U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of
the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 3
of 3--Subsurface Contamination From Point Sources, Water-Resources
Investigations Report 99-4018C
KEYNOTE PAPER
Scale Considerations of Chlorinated Solvent Source Zones And
Contaminant Fluxes: Insights From Detailed Field Studies
By Beth L. Parker and John A. Cherry, Department of Earth Sciences
University of Waterloo, Waterloo, Ontario, N2L 3G1
Chlorinated solvents are the most common contaminants of industrial
origin found in groundwater in industrialized countries. They pose
exceptional challenges for monitoring and remediation because they
commonly penetrate deeper than other contaminants and they are
persistent and mobile. The increasing dependence on risk
assessment and natural attenuation for site management decisions
and mass removal or in situ mass destruction for groundwater
remediation has created an urgent need for improved information on
the subsurface distribution of solvent mass and the contaminant
mass flux in groundwater. Since 1981 the University of Waterloo
has used the Borden research site situated 50 km northwest of
Toronto, Ontario, for many experiments involving the release of
chlorinated solvents, including experiments in which free-phase
solvents (DNAPLs) are released directly into the shallow sandy
aquifer. Visual examination of solvent DNAPL distributions (TCE
and PCE) in excavations at Borden and in aquifer cores indicates
strong influence of small-scale or subtle variations in sediment
texture and at greater depth, the influence of fractures and thin
discrete sandy beds in the clayey aquitard. These observations in
DNAPL zones demonstrated the need for data acquisition at much
smaller spatial scales than was necessary when Borden experiments
involved only aqueous-phase releases for attenuation studies. The
Borden site however offers only a narrow set of geological
conditions, and a short time frame of contamination relative to
actual contaminated sites.
In a new phase of research initiated three years ago, with continued
focus on the distribution and fate of chlorinated solvents in
groundwater, we have expanded the field studies to include twelve
contaminated industrial sites in Canada and the United
States. Also, a second site for solvent release experiments has
been established on fractured clay (the Laidlaw site in southern
Ontario), while use of the Borden site for sand aquifer
experiments continues. Three of the industrial sites are on
fractured clay, three on fractured sedimentary rock (sandstone,
shale and dolomite) and the rest on surficial sand aquifers of
various origins underlain by clayey aquitards. Therefore a
portfolio of sites are available to us and our team of graduate
students, research associates and technicians; each site offers
sufficient hydrogeologic simplicity and monitoring feasibility for
studies to answer specific questions in practical time
frames. Except for the fractured rock sites, all of the solvent
contamination occurs at depths less than 30 m in deposits easily
penetrated by direct-push equipment for continuous coring and
groundwater sampling. To expand the usefulness of the sites and to
accomodate the variety of site conditions, existing sampling
equipment is modified and new types of equipment are developed. In
site selection priority is given to hydrogeologic simplicity and
also their simplicity of contaminant types and input histories,
which greatly enhances prospects for meaningful data
interpretation. However even with this relative simplicity, the
number of factors influencing the contaminant distribution and
behaviour is large. The potential to accomodate more complexity in
site conditions is increasing with experience and equipment
improvements. The sites also must have cooperative site owners,
minimal litigation activity, and good terrain access. The field
studies are complemented by lab measurements of parameters such as
porosity, hydraulic conductivity, effective diffusion
coeffieients, fraction organic carbon and other sorption-related
parameters. Large column experiments (0.3 - 0.5 m diameter) of
relatively undisturbed sand or fractured clay are used to assess
particular affects of geologic structure on DNAPL and solute
behaviour.
At solvent DNAPL sites nearly all of the DNAPL normally resides below
the water table, which causes a plume of dissolved-phase
contamination to emanate from the source zone. However only in
rare cases does a site offer good prospects for investigation of
both the source zone and the plume. Therefore at some sites
studies emphasize the source zone and at other sites the
plume. Ultimately the smallest spatial scale relevant to the
understanding of the behaviour and fate of solvents is the
diffusion scale. This scale becomes most important for the time
scales relevant to most contaminated industrial sites where DNAPL
entered the subsurface decades ago. The field studies have in
common the collection of thousands of samples of water and
sediment or rock for chemical analyses. The samples are obtained
from vetical holes to provide detailed profiles of concentration
versus depth. The vertical space between samples for volatile
contaminant analysis (VOC) is tailored to sediment type and age of
the contamination, or in some cases sampling is done at scales as
small as is feasible, which for groundwater sampling can be 0.2 -
0.4 m vertical spacing in sediment and several meters in fractured
rock. Smaller vertical spacing of groundwater samples is generally
not appropriate because of limitations imposed by disturbance of
in situ conditions caused by purging prior to sampling. Data
acquisition is accelerated by rapid on-site VOC
analyses. Subsamples from sediment or rock cores are commonly
taken at even smaller intervals, as close as a few millimeters in
some cases. The sediment and rock core VOC analyses provide total
concentrations that are connected to dissolved concentrations in
pore water and concentrations sorbed on solids. In some situations
the sample spacing is extremely small because the scale of
diffusion effects is uncertain a priori and therefore sampling
feasibility rather than prediction of contaminant distributions is
the overriding factor. Networks of conventional monitoring wells
exist at most of the sites and therefore comparisons are made
between three sampling scales: wells, discrete-sample groundwater
profiles and core subsampling.
