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Geostatistical Analysis of Effective Vertical Hydraulic
Conductivity and Presence of Confining Layers in the
Shallow Glacial Drift Aquifer, Oakland County, Michigan

By E.G. Bissell and S.S. Aichele

Scientific Investigations Report 2004-5167

ABSTRACT

About 400,000 residents of Oakland County, Mich., rely on ground water for their primary drinking-water supply. More than 90 percent of these residents draw ground water from the shallow glacial drift aquifer. Understanding the vertical hydraulic conductivity of the shallow glacial drift aquifer is important both in identifying areas of ground-water recharge and in evaluating susceptibility to contamination. The geologic environment throughout much of the county, however, is poorly understood and heterogeneous, making conventional aquifer mapping techniques difficult. Geostatistical procedures are therefore used to describe the effective vertical hydraulic conductivity of the top 50 ft of the glacial deposits and to predict the probability of finding a potentially protective confining layer at a given location.

The results presented synthesize the available well-log data; however, only about 40 percent of the explainable variation in the dataset is accounted for, making the results more qualitative than quantitative. Most of the variation in the effective vertical hydraulic conductivity cannot be explained with the well-log data currently available (as of 2004). Although the geologic environment is heterogeneous, the quality-assurance process indicated that more than half of the wells in the county’s Wellkey database (statewide database for monitoring drinking-water wells) had inconsistent identifications of lithology.

CONTENTS

Abstract

Introduction

Approach and Methods

Calculation of Effective Vertical Hydraulic Conductivity

Identification of Distinct Geologic Settings

Quality Assurance of Well Logs

Prediction of Effective Vertical Hydraulic Conductivity

Prediction of Confining Layer Presence

Error Analysis

Results

Conclusions and Limitations

Acknowledgments

References Cited

Glossary

FIGURES

1. Map showing surficial geology of Oakland County, Michigan

2–4. Graphs showing:

2. Sample variogram showing the nugget, sill, and range, and the random and structured parts of the variation

3. Driller-specific sample variogram showing spatial structure in well-log data

4. Driller-specific sample variogram showing lack of spatial structure in well-log data

5. Map showing till and outwash settings in Oakland County

6–9. Graphs showing:

6. Sample variogram and model variogram for outwash setting

7. Sample variogram and model variogram for till setting

8. Sample variogram and model variogram for indicator kriging of confining layers in outwash setting

9. Sample variogram and model variogram for indicator kriging of confining layers in till setting

10–13. Maps showing:

10. Predicted effective vertical hydraulic conductivity in the shallow glacial drift aquifer, Oakland County

11. Predicted probability of confining-layer (10 ft thick or greater) presence in the shallow glacial drift aquifer, Oakland County

12. Uncertainity associated with prediction of effective vertical hydraulic conductivity in the shallow glacial drift aquifer, Oakland County

13. Uncertainty associated with prediction of confining-layer presence in the shallow glacial drift aquifer, Oakland County

TABLES

1. Results of Student-Newman-Keuls Multiple Comparison Test

2. Cross-validation results for ordinary kriging and indicator kriging



AVAILABILITY

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

Bissell, E.G., and Aichele, S.S., 2004, Geostatistical analysis of effective vertical hydraulic conductivity and presence of confining layers in the shallow glacial drift aquifer, Oakland County, Michigan: U.S. Geological Survey Scientific Investigations Report 2004-5167, 19 p.


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