A numerical model was used to simulate pond-aquifer interactions under natural and stressed conditions near Snake Pond, Cape Cod, Massachusetts. Simulation results show that pond-bottom hydraulic conductivity, which represents the degree of hydraulic connection between the pond and the aquifer, is an important control on these interactions. As this parameter was incrementally increased from 10 to 350 feet per day, the rate of ground-water inflow into the pond under natural conditions increased by about 250 percent, the associated residence times of water in the pond decreased by about 50 percent, and ground-water inflow to the pond shifted closer to the pond shore. Most ground-water inflow (90 to 98 percent) was in the upper model layer, which corresponded to shallow, near-shore areas of the pond, over the entire range of pond-bottom hydraulic conductivity. Ground-water flow paths into the pond became more vertical, the contributing area to the pond became larger, and the pond captured water from greater depths in the aquifer as the hydraulic conductivity of the pond bottom was increased. The pond level, however, remained nearly constant, and regional ground-water levels and gradients differed little over the range of pond-bottom hydraulic conductivity, indicating that calibrated models with similar head solutions can have different pond-aquifer interaction characteristics.
Hydrologic stresses caused by a simulated plume-containment system
that specifies the extraction and injection of large volumes of
ground water near the pond increased the pond level by about 0.4
foot and ground-water inflow rates into the pond by about 25 percent.
Several factors related to the operation of the simulated containment
system are affected by the hydraulic conductivity of the pond
bottom. With increasing pond-bottom hydraulic conductivity, the
amount of injected water that flows into Snake Pond increased
and the amount of water recirculated between extraction and injection
wells decreased. Comparison of simulations in which pond-bottom
hydraulic conductivity was varied throughout the pond and simulations
in which hydraulic conductivity was varied only in areas corresponding
to shallow, near-shore areas of the pond indicate that the simulated
hydraulic conductivity of the pond bottom in deeper parts of the
pond had little effect on pond-aquifer interactions under both
natural and stressed conditions.
Abstract
Introduction
Purpose and Scope
Hydrogeologic Setting
Conceptual Model of Pond-Aquifer Interactions
Kettle-Hole Ponds on Western Cape Cod
Variables That Affect Pond-Aquifer Interactions
Site History and Specifications of the Simulated
Plume-Containment System
Numerical Ground-Water Flow Modeling
Model Development
Model Grid
Boundary Conditions
Hydraulic Properties
Ponds
Model Analysis
Pond-Aquifer Interactions under Natural Conditions
Pond and Ground-Water Levels
Hydraulic Gradients
Inflow to and Outflow from Snake Pond
Rates and Distribution of Ground-Water Inflow
Residence Time of Water in Snake Pond
Area and Volume of the Aquifer Contributing
Water to and Receiving Snake Pond
Comparison of Uniform and Spatially Variable
Pond-Bottom Hydraulic Conductivity
Pond-Aquifer Interactions under Stressed Conditions
Effects of Simulated Plume-Containment System
on Snake Pond
Changes in Pond and Ground-Water Levels
Changes in Ground-Water Inflow Rates and Residence
Times in Snake Pond
Inflow of Injected Water into Snake Pond
Effects of Snake Pond on the Simulated Plume-Containment
System
Drawdown and Mounding near Extraction and Injection
Wells
Recirculation of Water between Extraction and
Injection Wells
Comparison of Uniform and Spatially Variable
Pond-Bottom Hydraulic Conductivity
Summary and Conclusions
References Cited
1. Map showing regional water table, surficial geology, and location of Massachusetts Military Reservation and Snake Pond on western Cape Cod, Massachusetts
2. Diagram illustrating the interactions between a ground-water flow-through pond and the surrounding aquifer in an unconfined hydrogeologic environment similar to western Cape Cod
3-5. Maps showing:
3. Fuel Spill-12 (FS-12) source area, extent of FS-12 plume in 1996, and location of simulated extraction and injection wells near Snake Pond
4. Extents of regional and subregional models for the Snake Pond area
5. Extent of subregional model grid, boundary
specifications, and location of simulated ponds and
extraction and injection wells
6. Schematic cross section showing vertical
model discretization and discretization of Snake Pond along
model column 67, and diagram of model representation of the pond-aquifer
connection and flow components between pond and aquifer cells
7, 8. Maps showing:
7. Model-calculated heads near Snake Pond under natural conditions for pond-bottom hydraulic conductivities of 350 and 10 feet per day
8. Location of simulated vertical pond-bottom hydraulic gradients at the pond bottom of Snake Pond under natural conditions for a pond-bottom hydraulic conductivity of 350 feet per day
9, 10. Graphs showing:
9. Sensitivity of (A) magnitude of the pond-bottom hydraulic gradient and (B) directions of the hydraulic-gradient and velocity vectors at a representative model cell (row 91, column 61, fig. 8) along the northern shore of Snake Pond to changes in pond-bottom hydraulic conductivity for simulation of natural conditions
10. Sensitivity of (A) ground-water and total inflow to Snake Pond and (B) distribution of ground-water inflow to the pond by model layer to changes in the pond-bottom hydraulic conductivity for simulation of natural conditions
11. Map showing area at the water table that contributed water to Snake Pond and area of the aquifer that received water from Snake Pond for simulation of natural conditions and pond-bottom hydraulic conductivities of 350 and 10 feet per day
12.Vertical section along model column 67 showing the vertical extents of the aquifer volumes that transmit water to and from Snake Pond for simulation of natural conditions and pond-bottom hydraulic conductivities of 350 and 10 feet per day
13. Map showing change in water-table elevations (hydraulic head in layer 1) caused by operation of simulated extraction and injection wells near Snake Pond for pond-bottom hydraulic conductivities of 350 and 10 feet per day
14, 15. Graphs showing:
14. Sensitivity of ground-water inflow rates and the change of inflow rates from natural conditions to changes in pond-bottom hydraulic conductivity for simulations of stressed conditions owing to operation of the simulated plume-containment system
15. Sensitivity of (A) rate of treated-water inflow to Snake Pond and (B) the fraction of ground-water inflow to Snake Pond that was composed of treated and injected water to pond-bottom hydraulic conductivity for simulations of stressed conditions owing to operation of the plume-containment system
16. Map showing maximum changes in ground-water heads around the well screens (in model layer 12) caused by operation of the simulated extraction and injection wells near Snake Pond for pond-bottom hydraulic conductivities of 350 and 10 feet per day
17. Sensitivity of the fraction of water extracted from the simulated extraction wells that was composed of injected water from (A) the lakeside injection wells and (B) the southeastern injection wells to changes in pond-bottom hydraulic conductivity for simulations of stressed conditions owing to operation of the simulated plume-containment system
1. Screen altitudes for extraction and injection wells in the simulated plume-containment system
2. Comparison of simulated steady-state hydrologic budgets for coincident areas of the regional model of western Cape Cod, Massachusetts, and subregional models of the Snake Pond area for low and high values of pond-bottom hydraulic conductivity
3. Hydraulic conductivity and horizontal-to-vertical anisotropy for simulated pond-bottom sediments
4. Simulated ground-water inflow and direct recharge for Snake Pond under natural and stressed conditions for pond-bottom hydraulic-conductivity
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The citation for this report, in USGS format, is as follows:
Walter, D.A., Masterson, J.P., LeBlanc, D.R., 2002, Simulated Pond-Aquifer Interactions under Natural and Stressed Conditions near Snake Pond, Cape Cod, Massachusetts Water-Resources Investigations Report 99-4147, 35 p.
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