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By Michael J. Wade
Wade Research, Inc., Marshfield, Mass.
This paper reports the results of an assessment of groundwater
contamination at a study site the New England area of the
United States of America (USA). Historical monitoring efforts
found contaminated groundwater from gasoline range
hydrocarbons at monitoring wells at the study site as well as
in groundwater monitoring wells hydraulically down gradient
from the study site. Available chemical data consisted only of
groundwater concentrations of BTEX compounds (benzene,
toluene, ethylbenzene and the xylene isomers) and the
oxygenate additive methyl t-butylether (MtBE). No other
chemical data were available and no possibility of collecting
additional samples for chemical analysis existed. Laboratory
data were collected by different contractors over an eight
year time period from 1988 to 1996 using different standard
analytical techniques.
Background Information on the Chemical Fate of
Gasoline-Range Hydrocarbons in Groundwater
Detailed chemical analyses of component changes can be used to
determine different source(s) for petroleum hydrocarbons and
also help to estimate the time of onset of all identified
sources. In most subsurface investigations in the United
States, reporting of the groundwater concentrations of
selected monoaromatic hydrocarbons benzene, toluene,
ethylbenzene and the three isomers of xylene (o-xylene,
m-xylene, and p-xylene) collectively referred to
as the BTEX compounds, using standard analytical methods is
required by regulatory agencies. Much of the time, BTEX data
are all the chemical data that are available for assessment
without going to the expense of additional analytical monies,
which can be considerable depending on the analytical testing
selected. When BTEX component data are all that are available,
relatively simple data analysis techniques can be useful in
the determination of separate sources and different times of
release of each identified source. Such data can guide
selection of types and locations for any further sampling and
detailed high-precision chemical analyses of gasoline that can
be used to confirm initial findings from the BTEX data. More
often than not, however, additional analytical testing does
not contribute significantly to an advancement of knowledge
over and above what can be gained using the BTEX data alone.
Gasoline hydrocarbons released to the subsurface environment
undergo numerous physical and chemical interactions over time.
Published research on the behavior of petroleum components in
the environment has identified the principal governing factors
that influence changes in petroleum composition (TRC
Environmental Consultants, Inc. 1985; Thomas et al.
1998; Miralles-Wilhelm et al. 1993.; Hubbard et al.
1994; Mormile et al. 1994). Component changes in
petroleum products are made from vaporization, photolysis,
hydrolysis, solubilization, adsorption/desorption, and
microbial degradation. Once petroleum is released into the
environment, these processes begin to alter the composition of
the petroleum and over time their effects can be observed.
This process is collectively referred to as petroleum product
weathering.
In the subsurface environment, the effects of photolysis and
vaporization are minimal. Hydrolysis of individual petroleum
components is dependent upon the chemical structure of
individual components. Solubilization of petroleum products is
dependent upon the partitioning of individual petroleum
components between soil, water and petroleum matrices.
Distribution and redistribution of petroleum products and
major components within each petroleum product is most heavily
influenced by adsorption/desorption reactions between soil,
groundwater and petroleum products themselves. The most
important process governing the composition of petroleum
products in the subsurface is microbial activity.
The type of microbial degradation of petroleum hydrocarbons in
the subsurface is governed by the availability of oxygen.
Oxygen-governed microbial degradation is referred to as
aerobic degradation. Having ample supplies of dissolved oxygen
in the subsurface will speed degradation of petroleum
components. In the absence of appreciable concentrations of
oxygen, other species such as nitrate, sulfate, and metals
such as iron will be used as a source of oxygen (or electrons)
for oxidation/reduction reactions. Such degradation of
petroleum hydrocarbons in the subsurface in the absence of
oxygen is termed anaerobic degradation. Anaerobic hydrocarbon
degradation is generally regarded as a slower process compared
to aerobic hydrocarbon degradation.
Scientific studies of gasoline degradation in the subsurface
have focused on the effects of changes on major components
such as the BTEX compounds over time under controlled
circumstances. Review of such studies following concentrations
of BTEX components over distance and time provides an
understanding of the basic changes that occur among the BTEX
compounds as gasoline weathers. A controlled subsurface
contamination experiment was completed in an aerobic
unconfined shallow sand aquifer in Ontario, Canada (Hubbard
et al., 1994). The final report provided chemical data on
the transport and fate of BTEX compounds.
