EPA/600/R-17/293
September 2017
xvEPA
U nited States
Environmental Protection
Agency
u
Best Practices for Environmental Site Management:
A Practical Guide for Applying Environmental Sequence
Stratigraphy to Improve Conceptual Site Models
Michael R. Shultz1, Richard S. Cramer1, Colin Plank1, Herb Levine2, Kenneth D. Ehman3
BACKGROUND
CONTENTS
Background
1
1. Introduction - The Problem of Aquifer
Heterogeneity
3
Impact of Stratigraphic Heterogeneity on
Groundwater Flow and Remediation
4
Sequence Stratigraphy and Environmental
Sequence Stratigraphy
4
II. Depositional Environments and
Facies Model!
7
Facies models for fluvial systems
10
Glacial geology and related depositional systems
10
III. Application of Environmental Sequence
Stratigraphy to More Accurately
Represent the Subsurface
12
Phase 1: Synthesize the geologic and
depositional setting based on regional geologic
work
12
Phase 2: Formatting lithologic data and
identifying grain size trends
16
Phase 3: Identify and map HSUs
19
Conclusions
22
References
24
Appendix A: Case Studies
A1
Appendix B: Glossary of terms
61
This document was prepared under the U.S. Environmental Protection Agency
National Decontamination Team Decontamination Analytical And Technical Service
(DATS) II Contract EP-W-12-26 with Consolidated Safety Services, Inc. (CSS),
10301 Democracy Lane, Suite 300, Fairfax, Virginia 22030
'Burns & McDonnell
2U.S. EPA
^Chevron Energy Technology Company
This issue paper was prepared at the request of the
Environmental Protection Agency (EPA) Ground Water Forum.
The Ground Water, Federal Facilities, and Engineering Forums
were established by professionals from the United States
Environmental Protection Agency (USEPA) in the ten Regional
Offices. The Forums are committed to the identification
and resolution of scientific, technical, and engineering
issues impacting the remediation of Superfund and RCRA
sites. The Forums are supported by and advise the Office
of Land and Emergency Management's (OLEM) Technical
Support Project, which has established Technical Support
Centers in laboratories operated by the Office of Research
and Development (ORD), Office of Radiation Programs, and
the Environmental Response Team. The Centers work closely
with the Forums providing state-of-the-science technical
assistance to USEPA project managers. A compilation of issue
papers on other topics may be found here:
http://www.epa.gov/superfund/remedytech/tsp/issue.htm
The purpose of this issue paper is to provide a practical guide
on the application of the geologic principles of sequence
stratigraphy and facies models (see "Definitions" text box,
page 2) to the characterization of stratigraphic heterogeneity
at hazardous waste sites.
Application of the principles and methods presented in this
issue paper will improve Conceptual Site Models (CSM)
and provide a basis for understanding stratigraphic flux and
associated contaminant transport. This is fundamental to
designing monitoring programs as well as selecting and
implementing remedies at contaminated groundwater sites.
EPA recommends re-evaluating the CSM while completing the
site characterization and whenever new data are collected.
Updating the CSM can be a critical component of a 5 year
review or a remedy optimization effort.
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DEFINITIONS
Sequence Stratigraphy: The study of
sedimentary deposits in the context of their
depositional environments and changes
in relative sea-level, sediment supplyand
available sediment storage areas.
Fades Model: Conceptual construct describing
the processes acting in a particular depositional
environment to transport, deposit, and
preserve sediment, usually presented as a
three-dimensional block diagram illustrating
the organization of sedimentary bodies in the
stratigraphic record.
These methods are applicable to sites underlain by
clastic sedimentary aquifers (e.g., intermixed gravels/
sands/silts/clays). The scientific principles and
methods presented in this document bring clarity
to the challenges posed by lithologic heterogeneity
thereby facilitating successful site management
strategies. Lithologic heterogeneities can be
characterized by the use of high resolution site
characterization (HRSC) techniques (http://www.
cluin.org/characterization/technologies/hrsc/). The
application of sequence stratigraphy can be applied
to new site investigations as well as existing site data
to update the Conceptual Site Model (CSM). These
methods allow the practitioner to place environmental
subsurface data in a geologic and hydrogeologic
context, and predict the geology where subsurface
data are absent.
Application of Environmental Sequence Stratigraphy
and facies models benefit groundwater remediation
projects by improving the ability to:
1. Interpret lateral continuity between borehole
data and correlate site data in three dimensions;
2. Identify groundwater flow paths and
preferential contaminant migration pathways;
3. Map and predict contaminant mass transport
(high permeability) and matrix diffusion-
related storage (low permeability) zones;
4. Identify data gaps and assess the need and
cost benefit of high resolution site
characterization;
5. Determine appropriate locations and screen
intervals for monitoring and remediation wells,
and;
6. Improve efficiency of remediating and
monitoring of contaminated groundwater.
The first two sections (I and II) present the technical
basis of Environmental Sequence Stratigraphy.
Section III presents a three phase process for practical
application of Environmental Sequence Stratigraphy,
ending with stratigraphic "rules of thumb" developed
through experience in a wide variety of environments
of deposition from outcrop and subsurface data sets
worldwide. Appendix A presents six case studies
of various applications of Environmental Sequence
Stratigraphy and Appendix B is a glossary of terms
used in the document.
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I. INTRODUCTION - The Problem of Aquifer Heterogeneity
Permeability heterogeneity is inherent in the
subsurface and interacts with regional groundwater
gradient to control groundwater flow and contaminant
transport. In clastic sedimentary aquifer systems (i.e.,
gravel, sand, silt, and clay deposits), this permeability
heterogeneity is primarily due to lithologic and
grain-size heterogeneity in three dimensions, termed
"stratigraphic heterogeneity", with post-depositional
changes (bioturbation, compaction, cementation,
alteration, etc.) as contributing factors. Stratigraphic
heterogeneity is imparted by the physical processes
acting to transport, deposit, and bury sediments and
is present at all scales from pore (microscale) through
regional (macroscale) (Figure 1).
While the impacts of stratigraphic heterogeneity on
groundwater flow and contaminant transport have
long been recognized {e.g., Koltermann and Gorelick,
1996; Puis and Barcelona, 1996; EPA, 2004, Weissman,
et al., 1999), the treatment of aquifers as isotropic and
homogeneous porous media remains commonplace
in groundwater remedy design and implementation.
At many legacy sites, pump and treat remedies were
applied, which served to recover contaminant mass
and provided a degree of hydraulic containment of
same. However, stratigraphic heterogeneity and
associated issues including matrix diffusion,
(e.g., Sale and Newell, 2011) make groundwater
site cleanups particularly challenging. For sites
where sedimentary aquifers are impacted, detailed
understanding of stratigraphic heterogeneity at all
scales is required to inform future site management
decisions.
As with the groundwater remediation industry,
problems related to subsurface fluid flow arising from
stratigraphic heterogeneity have long challenged the
petroleum industry, impacting exploration success and
field production. Tools such as sequence stratigraphy
and facies models were developed to address these
problems and to make predictions in between
individual wells regarding reservoir continuity and
heterogeneity. This paper provides instruction on
application of these tools to contaminated aquifers.
The concepts presented herein are equally applicable
to the unsaturated zone, including prediction of
contaminant and vapor migration pathways.
AQUIFER HETEROGENEITY
MICROSCALE
MESOSCALE
MACROSCALE MEGASCALE
GRAINS
LAMINATIONS
BEDS, CHANNEL PARASEQUENCE,
FILLS CHANNEL COMPLEX SEQUENCE
-FORMATION-
OIL RESERVOIRS-
REGIONAL AQUIFERS-
-REMEDIATION SITES
< inches < feet < hundreds of feet > miles
Figure 1. Scales of stratigraphic heterogeneity in clastic aquifers. Facies models and sequence
stratigraphy are applicable across all scales. (Modified from Krause et al., 1987). AAPG®19S7
Reprinted by AAPG whose permission is required for further use.
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Impact of Stratigraphic Heterogeneity on
Groundwater Flow and Remediation
As emphasized in this paper, groundwater flow
directions can vary greatly from regional groundwater
gradient due to anisotropy resulting from lithologic
heterogeneity. In many cases, sand and clay elements
are not deposited as a "layer cake" with one unit
stacked upon the other, but rather deposited in a
"shingled" or "laterally offset" fashion. This common
phenomenon spans a wide range of depositional
environments (see Table 1), and imparts a first-order
control on groundwater flow (see Figure 2).
Regardless of their geographic location, sites with
similar depositional environments also share
characteristic distribution of lithologic units.
Coarse-grained (sand-rich) lithologic units (e.g.,
point bar deposits, channel fills, alluvial fans)
typically define the primary groundwater flow
pathways, and are referred to herein as permeable
"hydrostratigraphic units" ("HSUs"). Because HSUs
behave as the subsurface "plumbing", one goal of site
characterization is HSU identification and mapping.
Once HSUs are defined, well screen positions (in X,
Y, and Z coordinates) can be related specifically to
them. This approach provides a superior tool for
contaminated site management and remediation
compared to contouring groundwater elevations
and contaminant plumes in aquifer zonations based
primarily on depth. Commonly, aquifer zonations used
in groundwater remediation project areas are found
to be poor predictors of subsurface architecture and
contaminant fate and transport.
Sequence Stratigraphy and
Environmental Sequence Stratigraphy
The science of sequence stratigraphy was initially
developed based on basin-scale reflection seismic
studies, and identification of termination of seismic
reflectors on continental margins as related to global
sea level changes for petroleum exploration purposes
(e.g., Mitchum et al., 1977). However, during the
decades since this seminal work the concepts have
been applied at increasingly finer scales on well
logs and cores, outcrops, petroleum reservoirs,
and aquifers (Van Wagoner et al., 1990). Sequence
stratigraphy and facies models are applied as a best
practice in the petroleum industry for delineating
reservoir geometry and continuity. These methods
are equally applicable to groundwater systems and
related groundwater contaminated sites.
The deposition of sediment in a particular location
is controlled not only by the depositional processes
operating, but also by the interplay of multiple factors.
These factors include sea-level change (magnitude and
rate), amount of sediment being delivered, climate,
and tectonic history of an area (e.g., Miall, 2000).
As these factors change with time, depositional
environments shift laterally or may change altogether.
During a transgression, for instance, as sea-level
rises, the shoreline moves landward, placing marine
deposits atop terrestrial deposits. Conversely, during
a regression, the shoreline moves seaward, often
leading to erosion of sediments. The science of
sequence stratigraphy is concerned with how the
factors above interrelate, their impact on processes
which operate to transport, deposit, and preserve
sediments, and the organization of the resultant
deposits (e.g., Posamentier and Allen, 1999). For
more information regarding the interaction of these
factors, and the impact on sedimentary geometry,
the reader is referred to the Society of Sedimentary
Geology (SEPM) website: http://www.sepmstrata.org/
page.aspx?pageid=l
"Environmental Sequence Stratigraphy", or "ESS"
as used herein, refers to the application of both the
concepts of sequence stratigraphy and facies models
(discussed below) to the types of datasets collected
for environmental groundwater investigations, which
are typically at the outcrop scale (tens to hundreds
of feet vertically, hundreds to thousands of feet
laterally). In order to develop this environmental
application of sequence stratigraphy, some liberty
was taken in generalizing the science of sequence
stratigraphy so that it may be of use to practitioners
with varying levels of expertise in the field. Although
the application to the environmental industry is not
focused on changing sea level as it is in the petroleum
industry, it does satisfy a key aspect of the sequence
stratigraphic approach which is to encourage the
integration of data sets and research methods, and it
focuses on changes in depositional trends and their
correlation across the study area.
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250 m
Figure 2. Unannotated (top) and annotated (bottom) photograph illustrating stratigraphic heterogeneity in
outcropping strata. In this meandering river deposit, laterally offset-stacked, or "shingled" sand units (point bar
deposits, light colored) are separated by clay units (dark colored) (Upper Cretaceous, Alberta, Canada). Bottom
photo highlights clay beds dipping from upper left to bottom right (red lines). Blue rectangles indicate hypothetical
well screens in the "first encountered saturated sand" (commonly referred to as the "A sand" during groundwater
remediation investigations). Though screened at a similar depth, and in a similar sandy lithology, the wells are
screened in distinctly different hydrostratigraphic units separated by the laterally continuous, dipping clay beds.
Thus, they are not in hydraulic communication and contaminant concentration data from any one well is only
representative of the hydrostratigraphic unit in which it is screened. Hydraulic conductivity into and out of the
photograph plane (and in the direction of dip) may be orders of magnitude higher than that from left-hand side
to right-hand side. Also of note is that "high resolution" subsurface data logs for these three locations would look
identical, and, without knowledge of the depositional environment and stratigraphy, the lateral shingling would not
be identified. Facies models predict such heterogeneity and hydrostratigraphic unit delineation in a meandering
fluvial setting. (Photo courtesy S. Hubbard, University of Calgary, personal communication [2/3/2015])
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ESS analyses have been applied to groundwater
remediation and water resource studies since the
1990s (Ehman and Cramer, 1996; Ehman and Cramer,
1997), and the importance of advanced stratigraphic
methods for understanding aquifer heterogeneity has
been emphasized by numerous authors
(e.g., Koltermann and Gorelick, 1996; Weissmann and
Fogg, 1999; Biteman, et al., 2004; Ponti, et al., 2007;
Payne, et. al., 2008; Scharling, P. B., et. al., 2009).
Most groundwater basins have had regional scale
stratigraphic analysis undertaken which can greatly
benefit site characterization if carefully integrated into
remediation studies (e.g., USGS Water Supply Papers
at https://pubs.er.usgs.gov/browse/usgs-publications/
WSP). However, the number of studies which have
applied these concepts to data from remediation sites
is very limited (e.g., Ehman and Cramer, 1996).
Examples of Benefits of Applying ESS
The following are some examples where applying ESS methodology to existing data sets on sites with heterogeneous
aquifers have provided significant benefits to groundwater remediation projects (see Appendix A).
• In fluvial channel and point bar deposits in the Santa Clara Valley (Silicon Valley), fining-upward sequences
bounded by paleosol units were correlated and mapped using existing boring log data. Detailed examination of
well screen intervals and integration with facies maps allowed separation of distinct hydrostratigraphic units
with different three-dimensional arrangements. Contaminant fingerprinting validated that coarse-grained units
represent hydrostratigraphic units (contaminant pathways). This mapping allowed the responsiblie party at this
multiparty site the ability to separate onsite vs offsite-derived contamination, providing a basis for modification
of cleanup metrics and re-negotiation of proportional liability. (Case Study #1)
• In a glacial outwash fluvial channel system, a keen understanding of the glacial sub-environment and associated
stratigraphic "rules of thumb" for correlation results in a significantly different stratigraphic framework to
understand and manage dense non-aqueous phase liquid (DNAPL) occurrence. (Case Study #2)
• In an aquifer composed of glacial deposits, a site-specific depositional model identified contaminant migration
pathways from a DNAPL release that was not apparent using groundwater contour maps and isoconcentration
maps. This provided the blueprint for optimized site characterization, groundwater monitoring, and
remediation design. (Case Study #3)
• In an aquifer composed of desert alluvial fan deposits, ESS defined dipping thin, continuous, clay layers that
compartmentalized the aquifer into several hydrostratigraphic units. This was critical for targeting and
monitoring the injections for the in-situ bioremediation program. (Case Study #4)
• In a perchlorate-impacted aquifer composed of alluvial (river) deposits, ESS defined channel-controlled
preferential pathways prior to the pilot test of a pump-and-treat / plume containment system resulting in a
system re-design and over 75% reduction in projected groundwater extraction and treatment volume.
