5.0 CONTAMINANT FATE AND TRANSPORT
This section discusses the physical and chemical processes that affect contaminant
migration in matrices at the Site. The properties of the chemicals detected beneath the Site are
reviewed, and the interactions of these chemicals within groundwater and surface water are
summarized. Difficulties and concerns associated with their presence in the subsurface also
are presented.
5.1 POTENTIAL ROUTES OF MIGRATION
Contaminant presence at the Site is apparently a result of operational activities at the
former location of the Untz Dry Cleaners during the years 1960 to 1975. During this period,
typical dry cleaning solvents were released to the environment. Results of sampling of several
matrices, including soil, sediment, surface water, and groundwater, by NCDEHNR in 1992 and
EPA in 1996 have indicated that contaminants associated with dry cleaning solvents (i.e., PCE)
and their potential degradation products (i.e., TCE, cis 1,2-DCE) are present in groundwater
beneath the site and have been detected in a local, unnamed stream northeast of the former
dry cleaner location. These constituents have not been detected in soil and sediment samples.
The reader is referred to Section 4 for a discussion of sample analytical results.
As indicated by previous sampling results, potential routes of migration for PCE and
associated degradation products within the area of the Site are principally through flow of water
within the groundwater. Continued use of groundwater by local residents as a water source
and natural hydraulic flow of groundwater will result in the continued migration and spreading of
contaminants. In addition, if groundwater is discharging to nearby streams, the possibility exists
that contaminants may be detected in the surface water bodies. The following paragraphs will
discuss the persistence of chlorinated hydrocarbons in the environment and factors affecting
the length of persistence.
5.2 CONTAMINANT PERSISTENCE
Although semi-volatile organic compounds, pesticides, and metals were detected in
groundwater samples from the Site, only volatile organic compounds were observed in
significant quantities above MCLs. Therefore, this discussion will focus on VOC properties.
5.2.1 Chemical Properties of Contaminants
From the perspective of groundwater and surface water contamination, the most
significant contaminant characteristic is solubility (Gorelick et. al., 1993). The solubility of a
solute is defined as the mass of the solute that will dissolve in a unit volume of solution under
specified conditions. The solubility defines the maximum possible concentration that commonly
occurs in groundwater or surface water, for any given contaminant.
North Belmont PCE Site
Remedial Investigation
North Belmont, Gaston County, NC
SESD Project No. 96S-058/June, 1997 5-1
, The most simple organic compounds are hydrocarbons, which consist solely of carbon
and hydrogen. Hydrocarbons can be divided into two classes, aromatic hydrocarbons, which
contain a benzene ring, and aliphatic hydrocarbons, that don't contain a benzene ring (Fetter,
1993). Hydrocarbons such as benzene, toluene, ethylbenzene, and xylene (BTEX) consist of
hydrocarbon molecules combined with aromatic compounds.
Halogenated organic compounds are characterized by a variable number of single,
double, and triple bonds and the presence of chlorine, bromine, fluorine, or iodine. These
compounds are widely used and have been frequently found as contaminants in groundwater.
Tetrachloroethene (PCE), trichloroethene (TCE), and 1,1,1-trichloroethane are examples of
these compounds. Each of these chemicals has a low flammability and a high vapor density,
which makes them very useful as solvents for the degreasing of metal parts. PCE and TCE are
denser than water, and if spilled on the ground in quantities great enough to overcome the
residual saturation, may migrate vertically downward through an aquifer (Fetter, 1993). They
are also soluble in water and can migrate in a dissolved phase in the direction of groundwater
flow.
Regardless of whether a liquid is composed of a single type of molecule, such as TCE,
or a mixture, it is the nature of the intermolecular bonding in the liquid that contributes to its
generally low solubility in water. Liquids with infinite solubilities (e.g., acetone) are referred to
as being miscible with water. Liquids with finite solubilities (e.g., trichloroethene) are generally
referred to as immiscible with water, even if the solubility is high.
Low density immiscible liquids, or light non-aqueous phase liquids (LNAPLs), will float
on the surface of the higher density groundwater and surface water. High density liquids, or
dense non-aqueous phase liquids (DNAPLs), sink through water until they reach the aquifer or
surface water bottom. Gasoline is an example of an LNAPL, and PCE and TCE are examples of
DNAPLs. While these liquids do not go completely into solution in groundwater, they do contain
compounds with limited solubilities in water (Gorelick et. al., 1993).
5.2.2 Site-Specific Chemical Properties
The various VOCs detected in soils and groundwater at the Site are classified as
halogenated aliphatic compounds (HACs); i.e., PCE, TCE, and 1,2-DCE. Table 5-1 lists
physical properties for the various organic contaminants detected in soils (ranked by their
aqueous solubility). Table 5-2 lists physical properties that can affect fate and transport of
contaminants in the surface and subsurface, as discussed in the previous sections.
HACs are characterized by open-chain structures; a variable number of single, double,
and triple bonds; and the presence of chlorine, bromine, fluorine, or iodine. HACs have many
applications, such as solvents, degreasers, dry cleaning agents, refrigerants, and organics
synthesis agents (Moore et al, 1984). PCE was the most frequently detected HAC in North
Belmont groundwater samples.
