13    THE EFFECT OF VARYING WATER CONTENT ON PASSIVE
VOLATILIZATION OF GASOLINE FROM SOIL
Michelle Smith, Undergraduate Student
Warren H. Stiver, Assistant Professor
Richard G. Zytner, Assistant Professor
School of Engineering, University of Guelph,
Guelph, Ontario, Canada
INTRODUCTION
Various regulations were implemented in the late 1980s to address the problem of leaking underground storage tanks. As a result, numerous sites have been identified as requiring remediation, with
the majority of these sites involving releases of gasoline.1 Since gasoline is a mixture of over 200
chemicals, the majority of which are generally highly volatile and toxic to plants and animals,2
prompt remediation is imperative. If not, the gasoline release will remain a potential source of
contamination for the atmosphere, surface waters and groundwater.Various remediation options for
clean-up of gasoline contaminated sites are available,3 including excavation and landfill,4 in situ
bioremediation,5 surfactant flushing,6 soil vapour extraction7 and passive volatilization.2
Passive volatilization is a low cost technique that relies on natural mechanisms to dissipate the
gasoline into the atmosphere. The rate at which this occurs depends on the concentration of gasoline
in the soil, the rate of gasoline wicking, the vapour pressure of each gasoline component, air movement on the soil surface and the condition and type of soil. Unfortunately, quantifying the impact of
these factors is difficult. Arthurs et al.8 investigated the passive volatilization of synthetic gasoline in
dry soils and observed that wicking was a major factor. However, when considering the use of passive
volatilization at typical sites, the impact of moisture in the soil must be considered. Consequently, the
objective of this research was to quantify the impact of soil water content on the rate of passive
volatilization.
To quantify the relationship between soil water content and the rate of passive volatilization of
gasoline, several experiments were conducted in the laboratory. A silt loam soil, with three moisture
contents, was contaminated with a six component synthetic gasoline mixture at a concentration
sufficient to ensure the presence of an immiscible phase. The rate of gasoline volatilization for both
total and individual components was then monitored over time. This paper will present the findings
and discuss the implication on remediation.
MATERIALS AND METHODS
The experiments have been conducted using a typical Ontario soil, Elora Silt Loam. The sand, silt,
clay and organic matter fractions are respectively, 34%, 50.1%, 15.9% and 2.5%. Prior to use, the
soil was sieved through a 2.0 mm mesh sieve and thoroughly mixed to improve consistency. The
saturated hydraulic conductivity was measured at 3.7 x 10"6 m/s.
The gasoline used to contaminate the soil was a six component synthetic gasoline. It was used to
simplify both chemical and data analysis and improve reproducibility. The mixture composition and
the chemical properties are summarized in Table I. The aromatic compounds are at concentrations
typical of commercial gasoline; while, the remaining bulk of the synthetic gasoline is comprised of
heptane and octane. n-Hexadecane was included as a low volatility control to indicate bulk gasoline
phase movement in the soil.
The volatilization studies were conducted in the apparatus depicted in Figure 1. These columns have
been used previously for chemical retention capacity experiments.10 The acrylic columns consisted of
an inner and outer sleeve. The outer sleeve provides structural support, while the inner sleeve is
segmented to provide easy and rapid sampling at various depths. The columns consisted of an 250 mm
49th Purdue Industrial Waste Conference Proceedings, 1994 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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