(Case Study #5)
• In a chlorinated volatile organic compound (CVOC)-impacted aquifer composed of incised valley fill deposits,
ESS identified channel-controlled preferential migration pathways that are perpendicular to the regional
groundwater gradient, which helped to understand the performance of the remediation injection and extraction
programs. (Case Study #6)
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II. Depositional Environments and Facies Models
The geographic areas where sediments accumulate
over geologic time spans are referred to as
"depositional environments" (Figure 3).
In each depositional environment, characteristic
processes operate to erode, transport, distribute,
and deposit sediment. Due to these processes,
each depositional environment leaves characteristic
building blocks of sediment in the geologic record.
These building blocks are commonly referred to as
"architectural elements", and have characteristic
vertical grain size profiles, dimensions, lithology, and
facies associations (see Table 1). These architectural
elements fit together in three dimensions to form the
"stratigraphic architecture" of a sedimentary unit.
Observations of sedimentary deposits in modern
environments, outcropping systems, and subsurface
systems have been distilled over decades of research,
and conceptualizations of how these processes
interact and the three dimensional organization of
architectural elements they produce exist for virtually
all depositional environments. These conceptual
models are referred to as "depositional models" or
"facies models" (See Figure 4).
Fluvial environment
Alluvial
Eolian fan
Playa lake (dunes)
Swamp
Barrier island
Shallow marine
Beach
Estuary
Continental
\ Shelf
Continenta
slope."""
Deep marine
Figure 3. Block Diagram illustrating typical sedimentary depositional environments (from Jones, 2001).
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Table 1. Table showing vertical grain size profiles typical of a variety of depositional environments, major aquifer
and aquitard elements and their common dimensions, impact on CSMs, and implications for required data
resolution for characterization of groundwater remediation sites.
Depositional
environment
and typical
grain size
profile
Major aquifer
elements and
their common
dimensions
Major aquitard
elements and
their common
dimensions
Impact on CSM
Data resolution
needs
Alluvial Fan
tine coarse
>
7
coarsening-upward
Proximal fan channels,
mid-fan sheet sands,
distal fnnge sands
X: 102 m- 104 m
Y: 10'm- 103 m
Z: 10-' m- 10's m
Playa lake deposits or
paleosol formations
commonly vertically
separate fans. Debris-flow
deposits also commonly
clay-rich.
X: 102 m- 103 m
Y: 102 m - 103 m
Z: 10-1 m - 10's m
Laterally extensive paleosol or playa lake deposits may be thin (10's of cm to
meters), but can vertically compartmentalize aquifers. Such thin aquitards may
not be recognized by non-continuous sampling methods due to their thin
nature. Fans have a primary stratigraphic dip basinward at 1-6 degrees, and
are laterally offset stacked ("shingled'). Fans are constructed primarily by
channels encased in sheet-flood deposits. Channels are radial from a point
source and represent permeable pathways. Channel density decreasing down-
fan.
High in vertical sense,
need for lateral resolution
decreases down-fan
where channels are less
predominant.
Meandering
Fluvial
fining-upward
Channel axial fill, point
bar, crevasse splays
X: 1 m - 10 s m
Y: 102 m- 103 m
Z: 10"1 m - 10 m
Floodplain deposits, levee
deposits, clay drapes on
lateral accretion surfaces,
plugs filling abandoned
channels.
X: 102 m - 103 m
Y: 102 m- 103 m
Z: 10*1 m - 10"s m
Channel and point-bar deposits are encased in fine-grained floodplain deposits
and represent the groundwater flow pathways. Traditional potentiometric
surface maps are poor predictors of specific groundwater flow paths and
contaminant migration pathways. Coarse-grained "lags' at the bases of
channels and point bars represent high-permeability pathways. Lateral
accretion drapes can form "shingled" aquifer units. Clay plugs filling
abandoned channels ("oxbow lakes') common and provide barriers to
groundwater flow and contaminant fate and transport.
High both laterally and
vertically
Braided
Fluvial ^
blocky
Channel axial fill, bar
complex
X: 1 m-101 m
Y: 10' m-102 m
Z: 10"' m-Wm
Floodplain deposits, silt
and clay plugs filling
abandoned channels.
X: 102 m - 103 m
Y: 10 * m- 102 m
Z: 10*1 m-1'sm
Low-sinuosity high-permeability streaks encased within an overall permeable
matrix may dominate groundwater flow and contaminant migration Laterally
discontinuous silt and clay units may be significant at the plume scale, and are
more continuous in the down-channel direction compared to the cross-channel
direction.
High both laterally and
vertically, greater lateral
resolution required
perpendicular to
depositional axis (i.e.,
cross-channel transects)
versus parallel to
depositional axis (down-
channel)
Marine ?
or
Lacustrine ">
symmetrical or bow
Offshore bar, shelf,
transgressive sand
X: 10' m-102 m
Y: 102 m- 10s m
Z: 10-'m-10m
Fair-weather fine-grained
draping shales
X 101 m- 102 m
Y 102 m- 103 m
Z ; 10-1 m-10 m
Gradational base and top related to shifting sea-level or environment. High
degree of lateral continuity. Interbedded storm deposits (coarser grained) with
fair-weather deposits (finer-grained) lead to high degree of vertical
heterogeneity and "layer cake" stratigraphy
Low in lateral sense, high
in vertical
Near-
shore, i
Deltaic
coarsening-upward
Shoreface (beach),
distributary channels in
upper part, prodelta in
lower part
X: 10'm -102 m
Y: 102m-103m
Z: 10"' m- 10 m
Marine flooding shales
capping sequences,
interdistributary fluvial
overbank in upper parts
X: 101 m - 102 m
Y 102 m-103 m
Z: 10*1 m- 10 m
Lateratiy extensive, sand-rich near-shore units in upper parts of sequences
and delta-plain channels. High degree of interbedding of coarse and fine-
grained units in lower parts. Sitt and clay flooding shales capping sequences
dip basinward, may lead to erroneous correlations at distances of hundreds of
meters to kilometers.
Low in lateral sense, high
in vertical. Higher
resolution required in
upper parts of sequences
due to the presence of
distributary channels.
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AMALGAMATED
ESTUARINE
CHANNEL FILL
NESTED
AMALGAMATED
TIDAL CHANNEL
FILL
SHINGLED STACKING
COASTAL PLAIN
BREACHED BEACH
RIDGE WASHOVER
SPLAY SANDSTONE
& MUDSTONE
BURROWS
COAL
ROOTS
PLANAR BEDS
TROUGH
CROSS BEDS
CONTORTED BEDS
HUMMOCKY BEDS
CURRENT RIPPLE BEDS
WAVE RIPPLE BEDS
PROGRADING BARRIER SHORE
NESTED OFFSET STACKED "WINGED' EBB CHANNELS.
TIDAL DELTA LOBES, &
Shetf ^ DISCONTINUOUS BEACH SAND SHEETS
Christopher G. St. C. Kendall 2007
^— Coastal Plain Shoreface
COASTAL PLAIN
FORESHORE
& UPPER
SHOREFACE
SANDSTONES
SWAMP
SANDSTONE &
MUDSTONE
LOWER SHOREFACE «
DELTA-FRONT SANDSTONES
SHELF
MUDSTONES
Figure 4. Three dimensional facies model of a prograding barrier shoreline developed through integration of
observations of modern barrier island systems, outcropping ancient systems, and subsurface datasets worldwide.
"Prograding" refers to the shoreline system migrating seaward, due to an abundance of sediment being supplied,
falling sea level, or both (the converse is referred to as "retrograding"). Note the "sheets" of the barrier island
beaches, the "lobes" of the ebb tidal deltas, the "stacked and amalgamated channel fill" and " shingling" of the
washover splay sandstones. Each sub-environment has corresponding vertical grain size trends. Scale has been
intentionally omitted, as a variety of scales exist for each sub environment. This depositional model applies to many
remediation sites located in coastal areas of the Atlantic and Gulf Coasts of the United States as well as coastal
regions worldwide. From http://www.sepmstrata.org/CMS_Files/553_lecturel_introduction.ppt
Used under Creative Commons fair use policy with thanks to Dr. Christopher G. St. C. Kendall.
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In the prograding barrier island facies model example,
a variety of sub-environments are present and
produce architectural elements (e.g., ebb tidal delta
lobes), which also have characteristic vertical grain
size trends. Thus, with the knowledge that a site
lies within a prograding barrier island depositional
environment, vertical grain size profiles can be used
to predict lateral relations away from the known data
points, and the facies model serves as a guide to
interpreting depositional elements and correlating site
data in three dimensions.
Facies models for fluvial systems
Extensive areas of the United States located in
river valleys and on alluvial plains are underlain by
aquifers that were deposited in channelized (fluvial)
depositional environments. As such, a brief overview
of fluvial classification and facies models is presented
herein. The reader is referred to the extensive
literature (e.g., Walker and James, 1992; Miall, 2000)
for detailed information. While a continuum between
styles exists, fluvial systems can be broadly subdivided
into braided, meandering, and, less commonly,
anastomosing (Figure 5).
Sinuous, meandering-type rivers are common in the
Eastern United States due to the abundance of clay
and sand-sized material in the river system, relatively
low topographic gradient, perennial river discharge,
and abundant vegetation. Meandering river processes
result in deposition of "point bars", which are laterally
accreted sand units deposited on the inner bend of
channels as the outer bend of the channel (cutbank)
erodes the older deposits and the channel axis
migrates towards the outer bend of the meander
(Figures 5 and 6). While the vertical grain size trend
of a meandering fluvial deposit is the "classic" fining-
upward point bar sequence, additional architectural
elements are present with characteristic grain size
trends and architecture (Figure 6).
Another common element of meandering fluvial
systems is the "clay plug" which is deposited in
abandoned meanders, or oxbow lakes (e.g., Walker
and James, 1992).
In contrast to the Eastern United States, arid regions
such as much of the Western United States are
often near mountainous areas, and their rivers have
abundant coarse sediment supply, ephemeral flashy
runoff, and less vegetation on riverbanks. This leads
to riverbank instability and rapid shifting of the active
channel, and streams tend to take on braided-type
morphology (Figure 5).
Glacial geology and related depositional
systems
Because sequence stratigraphy seeks to identify
genetically related packages of sediment, reflective
of a depositional event or series of depositional
events, its concepts are applicable universally.
While a sequence-based approach has not been
applied widely to glacial sediments to date, it is
applicable. Glacial advance and/or retreat is
distinctly recognizable in the stratigraphic record
in the Midwest USA via predictable successions of
facies, many consisting of the depositional systems
detailed in this document. For example, successions
of subaqueous fans and lacustrine sediments grading
upward in to subaerial fan deltas and outwash, basal-
till and ice contact deposits provide a clear record of
glacial advance that is recognizable in both lithologic
and geophysical logs. Case Study 3 provides a
remediation-scale example of how lithologic data was
used to recognize and reconstruct distinct facies in a
glacial setting.
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River Type
Aerial Image
Sand
Distribution
Cross
Section
Log
Signature
Channel
Stacking
Braided
(f
I
Meandering
(\
Anastomosing
Figure 5. General classification of fluvial systems and their deposits (modified from http://wvyw.beg.utexas.edu/agi/
mod03/graphics/9180.gif). Courtesy of the Bureau of Economic Geology, University of Texas at Austin).
ID Floodplain
J Levee
| Splay
~ Channel fill
SP log scale
100n
-30
Floodplain
Splay
Levee
Channel fill, point bar
Figure 6. Meandering fluvial sub-environments. Within a meandering fluvial environment, many sub-environments
are present and can be differentiated on the basis of geophysical log signatures that represent vertical grain size
patterns associated with the sub-environments (log plots showing increasing grain size to the right). Deposits of
different sub-environments have characteristic dimensions, orientations, and impact on groundwater flow. Figure
courtesy of Bureau of Economic Geology, University of Texas at Austin (http://www.beg.utexas.edu/agi/mod08/
m08-step2-02.htm).
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III. Application of Environmental Sequence Stratigraphy
ESS methodology begins with an understanding of
the depositional environment and the use of existing
lithology data to elucidate vertical grain size patterns.
Work products include geologic cross sections
and facies maps that form a basis for integration
and interrogation of groundwater chemistry and
hydrogeology data.
The application of ESS to contaminated groundwater
sites can be broadly subdivided into three general
phases.
Phase 1: Synthesize the geologic and
depositional setting based on regional
geologic work, and identify facies models
which are applicable to the site.
Phase 2: Review the existing CSM and
site lithology data in light of Phase 1
findings and format existing lithology
data to highlight vertical grain-size
patterns (sequences) as a basis for
correlations honoring stratigraphic
"rules of thumb" (presented later in
this paper).
Phase 3: Construct a hydrostratigraphic
CSM consisting of maps and cross sections
that depict the HSUs present as a basis
to integrate and interrogate hydrogeology
(e.g., water levels, pump test, slug test)
and chemistry data (e.g., constituents,
concentrations).
Subdividing an "ESS Methodology" into three phases
outlined above may facilitate implementation, but it
is an integrated, iterative process and is meant to be
revised when additional data are collected.
Phase 1: Synthesize the geologic and
depositional setting based on regional
geologic work
Phase 1 analysis is focused on developing a thorough
understanding of the depositional environments
present, identifying applicable facies models against
which the HSU framework can be evaluated, and
developing a conceptualization of the series of
erosional and depositional events which formed the
aquifer. While a short description of geologic setting
is commonly included in historic site characterization
documents, this material typically relies on
previous work, seldom incorporates a discussion of
depositional environments, and typically does not
incorporate facies models into correlation strategy
or a discussion of permeability heterogeneity. Often
overlooked are local and regional geologic mapping
and studies that may be directly applicable to
remediation sites.
In tectonically active areas or older sedimentary
deposits, review of geologic maps and identification
of structural dip (tilting by tectonic forces such as
near faults) is especially important. As most natural
sedimentary deposits are highly vertically anisotropic
(i.e., Kh»Kv), structural dip will impose a strong
lateral anisotropy in the subsurface and impact fluid
flow accordingly.
