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Simulated Pond-Aquifer Interactions under Natural and Stressed Conditions near Snake Pond, Cape Cod, Massachusetts

Water-Resources Investigations Report 99-4174

By Donald A. Walter, John P. Masterson, and Denis LeBlanc

ABSTRACT

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.


CONTENTS

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

FIGURES

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

TABLES

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.

For more information about USGS activities in Massachusetts-Rhode Island District, visit the USGS Massachusetts-Rhode Island Home Page.


U.S. Department of the Interior, U.S. Geological Survey
Persistent URL: http://pubs.water.usgs.gov/wri994174
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Last modified: Friday, September 16 2005, 04:22:52 PM
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