Principles of Contaminant Hydrogeology

Christopher M. Palmer

• Publisher: CRC Press
• Year: 2019
• Print ISBN:
  9780367448516
  9781566701693
• eBook ISBN:
  9780429606427
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Environmental Science

This second edition features new and expanded coverage of contaminant hydrogeologic investigations. It presents a practical approach to completing investigations for environmental compliance, emphasizing the use of geologic principles in assessment to move sites toward cleanup. Stressing the basics of collecting data that can withstand regulatory scrutiny and achieve remediation, Principles of Contaminant Hydrogeology, Second Edition demonstrates how to solve a client's site contamination problem while maximizing cost effectiveness. It focuses on small- and medium-sized firms, for which speed, accuracy, and cost are all crucial factors in the site assessment and closure process. Based on "real world" problems, the book takes you step-by-step through the investigation and includes client-consultant-regulator interaction, budgets, ethics, and data extrapolation for solving problems. It introduces concepts such as field logistics, drilling techniques, sampling protocols, contaminant movement, and remediation. Regulatory personnel, hydrogeological consultants, drilling contractors, remediation contractors, university instructors, and students will benefit from the wealth of information provided in this new edition.

CHAPTER 1

Geologic Frameworks for Contaminant Hydrogeologic Investigations

INTRODUCTION

Subsurface investigations into the presence and extent of contaminated groundwater are primarily geological investigations. The site subsurface geology forms the physical framework through which groundwater and contaminants move. Understanding the site geology provides the fundamental basis for understanding site hydrogeology and defining contaminant movement. This information is of paramount importance when preparing models for groundwater flow, contaminant transport and fate, and the site remediation plan. Indeed, the first model erected in the study is the geologic model, which is overlain by the hydrogeologic model, contaminant distribution model, and so forth. Geologic environments will vary depending on where the geologist is working, but the basic hydrogeologic questions of depth to groundwater, aquifer contacts, and so on need to be answered.

The investigations also integrate aspects of soil engineering, applied chemistry, and environmental engineering disciplines. Although some surface geophysical techniques may aid subsurface exploration, site information is typically required to be collected from exploratory boreholes for soil, rock, sediment, soil vapor, and groundwater sampling. In fact, some of these requirements have been codified in portions of state and federal regulations and government guidance documents as the industry has matured in the past years.

Site investigations are conducted to gather the following information regardless of the site size or potential contaminant problem in the unsaturated (vadose) zone and saturated zone. Current regulations and cleanup are directed to both the groundwater and overlying soil and sediment that could have a continuing impact on groundwater quality. Thus, the investigation should address geology and operational processes of the vadose zone and aquifer. This will yield information on the general site geology, hydrogeology, aquifer geometry, groundwater occurrence and flow direction, gradient, physical and chemical testing programs, and aquifer pump testing. In this way, the consulting geologist and hydrogeologist can meet the primary goal of identifying and tracking contaminants for site cleanup.

GROUNDWATER OCCURRENCE AND GEOLOGY OF AQUIFERS

Groundwater occurs in subsurface rock and strata called aquifers, which comprise porous and permeable material (see Figure 1). Aquifers may be composed of alluvium, sedimentary rocks, and fractured crystalline or metamorphic rocks. The aquifers can be bounded by relatively “impermeable” bodies called aquitards, which do not readily transmit water. The discussions throughout most of the rest of the text will treat aquifers in a stratigraphic sense — that is, products of sediment deposition with sand and gravel as aquifers and silt and clay as the aquitard (or units of interest grouped by composition as aquifers and aquitards). Fractured rock aquifers will be discussed separately below. Aquifers and aquitards will be conceptually presented as tabular bodies that, although it is a strong generalization, will illustrate subsurface exploration and movement of water and contaminants. When case histories deal with specific hydrogeologic conditions, the text will so state.

A BRIEF REVIEW OF UNSATURATED AND SATURATED GEOLOGIC ENVIRONMENTS

Groundwater occurs in almost every type of geology. Therefore, an exhaustive review of all geologic and hydrogeologic environments is beyond the scope of this book. Groundwater occurrence has been described in the standard texts and the reader is referred to those for detailed discussions (see, for example, Davis and DeWeist, 1966; Fetter, 1988; Freeze and Cherry, 1979; Heath, 1982). A review of the unsaturated zone will be presented first, followed by a review of the general concepts of groundwater occurrence and flow.