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4 '
i s
r "s
Geomorphology of Modern Landforms
as Predictors of HSUs
Geomorphic features at or near the project
site provide insight into aquifer heterogeneity
and site hydrostratigraphy. Consult Google
Earth, current and historic aerial photographs,
and topographic maps and identify surface
features {e.g., scroll bars in meandering river
systems) that indicate depositional trends and
HSU orientations (Figure 7),
Satellite imagery and geologic maps are
extremely valuable for interpretation of
subsurface conditions in coastal depositional
environments (Figure 8).
Figure 7. Satellite image of an
industrial facility in the Ohio River
Valley constructed on point bar
deposits. Depositional grain is
visible (meander loop migration
to north) and suggests subsurface
anisotropy with relatively lower
permeability across point-bar
deposits.
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Outer bend of
meander belt
^,(cutbank)
Industrial
III v 1 1 ^<1 B
Facility^
"Scroll bars"
representing
point bar deposits
-------
Barrier Front Environments;
Offshore /Lower shoreface deposits
Beach Face and Swashbar Sands
Eolian sands
Limited fines in Beach Ridge Runnels
Mid Barrier Environments;
Overwash Fans
Flood Tidal Deltas
Tidal Chanel Fills and Point bars
Marsh/Mangrove Swamp organics
Back Barrier Environments;
Tidal Chanel Fills and Point bars
Marsh/Mangrove Swamp organics
Lagoon and Tidal Rat deposits
Figure 8. Block diagram showing subsurface nature of a Department of Defense facility in the Gulf Coast as predicted from boring logs and surface
geomorphology. Satellite imagery shows an active barrier island system (far left) and a relict barrier island system separated by a lagoon. The relict barrier
island system passes landward into a mid- and back-barrier island system, respectively. Facies models for barrier island systems predict high continuity of
washover fan and tidal deltas in the mid-barrier environment, and discontinuous tidal channel fill units in the back barrier environments.
(Yellow = beach/eolian sands; Gold = tidal sand channel fills and point bars; Black = clay; Green = silt/clay deposits).
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Impact of Sea-Level Change in Quaternary
Systems (2.5 million years ago to present)
In coastal regions, fluctuations in sea-level resulting
from Quaternary glacial and interglacial cycles
resulted in a series of erosional and depositional
events, and caused depositional environments to
shift landward and seaward with rising and falling sea
levels, respectively. During glacial periods, seawater
was sequestered in continental ice sheets. As a
result, sea-level was lowered by as much as 400 feet,
exposing the modern continental shelves to erosion.
During these times, river systems entering the ocean
carved erosional valleys known as "incised valleys,"
which are prevalent in coastal regions worldwide.
During deglaciation, these valleys were filled with
fluvial deposits or flooded by rising sea level.
Interplay of marine incursions with sediment delivery
via rivers produced characteristic incised valley-fill
sequences which have been extensively studied and
documented in coastal areas of the East and Gulf
Coasts of the United States (e.g., Anderson et al.,
2004), and worldwide (e.g., Posamentier and Allen,
1999). Many remediation sites are located within
areas where these studies have been undertaken.
In such settings, the known Quaternary sequence
stratigraphic architecture controls permeability
architecture, and integrating site stratigraphy with this
information is critical to subsurface interpretation.
Leveraging Nearby Off-Site Data to Augment
Site Data
In areas with a high density of remediation sites within
the same geologic setting, data from nearby sites can
augment site data. For instance, a nearby site may
have data of different types or resolution that provide
insights into vertical sequences, channel dimensions
and orientations, the nature of stratigraphic contacts,
and hydrogeologic parameters directly applicable
to the site in question. Regional data are in most
cases publicly available through state and/or local
regulatory agencies, and represent additional site
characterization data available to project teams at
minimal cost.
As stated, Phase 1 analysis focuses on identifying
existing resources to develop a deeper understanding
of the depositional environments present, and
identifying applicable facies models against which the
HSU framework can be evaluated. At the conclusion
of Phase 1, project teams will document key findings
and working hypotheses. These can include but are
not limited to the following:
• sources of geologic information;
• interpretation of depositional environments;
• preliminary selection of analogs and facies
models;
• expected dimensions and types of
heterogeneities observed (e.g., channel
occurrence and scale); and
• data resolution required to evaluate
heterogeneities observed and predicted from
facies models.
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Phase 2: Formatting lithologic data and
identifying grain size trends
Phase 2 is focused on formatting existing lithologic
datasets to accurately represent data density, vertical
resolution, and vertical and lateral grain size trends.
The following briefly summarizes typical subsurface
lithology data types at groundwater remediation
sites and recommends a method of formatting and
displaying vertical grain size patterns to maximize
the value of these existing data for stratigraphic
interpretation. While, in general, coarser-grained
units are expected to have higher permeability, other
characteristics, such as sorting, can have a significant
influence on the permeability. Where differences in
sorting are noted in boring logs and can be shown to
be consistent among various site datasets (e.g., SP
vs SW), then a higher permeability is expected in the
well sorted (poorly graded) deposits. Such patterns
may be equally important as grain size differences and
should be considered.
Existing Lithology Data
Borehole logs typically capture lithologic
descriptions in terms of color and grain size and, in
the environmental industry, are typically classified
according to the Unified Soil Classification System
(USCS). The USCS was developed for engineering
or geotechnical purposes with little emphasis on
identifying geologic features indicative of depositional
processes or environments. While we advocate
a "facies-based" approach for describing strata
instead of USCS classification (see "Facies-based"
Description of Sedimentary Deposits, herein), a wealth
of information beyond the USCS classification is
often present in legacy boring log data, which can be
extracted to reveal trends in grain sizes and used to
make stratigraphic interpretations (see Graphic Grain
Size Logs section below). The detail of the borehole
log descriptions may vary widely depending on drilling
and sampling methods and the experience and biases
of the geologist logging the borehole. Logging biases
as well as data quality are variable among different
generations of boring log descriptions. Descriptions
may be from continuously cored boreholes (e.g.,
direct-push sampling, hollow stem auger, mud rotary,
air rotary, or sonic drilling methods), or from depth-
discrete samples (e.g., 18-inch samples collected at
5-foot intervals using a hollow-stem auger drill rig). In
some cases drill cuttings logs from air or mud rotary
drilled boreholes are available. The quality of the
lithologic description is dependent on these factors
and needs to be considered when evaluating existing
lithology data. Formatting legacy lithologic datasets
in a way that emphasizes relative vertical grain size
trends as described herein serves to normalize
seemingly inconsistent lithologic datasets.
Cone penetrometer testing (CPT) logs may be
available and provide continuous tip resistance, sleeve
friction, and pore pressure data that serve as a proxy
of formation grain size and relative permeability. The
CPT typically has a maximum depth of penetration of
approximately 100 feet. It is best practice to collect
at least one continuous core next to a CPT boring to
calibrate the CPT response to the lithology. While CPT
is historically widely used for lithologic data collection,
a wide variety of other direct-push characterization
tools are currently in use (ITRC, 2015), and may
provide valuable information depending on the
specific site characteristics.
Downhole geophysical logs provide a continuous
representation of formation properties (see Figure
A24). As with the CPT logs, it is a best practice
to calibrate the geophysical log response with
continuously cored lithology description. Electrical
conductivity logs from MIP or other direct push
characterization programs provide lithologic
information as well. Such continuous resolution
provided by geophysical logs provides data on degree
of interbedding and grain size patterns.
Lithologic Data Formatting to Identify Grain-
Size Trends: Graphic Grain-Size Logs
Borehole log data are commonly represented on
cross sections of remediation sites as vertical "strip
logs" with USCS classification indicated. However,
to maximize the value of existing lithology data for
stratigraphic interpretation, borehole log data can
be formatted as graphic grain-size logs to emphasize
vertical grain size patterns. Graphic grain-size logs
are constructed by plotting the maximum grain size
described in the boring log (Figure 9a). The coarser
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grained sands and gravels plot away from the axis
while the silts and clays plot closer to the axis, with
the goal being to create a vertical grain size profile.
As maximum grain size provides an estimate of the
energy level (e.g., current velocity) in the depositional
system, this provides a superior indicator of
depositional environments. This representation of the
lithology data offers the following advantages vs. (JSCS
strip logs (refer to Figure 9a).
• Grain size details are identified that are
otherwise masked by the (JSCS classification,
such as the shallow unit at 0 to 20 ft that is
described as silty sand (SM) is composed of fine
to medium-grained silty sand, whereas the SM
unit at 30 ft is composed of fine to coarse-
grained silty sand with gravel.
• The sample density (i.e., vertical resolution
of data) is represented, showing that samples
are collected every 5 ft, and not continuously.
• The vertical grain size pattern clearly shows
two fining upward sequences representative of
separate channel complexes. These sequences
are not apparent within the (JSCS strip log.
When evaluated on the ESS cross section (Figure 9c),
the two channel-fill cycles correlate with surrounding
logs and help to define sand/gravel-filled channels.
These channels provide preferential contaminant
transport zones within this transect, and represent
a starting point for channel mapping. Using USCS-
based cross sections, no such patterns are apparent,
and these channel features had not been previously
identified at the site. Case Study #1 is another good
example of the value that graphic grain-size logs can
extract from existing borehole logs.
As is the case with graphic grain-size logs, CPT and
geophysical log data are posted as curves on cross
sections to represent vertical grain size and guide
correlations.
At the conclusion of Phase 2, project teams will have:
• identified lithologic datasets and reviewed them
for quality, drilling methods, data resolution,
and consistency;
• created a sufficient number of graphic grain-size
logs to identify grain size trends
and have developed ideas regarding
• depositional environments (see Table 1, and
Appendix A for multiple case studies)
• degrees and orientations of heterogeneity
• their potential to impact groundwater flow
and contaminant migration.
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a
5M8MW09
I
tan/brown = siity/sandy lithofacies
~ yellow = sand-rich lithofacies
~ .
¦
I | gray = bedrock
1,550
1,500
£
.2
J?
t" 1,450
USCS-Based Cross Section
HfUl
SC
500
I
1500
ESS-Based Cross Section
I
2500
I
3000
tii.
&
~
i
i
i
xz. ¦
. I—
i
L '
J
1
N
Channel (typ.)
Bedrock
1,400
1,000
2,000
3,000
Figure 9. Graphic Grain-Size Logs Used to Define Channel Occurrences, a) A graphic grain-size log prepared
from boring log data to emphasize vertical grain size trends. Two fining-upward sequences representing channel-
fill cycles are visible in this log, which are not visible in traditional USCS "strip logs". Two channel-fill cycles are
correlated on the lower ESS cross section (c) to define channel occurrence. Note that these channel features are
not apparent on the USCS-based cross section (b) and would not be identified by kriging algorithms.
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Phase 3: Identify and map HSUs
With a detailed understanding of site lithology data
and grain size trends in vertical boreholes obtained
during Phase 2, Phase 3 is the data integration
and interpretation step aimed at identification of
depositional elements which may comprise HSUs.
Candidate HSUs can be tested and validated by
integrating hydrogeology data and groundwater
chemistry data. This is accomplished by creating a
series of cross sections, identifying candidate HSUs
and their bounding surfaces, attempting to map said
units in three dimensions, and examining water levels
and analytical results in the context of the working
interpretation. This is an iterative process, arriving at
a best fit between all data.
Cross sections in traditional CSMs are typically
oriented parallel and perpendicular to general
groundwater gradient and through contaminant
source areas. However, cross sections oriented
parallel and perpendicular to depositional trends as
identified in Phase 1 are also valuable with respect
to contaminant fate and transport. For example, in
a braided stream environment, cross sections should
be oriented parallel and perpendicular to channel
orientations. As cross sections are constructed
and candidate HSUs are identified, additional cross
sections and maps may be required to validate the
interpretation. Maps required may include facies
maps showing lateral changes within HSUs, isopach
(equal thickness) maps, clay or paleosol continuity
maps, or other maps depending on local conditions.
Integration of other available data (e.g„ hydrogeology
and groundwater chemistry data) in the context of
the depositional environment provides the multiple
lines of evidence for a geologically defensible HSU
interpretation.
Interpretation Methodology and Stratigraphic
"Rules of Thumb"
While there is no substitute for experience
in application of facies models and sequence
stratigraphy for accurate stratigraphic interpretation,
the following generalized "rules of thumb" are
presented to assist practitioners in the groundwater
remediation community to improve subsurface
correlations and prediction.
1. Identify a suite of applicable facies models
for a particular site and use them as a guide
in correlation of sand units. Interpret
depositional elements (e.g., channel axis,
margin, and overbank) as potential HSUs
and define the criteria used to classify
elements. Develop hypotheses regarding
site specific conditions and how they might
cause the site stratigraphy to vary from the
facies models identified.
2. Vertical patterns in grain size (i.e., sequences)
are indicative of the relative energy level
present in the depositional environment and
hence are better indications of correlative
units than tops or bases of sand units.
3. Aquifers are usually correlated in a "top down",
lithostratigraphic fashion; however, sediments
were originally laid down from the bottom up
or in laterally-offset fashion (Appendix A,
desert alluvial fans Case Study #4, Figure A22),
and are often eroded by younger units. Thus a
conceptual model of how sedimentation
and erosional events occurred is necessary for
accurate stratigraphic correlation.
4. Correlate clay units first. In "channelized"
fluvial (riverine) settings, channel bases are
often erosive and irregular in elevation,
whereas floodplain clays and paleosols
capping channel sequences tend to be more
horizontal (see Appendix A, glaciofluvial
Case Study #2). However, in some cases,
clay-filled channels may be encountered as a
result of channel abandonment and passive
filling by fine-grained materials. This is
especially prevalent in meandering stream
deposits.
5. Paleosols commonly form within
floodplain deposits, and form superior
correlation markers to bases or tops of
individual channel sands, and are likely to be
continuous over large areas. Paleosols may be
identified in borings logs by mention of soil
nodules such as caliche or siderite, relative
hardness as identified by blow count or
changes in drilling conditions, or zones of high
tip resistance in CPT logs.
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6. For channel deposits, identify channel bases
that are generally erosive and irregular in
topography, and contain the coarsest grain-size
fraction present in the overall system,
representing potential high-permeability
zones which may move a large proportion of
the groundwater and contaminant mass
through a very small percentage of the overall
cross-sectional area. Bases of channel
complexes are more likely to be hydraulically
connected than the upper portion of individual
channel deposits.
7. Channel systems that are characterized
by a relatively high clay content overall
(e.g., meandering river systems) are likely
to be more compartmentalized than sand-rich
(e.g., braided) systems, and correlations of
channel packages may, therefore, have greater
uncertainty. Clay-filled abandoned channels,
or "plugs", resulting from cutoff meanders are
common in meandering systems. These
arcuate features can serve as barriers to
groundwater flow and can dramatically affect
hydraulic gradients.
8. The degree of lithologic heterogeneity
(interbedding) observed in a vertical log is
generally a good first-pass indicator of lateral
heterogeneity. However, thin clay beds present
in an otherwise sand-rich aquifer system may
be laterally continuous for long distances
(hundreds to thousands of feet or more) and
may form effective barriers to groundwater
flow. At sites where data have been collected
at 5' intervals by split spoon or other methods,
these clay units may not be represented
in many borings. The potential for thin,
laterally continuous clays to be present is high
in marginal marine, playa lake, fluvial overbank,
and glacio-lacustrine depositional
environments (see Appendix A, desert
alluvial fan Case Study #4).
