. Inorganic and organometal-
lic chemicals generally are outside the tool's domain. Future work may be focused on the ex-
pansion of the typology table database to a larger database to simulate mixtures that consist
of more chemicals. In addition, further research is needed to update the xlUNIFAC param-
eters with the latest available data. For example, as the largest database, the commercial
UNIFAC still lacks parameters for some halogenated compounds and new pharmaceutical
compounds.
This study is focused on the mixture source zone property estimate. Therefore, the chem-
ical property parameters were calculated based on the assumption that the NAPL and
groundwater reach equilibria for individual components. The kinetic process of the inter-
action zone was not considered, and the interaction between the NAPL source zone and the
dissolved plume in groundwater was not modeled in this study. For example, half-life time
was produced by the tool for anaerobic and aerobic conditions from the typology table,
which does not represent site-specific decay. Although the tool generates property parame-
ters with a factor of 5.0, field assessment is necessary to further calibrate the tool for modi-
fication to simulate the field spill scenarios. The tool will not be able to assess the property
changes with the temporal NAPL composition changes or the decay in the downgradient
groundwater. For example, the rapid transfer of ethanol from gasoline into the water in the
vadose zone (e.g., small volume spills) may not change the gasoline bulk transport proper-
ties (e.g., interfacial tension and viscosity) as predicted in this study. Furthermore, the quick
degradation of ethanol in the groundwater will alter the cosolvency power, as well as the
biodegradation of other components. Improved tool modification is required to integrate
the mixture degradation in the field, especially the ethanol effect on the biodegradation of
BTEX in the downgradient of the plumes.
A screening model has been incorporated within the tool for the users to simulate the fate
and transport in groundwater. However, the screening model results by HSSM and MOFAT
indicate that the mixtures have dramatically different transport in the unsaturated zone due
to changes in the density, interfacial tension, and viscosity. The behavior of the mixture in
the unsaturated zone will significantly affect the groundwater fate and transport. Therefore,
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combined with the existing groundwater model, further research is necessary to design a va-
dose zone screening model to simulate the mixture fate in subsurface based on the spill sce-
narios (e.g., spill volume, mixture component, and site-specific hydrogeologic setting). A
user-friendly unit conversion may be included within the tool to export the tool results directly
to the required form of screening models.
Although the color-coding function is designed in the tool to compare the fate and trans-
port characteristics of pure chemicals versus chemical mixtures in order to rank the relative
impacts to soil and groundwater, future research is needed to compare the concentrations
in subsurface to EPA clean up levels and to consider the cost and time frame of active reme-
diation compared to natural attenuation. This module may be designed to estimate the cost
of the most commonly used remedial approaches (e.g., groundwater pump-treat, air sparg-
ing, soil vapor extraction, chemical oxidation, and enhanced natural attenuation) at different
time frames after the incident spills.
The current version of the chemical mixture tool, provided on CRP-CD-90, was designed
and tested to work with the PC version of Microsoft Excel. Additional research may be
needed to modify the tool to work with a Mac system.
