CHAPTER 3 GROUND WATER FLOW AND CONTAMINANT MIGRATION

JurisdictionUnited States
Ground Water Contamination
(May 1991)

CHAPTER 3
GROUND WATER FLOW AND CONTAMINANT MIGRATION

Robert ?. Ramsey, P.G.
James M. Montgomery
Consulting Engineers, Inc.
Salt Lake City, Utah

TABLE OF CONTENTS

SYNOPSIS

Page

I. INTRODUCTION

II. FUNDAMENTALS OF GROUND-WATER FLOW

Occurrence

Aquifers

Ground-Water Flow

III. SOURCES OF GROUND-WATER CONTAMINATION

IV. BEHAVIOR OF CONTAMINANTS IN GROUND WATER

Mass Transport

Mass Transfer

Nonaqueous Phase Contaminant Migration

V. MONITORING GROUND-WATER CONTAMINATION

Contaminant Investigation Programs

Contaminant Detection Methods

Sampling Networks and Reliability of Sampling Results

VI. CASE STUDY: OPERABLE UNIT 1 REMEDIAL INVESTIGATION, HILL AIR FORCE BASE, UTAH

Project Background

Purpose and Scope of the Remedial Investigation

Findings and Results

Conclusions

REFERENCES

———————

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LIST OF TABLES

TABLE NO. TITLE PAGE
3-1 Sources of Ground-Water Contamination 3-10
3-2 Frequently Encountered Organic Compounds and Their Major Industrial Uses 3-12
3-3 Types of Information Typically Required for Ground-Water Contamination Investigations 3-21
LIST OF FIGURES
FIGURE NO. TITLE FOLLOWING PAGE NO.
3-1 The Hydrologic Cycle 3-2
3-2 Classification of Subsurface Water 3-3
3-3 Schematic Diagram of Confined and Unconfined Aquifers 3-4
3-4 Relationship of Water Levels in Confined and Unconfined Aquifers 3-6
3-5 Diagram showing Potentiometric Contours and Ground-Water Flow Directions 3-7
3-6 Porosity Relationship between Material Type, Size and Texture 3-7
3-7 Theoretical Contaminant Migration in an Isotropic Aquifer 3-13
3-8 Conceptualization of Contaminant Mass Transport in a Ground-Water System 3-14
3-9 Contaminant Mass Transport by the processes of Advection and Dispersion 3-15
3-10 Downward Migration of a Light Nonaqueous Phase Liquid (DNAPL) through the Unsaturated Zone 3-17
3-11 Secondary Contamination due to Dissolution and Volatilization 3-18
3-12 Migration of a Dense Nonaqueous Phase Liquid through the Unsaturated and Saturated Zones 3-19
3-13 Sampling Networks 3-25
3-14 Location Map of Hill AFB 3-27
3-15 Location of OU 1 and Waste Disposal Sites within the Unit 3-27
3-16 Generalized Geologic Cross Section of OU 1 3-30
3-17 Soil Gas Contours for Benzene 3-31
3-18 Soil Gas Contours for Toluene 3-32
3-19 Soil Gas Contours for Trans 1,2-Dichloroethene 3-32
3-20 Preliminary Ground-Water Contour Map 3-33
3-21 Distribution of Light-NAPL Contamination 3-34
3-22 Preliminary Isoconcentration Contours of DCE in Ground Water 3-35

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GROUND-WATER FLOW AND CONTAMINANT MIGRATION

I. INTRODUCTION

Ground water is one of this country's most valuable resources and is a critical component of the natural environment. In the United States, nearly one out every two people depend on ground water for their drinking water supply. Over the past three decades, the development and use of ground water has increased by more than 250 percent (van der Leeden, 1990). Much of this development has occurred in the arid western United States where surface water supplies are relatively sparse and undependable.

Prior to the 1950's, the primary concern with ground water was exploration and development of new supplies. In general, there was little concern for overdevelopment of ground-water resources or the potential for ground-water contamination. The tremendous population growth since the end of World War II has resulted in substantial increases in agricultural and industrial production. These factors have created huge volumes of solid and liquid waste which require proper disposal. In the past, disposal methods were chosen based on cost and expediency with minimal consideration for the potential impacts to surface and subsurface environments. It was not until the late 1960's and 70's that the effects of waste disposal on surface water and ground-water supplies became a national concern. With the increasing dependence on ground water for drinking water supplies, the need for protection and remediation of these vital resources has come to the forefront in recent years.

