1 Groundwater Modeling

2 Introduction

3 Lecture Outline What Is A Model? Modeling Axioms Guiding Thoughts and Protocol Governing Equations Practical Applications and Examples Model Documentation Concluding Remarks

4 Groundwater Models

5 What is a Model? “A Model Is Any Device That Represents An Approximation Of A Field Situation” (Anderson and Woessner, 1992, p. 2).

What is a Model? 6 It is an Imperfect Tool

What is a Model? 7 Its Purpose Is To Predict The Value Of A Particular Variable Or Set Of Variables

What is a Model? 8 A Flow Model Predicts Groundwater Heads A Contaminant Fate and Transport Model Predicts Changes In Contaminants and Their Concentrations

Modeling Axioms 9 There Should Be No Such Thing As A Model For an Area, Only For A Specific Problem A model should be designed to achieve a specific objective or set of objectives. No one, all-inclusive model exists for an area.

Modeling Axioms 10 Do Not Use A Model For A Purpose Other Than That For Which It Was Intended Rather, change the model to solve the “new” problem.

Modeling Axioms 11 No One Understands The Compromises Made During Development, Nor The Nuances Of A Model Better Than The Developer Consequently, a model should not be developed by one party and then used by another.

Modeling Axioms 12 Changing Software Without Updating And Revising A Model (Starting With The Conceptual Model) Gains Little Beyond The Ability To Use Different Software The confidence in a model and its predictions will not be significantly increased by a software change alone.

Guiding Thoughts 13 Plan – Think Through The Steps And Decisions To Build The Model Keep It Physically-Based In Reality Seek And Use Stakeholder Advice Define The Questions To Be Answered By The Model

Guiding Thoughts 14 Develop Modeling Objectives Understand Expectations Conduct Technical Working Meetings (Early and Often)

Questions 15 On Which Contaminants Should The Model Focus? Can An Action Reach Conclusion In A Reasonable Time?

Questions 16 Can An Action Be Improved Or Revised To Shorten Its Life Cycle? What Would Be The Potential Effect(s) Of Reducing The Level Of Effort On An Action? Of Increasing The Level Of Effort?

Modeling Protocol 17 1.Establish A Purpose For The Model 2.Develop A Conceptual Model 3.Select The Governing Equation And A Computer Code 4.Design The Model (Conceptual and Numerical) 5.Calibrate The Model Anderson and Woessner, 1992

Modeling Protocol 18 6.Perform Calibration Sensitivity Analysis 7.Perform Model Verification 8.Run Model to Predict Outcomes 9.Perform Predictive Sensitivity Analysis 10.Present Model Design and Results Anderson and Woessner, 1992

Modeling Protocol Conduct Post-audits 12.Revise Model Based On Post-audit Results Anderson and Woessner, 1992

20 Governing Equations

Flow Equation Harbaugh, A et al., 2000

Fate and Transport Equation Zheng and Wang, 1999

Common Types of Numerical Models 23 Finite Difference Finite Element Stochastic

Common Types of Numerical Models 24 Wang and Anderson, 1982

Common Types of Numerical Models 25 Wang and Anderson, 1982

Common Types of Numerical Models 26 Wang and Anderson, 1982

Common Types of Numerical Models 27 Wang and Anderson, 1982

28 Practical Applications

Developing a Conceptual Model 29 Regional Geologic Formations Alluvial and Fluvial Deposits from the Sierra (Modesto and Riverbank Formations) About 150 Feet Thick in Center of Basin Discontinuous lenses of Fluvial Sand and Silt with Clay and Gravel (Laguna Formation). About 400 to 1,000 Feet Thick. Indurated Andesitic Sand to Sandstone with Conglomerate, Siltstone, and Claystone (Mehrten Formation). About 400 Feet to 600 Feet Thick.

Developing a Conceptual Model 30 Conceptual Depositional Environment

Developing a Conceptual Model 31 Local Geologic Model

Developing a Conceptual Model 32 Regional Hydrogeology Located in the San Joaquin River Groundwater Basin/East San Joaquin Subbasin Sources of Groundwater Recharge – Infiltrating Rainwater – Losses from Rivers and Sloughs – Deep Percolation of Irrigation Water

Developing a Conceptual Model 33 Hydrogeologic Features

Developing a Conceptual Model 34 Multiple Sources of Contaminants Reaching Groundwater

Developing a Conceptual Model 35 Schematic

Developing a Conceptual Model 36 Numerical Model Grid

37 Practical Examples

38 Conceptual Model Film Loops Particle Tracking Transport Film Loops

39 Uncertainty

Sources of Uncertainty 40 Water Level, Flow and Chemical Measurement Errors Input Parameter Errors Outside Influences (Extraneous Pumping) Mathematic Approximations of Model Code Aquifer and Aquitard Heterogeneities

41 Model Documentation

Example Report Outline 42 Title Introductory Material Hydrogeologic Setting and Conceptual Model Model Design, Calibration and Sensitivity Analysis Results Model Limitations Disclaimer Summary, Conclusions, and Recommendations

43 Concluding Remarks

44 Summary All Models are Wrong Must Use Professional Judgment Identify Uncertainties and Acknowledge Them Always Maintain Your Ethics

45 References Anderson, M.P. and W.W. Woessner, Applied Groundwater Modeling: Simulation Of Flow And Advective Transport. Academic Press. San Diego, California. 381 p. Harbaugh, A., E. Banta, M. Hill, and M. McDonald, 2000, MODFLOW-2000, The U.S. Geological Survey Modular Ground- Water Model – User Guide to Modularization Concepts and the Ground-Water Flow Process. U.S. Geological Survey Open-File Report p. McDonald, M. and A. Harbaugh, A Modular Three- Dimensional Finite-Difference Ground-Water Flow Model. U.S. Geological Survey. Techniques of Water-Resources Investigations. Book 6. Chapter A p.

46 References Mercer, J. and C. Faust, 1980a. Ground-Water Modeling: An Overview. Ground Water v. 18. n. 2. p Wang, H and M. Anderson Introduction to Groundwater Modeling. Finite Difference and Finite Element Methods. Academic Press. San Diego, CA. 237 p. Zheng, C. and P. Wang, MT3DMS: A Modular Three- Dimensional Multispecies Transport Model for Simulation of Advection, Dispersion, and Chemical Reactions of Contaminants in Groundwater Systems, Documentation and User’s Guide. U.S. Army Corps of Engineers. Engineer Research and Development Center. Washington, D.C. 169 p. Plus Appendixes.

Developing a Conceptual Model 47 Take a Set of Drilling Logs, Now Make Some Interpretations

Developing a Conceptual Model 48 An Engineer Cannot As There Are Not Enough Data Available

Developing a Conceptual Model 49 A Very Conservative Geologist’s Interpretation

Developing a Conceptual Model 50 A Conservative Geologist’s Interpretation

Developing a Conceptual Model 51 An Optimistic Geologist’s Interpretation

Developing a Conceptual Model 52 A Very Optimistic Geologist’s Interpretation

Developing a Conceptual Model 53 An Extremely Optimistic Geologist Interpretation

Developing a Conceptual Model 54 A Geophysicist’s Interpretation

Developing a Conceptual Model 55 A Modeler’s Interpretation