Unsaturated (Vadose) Zone

The vadose zone overlies the saturated zone, or, for our purpose, any aquifer. Typically there is a contaminant release; the contaminants must pass through this region to get to the aquifer. Although this region is not saturated (that is, having all available pore space filled with fluid), areas may be locally saturated, while elsewhere fluid moves in response to tension and capillary forces. The vadose zone is also typically where volatile contaminant gases or vapors will arise and move from contaminant releases (see Figure 2).

The vadose zone is geologically a very heterogeneous region. Typically soil formation occurs at the surface and soils may be buried sequentially in fluvial and alluvial depositional processes. Unsaturated zones may vary in thickness from several feet to several hundred feet. Very thick vadose zones typically occur in the alluvial basins of the western U. S. The term “soil” has become a catchall to describe anything that is not a rock, and may cause a somewhat geologically misleading interpretation. For example, the site of interest may be soil-covered fractured rock, which contains soil, colluvium, and rock. A series of soils (or paleosols) may be buried sequentially, thus resulting in a crudely horizontally bedded deposit many feet thick, below the organic rich surface soil and soil profile. Further, because most contamination problems occur in an urbanized setting, land development such as digging basements, pipelines, or foundations may have altered the subsurface. The range of particle sizes, bedding, presence of buried structures, and so on may have a profound effect upon soil vapor and liquid contaminant movement (Morrison, 1989). Accurately ascertaining the geologic makeup of the vadose zone is vital to understanding the migration pathways toward the groundwater. A substantial investigative effort is required to locate the contaminant and understand the local geologic implications.

Figure 1 Hydrologic cycle with unconfined and confined aquifers (Modified from Cal. Bull., 118, 1974.)

Figure 2 A. Vadose zone soil model showing mass transport and retardation mechanisms. (From Gierke, Hutzler and Crittenden, 1985. With permission.) B. Relationship between soil moisture, matric suction, and hydraulic conductivity. (From Freeze and Cherry, 1979 in EPA, 1993. With permission.)

Unsaturated fluid movement may be affected by one or more processes depending on local conditions. Four processes which affect movement are hysteresis, macropore flow, capillary movement, and saturated (or Darcian) flow.

Hysteresis

The unsaturated region is not devoid of moisture, but what moisture does occur adheres to soil particles and grains. Flow that occurs between grains (pore throats) moves through micropores. A general equation for unsaturated flow (Fetter, 1988) is

$$\text{phi}=\text{psi}\left(\text{ϕ}\right)+\text{Z,}\text{where}$$

phi - total potential, unsaturated flow

psi(ϕ) - moisture potential, measured as suction

Z - elevation head

Fluid movement is governed by fluctuations in local pressure gradients. The suction (usually measured in centibars per cubic centimeter) is the effect of a negative pressure head of soil and water on the unsaturated conductivity (Hillel, 1980; Morrison, 1989; see Figure 2). As the soil becomes saturated, the soil pressure or suction declines toward zero as the available porosity becomes totally filled with water. As the soil drains, suction increases until the remaining water is held in tension on or between soil particles. As moisture increases or decreases in the soil, the suction changes. This would also be the case for contaminants migrating into and through the soil. For contaminant studies, the most important difference between saturated and unsaturated flow is that the unsaturated hydraulic conductivity (K) is not a constant for the different soil moisture contents (Morrison, 1989).

Macropore Flow

This occurs when surface water enters a crack or macropore and literally flows under gravity down into the crack. The vertical movement continues until the crack fills and local saturated conditions arise migrating from the crack (see Figure 3). Openings and channels are common in vadose soil and buried sediment resulting from roots, worms, burrowing animals, or man-made conduits and foundations. Desiccation cracks in clay may be open to depths of several feet, or even tens of feet, and could be infilled with granular sediment. When the clay is remoisturized, the sand in the crack may allow enhanced flow. This could allow rapid and deep penetrations of fluid through an otherwise “impermeable” layer with the obvious potential movement toward the aquifer. Lateral saturation from the vertical flow would occur at a slower rate into the soil around the macropore (Morrison, 1989; U.S. EPA, 1987). The implication that soil logging techniques must allow for recognition of these structures in the site investigation is obvious.