This paper focuses on the fundamentals of ground-water flow, sources of ground-water contamination, behavior of contaminants in ground water and methods for evaluating the nature and extent of contamination. A case study is presented to illustrate these principles.

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II. FUNDAMENTALS OF GROUND-WATER FLOW

Occurrence

It is important to recognize that ground water is but one component of the much larger hydrologic cycle. The hydrologic cycle is the phenomenon of continuous circulation of water between the atmosphere, land and the ocean. The cycle involves movement of water in the form of evaporation, precipitation, surface runoff and ground-water flow. It also involves storage of water within lakes, aquifers and the ocean. Figure 3-1 is a conceptualization of this dynamic process in which water is in constant movement, but within the global context is neither gaining or losing significant volume.

The importance of ground water in the hydrologic cycle, especially as a water resource, should not be underestimated. Consider that 94 percent of the earth's water resides in the ocean at high levels of salinity and that freshwater makes up the remaining six percent. Of this total available freshwater, two percent is contained in the world's ice caps and glaciers. Ground water

Figure 3-1 The Hydrologic Cycle (modified after Davis, 1966).

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constitutes about 95 percent of the remaining four percent of earth's freshwater (Freeze and Cherry,1979).

Ground water1 is generally defined as subsurface water that occurs at or beneath the water table where the pores or voids in the soil or rock are fully saturated. Below the water table, ground water is at greater than atmospheric pressure and moves under the force of gravity. The area above the water table is referred to as the vadose or unsaturated zone. This water reacts to capillary forces in addition to gravity. Figure 3-2 illustrates these two types of subsurface water. The water table is the theoretical surface that separates ground water from vadose water. It is also the surface along which atmospheric pressure is equal to hydrostatic pressure. The water table can range from ground surface to several hundred feet below ground. In practice, the water table is determined by measuring the elevation of water surfaces in wells or piezometers that penetrate a short distance into the saturated zone (Davis and De Wiest, 1966).

Figure 3-2 Classification of Subsurface Water (modified after Davis, 1966).

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Aquifers

Until recently, an aquifer was generally defined as a water-bearing formation that transmits and yields water to wells in sufficient quantity to be important economically. However, with the increased concern about ground-water contamination, an aquifer is now commonly defined as any saturated water-bearing formation regardless of its ability to transmit or yield water. This expanded definition reflects the concern that contaminants can migrate into and through fine-grained formations and serve as sources of contamination to underlying water supplies. Aquifers exist in a variety of geologic materials and formations. The most common aquifer material in the intermountain west consists of unconsolidated sand and gravel. These ubiquitous materials occur in valleys, stream channels, and desert plains. Aquifers also occur in consolidated formations such as sandstone, limestone, and basalt where water moves through void spaces, fractures and solution channels.

Aquifers are divided into two categories: unconfined and confined (Bouwer, 1978). These are schematically depicted in Figure 3-3. Unconfined or water-table aquifers commonly occur in the sand and gravel of alluvial valleys.

Figure 3-3 Schematic Diagram of Confined and Unconfined Aquifers (from Davis, 1966).

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The water surface or water table is able to freely rise and fall as a function of recharge to and discharge from the aquifer. Thicknesses of unconfined aquifers can range up to a thousand feet or more. An unconfined aquifer is generally bounded below by materials of lower permeability2 , such as silt, clay or indurated rock. In cases where an unconfined aquifer is underlain by a confined aquifer, they are separated by a relatively low permeability layer known as an "aquitard". This layer is frequently a fine-grained material such as a clay or silt, or low permeability rock unit. Although these materials have much lower permeability than an aquifer, they can transmit some water or contaminants, albeit very slowly.

Confined aquifers occur where they are bounded by less permeable layers both above and below. These aquifers frequently occur beneath unconfined aquifers. Occasionally, near surface soils are sufficiently fine grained to create a shallow confined aquifer without the presence of an overlying unconfined aquifer. Confined aquifers are also characterized by not having a free water table. Rather, this water exists under hydrostatic pressure creating a condition called a "potentiometric surface3 ". This pressure can cause water levels in piezometers4 or wells that penetrate such an aquifer to rise to an elevation above the upper surface of the confined aquifer. Under certain conditions, the pressure can be sufficient to cause water to flow freely at the surface from wells penetrating the confined aquifer. These are commonly referred to as "flowing artesian wells". The cross section depicted in Figure 3-4 shows the relationship of water levels in both confined and unconfined aquifers. As will be discussed later, this relationship can have a profound effect on the migration of contaminants in ground water.

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Figure 3-4 Relationship of Water Levels in Confined and Unconfined Aquifers (modified after Davis...

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