9. In coastal areas, during the relative highstands
of sea level, the incised valleys of the Gulf
coast and Atlantic coast of the United States
became inundated with marine waters. This
caused depositional environments to shift
inland and resulted in deposition of laterally
continuous marine clay deposits referred to as
maximum flooding surfaces. There are
industrial facilities that are located in incised
valleys of the Gulf and Atlantic coast areas
(see Appendix A, incised-valley fill Case Study
#6). These maximum flooding surfaces can be
identified by high gamma-ray counts and
relatively pure clays and have high potential to
compartmentalize aquifers in incised valleys.
10. Clay units correlated in a way which shows
"mounding" or positive topography are
suspect unless tectonic deformation has
impacted the site (Appendix A, glaciofluvial
deposits Case Study #2), or deep burial and
extensive compaction of the sedimentary
sequence has occurred, which can lead to
"compaction folds" resulting from clays being
more prone to compaction than sands. This is
uncommon within the upper several hundred
feet from the surface.
11. Vertical stacking of facies or "pillars" (see
Figure 10) is a common mistake in
groundwater CSMs and results from variability
in boring log data quality or logging bias
(e.g., one geologist may log a facies as an SM,
and another may log the same facies as SP).
Some interpreters may take this information
literally without considering the potential
for logging bias and hence may place a facies
change between every well, resulting in a
"pillar" style interpretation of facies. Such an
interpretation is non-geologic, and is of
limited value in understanding subsurface
conditions or planning remediation.
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MORtWWIST
SOWHWBT.
LEGEND
SP
(ftrve sandy
r sM-ML
isifty sand and sandy silt)
ml-cl
(silt And day)
\2 ML-CL-SM
(intertoyered sill day,
and silty sand)
P Wetl Sawn
Groundwater Elevation
(2004)
A-ALT1
A-ALT1
Figure 10. Cross section showing a common mistake in correlating subsurface data. Interpreted vertical fades
patterns ("pillars") corresponding to individual borehole locations with interfingering facies changes laterally. This
cross section reflects biases in USCS classification between different geologists or vintages of data collection, is not
geologically defensible, and is of extremely limited utility in understanding subsurface conditions.
As stated, defining the HSU includes integrating
hydrogeology and chemistry data, which provide
further evidence for hydraulic continuity. This
iterative process further interrogates and refines
the CSM with multiple lines of evidence. The HSU
interpretation is revised and refined as additional site
data are collected and updated as necessary to be
consistent with all available data.
At the conclusion of Phase 3, the project team will
have:
• an improved understanding of vertical and
lateral trends in grain sizes, and a clear
understanding of existing lithologic data types
and resolution;
• a network of correlated cross sections
which tie together in three dimensions,
consistent with the facies models applicable to
the site;
(Correlations generated by kriging or other
computer programs are to be used with
caution as they do not abide by the "rules of
thumb" presented herein [see Case Study #4].)
• identified and mapped candidate
HSUs as a basis for integrating hydrogeology
and chemistry data to validate their impact
on groundwater flow and contaminant fate and
transport.
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CONCLUSIONS
The uncertainty with respect to fluid flow controlled
by geology is the primary risk to the success of
groundwater cleanup programs. Fluids in both the
vadose and saturated zone flow along preferential
pathways controlled by the stratigraphy. This is
important because it often results in contaminant
transport directions that diverge significantly from
groundwater gradients inferred from groundwater
elevation data. This fact presents challenges for
characterization, monitoring, and remediation of
contaminants in the subsurface, as evidenced by
challenges historically encountered for groundwater
remediation projects.
The conceptual tools of sequence stratigraphy and
facies models developed in the petroleum industry
represent a step-change in our ability to manage
subsurface heterogeneity, and are directly applicable
to groundwater remediation projects. These tools are
founded on an understanding that each depositional
environment has characteristic processes which act
to transport, deposit, and preserve sediment, and
therefore leave characteristic vertical and lateral
grain size trends in the sedimentary record. Thus,
aquifers that were laid down in the same depositional
environments, regardless of their geographic
location, share a host of characteristics impacting
fluid flow. Therefore, an appreciation of depositional
environments corresponding to a particular aquifer
allows for a great number of predictions to be made
regarding heterogeneities and acts as a guide to
subsurface data correlation.
This paper provides suggested data presentation
methods for identifying grain-size trends in existing
data, generalized stratigraphic methods and rules of
thumb for correlation of subsurface logs, and a three-
phase approach to applying "Environmental Sequence
Stratigraphy". Practical guidance presented herein
will help move projects away from a homogeneous
and isotropic subsurface CSM to a more geologically
defensible CSM which takes advantage of facies
models and sequence stratigraphy to identify
contaminant pathways. Case studies presented in
Appendix A highlight benefits of using facies models
to guide well log correlations, and the benefits of
stratigraphic correlations to CSMs for groundwater
remediation, resulting in robust CSMs that guide
groundwater remediation project success.
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"Facies-based" Description of Sedimentary Deposits
Strategically located continuously cored boreholes provide a direct observation of the geology that can be used to
calibrate proxy representations of lithology (e.g., direct-push HRSC tools, geophysical tools). Another advantage
of strategically located continuous core is it can be sampled and analyzed to determine relative concentrations
of contaminants in coarse- and fine-grained strata to evaluate the potential for fine-grained strata to act as long-
term sources of contamination to groundwater (i.e., matrix diffusion [Sale and Newell, 2011]), and better evaluate
mass flux across transects. The Unified Soil Classification System ((JSCS) has traditionally been used to describe
sedimentary deposits of contaminated sites. The (JSCS was developed for geotechnical investigation and is focused
on the engineering properties of the deposits and is of limited use for identifying depositional processes or
environments recorded in the strata. A "facies-based" description of strata is focused on depositional characteristics
and provides a better way to interpret depositional processes and environments.
• When drilling, care should be exercised to recover a continuous, intact core. While drilling methods commonly
used in environmental investigations may disturb the materials, scraping the surface with a knife and/or or
spraying the coating away with a spray type water bottle often allows for a clearer view of sedimentologic
features such as laminations, cross-bedding, degree of interbedding and bed thickness, etc. If rotosonic
methods are required, work closely with the driller to minimize liquefaction and destruction of sedimentologic
features.
• A "facies-based description" of core materials includes information that can be used to determine
depositional processes. If possible, slice the core in two down the vertical axis to expose a planar surface
to examine features. If the core is to be used for analytical sampling as well, care should be taken to sample
appropriate intervals prior to complete core splitting or to slice the core across the barrel width using a core
guillotine-type device to avoid transposing contaminants vertically with the motion of a knife up or down the
core barrel.
• While a USCS-based description of subsurface core materials might be "SM, very dark gray (5Y 3/1), 70% fine
sand, 30% silt, loose, dry", a facies-based description of the same interval might be "fine-grained, current
ripple-laminated sand with interbedded silty sand".
• Additional description would include bed thickness, ripple morphology, bounding surfaces or erosional
surfaces present, root casts or other biogenic structures, soil formation features, etc. The reader is referred
to sedimentology and stratigraphy texts for additional information (e.g., Walker and James, 1992; Miall, 2000).
Facies-based descriptions provide identification of the depositional features which can be used to interpret
depositional environments. The reader is referred to stratigraphy and sedimentology textbooks for information on
mechanics of sediment transport and deposition and resultant sedimentary structures as well as paleoenvironmental
interpretation.
Limitations of Kriging and Overreliance on Visualization Tools
Three-dimensional computer-generated graphical displays of subsurface data are an important data visualization
and interrogation tool, but should not be mistaken for a conceptual site model. High resolution lithologic data are
valuable for site characterization, but it is recommended that they be interpreted in the context of the depositional
environments. Often, such data are acquired and used to generate 3D computer models which are thought to
represent a highly quantitative CSM. However, these models rely on kriging, which provides an oversimplified and/
or unrealistic view of the subsurface geologic architecture. This can be especially problematic in geologic settings
characterized by a primary depositional dip and/or laterally continuous thin clay beds. The alluvial fan Case Study #4
Figure A23 shows an example of this in alluvial fan environments with thin draping clays where a kriged model does
not correlate thin clay beds. This can be misleading when in fact the system is highly compartmentalized and the
compartments are oriented systematically and predictably when a facies model is considered. Modeling approaches
and geologic analysis are merging in academia and the petroleum industry, but not so in the environmental industry.
Current research in computer modeling of aquifers (Michael and Gorelick, 2010), and oil reservoirs (e.g., Pyrcz,
and Deutsch, 2014) has focused on generating models which use "training images" (Mariethoz and Caers, 2014) or
geologic "rules" to produce more geologically realistic simulations and improve predictions over traditional kriging-
based models. The takeaway here is to ensure that geologic cross sections are constructed by geologists, not
computer software.
Best Practice for Improving Conceptual Site Models
A Practical Guide for Applying Advanced Stratigraphic Concepts to Contaminated Groundwater Sites
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REFERENCES
Anderson, J.B., Rodriguez, A., Abdulah, K.C., Fillon, R.H., Banfield, LA., McKeown, H.A., Wellner, J.S., 2004. Late Quaternary
stratigraphic evolution of the Northern Gulf of Mexico margin: a synthesis. In Anderson, J.B., Fillon, R.H., eds., Late
Quaternary stratigraphic evolution of the Northern Gulf of Mexico margin, SEPM Special Publication no. 79, p. 1-23.
Biteman, S. E., D. W. Hyndman, M. S. Phanikumar and G. S. Weissmann (2004). "Integration of sedimentologic and
hydrogeologic properties for improved transport simulations; Aquifer characterization." Special Publication -Society for
Sedimentary Geology 80: 3-13.
Ehman, K.D. and R.S. Cramer, 1996. Assessment of potential groundwater contaminant migration pathways using sequence
stratigraphy, Proceedings of the 1996 Petroleum Hydrocarbons and Organic Chemicals in Ground Water, November 13 -15,
1996.
Ehman, K. D. and R.S. Cramer, R. S., 1997. Application of Sequence Stratigraphy to Evaluate Groundwater Resources. In
Kendall, D. R., editor, Proceedings of the American Association of Water Resources Symposium, Conjunctive Use of Water
Resources: Aquifer Storage and Recovery, American Water Resources Association, Herndon, Virginia, TPS-97-2, p. 221-230.
http://www.beg.utexas.edu/agi/mod08/m08-step2-02.htm
http://www.beg.utexas.edu/agi/mod03/graphics/9180.gif
http://www.cluin.org/characterization/technologies/hrsc/
http://www.epa.gov/superfund/remedytech/tsp/issue.htm
https://pubs.er.usgs.gov/browse/usgs-publications/WSP
ITRC, 2011. Technical/Regulatory Guidance for Integrated DNAPL Site Strategy, Interstate Technology & Regulatory Council,
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Jones, N.W., 2001. Laboratory manual for physical geology, 3rd edition, McGraw-Hill.
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Knauer, L.C., R. Horton, and A. Britton, 2003. Analysis of Low Permeability Intervals in a Heavy-Oil Braided Stream Deposit
Using a Combination of Core and Log Analysis, Kern River Field, California, poster session presented at the AAPG Convention,
Sit Lake City, Utah, May 2003.
Koltermann, C.E. and S.M. Gorelick, 1996. Heterogeneity in sedimentary deposits: A review of structure-imitating, process-
imitating, and descriptive approaches. Water Resources Research, Vol. 32, No.9, p. 2617-2658.
Krause, F. F., H. N. Collins, D. A. Nelson, S. D. Machemer, and P. R. french, 1987, Multiscale anatomy of a reservoir: Geological
characterization of Pembina-Cardium pool, west-central Alberta, Canada: AAPG Bulletin, v. 71, no. 10, p. 1233-1260.
Mariethoz, G. and J. Caers, 2014. Multiple-point geostatistics: stochastic modeling with training images, Wiley-Blackwell,
376 p.
Miall, A., 2000. Principles of Sedimentary Basin Analysis, Springer Publishing, 616 p.
Michael, H. A., Li, H., Boucher, A., Sun, T., Caers, J., & Gorelick, S. M., 2010. Combining geologic-process models and
geostatistics for conditional simulation of 3-D subsurface heterogeneity. Water Resources Research, 46 p.
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depositional sequence as a basic unit for stratigraphic analysis. In: Payton, C. E. (ed.), Seismic Stratigraphy - Applications to
Hydrocarbon Exploration. American Association of Petroleum Geologists Memoir 26, 53-62.
NRC, 2013. Alternatives for managing the nation's complex contaminated groundwater sites. National Research Council, The
National Academies Press, 320 p.
Payne, F.C., J.A. Quinnan, and S.T. Potter, 2008. Remediation Hydraulics, CRC Press, 432 p.
Ponti, D. J., K. D. Ehman, B. D. Edwards, J. C. Tinsley, T. Hildenbrand, J. W. Hillhouse, R. T. Hanson, K. McDougall, C. L. Powell,
E. Wan, M. Land, S. Mahan and A. M. Sarna-Wojcicki (2007). "A 3-Dimensional Model of Water-Bearing Sequences in the
Dominguez Gap Region, Long Beach, California." Open-File Report -U.S.Geological Survey(Reston, VA).
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Posamentier, H.W. and G.P. Allen, 1999. Siliciclastic sequence stratigraphy - concepts and applications. SEPM concepts in
Sedimentology and Paleontology, no. 7, Tulsa, Oklahoma, USA, 204 p.
Puis, R.W. and M.J. Barcelona, 1996. Low-flow (minimal drawdown) ground-water sampling procedures, Office of Solid
Waste, Washington, DC, EPA/540/S-95/504,12 p.
Pyrcz, M.J. and C.V. Deutsch, 2014. Geostatistical reservoir modeling, Oxford University Press, 433 p.
Sale, T. and C. Newell, 2011. A guide for selecting remedies for subsurface releases of chlorinated solvents. Environmental
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Scharling, P. B., E. S. Rasmussen, T. O. Sonnenborg, P. Engesgaard and K. Hinsby (2009). "Three-dimensional regional-
scale hydrostratigraphic modeling based on sequence stratigraphic methods: a case study of the Miocene succession in
Denmark." Hydrogeology journal 17(8): 1913-1933.
Steel, R.J. and T.G. Gloppen, 1980: Late Caledonian (Devonian) basin formations western Norway - signs of strikeslip
tectonics during infilling. In: H. Reading & P. F. Ballance, eds, Sedimentation in oblique-slip mobile zones. Spec. Publ. Int. Ass.
Sediment., 4, 79-103.
U.S. EPA, 2004, Performance Monitoring of MNA Remedies for VOCs in Ground Water, EPA/600/R-04/027, April 2004, 8-13.
