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Simulation of Advective Flow under Steady-State and Transient Recharge Conditions, Camp Edwards, Massachusetts Military Reservation, Cape Cod, Massachusetts

By DONALD A. WALTER and JOHN P. MASTERSON

Water-Resources Investigations Report 03-4053


ABSTRACT

The U.S. Geological Survey has developed several ground-water models in support of an investigation of ground-water contamination being conducted by the Army National Guard Bureau at Camp Edwards, Massachusetts Military Reservation on western Cape Cod, Massachusetts. Regional and subregional steady-state models and regional transient models were used to (1) improve understanding of the hydrologic system, (2) simulate advective transport of contaminants, (3) delineate recharge areas to municipal wells, and (4) evaluate how model discretization and time-varying recharge affect simulation results.

A water-table mound dominates ground-water-flow patterns. Near the top of the mound, which is within Camp Edwards, hydraulic gradients are nearly vertically downward and horizontal gradients are small. In downgradient areas that are further from the top of the water-table mound, the ratio of horizontal to vertical gradients is larger and horizontal flow predominates. The steady-state regional model adequately simulates advective transport in some areas of the aquifer; however, simulation of ground-water flow in areas with local hydrologic boundaries, such as ponds, requires more finely discretized subregional models. Subregional models also are needed to delineate recharge areas to municipal wells that are inadequately represented in the regional model or are near other pumped wells.

Long-term changes in recharge rates affect hydraulic heads in the aquifer and shift the position of the top of the water-table mound. Hydraulic-gradient directions do not change over time in downgradient areas, whereas they do change substantially with temporal changes in recharge near the top of the water-table mound. The assumption of steady-state hydraulic conditions is valid in downgradient area, where advective transport paths change little over time. In areas closer to the top of the water-table mound, advective transport paths change as a function of time, transient and steady-state paths do not coincide, and the assumption of steady-state conditions is not valid. The simulation results indicate that several modeling tools are needed to adequately simulate ground-water flow at the site and that the utility of a model varies according to hydrologic conditions in the specific areas of interest.


CONTENTS

Abstract

Introduction

Purpose and Scope

Geologic Setting

Hydrologic Setting

Site Description and History

Development of Models

Steady-State Regional Models

1993 Regional Model

1998 Regional Model

2000 Regional Model

Steady-State Subregional Models

Camp Edwards Subregional Model

Grid and Boundaries

Aquifer Properties and Stresses

Southern J-Ranges Subregional Model

Grid and Boundaries

Aquifer Properties and Stresses

Transient Regional Models

Simulation of Transient Stresses

Long-Term Recharge and Pumping Stresses

Seasonal Recharge and Pumping Stresses

Limitations of Models Simulating Time-Varying Recharge

Simulation of Transient Streamflow

Comparison of Transient Model Results to Measured Heads

Particle-Tracking Methodology

Steady-State Simulations

Regional Modeling

Delineation of Ground-Water Flow and Advective Transport of Contaminants

Delineation of Ground-Water-Flow Patterns

Simulation of Advective Transport to Support Field Investigations

Comparison of Particle Tracks From 1993, 1998, and 2000 Regional Models

Areas Contributing Recharge to Municipal Wells

Subregional Modeling

Camp Edwards Model

Southern J-Ranges Area

Transient Simulations

Effects of Transient Recharge on Heads and Gradients

Effects of Long-Term Transient Recharge on Advective Transport

Demolition Area 1 and Impact Area

J-Ranges Area

Implications for the Use of Transient and Steady-State Models

Summary and Conclusions

References Cited

FIGURES

1, 2. Maps showing:

1. Location of Camp Edwards and its firing ranges, also known as the Impact Area, on the Massachusetts Military Reservation, western Cape Cod, Massachusetts

2. Surficial geology of western Cape Cod, water-table-altitude contours from March 1993, and the locations of the Hatchville precipitation gage and selected long-term monitoring wells

3. Schematic showing generalized vertical section (vertically exaggerated) illustrating hydrologic boundaries and general flow lines in the ground-water system of western Cape Cod

4-8. Maps showing:

