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Characterization and Modeling of the Cape Cod Aquifer EE&S 816. James K. Henderson May 5, 2005. What are Lead Scavengers?. The lead scavengers 1,2-dibromoethane (ethylene dibromide, or EDB) and 1,2-dichloroethane (DCA) were added to leaded gasoline to prevent lead buildup in engines
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Characterization and Modeling of the Cape Cod AquiferEE&S 816 James K. Henderson May 5, 2005
What are Lead Scavengers? • The lead scavengers 1,2-dibromoethane (ethylene dibromide, or EDB) and 1,2-dichloroethane (DCA) were added to leaded gasoline to prevent lead buildup in engines • EDB and DCA have low allowable Maximum Contaminant Levels (MCLs) and are suspected carcinogens • Almost 20 years after leaded gasoline was banned, available field evidence suggests that EDB and DCA persist at some sites where leaded gasoline was released
MMR EDB Plumes • The Massachusetts Military Reservation is one of the few sites where EDB plumes have been delineated • How are EDB plumes able to persist in the subsurface at low levels years after their release?
Objectives • Characterize Cape Cod geology and hydrogeology • Model EDB plume behavior
Cape Cod Overview • Cape Cod is a hook-shaped peninsula that extends eastward from the Massachusetts mainland and curves north over a total length of approximately 110 • The population of Cape Cod is 229,545 • Over 4.6 million visitors come to Cape Cod every year • Dave Hisz is from there
Map of Cape Cod Dave Dave’s House
Geology • Four broad classes of geologic materials are present on Cape Cod: • sand and gravel in glacial outwash plains • fine-grained silt and clay layers that underlie outwash plains • glacial till overlying bedrock • crystalline rock
Hydrostratigraphy • Six interbedded lenses of highly permeable glacial outwash sand and gravel are the principal hydrostratigraphic unit on the Cape • The Cape Cod aquifer is relatively homogeneous and mildly anisotropic
Conceptual Model Recharge Streams Cape Cod Bay Fine-grained Sand/Silt Groundwater Flow Paths Saline Groundwater Bedrock Glacial Till Freshwater/Saltwater Interface South North
Fuel-Spill 28 Plume • Discovered in 1992; source unknown • Highest concentration of EDB is 0.031 ug/l • Top of plume 60 to 100 feet below ground surface • Groundwater upwells into bogs
FS-28 Plume Conceptual Model Water Table Coonamessett Pond EDB Plume Coarse-grained sand/gravel Fine-grained silt/sand Bedrock Graphic design rendering support provided by M. Iannacone and J. Soberaski
Release Simulation • Simulate release of aviation fuel as contaminated recharge • Run the model for 20 years • Expect to reproduce groundwater upwelling into bogs
Model Development Overview Model of Cape Cod Calibration Refinement (TMR) Transport Simulation (MT3D)
Overview Model • Based on three DEMs • 64 rows by 72 columns • Each cell is one square kilometer • Of 4608 total cells, 2860 were imported as constant head using StreamBC
Overview Model, contd. • Overview model reproduced six groundwater mounds • Calibration using actual head targets unsuccessful • Calibration achieved by adding three head targets and one flux target • Calibrated K = 224 m/d • Calibrated R = 0.002 (compare to 0.001, CUHydrograph)
MMR Model • TMR used to refine overview model • 100 by 100 grid used • Each cell is 220 by 240 meters • Focus is FS-28 release area • MT3D used to simulate EDB transport
Model Layers Ground Surface
Source Determination • Source of FS-28 unknown • Location of bogs chosen as discharge point of FS-28 • ModPath run in reverse (layer by layer) until flow ended at a single cell (R50, C32). • This cell selected at source of EDB plume
ModPath Results (forward model) Coonamessett Pond
ModPath Results, contd. Coonamessett Pond
