Aquifer - System Compaction and Land Subsidence: Measurements, Analyses, and Simulations - the Holly Site, Edwards Air Force Base, Antelope Valley, California

By Michelle Sneed and D.L. Galloway

U.S. Geological Survey
Water Resources Investigations report 00-4015

Prepared in cooperation with the U.S. Department of the Air Force
Sacramento, California 2000

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Abstract

Introduction

Purpose and Scope

Location

Development of the Ground-Water Resource and Resulting Land Subsidence

Previous Studies

Hydrogeologic Setting

Geologic Framework

Aquifer Systems

    Ground-Water Levels

    Aquifer-System Compaction

Analytical Approach

Principle of Effective Stress

    Preconsolidation Stress

    Elastic and Inelastic Compressibility (Specific Storage)

Aquifer-System Storage Coefficients

Theory of Hydrodynamic Consolidation

Parameter Range Estimates

Vertical Hydraulic Conductivity

Preconsolidation Stress

Elastic and Inelastic Storage Coefficients (Specific Storage)

Holly Model

Formulation of Numerical Model

    Spatial and Temporal Discretization

    Boundary Conditions

    Initial Conditions

    Convergence Criteria and Mass Balance Criteria

    Assumptions and Limitations

Simulations

History Matching

Sensitivity Analysis

Analyses

Simulated Compaction

Future Compaction Scenarios

Summary

References Cited

1-4. Maps showing:

1. Location of Antelope Valley and the Holly site (8N/10W-1Q)

2. General surficial geology and location of ground-water subbasins and geologic section, Antelope Valley, California

3. Location of selected well fields, observation wells, and bench marks near the Holly site (8N/10W-1Q) in the south-central part of Edwards Air Force Base, Antelope Valley, California

4. Ground-water levels in the Lancaster subbasin, Antelope Valley, California, 1915

5. Lithologic and geophysical logs of the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

6. Generalized geologic section showing relation of lacustrine deposits to younger and older alluvium and aquifers in the Lancaster and North Muroc subbasins, Edwards Air Force Base, Antelope Valley, California

7. Conceptual model of the aquifer systems at the Holly site (8N/10W-1Q) including the piezometers and the borehole extensometer, Edwards Air Force Base, Antelope Valley, California

8. Map showing change in ground-water levels in the Lancaster subbasin, Antelope Valley, California, 1915-91

9. Map showing water-level surface of the confined aquifer system, spring 1992, location of selected wells, and hydrographs of wells 8N/10W-28B1 (1951-97) and 9N/10W-24C1 (1952-99), Edwards Air Force Base, Antelope Valley, California

10. Graphs showing water levels in piezometers HO-1, HO-2, HO-3, and HO-4 at the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California, 1990-97

11. Graphs showing water-level fluctuations in piezometers HO-1 and HO-3 at the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

12. Map showing area of flowing wells, land subsidence, and two fissures in Antelope Valley, California, for the period about 1930-92, and graphs showing ground-water levels in wells 7N/10W-05E1 and 7N/12W-15F1 and subsidence at bench marks 1171A and 474

13. Schematic diagram showing the borehole extensometer at the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

14. Graph showing aquifer-system compaction and water levels measured at the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

15. Diagram showing principle of effective stress, as applied to land subsidence

16-19. Graphs showing:

16. Compression of highly compressible clay and sand samples

17. Change in land-surface elevation for various time intervals for selected bench marks, and estimated water levels (1908-97) for the middle aquifer near the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

18. Combined water levels for piezometers HO-2 and HO-3 and cumulative vertical compaction at the Holly extensometer, and applied stress and compaction at the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California, 1990-97

19. Sample time series used in stress/strain analyses of the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

20. Diagram showing relation of model domain to the conceptual model and short normal resistivity log of the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

21-31 Graphs showing:

21. Measured or estimated depth to water for piezometers HO-1, HO-2 and HO-3 (combined), and HO-4 used in the 1990-97 model simulation of the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

22. Water levels used for input in the historical model for the upper, middle, and lower aquifers at the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

23. Simulated compaction and measured or estimated land subsidence at bench marks P1155 and LS42 near the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California, 1908-97

24. History matches for 1990-97 for simulated and measured compaction and error and simulated and measured displacement-stress trajectories for the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

25. Distribution of error computed between measured and simulated compaction for 35 selected values of K'_v and S'_skv of the aquitards at the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

26. Simulated compaction by aquifer-system component, 1908-97, at the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

27. Simulated deformation by aquifer-system component, 1990-97, at the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

28. Hydraulic-head profiles of the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California, for 1908-97 and 1990-97

29. Future scenario--1996 pumping rates for 1997 to 2026 at the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

30. Future scenario--1997 aquifer water levels held constant through year 2096 at the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

31. Future scenario--no pumping for 1997 to 2096 at the Holly site (8N/10W-1Q), Edwards Air Force Base, Antelope Valley, California

Abstract

Land subsidence resulting from ground-water-level declines has long been recognized as a problem in Antelope Valley, California. At Edwards Air Force Base (EAFB), ground-water extractions have caused more than 150 feet of water-level decline, resulting in nearly 4 feet of subsidence. Differential land subsidence has caused sinklike depressions and earth fissures and has accelerated erosion of the playa lakebed surface of Rogers Lake at EAFB, adversely affecting the runways on the lakebed which are used for landing aircraft such as the space shuttles. Since 1990, about 0.4 foot of aquifer-system compaction has been measured at a deep (840 feet) borehole extensometer (Holly site) at EAFB. More than 7 years of paired ground-water-level and aquifer-system compaction measurements made at the Holly site were analyzed for this study. Annually, seasonal water-level fluctuations correspond to steplike variations in aquifer-system compaction; summer water-level drawdowns are associated with larger rates of compaction, and winter water-level recoveries are associated with smaller rates of compaction. The absence of aquifer-system expansion during recovery is consistent with the delayed drainage and resultant delayed, or residual, compaction of thick aquitards.

A numerical one-dimensional MODFLOW model of aquitard drainage was used to refine estimates of aquifer-system hydraulic parameters that control compaction and to predict potential future compaction at the Holly site. The analyses and simulations of aquifer-system compaction are based on established theories of aquitard drainage. Historical ground-water-level and land-subsidence data collected near the Holly site were used to constrain simulations of aquifer-system compaction and land subsidence at the site for the period 1908Ð90, and ground-water-level and aquifer- system compaction measurements collected at the Holly site were used to constrain the model for the period 1990Ð97.

Model results indicate that two thick aqui- tards, which total 129 feet or about half the aggregate thickness of all the aquitards penetrated by the Holly boreholes, account for most (greater than 99 percent) of the compaction measured at the Holly site during the period 1990Ð97. The results of three scenarios of future water-level changes indicate that these two thick aquitards account for most of the future compaction. The results also indicate that if water levels decline to about 30 feet below the 1997 water levels an additional 1.7 feet of compaction may occur during the next 30 years. If water levels remain at 1997 levels, the model predicts that only 0.8 foot of compaction may occur during the same period, and even if water levels recover to about 30 feet above 1997 water levels, another 0.5 foot of compaction may occur in the next 30 years. In addition, only a portion of the compaction that ultimately will occur likely will occur within the next 30 years; therefore, the residual compaction and associated land subsidence attributed to slowly equilibrating aquitards is important to consider in the long-term management of land and water resources at EAFB.