Vadose Zone Hydrology

Daniel B. Stephens

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9780873714327
• eBook ISBN: 9781351405799
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Environmental Science
• Index: Biological Sciences

Vadose Zone Hydrology describes the elements of the physical processes most often encountered by hydrogeologists and ground-water engineers in their vadose zone projects. It illustrates the application of soil physics to practical problems relevant to the characterization and monitoring of the vadose zone. It includes an introduction to physical processes, including basic flow theory, and provides examples of important field-scale processes that must be recognizable by hydrogeologists. Considerable attention is given to the concepts of recharge, including how it is most accurately evaluated in the vadose zone. Field and laboratory methods for characterizing hydraulic properties in the vadose zone are also covered, and case studies illustrating these methods are provided. New and emerging technologies for monitoring the vadose zone, particularly for the purpose of detecting contaminants, are highlighted. In the last section of the book, additional case studies are presented, demonstrating applications related to seepage detection, landfill monitoring, and soil gas investigations.This book is written from the perspective of hydrogeologists and is designed to be directly applicable and to maintain continuity and consistency between chapters. It will be an invaluable primer for environmental or geotechnical consultants, regulators, or students who have no prior formal academic training in unsaturated flow concepts. Because the text contains some of the latest advances in this field, it will be an excellent reference for geologists and engineers currently working on problems of vadose zone hydrology.

CHAPTER 1

Basic Concepts and Theory

Most hydrogeologists and engineers concerned about the vadose zone are typically faced with some aspect of characterizing or monitoring it. Any rationale for characterizing and monitoring in the vadose zone must be based upon fundamental concepts governing vadose zone processes and theory of flow and transport. In many instances, the motivation for vadose zone characterization is to provide quantitative information and parameters to make predictions with mathematical models. For instance, if seepage from a landfill should occur, models can be used to predict likely pathways, as well as the rate of migration and the sensitivity required for a monitoring system to detect such seepage. To gain a better understanding of what parameters require quantification during site characterization, in this chapter we present mathematical expressions governing flow and transport in the vadose zone. But first, we define the vadose zone and some basic terminology.

The vadose zone is generally defined as the geologic media between land surface and the regional water table (Figure 1). The upper part of the vadose zone commonly includes the plant root zone and weathered soil horizons. Within the vadose zone, soils and bedrock are usually unsaturated; that is, their pores are only partially filled with water.

In places, however, the vadose zone may become water saturated; consequently, the vadose zone is not synonymous with the unsaturated zone. One obvious location where this occurs is just above the regional water table, where capillary rise causes water to fill the pore spaces. Here, the pores are essentially saturated but the water is held under a tension, that is, the water pressure is less than atmospheric pressure. The thickness of this zone of tension saturation may be less than 10 cm for gravels to more than 2 m for clays. Saturated regions may also be found where the water is under positive pressures, such as above a low permeable layer where perched conditions may develop. A perched aquifer occurs within and is part of the vadose zone, and it is separated from the regional water table below by an unsaturated zone. These unsaturated zones above and below the perched aquifer are critical to the definition and field identification of a perched aquifer. In the vadose zone, saturated conditions also may develop locally beneath surface impoundments and drainages, as well as over more extensive areas near land surface during infiltration following precipitation events or flood irrigation.

Figure 1

Conceptual model of the vadose zone.

To many hydrogeologists, groundwater occurs at a depth where water first enters a well. But Figure 1 shows that, by our preferred definition, the groundwater system includes the vadose zone as well. The groundwater system also includes the geologic medium below the regional water table, the phreatic zone, which will be either an aquifer, aquiclude, or aquitard, depending upon the permeability, well yield, and water use. The elevation of the water level in wells perforated across the uppermost of these saturated units in the phreatic zone will define the top of the water table and also the base of the vadose zone.

Note in Figure 1 that flow paths that would be drawn beneath the local impoundment or perched zone begin in saturated zones, would traverse through unsaturated media, and then enter the saturated zone again at the top of the capillary fringe. Along this flow path, the hydraulic properties vary as a function of the degree of fluid saturation. Within otherwise homogeneous media, this spatial variation in hydraulic properties, such as hydraulic conductivity, is an important difference between the vadose zone and aquifers. More detail on the nature of vadose zone hydraulic properties is presented later in this chapter.

In summary, the vadose zone has the following general characteristics:

• The vadose zone lies below the land surface and above the regional water table.

• Water pressure within most of the vadose zone is usually less than atmospheric pressure, although some areas of complete saturation and positive fluid pressure may occur.

• Flow properties are dependent upon the degree of saturation in the pore space.

In the following sections, we review the basic theory and concepts that allow us to understand the physical processes acting on the vadose zone. Section A deals with driving forces, Sections B and C address fluid storage, and Section E discusses fluid transport properties.

I. ENERGY STATUS OF POREWATER

What causes water to flow in the vadose zone? Just as in aquifers, the vadose zone water flows from areas of high potential energy to areas of low potential energy. To determine the direction of the driving force acting on water in the vadose zone, one will need to quantify the total potential energy of the soil water. The gradient of this total soil-water potential, that is, the change in potential energy with direction in space, determines the magnitude of the driving force acting on the fluid.

Mathematically, the total soil-water potential is expressed as:

$\underset{\begin{array}{c}\text{total}\text{soil - water}\\ \text{potential}\end{array}}{{\varphi }_{\text{T}}}=\underset{\begin{array}{c}\text{soil - water}\\ \text{potential}\end{array}}{{\varphi }_{\text{SW}}}+\underset{\begin{array}{c}\text{elevation}\\ \text{potential}\end{array}}{{\varphi }_{\text{G}}}$

(1)

Notice that there are two primary compo...

Table of contents

• Cover
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Chapter 1 Basic Concepts and Theory
• Chapter 2 Soil-Water Budget
• Chapter 3 Physical Processes Relevant to Deep Soil-Water Movement
• Chapter 4 Recharge
• Chapter 5 Characterizing Hydraulic Properties
• Chapter 6 Vadose Zone Monitoring
• Chapter 7 Vadose Zone Monitoring Strategy and Case Studies
• References
• Index