Some preliminary conclusions can be drawn. Regardless of the
depositional origin of the sandy aquifer, DNAPL accumulation zones
(source zones) in sandy aquifers are comprised of many thin layers
of DNAPL rather than substantial free-product pools. In some cases
the DNAPL layers are associated with silt or clay layers and in
other cases the control by geologic layering is much more subtle
or not discernable. At locations where the DNAPL rests on top of a
thick clayey layer, diffusion-scale profiles from the DNAPL into
the clay are used to define the bottom of contamination and to
identify advection effects and also to estimate the time of
arrival of DNAPL at the location. Diffusion zones in clayey layers
situated within the aquifer can comprise a large percentage of the
total aqueous and sorbed mass within plumes. In sandy aquifers
vertical diffusion from suspended DNAPL layers combined with
lateral advection produces mixing that smooths the vertical
concentration distributions within plumes short distances
downgradient from the source zones. The tops and in some cases the
bottoms of plumes are abrupt over vertical distances of less than
a meter, which suggests diffusion control with advective influence
on apparent vertical transverse dispersion.
Vertical profiles in sandy aquifers along sections (transects) across
plumes at locations a short distance downgradient of the source
zones show extreme variability of solvent concentrations. They
show that much of the total contaminant plume mass flux occurs in
small zones. Estimates of total annual contaminant mass discharge
through these transects provides insight into the evolution the
the source zones during previous decades and their future
longevity. At sites where a network of conventional monitoring
wells were used prior to our investigations, plume characteristics
from these networks were generally indistinct or misleading when
compared to our delineation at the finer sampling scale.
In clayey aquitards, diffusion zones (haloes) along fractures and along
thin horizontal sandy layers indicate the pathways of DNAPL
flow. Natural fractures in clayey deposits are commonly so small
(10-50 µm apertures) that solvent DNAPL entering the
fractures disappears quickly from most fractures due to
diffusion. The diffusion-driven chemical mass transfer that puts
the solvent mass, as aqueous and sorbed phases, into the matrix
blocks between fractures. The matrix blocks, which have porosity
in the 30-50% range, offer large solvent mass storage capacity
relative to the fracture network. Small-scale mapping of diffusion
haloes along fractures provides information on channeling of DNAPL
flow and indicates variable DNAPL arrival times along fractures
within the same local zone.
In fractured sedimentary rock such as sandstone the porosity of the
matrix blocks (5-15%) is much smaller than the matrix porosity of
the clayey deposits. However over decades this rock porosity is
large enough for measurable diffusion haloes to form along the
fractures where DNAPL flow or solute transport has occurred. The
haloes and therefore the pathways are determined from analysis of
subsamples of rock core. This approach for pathway identification
has proven to be more precise and more reliable than the
conventional approaches of well sampling or open borehole packer
sampling that lump concentrations from various fractures, or that
are influenced by borehole cross-contamination that is usually
unavoidable due to natural hydraulic gradients. The rate of
expansion of plumes in fractured sandstone and other sedimentary
rocks can be severely restricted by the diffusion-driven chemical
mass transfer from fractures where active flow occurs to the
matrix blocks where the pore water is relatively
immobile. Site-specific proof of this retardation of plume
expansion lies in determination of the chemical mass distribution
in the rock matrix.
In most of the hydrogeological environments being studied, diffusion has
caused much solvent mass to enter low-permeability zones, such as
clayey layers within or below sandy aquifers, or matrix blocks
between fractures in clay or rock. This mass distribution causes
futility of groundwater restoration efforts involving
advection-based approaches such as pump-and-treat and chemical
flushing. Remediation possibilities are enhanced when the
technologies are capable of seeking out the solvent mass in the
diffusion-controlled zones.
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