PLUME
CLEAN-UP PROJECT Groundwater
Modeling to Assist in the Design of a Remediation
Purge Well System in East Bay Township, Michigan
Disclaimer: This project is
based on the 1985 U.S. Geological Survey study by F.R. Twenter, T.R. Cummings
and N.G. Grannemann. Some information
has been simplified, modified, or omitted to make the problem suitable for
educational application. The results and interpretations of this project are
not meant to be liable assessments of the real-world contamination
problem.
Background
A subsurface contaminant plume has been
identified in East Bay Township, a residential area adjacent to Traverse City,
Michigan and home to the Cherry Capital Airport. Prompted by complaints of
odors and foaming water in domestic water well supplies in the residential area
along Avenue E near East Grand Traverse Bay, the Department of Public Health
detected organic substances in the groundwater – including benzene, toluene,
xylene, and other compounds characteristic of solvents, cleaning agents, and
fuel substances. These chemicals are known to be harmful if ingested by humans.
Subsequent water quality analyses from
numerous groundwater monitoring wells indicate that the plume stretches over
4,000 feet from the bay to an area south of Parsons Road, in the vicinity of
the US Coast Guard Air Station – a property formerly owned by the U.S. Navy
(see Figure 1). A review of old Navy files and records and interviews with
long-term residents of the area identified several possible sources of
contamination on the Coast Guard property: a former U.S. Navy waste dump on the
eastern edge of the property; several former and currently operated fuel
dispensers and storage tanks; and storage and disposal of solvents and aviation
materials near the Hangar/Administration Building (see Figure 2).
Objectives
You are tasked with determining if the
plume has indeed originated from the former Navy property by conducting a study
of the hydrogeologic conditions near the plume and adjacent areas. Furthermore,
you will design and analyze a network of hydrologically suitable remediation purge
wells to remove the contamination from the groundwater. Your project analysis
should also include estimated costs of pumping contaminated groundwater and the
removal of contaminated soil at the Air Station.
Specific hydrogeologic questions that
should be addressed include:
- What
are the general flow patterns in the region? What controls these patterns? What
are the regional sources and sinks of water (e.g., major surface water bodies
receiving groundwater, upland areas recharging the water table, etc.)?
· What
are the local groundwater flow directions and velocities at the site and
downstream from the site?
· Where
does the water pumped by the domestic wells come from? Is the plume in the
contribution areas of the wells? Which ones?
· How
long does it take for the plume to reach domestic water wells downstream from
the site?
· How
many purge wells should be used; where should they be located; and at what
rates should they pump?
Tasks
- Develop a regional
model of East Bay Township and the adjacent areas. This model will provide
boundary conditions for a local model of the site and contaminated area between
Parsons road and East Grand Traverse Bay.
2. Develop a local
model of the site and the contaminated area between Parsons Road and East Grand
Traverse Bay.
3. Apply the local
model to determine the well capture areas of potentially contaminated wells.
4. Apply the local
model to predict the 3D movement of contaminants from the Coast Guard Property
(plan and cross-section visualizations).
5. Design and test
multiple groundwater clean-up scenarios (i.e., configurations of the purge well network);
6. Determine an optimal
design of the purge well network.
7. Estimate
costs/resources needed for pumping contaminated groundwater and for excavation
of contaminated soils.
8. Organize your
findings into a nicely written technical report. Be sure to clearly explain all assumptions and methods used in your analysis.
The following sections provide more
background information and modeling instructions needed to complete the tasks.
Overview of Site Hydrogeology
The Air Station is located on a relatively
flat surface that gently slopes to the northeast. The land continues sloping
downward north toward East Grand Traverse Bay. South of the site the land
surface rises before encountering Mitchell Creek, which drains the area to the
south and east of the station. Boardman River and Boardman Lake to the west of
the site direct flow to the West Grand Traverse Bay. The average annual precipitation in the area
is about 35 inches.
Underneath the station and adjacent areas
are thick glacial deposits of lacustrine origin (i.e., formed through
depositional processed associated with lakes). A small exception is the area
just south of Cherry Capitol Airport, which is underlain by glacial outwash (deposits
of sand and gravel carried by running water from the melting ice of a glacier).
The lacustrine deposits consist of an upper sand and gravel unit and an
underlying clay unit.
The upper portion (~20 ft) of the sand and
gravel unit consists mostly of fine to medium grained tan sands. Gravel and
coarse gray sands are predominant below. Based on wells installed for
groundwater chemical analysis and water level monitoring, the sand and gravel
unit vary in thickness, from a minimum of about 30 feet to a maximum of about
120 ft (see Figure 4).
The clay unit is relatively impermeable
and its thickness is not known because it has never been fully penetrated
within the area. Borehole lithologies suggest that the top of the clay surfaces
ranges from 575 to 585 feet near the west side of the site and to about 480
feet on the east.
Multiscale
Approach
Simulating the plume transport and
remediation operations requires detailed information about the groundwater flow
at the site. But the site-specific flow conditions are influenced by the
regional flow patterns, requiring some understanding of how groundwater head is
distributed in the area around (and especially upstream) of the site. The
proper way to handle this ‘multiscale’ nature of groundwater systems is to
simulate large-scale conditions with a larger, relatively coarse-grid model –
the Regional model. This model can then be used to inform a smaller, finer grid
model (the Local model).
Regional Model Development
Two modeling areas (domains) have been
created for your regional and local-scale modeling (see Figure 5). The model domains can be added into the
MAGNET modeling environment by uploading
the MAGNET file ‘ModelDomains_ForStudents’ available on in the Project Folder
on the MAGNET Curriculum Network,
or made directly available to you by your instructor.