Van Wagoner, J.C., R.M. Mitchum, K.M. Campion, and V.D. Rahmanian, 1990, Siliciclastic Sequence Stratigraphy in Well Logs,
Cores, and Outcrop, AAPG Methods in Exploration Series, No. 7
Walker, R.G., and N.P. James, eds., 1992, Facies models: response to sea level change, Ontario: Geological Association of
Canada.
Weissmann, G.S., Fogg, G.E., 1999. Multi-scale alluvial fan heterogeneity modeled with transition probability geostatistics in
a sequence stratigraphic framework. J. Hydrol., 226(1-2): 48-65.
Zaitlin, B.A., R.W. Dalrymple, and R. Boyd, 1994. The stratigraphic organisation of incised valley systems associated with
relative sea-level change. In: R.W. Dalrymple, R.J. Boyd and B.A. Zaitlin., Eds., Incised valley systems : Origin and sedimentary
sequences, - SEPM Spec. Pub. 51, 45-60.
ACKNOWLEDGEMENTS
We would like to thank the following who provided peer-review comments to help improve this paper:
Dr. Jessi Meyer (University of Guelph), Murray Einarson (Haley & Aldrich), Linda Fiedler and Ed Gilbert (US EPA),
Dr. Tomas Perina (CB&I), Dr. Chuck Newell (GSI Environmental), and Brian Lewis (California Department of Toxic
Substances Control). Graphics support was provided by Kathy Tynsky (CSRA, Inc.) and Cheryl Lee (Staff Tech,
EPA Region 9). The funding for this issue paper was provided by the USEPA/ORD/NRMRL/GWERD/Groundwater
Technical Support Center, Ada, OK. Project management and oversight was provided by Dr. David Burden,
Director of U.S. EPA's Groundwater Technical Support Center, Ada, OK.
Best Practice for Improving Conceptual Site Models
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APPENDIX A
Case Studies
- #1:
Fluvial channel deposits, Silicon Valley, California; Contaminant pathways related to
commingled VOC plumes
A2
- #2:
Glacial Outwash Channel Systems, Northeast US; DNAPL source for VOC groundwater impact
A12
- #3:
Glacial terrain, till and lacustrine deposits, Upper Midwest US; LNAPL and dissolved phase
impact at a manufacturing facility
A15
- #4:
Desert alluvial fan environments, Western US; Managing hexavalent chromium impacts to
groundwater at an industrial facility
A20
- #5:
Fluvial channel and overbank deposits, Southern California; Updated CSM for perchlorate
plume containment remedy
A24
- #6:
Incised-valley fills, Gulf Coast Region, US; Optimize VOC plume containment and in-situ
remediation
A29
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Case Study 1: Fluvial Channel Deposits, Silicon Valley, California
Introduction to the Site
This case study documents a site representative of
many contaminated groundwater sites in the Santa
Clara Valleyj or Silicon Valley" of northern California
(Figure Al). Historic contaminant releases related to
semiconductor and other electronics manufacturing
resulted in extensive groundwater contamination
(primarily VOCs) in the basin. The groundwater
table in the basin is relatively shallow (approximately
10' below ground surface [bgs]), contaminant
concentrations in groundwater may be high, and
the highly urbanized area is characterized by dense
commercial and residential construction. Thus, vapor
intrusion poses risks to human health.
The heterogeneous aquifers in the Silicon Valley are
composed of high-permeability sand and gravel
channel-fill deposits encased in low permeability clay
and silt floodplain deposits and/or paleosol horizons.
The sand channels result in complex groundwater
flow and contaminant migration pathways that are
not reliably discerned with groundwater gradient
maps. This results in challenges in contaminant plume
characterization (particularly with comingling plumes),
design of groundwater monitoring wells, and remedy
design, performance, and monitoring.
At this site, despite considerable source remediation
work over the past decades, increasing contaminant
concentrations were observed in monitoring wells
considered "•down-gradient" of the source area, and
a CERCLA five-year review recommended additional
source remediation. Using the ESS approach, two
channel deposits underlying the site were mapped,
one of which could be traced back to the on-site
source area, and another which was oriented oblique
to the presumed groundwater gradient and represents
a contaminant pathway from off-site sources.
Analysis of contaminant constituents associated with
these two pathways revealed differing "chemical
fingerprints" and indicate that these channel deposits
are in fact separate and distinct hydrostratigraphic
units (HSUs). These findings enabled the responsible
party to differentiate which monitoring wells were
representative of on-site-related contamination, and
those impacted by off-site sources. The multiple lines
Figure Al. Map showing location of the Santa Clara Valley
in the southern San Francisco Bay region, California. Alluvial
lowlands (yellow) are distinguished from bedrock uplands
(green). Principal faults are shown in black. Red box
indicates general location of case study site. (Modified
from Wentworfh et al., 2014)
of evidence provided by hydrostratigraphic mapping
and groundwater chemistry fingerprints indicate off-
site contaminant contributions to onsite wells.
This case study demonstrates that:
• Channel deposits control groundwater flow and
contaminant transport and represent
distinct HSUs
• Mapping of such HSUs is feasible with existing
boring log data
• In settings such as the Santa Clara Valley
where groundwater flow is highly channelized, a
hydrostratigraphic mapping approach is superior
to a depth-based aquifer zonation approach for
characterization, monitoring, and remediation
• Anomalies in isoconcentration maps such as
"bullseyes" of high concentration result from
well screens which penetrate multiple HSUs
which are transporting waters with different
contaminant concentrations
12? 30'
Llvermore
Valley
Monterey
Bay
KILOMETERS
San
Francisco
PACIFIC
OCEAN
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Depositional Setting and Fluvial Channel Facies Models
The Quaternary alluvial stratigraphic section which
comprises the impacted aquifers in the Silicon Valley
was deposited in channel and floodplain environments
by mildly sinuous (anastomosing or meandering-
type) streams draining the Santa Cruz Mountains and
flowing into San Francisco Bay (Figure Al). As these
channels migrated across the landscape, sand and
gravel were deposited in channel axes and possibly
as point bars. During flooding events, silts and clays
were deposited outside the channels in the floodplain,
and rivers periodically abandoned their previous
courses and formed new channels. Figure A2 presents
the various depositional components resulting from
an anastomosing river.
The resultant sedimentary deposit is characterized
by highly permeable sand and gravel channel
deposits encased in relatively low-permeability silt
and clay floodplain deposits. Groundwater flow
and contaminant transport occurs primarily within
the permeable channel deposits, and the variable
orientation of channels deflects contaminant
migration directions from the regional groundwater
gradients. This can cause plumes to appear to spread
laterally, and assume complex plan-view morphologies
(i.e., Figure A3). Due to this channelized groundwater
flow and large number of source areas in proximity to
one another, many plumes have become commingled,
creating challenges for plume management in the
Silicon Valley.
THE ANASTOMOSING RIVER
CI sit
SAND
| Fn|Md|Cr
Grv
Description
Anastomosed fluvial systems have facies
associations similar to those of meandering
systems (active channel, abandoned chan-
nel, overbank, splay) but in different pro-
portions and with different geometries.
~ Sand
Bedrock
LEGEND
HH Pearl
Clay
h ' I Silty Clay and Sandy Clay
5. Flood plain clays with mudcracks
and root traces. Coals may form
here. Cross-bedded sands (small
planar and trough) are
crevasse splay flood deposits.
4. Fine sands to silts; climbing
ripples common. Some root
traces.
3. Medium to fine sand; small
trough cross beds; rippled
surface.
Erosional
channel
2. Coarse to medium sand with large
trough cross beds AND/OR high
velocity laminations.
1. Lag gravels (in mud pebbles from
slumping banks) to medium sand
over an erosional base. Channel
erodes laterally by undermining
bank.
Figure A2. Depositional components of anastomosing river depositional environment including fining upward
vertical grain size pattern, representative of channel fill deposits.
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-
TCE >5 pg/L
TCE >100 pg/L
TCE >1,000 |Jg/L
J!
A *
aft*
'¦'C *
^ * ",0
Q(*fi
tfvacnsiui icni •* in?
Figure A3. TCE isoconcentration map of the Silicon Valley B l aquifer zone groundwater plume discussed in this
case study ("the plume"). Note 1) irregular plume morphology resulting from channelized groundwater flow
pathways and groundwater extraction, and 2) "bulls eyes" of isolated wells showing high concentration resulting
from well screens penetrating multiple channel deposits containing groundwater with relatively higher contaminant
concentrations.
Review and Format Existing Subsurface Data and Apply Stratigraphic "Rules of Thumb"
The database for this project consisted of boring
logs (from direct push, hollow-stem auger, and mud-
rotary drilling methods), well construction data, and
chemical analyses from groundwater samples. As
described in Section III, Phase 2 herein, graphic grain
size logs were constructed to highlight vertical grain
size patterns captured in the boring logs. As shown
in Figure A4, fining-upward channel fill sands encased
in floodplain silts/clays are apparent which allows for
mapping of individual channel deposits.
In order to address increasing contaminant
concentrations in areas downgradient of the onsite
source area, cross section A-A' (location shown on
Figure A5 and cross-section shown on Figure A6) was
prepared using data-formatting methods described in
Section III, Phase 2 herein.
The following rules of thumb were applied to correlate
the grain size patterns between boring logs, as
depicted in Figure A6.
• Channel deposits tend to have erosive bases
and relatively flat tops, and clays make superior
correlation markers (paleosol horizons)
• Gravels define channel bases and grain size
fines upward
• Channel margins are sharp and erosive, and
result in strong segregation of channel-fill sands
and gravels from floodplain clays
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GRAPHIC LOG DESCRIPTION
Silty CLAY (CL); brown mottled black; stiff;
30-40% silt; <5% very lina to Una sand; vary
low est K
• 4nl«r • cuttings)
Sandy SILT (ML); blue-gray; stiff; 5-t0% clay;
20-30% very fine to line sand; low est K
60
45 -
Fe02 staining between 38* and 38 1/2'
Gravelly SAND layer 1-2* thick below 38 1/2"
dnlof ¦ cuttings)
Gravelly SAND (SW); brown; madium donso;
<5% clay; 5-10% silt; ve7 line lo very coarse
sand; 30-40% fin# subangular gravel to 1/4"
high est K
Silty SAND (SM); blue-gray; medium dense;
5-10% clay; 20-30% silt: very fine to medium
sand; <5% fine angular gravel to 1/8'
diameter; mod est K
(Coraaa-dHnw *euttns»|
Sandy GRAVEL (GP); multicolored; very
dense; <5% fines; 10-20% medium to very
coarse sand; 40-50% fine subangular to
subrounded gravel* 30-40% coarse
subrounded to rounded gravel lo 1* diameter;
very high est K
5-10% fines; 30-40% medium to very coarsa
sand; 40-50% fine subangular gravel; no
coarse gravel below 54 3/4'
Clayey SILT (ML); very light gray; stiff lo very
stiff; 20-30% clay; 3-1 or. very tine lo fino
<2% fine subangular gravel; low est K
Sandy GRAVEL (GP); blue-gray; dense to very
dense; 5-10% day; 10-15% siK; 20-30% very
fine lo very coarse sand; fine subangular to
subrounded gravel to 1/2" diameter; high est K
Grain Size Log
f
ill
S*!1
iii
Clay
Silt-Sandy Silt
Rrie Sand w/fines
Fine Sand
Medium Sand w/fines
Medium Sand
Coarse Sand w/fines
Coarse Sand
Gravel
Figure A4. Data formatting for stratigraphic analysis. Portion of a boring log from the site illustrating a clear fining-
upward sequence from 55' to 41' bgs representing a channel-fill and abandonment deposit (see Figure A2). Basal
gravel lag and overlying fining-upward sequence occurs at 41' below ground surface (bgs). Lithologic contacts
were identified on the basis of sampling, cutting returns and drilling behavior. Graphic grain size log (at left) shows
this fining-upward sequence within a well screen interval.
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Site boundary
m Area of increasing
concentration, suspected
onsite
^nsite source
(fe-
source ,
V
area
0.1
0.005
400
APPROXIMATE SCALE IN FEET
800
Figure AS. Map showing a portion of the Bl aquifer zone TCE plume, onsite source area, area of increasing
contaminant concentrations, site property boundary, direction of presumed groundwater flow based on the
groundwater gradient inferred from groundwater elevation data (white arrow), and location of cross section A-A'.
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A
EAST
J
EAST
61 B1 B»
~ # /•
A'
ivfsr
81 B2 Bl
I
t
i
ON-SITE ' OFF-SITE
WEST
S —
HSU 2
HSU 1
Site boundary
m Area of increasing i -j 1
concentration, suspected OnSlt€
spnsite source . * soUTCe •
T"I»B \. • area
»' Vn . /
©;
* **
V
4
at
0.005
APPROXIMATE SCALE IN FEET
LEGEND
channel deposit consisting of
coarse-grained sand, gravelly sand typically
fining-upward from gravel-bearing base
channel margin or splay deposit consisting
of coarse-to fine-grained sand, silty sand
Floodplain deposits consisting of clay, silty
clay, and sandy clay, often with root
structures, caliche nodules (soil horizons)
Figure A6. Uninterpreted (top), and interpreted (bottom) cross section A - A' from study area. General groundwater gradient is to the north (out of the plane
of the cross section towards the viewer, and towards the left on the map view). "Bl" or "B2" at the top of the boring indicates aquifer zone designation
corresponding to the screened interval of each well. Note that several "Bl" wells are screened across multiple channel deposits (e.g., S005B1, S149B1,
SI 01 Bl), and that, while I 12C is designated a "B2" well, it is in fact screened in the same channel unit as "Bl" designated wells S005B1, S101 Bl, and S101 Bl.
See Figure A4 for legend for graphic grain size logs created from boring logs. Channel dimensions interpreted based on detailed mapping at the site and
closely-spaced high-resolution datasets at other nearby sites in the same stratigraphic interval.
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Inspection of cross section A-A' (Figure A6) reveals
that onsite groundwater monitoring wells designated
as B1 aquifer zone wells (T-8B, T-2B, T-17B) are
screened in a shallower, isolated channel complex
(indicated as HSU-1) relative to the offsite wells
S005B1, S100B1, S149B1, S101B1, and S048B1.
Onsite well T12C, which is designated as a B2 aquifer
zone monitoring well, is screened within the same
HSU as offsite wells designated as B1 monitoring
wells. This highlights the confusion related to depth-
based water-bearing zones for plume mapping in
channelized depositional environments and the
difficulty in interpreting plume maps which combine
multiple HSUs. Offsite well S005B1 is screened across
two distinct channel deposits, and TCE concentrations
are significantly higher in this well, suggesting that
groundwater in the shallower channel indicated as
"HSU-2" contains a relatively high concentration of
contaminants.
Extensive on-site contaminant source removal coupled
with in-situ bioremediation resulted in significant
decrease in VOC concentrations in groundwater near
the source area. Vinyl chloride (VC) was generated as
a daughter product. However, monitoring well T-9B
at the downgradient extent of the property showed
increasing VOC concentrations, up to 390 ng/L, an
order of magnitude higher than other on-site wells.
