a $1 UNIVERSITY OF CALIFORNIA Los Angeles Simulated Groundwater Tracer Study of the Alamitos Barrier Project, Los Angeles, California A thesis submitted in partial satisfaction of the requirements for the degree Master of Science in Civil Engineering by Joseph Christopher Pope DISTRIBUTION STATEMENT A Approved for Public Release 2006 Distribution Unlimited 20070910387 S U The thesis of Joseph Christopher Pope is approved. Terr Hiiogue Steven A. Margulis William W-G. Yeh, Con ittee Chair University of California, Los Angeles 2006 TABLE OF CONTENTS LIST O F TA B LES ................................................................................................ IV LIST O F EQ U A T IO N S .......................................................................................... V LIST O F FIG U RE S .............................................................................................. V I A C KN O W LED G M EN TS ................................................................................... VII A BSTR A CT O F TH E TH ESIS ............................................................................ V III 1 IN T R O DU C T IO N ............................................................................................... 1 2 L ITERA TU RE REV IEW .................................................................................. 4 3 M ETH O DO LO G Y .............................................................................................. 7 3.1 CONCEPTUAL M ODEL DESCRIPTION .......................................................................................... 7 3.2 GROUNDWATER FLOW AND TRANSPORT SIMULATION ................................................................. 9 3.3 SIMULATION SCENARIOS ............................................................................................................ 13 4 R ESU LTS ............................................................................................................. 14 4.1 TIME-STEP SENSITIVITY ANALYSIS ......................................................................................... 14 4.2 SIMULATED TRACER CONCENTRATION AT PRODUCTION W ELLS ............................................. 16 4.3 VELOCITY FIELD ANALYSIS ..................................................................................................... 20 5 D ISC U SSIO N ..................................................................................................... 24 6 RE FEREN C ES ................................................................................................... 27 iii LIST OF TABLES Table 1 -List of Spatial and Time Invariant Parameters ......................................... 12 iv LIST OF EQUATIONS Equation I -Courant Condition ................................................................................ 6 Equation 2 - Density-dependent flow model .......................................................... 9 Equation 3 - Advection-dispersion model ................................................................ 9 Equation 4 - Storage coefficient ............................................................................. 10 Equation 5 - Hydraulic conductivity tensor ........................................................... 10 Equation 6 - Moisture Content / Dispersion Relationship ..................................... 10 Equation 7 - Darcy flux equation ........................................................................... 10 Equation 8 - Relationship between concentration and density .............................. 10 Equation 9 -RMSE Calculation ............................................................................. 14 v LIST OF FIGURES Figure 1. Location of Alamitos Barrier Project ...................................................... 2 Figure 2. Alamitos Barrier Project Map ................................................................. 8 Figure 3. ABP Geologic Layers ............................................................................. 8 Figure 4 -Time-Step Size Sensitivity Analysis ...................................................... 15 Figure 5 -Root Mean Squared Error Analysis ...................................................... 15 Figure 6 - Location of Production Wells ................................................................ 16 Figure 7 - Concentration Breakthrough Curve for SCWC-LAYT ......................... 17 Figure 8 -Concentration Breakthrough Curve for SB-BEV ................................. 18 Figure 9 - Concentration Breakthrough Curve for SB-LEI .................................... 19 Figure 10 - Concentration Breakthrough Curve for RUIZ-6F 1 ............................. 20 Figure 11 - B-Aquifer Velocity Field .................................................................... 21 Figure 12 -A-Aquifer Velocity Field .................................................................... 22 Figure 13 -I-Aquifer Velocity Field ....................................................................... 23 Figure 14 -Main Aquifer Velocity Field ................................................................ 24 vi ACKNOWLEDGMENTS Firstly, I would like to acknowledge the United States Navy for funding my graduate education under their Civilian Institutions, fully funded graduate education program. Secondly, I must give a special thanks to my fellow Humboldt State Lumberjack alumnus, Mr. Ben S. Bray for his time and efforts in assisting me with this project. Without him this task would have been impossible. I would also like to acknowledge my thesis committee for their time, patience, and skillful guidance. Finally, I give a special thanks to my wonderful wife Alice for her loving patience and support of all my professional and personal endeavors. vii ABSTRACT OF THE THESIS Simulated Groundwater Tracer Study of the Alamitos Barrier Project, Los Angeles County, California by Joseph Christopher Pope Master of Science in Civil Engineering University of California, Los Angeles, 2006 Professor William W-G. Yeh, Chair A simulated tracer study is performed on the Alamitos Groundwater Barrier Project (ABP), Los Angeles, California. The ABP consists of 43 injection wells that create a freshwater barrier to prevent intrusion of seawater into the coastal aquifer of southern Los Angeles County. The injection wells currently use a blend of recycled and imported water. Regional water quality regulations dictate the quantity of recycled water that can be injected into the aquifer. The regulations also require that the travel-time between the injection wells and the local production wells be greater than one year. The purpose of this study is to analyze the travel time of injected water into the ABP. A previously calibrated three-dimensional, finite element, coupled groundwater flow and transport model is used to simulate the movement of a viii conservative tracer in the Alamitos Barrier Project. The results of the simulations show that tracer travel times between the injection wells and the production wells typically exceed 60 years. The model results exhibit a high amount of numerical error when large time steps are used. Time-step sensitivity analysis indicates that a time step of one day or less will minimize model error. Further analysis of local head boundary conditions is recommended in order to provide a wider range of tracer travel times given variations in boundary hydraulic head levels. ix 1 INTRODUCTION Since the late 191h Century, groundwater has been an important source of water for the Southern California region. From the 1940's through the 1950's a rise in chloride levels was noticed in many coastal pumping wells. The rise in chloride levels was due to the intrusion of seawater into the coastal aquifers as a result of the significant depletion of fresh groundwater (Callison et al. 1991). In the late 1950's and early 1960's the Los Angeles County Department of Public Works (LACDPW) constructed and began operating three seawater barriers along the County's coastline: the West Coast Basin; Dominguez Gap; and Alamitos Barrier Project (ABP). The three barrier projects serve to protect inland groundwater reservoirs from contamination due to seawater intrusion and also act as a source of recharge water for the area aquifers. The ABP, the subject of this study, has been in operation since 1964 and consists of 43 injection wells and four extraction wells. The injection wells create a freshwater pressure ridge that prevents further intrusion of seawater, while the extraction wells create a trough between the coastline and the injection wells, further amplifying the effect of the freshwater pressure ridge (Water Replenishment District of Southern California 2006). The ABP is located between the southern coastal boundary of Los Angeles County and the northern coastal boundary of Orange County. An overview of the ABP location is shown in Figure 1. I