Analysis of petroleum components using ternary analysis has
been reported (Luhrs and Pyott 1992). In fact, use of ternary
analysis has been underway to show differences in soil and/or
rock types for decades (e.g., USGS Professional Papers
317-319, 1959; Hunt 1979). In this paper, ternary analysis has
been expanded, providing details on hydrocarbon interaction
over time and space.
While a similar general behavior was shown for all three
different types of gasoline, there were important differences
in the degradation of BTEX compounds in the 85% methanol
gasoline compared to the other two types of gasoline. A
continuation of individual data trends would eventually result
in the complete loss of toluene and ethylbenzene with the
enrichment of the _Xylenes number to the vicinity of 1.0 for
MtBE and PS-6 gasoline, while for 85% methanol gasoline the
proportions could be expected to be somewhat different. Based
on these data, older gasoline could be expected to have
elevated relative contributions of _Xylenes and proportions of
individual BTEX components would be skewed towards the bottom
right side of the ternary data plot. Such effects have been
reported previously (Wade, 1997; Siegel 1999)).
Over the 476 days of the experiment, the relative proportions
of ethylbenzene and total sum of all xylene isomers (_Xylenes)
were observed to decrease relative to the proportion of
toluene, which was shown to remain relatively constant at 0.9
relative to the other two components of this type of data
treatment. In terms of concentration (mass), all three of
these BTEX components were observed to decrease over time; it
is only with a ternary data display treatment that the
relative behaviors of individual BTEX components become
apparent.
Ternary Analysis of BTEX Components at the
Study Site
The BTEX groundwater data from samples taken at a study site
in the New England area of the United States of America using
a historical data set covering a span of eight years. Due to
site access and budget considerations, no additional data
collection efforts were possible. Accordingly, all existing
data were examined to assess the relative contributions of
individual BTEX compounds at all sampling points from the
first sample collection and laboratory analysis event in 1988,
to the newest set of groundwater analyses in April 1996. Data
were obtained on concentrations of groundwater BTEX compounds
and MtBE using historical data compiled by others starting in
1988 and ending in 1996. Confirming data from the analysis of
groundwater from monitoring well KOW-1 made in 1996 were also
reviewed.
Analysis of the entire data set, running from 1988 through
1996 showed a dramatic shift in the positioning on the ternary
plot of monitoring well KOW-7. The shift in the position of
the 1996 KOW-7 data identifies the time horizon when the
second gasoline arrived at monitoring well KOW-7, that is
sometime between sampling in 1995 and again in 1996.
Direct comparison of specific hydrocarbon degradation rates
between the study site data and the Hubbard et al.
(1994) data would not be appropriate because not only do
individual degradation rates differ at different sites, but
degradation rates within one site can differ from location to
location as well. However, a comparison of the relative
degradation state of BTEX hydrocarbons within the study
site is relevant. From the historical data, it is reasonable
to expect that the times of release of the gasoline found at
KOW-1 and KOW-3 in 1988 and 1989 were similar to each other,
with the gasoline hydrocarbons at KOW-1 and KOW-3 in 1994,
1995 and 1996 were indicative of the same gasoline only older
and in a more degraded state.
It was interesting to note that the gasoline found in
monitoring wells KOW-1 and KOW-3 in 1996 showed relative BTEX
distribution of degraded gasoline while at the same time
having the significant concentrations of the oxygenate
additive MtBE. The persistence of the oxygenate additive MtBE
to subsurface microbial degradation has been reported (Barker
et al., 1991; Mormile et al., 1994).
Consequently, data from monitoring wells KOW-1 and KOW-3 over
the time period of 1988 to 1996 clearly document the release
of MtBE-treated gasoline.
From the analysis of historical groundwater BTEX data, it was
concluded that the gasoline hydrocarbon distribution in the
vicinity of KOW-1 and KOW-3 was newer than the gasoline
represented by the hydrocarbon distribution in the vicinity of
KOW-6 and KOW-7. The microbial degradation state of gasoline
hydrocarbons at these KOW-1 and KOW-3 at the Site are
different, with more advanced degradation at KOW-6 and KOW-7.
Presence of the Oxygenate Additive Methyl t-butylether
(MtBE)
The conclusions from the ternary analysis of historical BTEX
data were substantiated from an analysis based on finding the
oxygenate additive methyl t-butylether (MtBE) at various
monitoring wells. During the 1970s and 1980s, automobile
gasoline formulation in the United States was undergoing a
period of important changes. The use of octane enhancing
chemicals such as alkylated lead and manganese-based compounds
was being curtailed, while the development of oxygenate
additive compounds such as methyl t-butylether (MtBE) and
others was being expanded. The timing of the use of MtBE in
gasoline markets around the United States provides an
age-dating approach for subsurface gasoline contamination,
provided such data are used together with data on the
site-specific degradation of gasoline hydrocarbons.