North Belmont PCE Site
Remedial Investigation
North Belmont, Gaston County, NC
SESD Project No. 96S-058/June, 1997 5-2
This section discusses the physical and chemical processes that affect contaminant
migration in matrices at the Site. The properties of the chemicals detected beneath the Site are
reviewed, and the interactions of these chemicals within groundwater and surface water are
summarized. Difficulties and concerns associated with their presence in the subsurface also
are presented.
5.1 POTENTIAL ROUTES OF MIGRATION
Contaminant presence at the Site is apparently a result of operational activities at the
former location of the Untz Dry Cleaners during the years 1960 to 1975. During this period,
typical dry cleaning solvents were released to the environment. Results of sampling of several
matrices, including soil, sediment, surface water, and groundwater, by NCDEHNR in 1992 and
EPA in 1996 have indicated that contaminants associated with dry cleaning solvents (i.e., PCE)
and their potential degradation products (i.e., TCE, cis 1,2-DCE) are present in groundwater
beneath the site and have been detected in a local, unnamed stream northeast of the former
dry cleaner location. These constituents have not been detected in soil and sediment samples.
The reader is referred to Section 4 for a discussion of sample analytical results.
As indicated by previous sampling results, potential routes of migration for PCE and
associated degradation products within the area of the Site are principally through flow of water
within the groundwater. Continued use of groundwater by local residents as a water source
and natural hydraulic flow of groundwater will result in the continued migration and spreading of
contaminants. In addition, if groundwater is discharging to nearby streams, the possibility exists
that contaminants may be detected in the surface water bodies. The following paragraphs will
discuss the persistence of chlorinated hydrocarbons in the environment and factors affecting
the length of persistence.
5.2 CONTAMINANT PERSISTENCE
Although semi-volatile organic compounds, pesticides, and metals were detected in
groundwater samples from the Site, only volatile organic compounds were observed in
significant quantities above MCLs. Therefore, this discussion will focus on VOC properties.
5.2.1 Chemical Properties of Contaminants
From the perspective of groundwater and surface water contamination, the most
significant contaminant characteristic is solubility (Gorelick et. al., 1993). The solubility of a
solute is defined as the mass of the solute that will dissolve in a unit volume of solution under
specified conditions. The solubility defines the maximum possible concentration that commonly
occurs in groundwater or surface water, for any given contaminant.
North Belmont PCE Site
Remedial Investigation
North Belmont, Gaston County, NC
SESD Project No. 96S-058/June, 1997 5-1
, The most simple organic compounds are hydrocarbons, which consist solely of carbon
and hydrogen. Hydrocarbons can be divided into two classes, aromatic hydrocarbons, which
contain a benzene ring, and aliphatic hydrocarbons, that don't contain a benzene ring (Fetter,
1993). Hydrocarbons such as benzene, toluene, ethylbenzene, and xylene (BTEX) consist of
hydrocarbon molecules combined with aromatic compounds.
Halogenated organic compounds are characterized by a variable number of single,
double, and triple bonds and the presence of chlorine, bromine, fluorine, or iodine. These
compounds are widely used and have been frequently found as contaminants in groundwater.
Tetrachloroethene (PCE), trichloroethene (TCE), and 1,1,1-trichloroethane are examples of
these compounds. Each of these chemicals has a low flammability and a high vapor density,
which makes them very useful as solvents for the degreasing of metal parts. PCE and TCE are
denser than water, and if spilled on the ground in quantities great enough to overcome the
residual saturation, may migrate vertically downward through an aquifer (Fetter, 1993). They
are also soluble in water and can migrate in a dissolved phase in the direction of groundwater
flow.
Regardless of whether a liquid is composed of a single type of molecule, such as TCE,
or a mixture, it is the nature of the intermolecular bonding in the liquid that contributes to its
generally low solubility in water. Liquids with infinite solubilities (e.g., acetone) are referred to
as being miscible with water. Liquids with finite solubilities (e.g., trichloroethene) are generally
referred to as immiscible with water, even if the solubility is high.
Low density immiscible liquids, or light non-aqueous phase liquids (LNAPLs), will float
on the surface of the higher density groundwater and surface water. High density liquids, or
dense non-aqueous phase liquids (DNAPLs), sink through water until they reach the aquifer or
surface water bottom. Gasoline is an example of an LNAPL, and PCE and TCE are examples of
DNAPLs. While these liquids do not go completely into solution in groundwater, they do contain
compounds with limited solubilities in water (Gorelick et. al., 1993).
5.2.2 Site-Specific Chemical Properties
The various VOCs detected in soils and groundwater at the Site are classified as
halogenated aliphatic compounds (HACs); i.e., PCE, TCE, and 1,2-DCE. Table 5-1 lists
physical properties for the various organic contaminants detected in soils (ranked by their
aqueous solubility). Table 5-2 lists physical properties that can affect fate and transport of
contaminants in the surface and subsurface, as discussed in the previous sections.
HACs are characterized by open-chain structures; a variable number of single, double,
and triple bonds; and the presence of chlorine, bromine, fluorine, or iodine. HACs have many
applications, such as solvents, degreasers, dry cleaning agents, refrigerants, and organics
synthesis agents (Moore et al, 1984). PCE was the most frequently detected HAC in North
Belmont groundwater samples.
North Belmont PCE Site
Remedial Investigation
North Belmont, Gaston County, NC
SESD Project No. 96S-058/June, 1997 5-2