High TCE and c/s-1,2 DCE concentrations are observed
in well S005-B1 compared to adjacent wells suggesting
that the upper channel across which the well is
screened represents a contaminant pathway. Thus, a
detailed ESS analysis was undertaken to map HSU-1
and HSU-2 and evaluate lithologic pathways from T-9B
area to the south (Figure A7).
As mentioned, on-site monitoring wells typically
contain VC, occurring as a daughter product of TCE.
Freon-113 is associated with the off-site source
and was not used in on-site operations. Thus, VC
is unique to the on-site source and Freon-113 is
unique to the off-site source. After completing the
ESS assessment, groundwater contaminant chemistry
data (trichloroethene [TCE], tetrachloroethene [PCE],
c/'s-l,2-dichloroethene [cDCE], vinyl chloride [VC], and
Freon 113 [freon]) were interrogated with respect to
the updated stratigraphic framework (i.e., HSUs) to
provide an independent line of evidence for off-site
related contamination (Figure A8).
Cross section B-B' (Figure A8) is oriented such that it
includes on-site wells along the path of HSU 1, and
then traverses to the south west to include the high-
concentration, deep HSU-2 channel in T-5B. Note that
the wells screened only across HSU 1 (T-10B, T-8B,
and T-2B) contain groundwater with TCE, c/s-1,2 DCE,
and VC, and lack Freon-113. The well that is screened
only across HSU 2 (T-5B) contains groundwater with
Freon-113, and lacks VC. Well T-9B is screened across
both HSU 1 and HSU 2 and thus contains mixed
groundwater with both indicator parameters (VC and
Freon-113).
A similar trend is observed in cross section C-C', which
illustrates the continuity of the HSU 2 channel sands,
which is corroborated by the chemistry fingerprint.
The wells that are screened solely in HSU 2 lack the
on-site source indicator VC and contain Freon-113
(T-4B has historically contained Freon-113, but not
during the 1-5 year average timeframe used to create
fingerprint graphs). Well T-9B is screened in both
HSU 1 and HSU 2 and contains groundwater that is
a mixture of HSU 1 and HSU 2, containing all four
analytes.
The chemistry fingerprint data provide an
independent line of evidence, and corroborate
the geologic interpretation that channel HSU 1 is
a contaminant pathway representative of the on-
site contaminant source and channel HSU 2 is a
contaminant pathway representative of the off-site
contaminant source.
This case study exemplifies why defining the details
of the subsurface geology is critical for distinguishing
hydrostratigraphic pathways, particularly when
there are multiple source areas for commingled
contaminant plumes. As shown in Figure A9, the
original CSM inferred contaminant migration pathway
based on the groundwater gradient interpreted
from groundwater elevation data, which assumes
that the subsurface conditions are homogeneous.
However, as presented here, the underlying geology
is heterogeneous due to the channelized depositional
environment. The updated ESS-based CSM defines
HSUs that are the primary control of contaminant
migration, as corroborated by multiple lines of
evidence (Figure A10). This realistic CSM provides
a basis for improved management of this complex,
commingled plume.
Best Practice for Improving Conceptual Site Models
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Channel map of HSU-1 (on-site channel)
a<*a
JSs-
A A® • •' ¦
10 , 0 <>¦<*»
Channel map ofHSU-2 (off-site channel)
A &r
•I
0.* ^
© :•
Mi
dtaKtt
*•
V
0.1
tp , 0 O-005
4
APPROXHMTI SCAlt M FtP
APPROMIWTI SCA4.E W fEET
WEST
LEGEND
channel deposit consisting of
coarse-grained sand, gravelly sand typically
finjngupwafd from gravel-bearing base
channel margin or splay deposit consisting
of coarse-to fine-grained sand sHty sand
Floodplain deposits consisting of clay silty
day, and sandy clay, often with root
structu res, caliche nodules (soil horizons)
Figure A7. Detailed mapping of HSUs, Maps of HSU l and HSU 2 channel axis facies (sand- and gravel-bearing, indicated by yellow outlines), and cross
section A-A' (lower figure). The deeper channel HSU-2 provides a direct lithologic connection and hence potential contaminant pathway from off-site
sources to well 1-9B. Note that the channel widths and morphology depicted on the cross sections are constrained by three dimensional facies mapping
of the channel complexes and floodplain deposits.
A9 GW Issue
Best Practice for Improving Conceptual Site Models
A Practical Guide for Applying Advanced Stratigraphic Concepts to Contaminated Groundwater Sites
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Channel map of HSU-1 (on-site channel) and cross section B-B'
•i
H5U- channel
Channel map of HSU-2 (off-site channel) and cross section C-C
j Ifll'lM •
•1
.
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% * 10 ¦ r
4
0.005
.
0-0&
APPROXIMATE SCALE W FEET
APPROXIMATE SCALE IN FEET
- (J
40 bgs
20' bgs
Cross Section B-B': Down-channel cross section of HSU-1 (on-site channel)
A.J
fr $ $ $
& V \ V «, \
V
J
¦¦
J I
<£" !&
?MEC
¦"Mr^ b
Cross Section C-C': Down-channel cross section of HSU-2 (off-site
channel)
ll Jl
20' bgs
40' bgs
nrwsa
Figure A8. Contaminant fingerprinting. Cross sections B-B' and C-C' oriented down the axes of channel HSU-1 and HSU-2 with contaminant fingerprint charts
corresponding to groundwater samples. Fingerprint charts post the log of the concentration of the different indicator contaminants, and as such are useful
for discerning the constituents. Fingerprint charts represent an average value of concentrations over the last five years.
Best Practice for Improving Conceptual Site Models
A Practical Guide for Applying Advanced Stratigraphic Concepts to Contaminated Groundwater Sites
GW Issue A10
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bnsfte
source
27.31
29.43
APPROXIMATE SCALE IN FEET
Figure A9. Original CSM based on simplifying assumption of homogeneous aquifer conditions. This interpretation
of contaminant migration is based on the groundwater gradient (groundwater elevation contours) and does not
focus on the geology and depositional environment. White arrows show interpreted groundwater flow directions
and contaminant transport directions from the on-site source to the down-gradient impacts at the property
boundary. Based on this assumption, additional source area remediation had been proposed.
* A
A/V
Site boundary
- HSU-2 channel
9 Area of increasing
concentration, suspected
^pnsite source
onsite
source
v iuufce »
\ « # area
v-1 Q/k • /
" * ©; •/
0.1
10
1.0
0.005
400
BOO
• APPROXIMATE SCALE IN FEET
Figure A10. ESS-based CSM focused on underlying geology to define HSUs. The HSU-2 channel (bounded by yellow
lines) controls the contaminant migration pathway (white arrow) showing that, unlike Figure A9, an off-site source
is contributing to the impact occurring at the property boundary. At this complex site, groundwater flow is strongly
influenced by lithology, and contaminant transport directions deviate significantly from those predicted from the
potentiometric surface maps.
REFERENCES
Wentworth, C.M., Jachens, R.C., Williams, R.A., Tinsley, J.C., and Hanson, R.T., 2015, Physical subdivision and description
of the water-bearing sediments of the Santa Clara Valley, California: U.S. Geological Survey Scientific Investigations Report
2015-5017, 73 p., 2 plates, http://dx.doi.org/10.3133/sir20155017.
All GWIssue
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A Practical Guide for Applying Advanced Strafigraphic Concepts to Contaminated Groundwater Sites
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Case Study 2: Glacial Outwash Channel Systems, Northeast US;
DNAPL Source for VOC Groundwater Impact
Introduction to the Site
Case Study 2 relates to a former manufacturing
site impacted by dense non-aqueous phase liquid
(DNAPL) and related VOC impacts to groundwater.
This example shows that an understanding of
the depositional environment and associated
stratigraphic "rules of thumb" for correlation results
in a significantly different CSM for groundwater
management.
During relative lowstands of sea level in the
Quaternary, large erosional valleys known as "incised
valleys" were formed in coastal regions due to erosion
by fluvial (river) systems issuing from glaciers. In
northern parts of the USA, many of these river valleys
were filled by glacial outwash fluvial systems prior
to flooding during sea level rise (e.g., Chesapeake,
Delaware, and Hudson River Valleys). This case
study documents glacio-fluvial outwash channels
and emphasizes a stratigraphic "rule of thumb" that
clay units tend to be flat and make better correlation
markers than sand channels, which tend to have
erosive, irregular bases. The conceptual site model
at this site consisted of a three-aquifer system
separated by two aquitard units (Figure All). Note
the convex-up morphology of the lower clay aquitard
(a.k.a . "mounded clay").
O.
IT
t
pk
VJ
tip pore
resistance pressure
(coarser) (higher)
ip » YERTiCAl EXAQCW'iQN
VERTICAL SCALE
10' 0 10'
HORIZONTAL SCALE
100"
100'
brown = silt/clay lithofacies
I | yellow = sand-rich lithofacies
~ orange = gravel-bearing channel lithofacies
Figure All. Existing CSM depicting three aquifer units (yellow) with gravel-bearing channel zone (orange)
separated by aquitard units (brown). Lower aquitard unit shows convex-up morphology ("mounded").
Best Practice for Improving Conceptual Site Models
A Practical Guide for Applying Advanced Stratigraphic Concepts to Contaminated Groundwater Sites
GW Issue AI2
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Research and experience in subsurface and
outcropping channel deposits indicate that channel
bases, due to the erosive nature of energetic river
systems, tend to be irregular, and that floodplain units
(silts and clays) tend to be horizontal in nature (Figure
A12).
Figure A13 shows an alternative interpretation of the
site data based on stratigraphic "rules of thumb" and
supported by the details of the CRT data collected.
The fundamental difference in interpretation of the
continuity of the aquitard unit between the previous
CSM and the ESS CSM has important implications
for risk of contaminant migration from source to
potential receptors. This highlights the importance
of objectively evaluating lithoiogic data according to
established stratigraphic concepts and facies models,
and is a caution against artificially forcing subsurface
data into a previously-established hydrogeologic
framework.
Figure A12. Photograph of outcropping fluvial channel (unannotated above, annotated below) showing light-
colored sand channel fill encased in floodplain deposits (dark colors). Note erosion at base of channel (blue arrows
indicate truncated beds). The top of the channel-fill is completely flat (although it appears slightly rounded due to
perspective of the photograph (looking upward at outcrop)). Erosive base and flat top is a common relationship in
fluvial depositional environments and calls into question the interpretation in Figure AI I.
A13 GW Issue
Best Practice for Improving Conceptual Site Models
A Practical Guide for Applying Advanced Stratigraphic Concepts to Contaminated Groundwater Sites
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resistance pressure
(coarser) (higher)
10 x VERTICAL EXAGGERATION
VERTICAL SCALE
10" 0 10*
HORIZONTAL SCALE
~ brown = silt/clay lithofacies
~ yellow = sand-rich lithofacies
~ orange = gravel-bearing channel lithofacies
_
tj> pore
resistance pressure
(coarser) (higher]
10 X VERTICAL EXAGGERATION
VERTICAL SCALE
10* 0 10"
HORIZONTAL SCALE
100"
i
-
?
— —S«- — — — — — —
4-• —
1
c
«— fN
tip pore £ £ £
rtnmance pressure U U U
(coarser) fhk>her)
10 x VERTICAL EXAGGERATION
VERTICAL SCALE
HORIZONTAL SCALE
100* 0 100'
High pore pressure response
Indicative of clay-rich flood plain
fades (verified by boring log
data}. Clay Is at the same
elevation as clay in CPT-3
not Hoodplai
High pore pressure response
indicative of clay-rich floodplai
fades (verified by boring log
data). Clay is at the same
elevation as clay in CPT-l
Fining-upward log
signature (not present
in CPT-l or CPT-3)
Insignificant pore pressure response
indicative of silty fades (verified by boring
log lithology data (. Considerably
shallower than days at CPT-l, CPT-3.
Suggests isolated intrachanrvel fines and
fades
Figure A13. a) The original CSM. fb,c) ESS CSM stratigraphic interpretation of CPT data showing a channel deposit
which has breached the principal aquitard unit through erosion. This interpretation is supported by the fining-upward
nature of the channel deposit in CPT-2, the low pore pressure response of CPT-2 relative to CPT-l and CPT-3, the
similarity in elevation of the floodplain fades in CPT-l and CPT-3, and the anomalous elevation of the silt unit in CPT-2.
Best Practice for Improving Conceptual Site Models
A Practical Guide for Applying Advanced Stratigraphic Concepts to Contaminated Groundwater Sites
GW Issue AI4
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Case Study 3: Glacial Terrain, Till, and Lacustrine Deposits, Upper Midwest
US; LNAPL and Dissolved Phase Impact at a Manufacturing Facility
Introduction to the Site
Case Study 3 relates to a former manufacturing site underlain by glacial deposits. Groundwater is impacted by
light non-aqueous phase liquid (LNAPL) and related dissolved-phase constituents. This case study illustrates
the importance of understanding the geologic evolution pertinent to a site and the value added from a review
of publicly available geologic resources. With a geologic context established for the site, the vertical grain-size
trends observed in boring logs were used to create interpretations of the subsurface which explain observed
phenomena and provide a basis for successful site management.
An ESS review of lithologic borings from a network of 30 monitoring wells was conducted in order to address an
anomalous divergent groundwater flow pattern moving away from the site. Migration of LNAPL and dissolved
contaminants moving with groundwater away from the facility was of primary concern. This site is located
on the north shore of Lake Ontario (Figure A14) and the original CSM predicted groundwater flow to follow a
pathway southward toward the lake. Kriging the hydraulic head data resulted in a divergent pattern (Figure A15)
that could not be explained with the existing CSM.
77.75°
Lake Ontario
43.75* 0 20 40
Oak Ridges Moraine
Kilometers
1. Oak Ridges Moraine 2. Niagara Escarpment 3. Streamlined Uplands 4. Broad Valleys
5. Plains (South of Moraine) 6. Glacial Lake Iroquois Shoreline 7. Incised Alluvial Valleys
Figure A14. General site location (blue circle) and some pertinent geological features including the shoreline
deposits of Glacial Lake Iroquois (red) and the plains (5) and streamlined uplands (3) known to include drumlin
landforms (Brennand, T.A., 1997: Surficial Geology of the Port Hope Area, NTS 30M/16, southern Ontario; Geological
Survey of Canada, Open File 3298, Scale 1:50,000),
A15 GW Issue
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Aroa A
LEGEND.
Area H
PROPERTY BOUNDARY
—, FENCE LIME
RAILWAY SPURS
¦ IB IRON BAR
® MWI-M PRE-KXJSTUG MONITORING WEU «Y OTHERS^
OMW«40 MONITORING WELL {AECCM, 2DTQQ
^MW212 MONITORING WEIL [AECOM 2012)
+-BH2Q5 BOREHOLE (ACCOM.2012)
193 2ft) GROUNDWATER ELEVATION
MARCH 2012
GROUNDWATER CONTOUR (0 1m INTERVAL)
INFERRED SHALLOW GROUNDWATER PLOW
DIRECTION
** MWSQf,
Figure A15. Divergent groundwater flow pattern observed at site based on computer contouring (kriging) of hydraulic
head data.