Figure 3 Hydraulic conductivity of selected consolidated and unconsolidated aquifers. (From Heath, 1982. With permission.)

Capillary Movement

Capillarity occurs due to available porosity immediately above the saturated zone as a response to water surface tension allowing vertical movement against gravity. The height of the capillary “fringe” above the saturated area is dependent on grain size. Hence, the height of capillarity into a clay or silt is higher than that of a sand or gravel. The pores of the clay and silt are much smaller and movement by tension occurs (Hillel, 1980).

Saturated Flow

This occurs where sufficient water collects on an impermeable layer to saturate the porosity. This results in a “perched” groundwater lens above the regionally recognized aquifer. These perched lenses may be small or very extensive, and form local “miniaquifers.” In this case, Darcian flow can occur and could be used to model the movement of fluid. If large quantities of water or contaminants are present, movement could be initially lateral and horizontal under unsaturated conditions, then vertical under saturated conditions.

Saturated Geologic Environments — Aquifers

This brief review of geologic environments that may be encountered in consulting hydrogeology may be classified broadly as igneous-metamorphic, sedimentary, and alluvial-filled basins. These three general classes will be used throughout the book as examples. Groundwater occurrence in these discussions is somewhat similar to the DRASTIC models developed by the U.S. Environmental Protection Agency for use in rating geology for contaminant investigations (Heath, 1984; Aller, et al., 1987a). The following summary is not an attempt to modify DRASTIC or other established hydrogeologic convention; the reader is referred to those references from which the following discussion draws. The intention is to introduce general groundwater occurrence in terms of porosity and permeability for different terrains (for an excellent review of this material, see Back et al., 1988).

The location and identity of drinking water aquifers are basic to contaminant hydrogeology since the goal is to protect that resource. Consulting work demands proper recognition of geologic materials and environments so that the appropriate investigation approach is selected to collect the proper and relevant data. Obviously, individual sites need to be evaluated for their specific hydrogeologic characteristics in all cases. Geologic materials vary in different rock types; Figure 3 presents diagrams and typical ranges of porosity and permeability of rocks and sediment (Heath, 1982; Morris and Johnson, 1967). Figure 4 shows idealized geologic units and groundwater flow.

Igneous–Metamorphic Rock

Crystalline igneous or metamorphic rocks, and some very well lithified or slightly metamorphosed sedimentary rocks, form these terrains. Usually these rocks are “dry” in a large-quantity water resource sense, yield little water to wells, and are considered impermeable. Groundwater flow is typically through joints, fractures, structural discontinuities, or tectonic fracturing, with very little porosity between minerals or metasediment grains. Where fractures are dense and recharge is abundant, locally high water yields may occur. In igneous and metamorphic terrains, fractures tend to be common in the upper 300 ft, and will contain and yield water. Overlying alluvial accumulations or weathered zones of rock add to the available porosity and water reservoir. While some fracture systems can be deep and extensive, typically there are fewer fractures and they close with depth and groundwater yield decreases accordingly. Although laminar flow assumptions may not be valid in fracture flow, it may not be a major hindrance to describing flow and transport in fracture systems (Schmelling and Ross, 1989).

Figure 4 Idealized distribution of geologic units and groundwater flow. Not to scale. Major valleys actually occupy 5–30% of large basins. (From Randall, 1988. With permission.)

Sedimentary Rock

Sedimentary rock contain lithified and semilithined sediments, as well as chemical sediments such as limestone. Porosity and permeability in sedimentary rocks tend to be greater than in igneous or metamorphic rocks. Groundwater resource occurrence is typically more abu...

Table of contents

• Cover
• Title Page
• Copyright Page
• Preface
• Author
• Acknowledgments
• Disclaimer
• Dedication
• Table of Contents
• Chapter 1 Geologic Frameworks for Contaminant Hydrogeologic Investigations
• Chapter 2 Contaminant Pathways — Subsurface Investigation and Monitoring Approach
• Chapter 3 Subsurface Exploration, Sampling, and Mapping
• Chapter 4 Groundwater Monitoring Well Design and Installation
• Chapter 5 Groundwater Monitoring Well Sampling
• Chapter 6 Introduction to Regulatory and Legal Framework
• Chapter 7 Introduction to General Groundwater Geochemistry
• Chapter 8 Aquifer Analysis
• Chapter 9 Remediation and Cleanup
• References
• Index