Oil & Natural Gas Projects
Exploration and Production Technologies
Improving Science-Based Methods for Assessing Risks Attributable to Petroleum
Residues in Soil Transferred to Vegetation
This project was funded through DOE's Natural Gas and Oil Technology Partnership
Program. The program establishes alliances that combine the resources and experience
of the nation's petroleum industry with the capabilities of the national laboratories
to expedite research, development, and demonstration of advanced technologies
for improved natural gas and oil recovery.
The overarching goal of this study was to improve risk-based decision making
at petroleum-contaminated sites. The specific goal was to directly measure soil-to-vegetation
bioconcentration factors for a series of polycyclic aromatic hydrocarbons (PAHs)
and high molecular weight n alkanes. These compounds are relevant to phytoremediation
sites and appropriate for establishing risk-based screening levels (RBSLs) at
hydrocarbon-impacted exploration and production sites. The ultimate objective
was to generate high-quality data to support the development and/or experimental
evaluation of models describing the potential accumulation of soil-borne hydrocarbon
residues in vegetation.
Lawrence Berkeley National Laboratory
University of California
Petroleum Environmental Research Forum (PERF 99-13)
ChevronTexaco Research & Technology Division
Over the course of this project, researchers constructed a highly controlled
chamber system for measuring the transfer of soil-borne contaminants into vegetation
over environmentally relevant time scales and plant-growth conditions. The system
was initially tested using wheat grass grown in agricultural soils spiked with
a range of PAHs and high molecular weight n-alkanes.
Because empirical data on the transfer of soil-borne organic contaminants to
vegetation are sparse and models for estimating this transfer are highly uncertain,
current methods for establishing risk-based screening levels typically do not
consider dietary exposure to food or feed grown on or near a contaminated site.
This project contributes relevant data and helps reduce uncertainties that severely
limit the reliability of existing plant uptake models. Through this effort,
researchers are improving the reliability of risk assessments in which the critical
and (often) most uncertain element is the accumulation of chemicals in plants.
Of particular value is the development of an experimental chamber system that
provides a unique opportunity to examine the combined role of uptake and loss
through multiple pathways over the life cycle of the plant or crop. The experimental
system will continue to provide an opportunity to improve regulatory models.
The data generated through this research should contribute to improved mechanistic
modeling capabilities for evaluating terrestrial food-chain exposure pathways.
The main benefits will be increased confidence in models that link soil residues
to risk, resulting in the increased use of risk and risk-benefit concepts by
industry, government, and public health organizations.
Both ecological and human health risk analyses rely on models that link soil
residue concentrations to exposure concentrations in vegetation. These models
are limited by a lack of quantitative information about direct (soil-to-plant)
and indirect (soil-air-plant) transfers of pollutants into vegetation. This
is particularly true for the contaminants and conditions that are relevant to
phytoremediation sites and upstream exploration and production sites. Existing
models use empirical relationships developed from a relatively small number
of plant species and chemicals where the uptake of pollutant into vegetation
is often assumed to follow a linear plant-soil partitioning relationship derived
from the octanol/water partition coefficient (Kow). Existing models ignore transformation
processes, variations among vegetation types, and competing uptake pathways.
As a result, linking chemical residues in soil to exposure concentrations for
food or feed has a high level of uncertainty. Without an improved mechanistic
understanding of plant uptake, the credibility of current risk-based analyses
of soil residues will remain limited.
The project researchers:
- Constructed a controlled environmental chamber system for plant uptake measurements.
- Developed laboratory capabilities for analyzing petroleum hydrocarbon mixtures
extracted from complex environmental matrices (plants and soil).
- Completed an initial chamber evaluation using wheat grass grown in agricultural
- Completed sample collection efforts for related field study.
As a first step in this project, researchers modified chambers at the Controlled
Environment Facility at the University of California-Davis to reduce ambient
levels of semi-volatile organic chemicals in the air. This made possible long-term
exposure studies. Then they collected soil from an organic farm and spiked the
soil with PAHs and n-alkanes at various concentration levels. The spiked soils
were allowed to age and then planted with wheat grass in pots and placed in
the exposure chamber using a grid layout, where the different concentration
levels were distributed uniformly across the growth area. The combination of
distributed soils, reduced ambient air concentrations, and the well-mixed air
compartment in the chamber made it possible to evaluate soil-to-plant transfer
separately from soil-to-air-to-plant pathway.
Preliminary findings for 12 PAHs indicate that:
- The concentration ratio between grass and soil are linear over a large concentration
range, demonstrating that uptake processes in the chamber had not reached
- The concentration ratios for the PAHs were not influenced by elevated levels
of n-alkanes in the soil.
- The measured concentration ratios were generally below those predicted by
an empirical Kow-based model-particularly for the 2- and 3-ring PAHs-and not
linear relative to the chemical's Kow. Measurements of the n-alkanes (C20-C30)
revealed no accumulation in the above-ground vegetation over a wide range
of contaminant concentrations in the soil. This indicates that this particular
class of chemical may not accumulate in vegetation from soil. However, the
uptake of n-alkanes has not been measured previously, and the current results
have not yet been corroborated, so these results should not be extrapolated
to other scenarios.
Current Status (August 2005)
This project was completed in April 2005. The results from the initial study
led to two subsequent proposals for work designed to use and improve the study
system to further address data limitations and modeling gaps for plant uptake
of organic chemicals from contaminated soils. A large set of chamber-generated
samples and field samples have been archived for future analysis, funding permitting.
Bimonthly progress reports submitted to DOE and available online at http://126.96.36.199/ngotp/ngotp.htm.
Maddalena, R.L., McKone, T.E., and Kado, N.Y., Exposure Chamber Measurements
of Mass Transfer and Partitioning at the Plant/Air Interface, Environmental
Science and Technology 2002, 36, 3577-3585.
Maddalena, R.L., and Sohn, M.D., Extending Sensitivity Analysis Methods beyond
the Input/Output Relationship, ISEA/ISEE Conference, Vancouver, BC, Canada,
Kobayashi,R.; Okamoto,R.A., Maddalena, R.L., and Kado, N.Y., Measurement and
Mechanism of PAH Uptake in Wheat Grains, poster presentation at Toxic Substances
Teaching and Research Program Annual Meeting, Oakland, CA, 2003.
Maddalena, R.L., Kobayashi, R., McKone, T.E., and Kado, N.Y., Controlled Chamber
Measurements of the Multipathway Uptake of PAHs from Soil into Wheat, paper
presented at the Society of Environmental Toxicology and Chemistry Annual Meeting,
Austin, TX, 2003.
McKone, T.E., and Maddalena, R.L., Soil-to-plant bio-concentration factor (BCF)
estimates: Experimental variability versus model uncertainty, SETAC North America,
November 14-18, 2004, Portland, OR.
Project Start: April 17, 2002
Project End: April 16. 2005
Anticipated DOE Contribution: $660,000
Performer Contribution: $75,000 (in kind support, about 11% of total)
NETL - Rhonda Jacobs (firstname.lastname@example.org or 918-699-2037)
LBNL - Tomas McKone (TEMcKone@lbl.gov or 510-486-6163)
Predicting exposure concentrations in vegetation using a multimedia modeling
framework requires bioconcentration ratios that account for the range of uptake
and loss pathways.
Controlled Environment Facility at the University of California-Davis, was fitted
with an air intake filtration system designed to remove volatile and semi-volatile
organic pollutants, providing an opportunity to track the long-term transfer
of soil-borne chemicals into vegetation and to monitor specific uptake pathways.