Best Practice for Improving Conceptual Site Models
A Practical Guide for Applying Advanced Sfrafigraphic Concepts to Contaminated Groundwater Sites
GW Issue AI6
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Buried/"Drowned" Drumlin Model
Inspection of Canadian geological survey maps
(Brennand, 1997) showed that the site was located
within the southernmost portion of a drumlin field
composed of the Bowmanville Till. In this area,
drumlins (mounded and lensoidal landforms created
during glacial retreat) are surrounded by the fine-
grained deposits of glacial lake Iroquois, a proglacial
precursor to modern day Lake Ontario.
The retreat of continental glaciers shaped tills and
left behind drumlin forms (Figure A16). These forms
persisted as islands of sand, gravel, and clay as glacial
meltwater filled the basin surrounding them. High
sediment loads of fine material shed from the glacier
and entrained in the melt water were deposited
regionally around the drumlins as layered, and
relatively flat lying silts and clays. Because they were
exposed to natural weathering and covered with
little to no vegetation in the immediate aftermath
of deglaciation, the drumlins would shed sediments
from their crests into the surrounding lake water as
small alluvial fans. Coarsening upward fans would
interfinger with lake derived clays around the drumlins
(Figure A17).
•X' 12
/
10a
Location
10a
too
/
%
y 3f
\ ¦ I i - - ¦
Glacial Deposits (till): sandy silt to sand
Glacial Lake Deposits: silt and clay
Glacial Lake Deposits: sand and gravel
Drumlin orientation
N
1 kilometer
Approximate scale
Figure A16. An excerpt from local surficial geology map (Brennand, 1997) showing the site is located on the crest
and western side of a drumlin (green area with axis of orientation shown in red) which is surrounded by fine (silts and
clays) lake deposits.
A17 GW Issue
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Figure A17. Conceptual summary cross section - Drowned Drumlin Model. Blue arrows are interpreted
groundwater flow.
Drowned Drumlin Model
Coarsening up Fans
Coarsening up Fans
Glacial Lake Iroquois
Lacustrine Clay
Bedrock
Graphic grain size logs from the site exhibit a lithologic
pattern that was consistent with the interpreted
geological scenario (Figure A18). Borings closer to the
mapped drumlin crest are composed almost entirely
of unstratified sandy and clayey sands and gravels
typical of the Bowmanville Till. Borings located on the
margin of the drumlin show a thick basal package of
laminated silts and clays, consistent with sediments
deposited in the low energy lake environment.
Coarsening upward sequences of sediments occur
atop, and in some cases interfingered with the fine
grained lake sediments. These were interpreted
as fans of material shed from the drumlin crests as
they sat exposed as islands surrounded by the glacial
meltwater of Lake Iroquois.
While site topography is generally flat, the site's
location on the crest of the drumlin, coupled with fans
of coarse materials extending radially off the drumlin
margins, produces the radial groundwater flow as
groundwater percolated down and away from the
drumlin crest (Figure A17).
This improved site understanding using existing
data to develop a CSM based on glacial depositional
model explained the groundwater flow and potential
contaminant migration pathways, saving the project
additional investigation costs. It also provides
a blueprint for optimized site characterization,
groundwater monitoring, and remediation design.
Best Practice for Improving Conceptual Site Models
A Practical Guide for Applying Advanced Sfrafigraphic Concepts to Contaminated Groundwater Sites
GW Issue A18
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Grain Size Log
~
~
LowTransmissivity
Lacustrine Silty Clay and Clays
(Glacial Lake Iroquois Deposits)
Fan Derived Silty Clays and Silts
(Eroded from Drumlin Core)
HighTransmissivity
Fan Derived Sands and Silty Sands
(Eroded from Drumlin Core)
~ Sand and Silty Sand with Gravel
(Newmarket Till, Core of Drumlin)
~
E
Artificial Fill material
(sand and gravel)
Figure A18. Example cross section from site, scale in meters. See Figure A15 for the map of cross section line.
A19 GW Issue
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A Practical Guide for Applying Advanced Stratigraphic Concepts to Contaminated Groundwater Sites
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Case Study 4: Desert Alluvial Fan Environments, Western US;
Hexavalent Chromium Impacts to Groundwater at an Industrial Facility
Introduction to the Site
Groundwater underlying this site was impacted by VOCs and hexavalent chromium. High-resolution lithology
data were collected using CPT to identify sand zones to design injection and monitoring wells. The CPT data were
correlated using a computer kriging software. This case study exemplifies the limitations ofkriging correlations,
even with high-resolution data, and the value of fades models to guide correlations. The impacts of stratigraphic
dip and thin clays bounding individual fan units were not recognized, and this limited remedy effectiveness and
led to byproduct generation, necessitating installation of additional remediation systems.
Alluvial Fan Fades Models
Alluvial fans form where coarse-grained material issues from mountain fronts onto basin floors. At this change
from higher to lower gradient, streamflow becomes less energetic and coarser material is deposited in the upper
(proximal) fan environment where current velocity is high. Finer-grained material is transported to the lower
(distal) fan where current velocity is low. The fan surface is concave-up, with a relatively steeper gradient at the
head and a flatter gradient at the distal end (Figure A19).
Composite
Alluvial
Last Active
Channel
Figure A19. Google Earth view of a large alluvial fan in Death Valley, CA with topographic profile shown below.
Best Practice for Improving Conceptual Site Models
A Practical Guide for Applying Advanced Stratigraphic Concepts to Contaminated Groundwater Sites
GW Issue A20
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With time, fans "prograde" {i.e., advance) out onto
the basin floor, placing coarse proximal-fan deposits
atop fine distal-fan deposits, producing coarsening-
upward profiles. This occurs in a punctuated,
stepwise fashion resulting in smaller multiple-stacked
coarsening-upward sequences separated by playa lake
or soil formation (paleosol) horizons. Multiple smaller
fans are stacked to form the larger fan and the overall
progradationa! pattern (Figure A20).
Coarser
Figure A20. Vertical profile through cyclic
alluvial fan deposits, showing the characteristic
coarsening-upward profiles of individual
packages which stack to form the overall alluvial
fan. (Redrawn from Steel and Gloppen, 1980)
Due to arid climate, active tectonics resulting in
topographic relief, and associated coarse-grained
sediment supply, alluvial fans are common in the
desert southwest of the USA. At a site in the US
desert southwest, CRT data were collected for the
purposes of identifying sand-rich zones for well screen
placement (Figure A21).
Qt (tip resistance)
0 1000
coarser
Figure A21. Cone penetrometer testing
data show two stacked coarsening-upward
sequences (red arrows) separated by thin clay
units.
A21 GW Issue
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Multiple, stacked coarsening-upward profiles are
seen within the saturated interval reflecting buried
alluvial fans. The fans are laterally offset-stacked
or "shingled" dipping and stepping basinward, and
are bounded by basinward-thickening clay deposits
representing paleosols or playa lake deposits which
are laterally continuous for hundreds of feet to miles
(Figure A22).
Clay units which separate individual fans, while
relatively thin, form effective barriers to groundwater
flow and reduce the hydraulic connectivity within the
system as seen in contaminant concentrations in wells
where a single fan is screened (e.g., HSU-B, Figure
A22). Thus, the fans represent HSUs. Most wells at
this site are screened across multiple fans, and thus
the water quality variation among individual fans is
unknown. Wells screened across multiple fans may
provide pathways for cross-contamination. Some
fans (i.e., HSU-B) are not in communication with
source areas and were not impacted with hexavalent
chromium. When reducing reagent was injected
(targeting hexavalent chromium) into unimpacted
zones, naturally occurring manganese byproduct was
released into solution.
It is illustrative to compare the stratigraphic cross
section to a cross section generated by a computer
model utilizing kriging of the same CPT data (Figure
A23). On the computer-generated cross section,
the thin clays do not appear to correlate due to the
primary stratigraphic dip of these units. The kriged
section gives an appearance of "randomness" of
facies distribution and compartmentalization is not
indicated. The compartmentalization and offset-
stacked shingling (stratigraphic dip) of alluvial fan
deposits was not recognized at this site prior to in-situ
remediation implementation, leading to compromised
remedy efficiency and by-product generation. This
case study highlights the risks posed by reliance on
kriging data for remedy planning without a facies
model to guide correlation.
Best Practice for Improving Conceptual Site Models
A Practical Guide for Applying Advanced Stratigraphic Concepts to Contaminated Groundwater Sites
GW Issue A22
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Basin ward
HSUD
Figure A22.
Stratigraphic cross
section with CPT logs
showing shingled,
dipping alluvial
fan deposits in the
saturated zone.
CPT Tip Resistance
coarser —>¦
Sand-Rich Alluvial Fan (Aquifer)
Screen interval
HSilt- and Clay-Rich Playa Lake and Paleosol (Aquitard)
with contaminant 64 6 1 -T
concentration 1 f
Hydrostratigraphic unit designation
1
West
East
Brown = silt/clay
White = sand/gravel
Figure A23.
Kriging of CPT data
presented in Figure A22
produces a cross section
that miscorrelates
thin clay beds, giving
an appearance of
randomness in lithology
and stratigraphic
architecture.
Water table
A23 GW Issue
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A Practical Guide for Applying Advanced Stratigraphic Concepts to Contaminated Groundwater Sites
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Case Study 5: Fluvial Channel and Overbank Deposits, Southern California;
Updated CSM for Perchlorate Plume Containment Remedy
Introduction to the Site
The site is a 1,000 acre former explosives manufacturing property underlain by a heterogeneous fluvial aquifer
impacted by VOCs and perchlorate. The original CSM was the basis for the groundwater containment system
approved by the regulatory agencies as the means to protect further impact to nearby groundwater production
wells. However, as part of the pilot study for the containment system, it was determined that the existing CSM
oversimplified the hydrostratigraphy underlying the site, bringing to question the efficacy of the proposed
containment system design. Using existing data, the CSM was revised by applying ESS methodology.
A sequence stratigraphic review of a 1,000 acre site in southern California was undertaken prior to
implementation of a pilot containment remedy for groundwater. The hydrostratigraphy underlying the site
consist of approximately 500 feet thick series of highly interbedded sands and clays corresponding to Plio-
Pleistocene fluvial channel and overbank deposits (Figure A24).
This study highlights the potential for relatively thin floodplain clay units to significantly reduce the hydraulic
connectivity within aquifers.
Kern River Analog
Kern River location
Silts & Muds ~ Sandstones
1 Base of major channel system or sand package
1 Sand-on-sand contact between channel system/sand packages
"holes in shales")
Figure A24. LEFT - Geophysical log suite calibrated to lithologic log showing highly interbedded nature of fluvial
channel and overbank deposits. RIGHT - Kern River analog, studies of nearby oil fields in the same depositional
setting show high continuity of higher-frequency floodplain fades, (from Knauer, et. al„ 2003)
Best Practice for Improving Conceptual Site Models
A Practical Guide for Applying Advanced Stratigraphic Concepts to Contaminated Groundwater Sites
GW Issue A24
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The original conceptual site model consisted of a layer-cake system of five hydrostratigraphic units
(Figure A25). The selected and regulatory-approved containment remedy called for extraction from
125' well screens (Figure A25, red box).
Aquifer (Sand and Gravels]
Aquitard (Clays and Silts)
Figure A25, Original CSM depicting five hydrostratigraphic units (HSU i, HSU ilia, HSU lllb, HSU V, HSU VII) and extraction well screened interval (red box).
A25 GW Issue
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Fluvial facies models,
and local knowledge
from nearby Kern
River oil field in the
equivalent stratigraphic
interval suggested
high continuity
of thin floodplain
facies (possibly
climate-driven) silt/
clay units, and a
stratigraphic analysis
was performed to
refine the conceptual
site model. Well logs
were correlated on
the basis of vertical
trends, and structural
dip removed {e.g., the
sections were datumed
on a major site-wide
floodplain clay so
fluvial architecture
could be interpreted)
(Figure A26).
This analysis was
carried out on a series
of 12 intersecting
cross sections, and
all units were loop-
tied to create a
high-resolution three
dimensional definition
of aquifer architecture.
i -j
r
> «»
r
i ~
NW
~ > *~
tit
fcif
n«v
l:i?.
SE
r <
-s»
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I
nir—
8a.
IP
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hofcontal Scale
11 f
it, -
h
i.
it:
¦
i.
w
IS
¦
a
*
I
c
: ?
i
T
i
¦
\b
IE
\
" Resistivity log
^Graphic grain
| Aquifer (Sands and Gravels) | | Aquitard (Clays and Silts)
~ Transitional (Silty Sands,
Sandy Silts)
StrMigraphK Ctom Section E-l"
WhUtakf ftermrte Site. S*nt« Onrils. Cslltemi*
Figure A26, a) Stratigraphic section where a key floodplain clay is used as a datum to correlate the lithology of fluvial facies
architecture based on stratigraphic rules for channel evolution, b) The same stratigraphic cross section datumed on mean sea
level elevation showing the structural dip.
Best Practice for Improving Conceptual Site Models
A Practical Guide for Applying Advanced Stratigraphic Concepts to Contaminated Groundwater Sites
GW Issue A26
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Targeted aquifer testing and a high-resolution groundwater sampling field program was implemented to validate
clay aquitard correlations and further refine the CSM and determine the extraction well design (Figure A27).
Resistivity (Ohrrvm)
DEPTH (ft bgs] 0
Proposed
EW-1A
PZ-1A
1,200 uq/L
EW-1A
PZ-TA2
Temp Wells
320
3r160 uq/L-
1,550 uq/L
44.3 [ig/L
EXPLANATION
Screen
interval
350
360
44.3 Perchlorate
concentration
RLN s resistivity long normal
RSN = resistivity short normal
Ohm-m = Ohm meter
Figure A27. Extraction Well Design - Strategic and Systematic. Based on the ESS CSM,
HSU designations of specific well screen intervals were evaluated based on strategic,
sequential aquifer tests and groundwater chemistry data to validate the stratigraphic
framework and optimize the groundwater extraction well design. Screens 1 are initial
pilot hole/piezometers for lithology and monitoring. Screen 2 represents temporary
wells to collect depth-discrete groundwater chemistry. Screen 3 represents
monitoring wells, and screen 4 is the final extraction well interval.
A27 GW Issue
Best Practice for Improving Conceptual Site Models
A Practical Guide for Applying Advanced Stratigraphic Concepts to Contaminated Groundwater Sites
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As shown in Figure A2S, the estimated extraction volume and cost for plume containment based on the original containment design was significantly
higher than the more optimized design based on the ESS-based stratigraphic framework. It showed that a single channel sequence approximately 35'
was impacted, not the entire 125' interval.
This is an example of how a relatively small upfront investment in expertise to reinterpret data from a stratigraphic perspective pays big dividends in
project lifecycle cost savings and risk reduction.