4. Location of sampling locations in and around Camp Edwards and wells in which Royal Dutch Explosive was detected

5. Regional model domains and simulated hydrologic boundaries in the 1993, 1998, and 2000 regional models of western Cape Cod 10

6. Extent of subregional model domains within the regional model of western Cape Cod, and the subregional model of the Camp Edwards Area 14

7. (A) Model-derived water-table contours and (B) vertical layering and horizontal discretization used in the regional model of western Cape Cod, and in the subregional model of the Camp Edwards area, Massachusetts Military Reservation

8. (A) Local hydrologic features and (B) vertical layering and horizontal discretization used in the subregional model of the southern J-Ranges Area, Western Cape Cod

9-11. Graphs showing:

9. Changes in precipitation at Hatchville for the period 1931–96 and changes in heads in well SCW253 for the period 1963– 96

10. Precipitation and estimated recharge rates at Hatchville for the period 1931– 96

11. Changes in measured and model-calculated heads in wells (A) BHW215, (B) SDW253, and (C) FSW167 for the periods 1962–96, 1975–96, and 1975– 96, respectively, western Cape Cod

12. Map showing model-calculated horizontal hydraulic gradients in the Camp Edwards area and the simulated location of the top of the water-table mound as determined from the 2000 regional model, western Cape Cod

13. Schematic showing flow paths calculated by the 2000 regional model along regional model row 45, and approximate locations of the northern J-Ranges Area and Demolition Area 1, western Cape Cod

14-15. Maps showing:

14. Advective flow paths and distribution of Royal Dutch Explosive (RDX) contamination in Demolition Area 1, 2000 regional model, western Cape Cod

15. Advective flow paths determined by the the 2000 regional models particle-trackingroutine from locations of Royal Dutch Explosive (RDX) detections at depth in the aquifer to possible source areas in the Central Impact Area, western Cape Cod

16. Schematic showing advective flow paths calculated by the 2000 regional model from two monitoring wells within the Central Impact Area, (A) MW44, and (B) MW59, and the approximate positions of nearby wells contaminated with Royal Dutch Explosive (RDX), western Cape Cod

17-23. Maps showing:

17. Model-derived advective flow paths from the water table at selected monitoring wells within Camp Edwards, western Cape Cod

18. Recharge areas to existing and proposed municipal wells downgradient of Camp Edwards, western Cape Cod

19. Areas at the water table across which water recharges the aquifer and discharges to South Sagamore Well 1 as simulated by the regional and subregional models, western Cape Cod

20. The effect of model discretization on simulated contributing areas to four municipal wells in close proximity to one another, by comparison of (A) regional and (B) subregional model results, western Cape Cod

21. Advective flow paths from selected monitoring wells in the southern J-Ranges Area and simulated water-table contours calculated by the regional and subregional models, western Cape Cod

22. Water-table contours and the position of the top of the water-table mound for steady-state, high-recharge (1955), and low-recharge (1965) conditions, western Cape Cod

23. Simulated differences in hydraulic heads and in magnitude and direction of the hydraulic gradient between high-recharge (1955) and low-recharge (1965) conditions on western Cape Cod

24. Graph showing relation between simulated differences in hydraulic-gradient directions and magnitudes for all active model cells for high-recharge (1955) and low-recharge (1965) conditions on western Cape Cod

25-28. Maps showing:

25. Changes in gradient direction between high-recharge (1955) and low-recharge (1965) conditions for the northern part of the western Cape Cod flow cell and steady-state advective flow paths from selected locations

26. Steady-state and transient advective flow paths of particles started in 1955, 1965, 1975, and 1985 from three locations in the Central Impact Area, western Cape Cod

27. Steady-state and transient advective flow paths of particles started in 1955, 1965, 1975, and 1985 from Demolition Area 1, western Cape Cod

28. Steady-state and transient advective flow paths for particles started in 1955, 1965, 1975, and 1985 from three locations the J-ranges Area, western Cape Cod


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The citation for this report, in USGS format, is as follows:

Walter, D.A. and Masterson, J.P., 2003, Simulation of Advective Flow under Steady-State and Transient Recharge Conditions, Camp Edwards, Massachusetts Military Reservation, Cape Cod, Massachusetts: U.S. Geological Survey Water-Resources Investigations Report 03-4053, 68 p.

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



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