Alternatively, you can manually add the
regional model domain using the ‘DomainRect’ tool (‘DomainDraw’ >
‘DomainRect’). Similarily, the local model area can be added using the ‘Zone
Poly’ tool (‘Zones’ > ‘ZonePoly’).
Load the model domains into the MAGNET
modeling environment and parametrize the regional model (i.e., assign model
inputs) to simulate horizontal (2D) flow in the surficial glacial aquifer
(i.e., the sand, gravel and clay aquifer). Run the model in steady-state model to
represent long-term average conditions. Use the available spatially-explicit
inputs on the Server to represent the aquifer elevations (top and bottom
surfaces), hydraulic conductivity, and recharge. The bottom surface follows the
bedrock surface, and it is assumed that there is negligible flow to/from the
bedrock units.
Local Model Development
A zone feature in the regional model was
included to delineate the local model domain. Make this zone ‘active’ to assign
boundary conditions from the regional model.
Run the local model in steady-state model
to represent long-term average conditions. For consistency, the local model
should apply the same K and recharge inputs as the regional model. The aquifer
top surface should still follow the Digital Elevation Model (DEM) available on
the Server. The bottom surface, however, is different for the local model. The
bottom surface should follow the clay layer surface. In other words, you will
only model flow in the sand and gravel layer, assuming negligible flow to/from
the clay layer. Scatter points of the elevations at which the clay layer is
first encountered are given in the ‘Available Field Data’ section below. These
data can be entered directly into the MAGNET scatter point interface for Bottom
Elevation of the local model zone feature.
Add particles at the locations of
potentially contaminated domestic wells and use reverse particle tracking to
determine the well source water areas.
Add a zone feature at the site to
represent the source of contamination. The substance occurring in the highest
concentrations will be used to simulate the plume migration: toluene (55,500
μg/L). For the purposes of this analysis, you may assume that the concentration
of toluene at the site does not change as the plume moves off site (i.e., it is
a continuous source). You may want to consider the conservative
situation of plume transport (i.e., pure advection of the ‘particles’
only).
Compare the simulated plume to the plume
extent estimated by USGS (see Figure 6). Figure 6 is available as a
georeferenced image that can be overlaid to the MAGNET modeling environment for
easy comparison. (Figures 2 and 3 are also available as georeferenced images).
Purge Well Network Design
Apply
the calibrated local model to evaluate hydrogeologically suitable locations of
the purge wells for removing the contaminated groundwater along the length of
the plume. Experiment with different locations/configurations and different
pumping rates. You may use any number of wells, but you should adhere to the
following constraints:
- pump
as little uncontaminated water as possible;
· minimize
the local drawdown (groundwater level decline) around each well to avoid
contaminating soils that go from saturated to unsaturated during pumping
(‘short-circuiting’ the removal mechanism).
· Minimize
costs associated with well installations and operations.
Also,
you must account for the restriction on purge well installations in the
residential area between Indian Trails Blvd and the bay.
Decide
on a final design and report the locations and pumping rates of the purge
wells. Also report expected water levels changes near the wells and estimate
the costs of purge well installations and operations. Suggest how the
contaminated water should be handled (e.g., how might you treat it and/or where
would you dispose of it). You should properly cite any outside resources used
in your cost analysis.
Site
Remediation
The final step in your analysis is to estimate
cost of excavating contaminated soil at the site. Also consider which potential
sources should be permanently removed from the site to prevent future
contamination. Suggest a monitoring plan for the following 5-10 years.
Available Field Data
i. Scatter
points, elevations at which clay layer first encountered:
K1D,-85.587227,44.745162,169.4688,-1
K3D,-85.578638,44.745722,169.7736,-1
K31D,-85.5807734,44.7446756,177.3936,-1
K33S,-85.5825351,44.745477,178.9176,-1
K5D,-85.5790499,44.7462372,173.4312,-1
K8D,-85.5795202,44.7469703,177.0888,-1
K45S,-85.5803632,44.7460995,177.3936,-1
K13D,-85.57605,44.749259,174.3456,-1
K32D,-85.573212,44.747024,149.9616,-1
K15D,-85.577063,44.749763,175.5648,-1
K11D,-85.581016,44.749994,176.4792,-1
K16D,-85.5741764,44.74964,170.9928,-1
K29D,-85.567555,44.751773,145.3896,-1
K23D,-85.5748527,44.7498207,173.4312,-1
K9D,-85.5797745,44.7472688,175.8696,-1
K25S,-85.5719898,44.7539879,170.688,-1
KT,-85.5811013,44.75005,176.784,-1
ClayReg1,-85.5557388715,44.7321159409,169.3592368968,-1
ClayReg2,-85.5985007738,44.7385844971,180.6766498728,-1
ClayReg3,-85.603258,44.673337,124.7527429248,-1
ClayReg4,-85.5417400396,44.7464487768,134.1697090032,-1
ClayReg5,-85.5850835561,44.7330264923,178.1070990048,-1
ClayReg6,-85.6075008772,44.7307707277,178.0999840584,-1
ClayReg7,-85.5752685988,44.7328878165,178.5397049976,-1
NOTE:
SPID=Scatter
point ID
X(Lon)=longitude
of scatter point
Y(Lat)=latitude
of scatter point
SPvalue=scatter
point value (elevation of clay surface)
Icolor=-1 …
default marker color applied when showing markers on map display