Starting in the 1970s, the U.S. Environmental Protection
Agency began an orderly phase down on the use of lead
antiknock additives in U.S. gasoline from 1.2 grams per gallon
(gpg) to a maximum of 0.5 gpg. There were some interruptions
in the leaded gasoline phase down and it was not a smooth
reduction. For example, as a consequence of the Arab Oil
Embargo, a one-year delay in the phase down was approved by
President Carter in 1979-1980 to allow major domestic
producers to market leaded gasoline containing 0.8 grams per
gallon until October 1, 1980 (Ethyl Corporation Annual Report,
1978). Also as a consequence of reduced gasoline supplies in
the U.S. during the late 1970s, a manganese-based antiknock
compound MMT was used in unleaded gasoline grades for the peak
gasoline driving time in 1979. Otherwise MMT was approved for
use only in leaded gasolines. Subsequently in the USA, use of
MMT was banned altogether later in the 1980s.
Use of MtBE in the United States automobile gasoline supply
for unleaded gasoline was approved by the U.S. Environmental
Protection Agency in March 1979 (Federal Register, Vol 44, No.
45, Tuesday, March 4, 1979). As millions of barrels of
gasoline were consumed daily in the USA during the 1970s and
1980s (Petroleum Supply Annual, 1992 and 1993), MtBE-treated
gasoline was not immediately available at all areas of the
country simply because of the approval by the EPA. In the USA
during the decade of the 1980s, various gasoline manufacturers
introduced MtBE-treated gasolines at different times. In fact,
it is important to know which gasoline supplier serviced which
areas of the USA in the decade of the 1908s when using MtBE
data to determine timing of subsurface gasoline releases.
Using such data, gasoline hydrocarbons containing MtBE found
in the groundwater at monitoring wells KOW-1 and KOW-3 in 1988
and 1989 had to have been released to the subsurface
environment in the time frame no earlier than in the early
1980s. And MtBE-formulated gasolines could have been released
later than this time frame if the gasoline were a formulation
of another gasoline retailer. In such cases, knowledge of the
gasoline supplier to any particular study site becomes
important. In this case, site-specific knowledge of gasoline
supplier ruled out one supplier of MtBE-treated gasoline in
favor of another supplier. Such information was consistent
with the finding of two different gasolines in the ternary
analyses.
Conclusions
A chemical data set from groundwater samples collected at a
coastal property in Massachusetts, including the BTEX
compounds and the oxygenate additive MtBE (covering a span of
eight years from 1988 to 1996) was examined. The historical
data were used to determine the type(s) of subsurface
contamination present, estimate the age of onset of any
identified subsurface petroleum contamination, and evaluate
data on an independent groundwater sample collected and
submitted to an independent analytical laboratory to confirm
or refute findings from the historical data.
Two different gasoline formulations were apparent from the
ternary analysis of BTEX compounds. Analysis results showed
the effect of hydrocarbon weathering resulting in the
conclusion that there are separate types of gasolines
represented in the historical data. Specifically, two
different gasolines were identified as having been released at
the Site: an older more degraded gasoline that did not contain
MtBE and a newer less degraded gasoline that contained higher
concentrations of MtBE.
Based on the composition of gasoline-range hydrocarbons and
their degradative states, it was estimated that the gasoline
at monitoring wells KOW-1 and KOW-3 was probably released in
the mid-to later-1980s after higher amounts of MtBE had begun
to be used in gasoline formulations. Based on the extent of
hydrocarbon degradation and the presence of MtBE at monitoring
wells KOW-1, KOW-3, and MW-1, it was concluded that in
subsequent years, this older gasoline had undergone weathering
reactions and had moved away from its apparent source.
Compared to BTEX concentrations at KOW-1 and KOW-3, much lower
hydrocarbon concentrations were found at monitoring wells
KOW-6 and KOW-7 throughout most of the data set with a
dramatic shift in the ternary position of KOW-7 in 1996 data.
The hydrocarbon distributions at these two off site wells
exhibited advanced hydrocarbon degradation patterns and
contained no MtBE. This type of contamination was indicative
of older gasoline, formulated before the early to mid-1980s,
with arrival of a second gasoline at KOW-7 sometime between
1995 and 1996.
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