Project
_ - _ ESS Evaluation as part of Plume
a r Containment Remediation Design Pilot
Program
1:1
125' extraction
interval; includes
non-impacted
strata
Remediation System Cost
(Before ESS)
• 12 extraction welis
• ~200 gpm per well
• 1,261 Million gal/yr
Capital cost = $7 MM
Treatment cost= $2.5MM/yr;
30 yr = $75 MM
Total cost = $82 MM
35' extraction
interval; impacted
strata only.
Remediation System Cost
(After ESS)
• 13 extraction wells
• 46 gpm per well
• 314 Million gal/yr
Capital cost = $2.5MM
Treatment cost= $800K/yr;
30 yr = $24MM
Total cost = $26.5 MM
Figure A28. Comparison of Site CSM used for pilot containment design (a) with refined CSM based on high-resolution sequence stratigraphic interpretation
(b) and projected water treatment and cost savings estimates.
Best Practice for Improving Conceptual Site Models
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GW Issue A28
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Case Study 6: Incised Valley Fills, Gulf Coast Region, US;
Optimize VOC Plume Containment and In-situ Remediation
Introduction to the Site
This case study documents a former manufacturing site where groundwater was impacted by VOCs and mercury.
Due to the complexity of the subsurface, multiple approaches to groundwater remediation were employed
including pump and treat, and in-situ bioremediation. The subsurface was defined by cross sections using
lithostratigraphic correlation of USCS strip logs and gamma ray geophysical logs. Groundwater and contaminant
migration was estimated based on groundwater gradient maps, assuming homogeneous/isotropic conditions.
Because of the limited success of the remedies that were employed, the CSM was revisited by applying the ESS
approach to the existing subsurface data to define the groundwater and contaminant preferential pathways.
The following exemplifies the value of understanding the sequence stratigraphy (sea-level changes and impact
on environment of deposition) and applying ESS concepts to improve groundwater cleanup.
In the area of the project site, sea level fell by as much as 400' relative to present sea level during Quaternary
glacial periods. This led to exposure of the continental shelf, downcutting of rivers by erosion, and formation of
"incised valleys" in coastal areas including the US Gulf Coast (Figure A29). The site lies within the well-studied
Mobile Bay incised valley.
The different environments of deposition which occupied the valleys, and the manner in which they evolved and
changed as sea-level rose are well documented for Mobile Bay, as well as other incised valleys of the Gulf and
East Coasts of the USA, as well as many other areas worldwide. However, this information was not considered
during initial site investigation or remediation.
STRATI GRAPHIC ORGANIZATION OF INCISED-VALLEY SYSTEMS
Highstand-Shoreline
Lowstand-Shoreline
.Coastal Plain
/ 1 / /ov Exposed
Shelf/
Marine
Alluvial Plain
Piedmont I.V. System
Coastal Plain '
Incised-Valley System
Figure A29. Block diagram depicting development of incised valleys during relative lows land of sea level
(glacial periods) modified from Zaitlin et al. (1994).
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Due to the relatively flat topography of terrace
deposits located within incised valleys, and proximity
to major shipping and commerce areas {e.g., Mobile
Bay), the flooded incised valleys are preferential sites
of industrial development and related groundwater
contamination. This case study shows how an
incised-valley depositional model can be used as a
tool for interpretation of site data and a predictor
of subsurface heterogeneity in incised valleys, and
highlights the need for stratigraphic characterization
prior to an in-situ injection program.
Site data consist of lithologic descriptions of borings
and a suite of gamma-ray logs run through casing
(Figure A30).
GAMMA COUNTS PER SECOND
fo m oo o w .
o o o o o o O '
D
m
V
H
I
i
m
rn
H
to
o
w
o
4*
o
U1
o
CO
©
o
Coarsening-upward,
gray sand with fossils
¦— "Hot" clay
Fining-upward
orange and tan
sand with clay
sand-rich,
no gravel
Clay break
Gravelly,
sand-rich
Basal Clay
Figure A30. Gamma ray log representing
grain size trends and significant clay spikes.
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An analog depositional model from a well-studied estuary
shows similar vertical facies trends and associations to the site
(Figure A31).
Gamma-ray and lithologic data from the site were interpreted
in the context of the incised valley fill sequences (Figure A32).
• Initially, an erosional surface (sequence boundary) was
developed during sea-level lowstand at approximately
-70' msl (mean sea level) (see IV-1 on Figure A32). Fluvial
gravels overlying the sequence boundary were deposited
by braided type rivers occupying the incised valley floor
(compare to Figure A31a). Injection treatments in this
interval were very effective at reducing contaminant
concentrations due to the well-connected, highly permeable
nature of these deposits (lowstand systems tract in
sequence stratigraphic terminology).
• As sea level rose, marine waters flooded the valley and a
marine transgressive clay was deposited at approximately
-25' msl (compare to Figure A31b). A series of channelized,
clay-rich estuarine sediments then accumulated (compare
to Figure A31b), Injection treatments in this interval have
been ineffective in reducing contaminant concentrations
due to difficulty in achieving distribution of reagent due to
the isolated, disconnected nature of estuarine channels in
this interval and the effects of back-diffusion of
contaminants out of estuarine clays.
• These deposits are overlain by a "hot clay" at 5' msl
characterized by high gamma counts representing the
"maximum flooding surface", or deposits of the relatively
highest sea level (compare to Figure A31c, note that the
micro-tidal setting of the gulf coast precluded development
of a tidal ravinement surface such as shown in Figure A31c).
This clay is widespread and likely forms an effective
hydrogeologic barrier.
• After this, a bay-head delta prograded across the site
producing the coarsening-upward, relatively sheet-like
"upper zone" at 0 to 10' msl (compare to Figure A31d).
• A second lowering of sea-level resulted in a second
erosional event and another incised valley system
(IV-2 on Figure A32). This second valley system is filled
with coarse fluvial gravels at its base. The contact between
this second incised valley and the older sediments provides
a barrier to groundwater flow and contaminant transport.
Interfluve
A uvial Plain
/
Incised Valley
Sequence
Boundary
Alluvial
Estuarine
Coastal Plain
Bayline
Transgressive
Surface
Flood Tidal Delta
Transgressive
Estuary
Tidal
Ravinement
Surface
Estuarine
Bay head Delta
i
Ravinement
Tidal Inlet
Figure A31. Block diagrams illustrating sequential
phases of fill of the incised valley during sea-level
lowstand and rise, (redrawn from Posamenfier
and Allen, 1999)
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CinuulUjIogi
ol qunbomblt quality
damn by lop
of q«slianiU« qualty
Pilinjs
Barge
(anal
i : : : :
r t t f i
III
¦z~t-
Figure A32. Cross section depicting the two incised valley sequences (IV-1 and IV-2).
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The Intermediate Zone is depicted on Figure A33 and
shows how the original CSM, based primarily on the
mapped groundwater gradient and on the assumption
of homogeneous, isotropic conditions (A33a and
A33b), does not take into account the permeability
architecture resulting from the incised valley
depositional system. The Groundwater Gradient
map (Figure A33a) and PCE Plume map (Figure
A33b) are both constructed without consideration
of the underlying heterogeneous geology. Figures
A33c and A33d show that the heterogeneous
Intermediate Zone is composed of high-permeability
sand channels that trend almost perpendicular to
regional groundwater gradient, which creates a
northwest/southeast preferential pathway that was
not identified before the stratigraphic evaluation. This
makes a strong case that with heterogenic geology
the regional groundwater gradient data alone cannot
be used to identify preferential flow pathways. The
details of the geology should be defined to assess
the potential impact of the geology on groundwater
flow and contaminant migration. In this example,
defining the channel features helped to understand
some of the issues encountered during the initial
in-situ injection for the bioremediation program.
The injection program was very successful in the
more homogeneous Lower Zone where there was
good distribution of the injectant. However, in
the Intermediate Zone there was poor distribution
and "daylighting" of injectant occurred due to the
heterogeneous nature of the channel deposits.
Understanding these inherent geologic permeability
pathways proved important to optimize future
groundwater containment and in-situ injection design.
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Figure A33. Original
CSM (prior to ESS
evaluation) is based
on the following:
A33a shows
groundwater
elevation contours
for the Intermediate
Zone with a northeast
groundwater flow
direction, and
A33b shows PCE
concentration
contours interpreted
prior to the lithofacies
interpretation and
based on this northeast
flow direction alone.
This interpretation of the
contaminant distribution
did not take into
account the underlying
geology. However,
as a result of the ESS
evaluation, A33c is a
lithofacies map of the
Intermediate Zone
(depicted on the
cross section A33d)
that shows higher
permeability sand
channels trend almost
perpendicular to the
groundwater gradient,
bringing to question the
chemistry concentration
contours interpreted in
Figure A33b. Potential
action would be to
collect groundwater
data along the high
permeability zones
to better define
contaminant extent.
Intermediate Zone
Groundwater Gradient
Intermediate Zone
PCE Plume
Low permeability
silts and clays
ATCOM
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APPENDIX B
Glossary of Terms
Aeolian (or Eolian): Of, relating to, or derived from the action of the wind. Generally refers to sand dunes and interdune
deposits either in coastal or desert environments.
Alluvial: Pertaining to or composed of clay, silt, sand, gravel or similar unconsolidated detrital material deposited by a stream
or running water.
Alluvial fan: A fan- or cone-shaped deposit of sediment built up by streams which shift laterally across its surface. Alluvial fans
typically form at the topographic change in slope where high-gradient mountain streams exit their confined canyons onto a
relatively broad, flat basin floor where they become unconfined and can spread laterally. If a fan is built up by debris flows it is
properly called a debris cone or colluvial fan.
Anastomosing: River system consisting of multiple interweaving channels. Anastomosing rivers typically consist of a network
of low-gradient, narrow, deep channels with stable banks, in contrast to braided rivers, which form on steeper gradients and
display less bank stability.
Aquifer architecture: Three-dimensional organization of permeable and relatively impermeable aquifer units.
Aquifer characteristics: Characteristics such as hydraulic conductivity, recharge, and aquifer boundaries.
Architectural elements: Component parts of sedimentary deposits with characteristic dimensions and properties, such as
channel fills, overbank splays, and floodplain clays.
Avulsion: Rapid abandonment of a river channel and the formation of a new river channel in a different location.
Braided river: One of a number of channel types that consists of a network of small channels separated by small and often
temporary islands (called braid bars). Braided streams occur in rivers with high slope and/or large sediment load, are typically
only a few feet deep.
Clastic sedimentary aquifer: Aquifer that consists of accumulations of transported and redeposited detrital material (e.g., clay,
silt, sand, gravel).
Clay plug: A clay- and organic matter-rich deposit which forms after an avulsion or "cut-off" of a meander loop in a
meandering stream. Clay plugs are arcuate (crescent-shaped) in map view, filling "oxbow lakes".
Deltaic: Of, or pertaining to, a delta environment where sediment load from a river is discharged into a body of standing water.
Deltas typically form a protuberance in the shoreline and can be dominated by fluvial processes, tidal processes, or wave
processes.
Depositional elements: Basic mappable components of both modern and ancient depositional systems and stages that can be
recognized in modern depositional environments, outcrops, and the subsurface.
Depositional models: See Facies Models
Depositional processes: Natural processes which transport, deposit, and preserve sediment, such as a stream shifting across
an alluvial plain.
Depositional System: A three-dimensional association or assemblage of facies (depositional environments) genetically linked
by active (modern) or inferred (ancient) environmental and sedimentary processes.
Facies: Bodies of sediment recognizably different from adjacent sediment deposited in a different depositional environment or
sub-environment (e.g., upper shoreface and lower shoreface facies of a barrier island environment).
Facies models: Conceptual construct summarizing the processes acting to erode, transport, deposit, and preserve sediments
in particular depositional environment. Also known as Depositional Models, they typically are represented as a three
dimensional block diagram showing component parts of buried strata (architectural elements), how they fit together, and a
map view showing the active depositional system and its key features.
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Fining (or coarsening) upward: Vertical trend in grain size related to a change in energy level within the depositional system
with time as the deposit accumulates.
Flooding surface: A general term that refers to surface that separates older rocks/sediments from younger rocks/sediments
and is marked by deeper-water strata resting on shallower-water strata.
Geomorphology: The scientific study of the origin and evolution of topographic and bathymetric features created by physical,
chemical, or biological processes operating at or near the earth's surface.
Glacial: Of, relating to, or derived from ice.
Hydrostratigraphic unit: A body of sediment saturated with groundwater with limited connectivity to adjacent sediments.
Clastic (sedimentary) aquifers typically are composed of multiple hydrostratigraphic units due to heterogeneous geology.
Immobile porosity: The portion of pore space (porosity) that does not allow for groundwater movement; contains stagnant
groundwater that serves as a reservoir for contamination; mainly in fine-grained sediments.
Lithology: A description of physical characteristics of a rock (or unconsolidated sediments) such as color, texture, grain size, or
composition.
Lithofacies: Lateral, mappable subdivision of a designated stratigraphic unit formed under common environmental conditions
of deposition, distinguished from adjacent subdivisions on the basis of lithology.
Loop-tied correlations: Using 2D information to aid in the construction of a valid 3D interpretation is called "tying" the
cross section interpretations together. To "tie the loop" (or loop tied) is to ensure that all geologic surfaces that affect an
interpretation have been tied around a loop along the cross section lines being constructed and are thus consistent in 3D.
Mobile porosity: Corresponds to portion of porosity where groundwater flow occurs; includes interconnected pore space that
acts as conduits for contaminant transport; mainly in coarse-grained sediments, (total porosity = mobile + immobile)
Outwash: Glacial sediments deposited by meltwater at the terminus of a glacier.
Overbank: An alluvial deposit consisting of sediment that has been deposited on the floodplain of a river or stream by flood
waters that have broken through or overtopped the banks.
Permeability heterogeneity: Diversity in a rock's ability to transmit fluids.
Point bar: An arcuate deposit of sediment, usually sand, that occurs along the convex inner edges of the meanders of channels
and builds outward as the stream channel migrates.
Sedimentary depositional environments: Specific depositional settings that are unique in terms of physical, chemical, and
biological characteristics (e.g., lake, stream, deep marine, glacier, etc.).
Sedimentary unit: Layers that are laid down by deposition of sediment associated with weathering processes, decaying
organic matters or through chemical precipitation.
Strata: Layers of sedimentary rocks or sediments.
Stratigraphic architecture: Structure of sediment/rock layers and layering.
Stratigraphic heterogeneity: Diversity in sediment/rock layers and layering.
Transgression: The migration of a shoreline onto land that can result in sediments characteristic of shallow water being
overlain by deeper water sediments.
Udden - Wentworth classification: A grade scale for classifying the diameters of sediments is widely used as the standard for
geology and the objective description of sediment.
USCS- Unified Soil Classification System: A soil classification system used in engineering and historically the environmental
industry to describe the texture and grain size of a soil to aid in the evaluation of its significant properties for engineering use.
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