Hydrogeology

Groundwater Science and Engineering

Alain Dassargues

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9780429894404
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

This text combines the science and engineering of hydrogeology in an accessible, innovative style. As well as providing physical descriptions and characterisations of hydrogeological processes, it also sets out the corresponding mathematical equations for groundwater flow and solute/heat transport calculations. And, within this, the methodological and conceptual aspects for flow and contaminant transport modelling are discussed in detail. This comprehensive analysis forms the ideal textbook for graduate and undergraduate students interested in groundwater resources and engineering, and indeed its analyses can apply to researchers and professionals involved in the area.

Chapter 1

General introduction

Water availability, along with land and food availability and other mineral resources, is an increasingly popular topic, a growing preoccupation as the world population continues to increase. This is not so much a global problem (de Marsily 2009) as it is a regional problem of availability to satisfy our needs for improving human health, food security, biodiverse natural ecosystems and effective energy production. Indeed, there are multiple feedback effects, interconnections, and couplings among these four main domains dependent on water resources. This relationship has been described as the “water—energy—food nexus” (Scanlon et al. 2017, Cai et al. 2018). The emergence of ‘nexus thinking’ comes from an increasingly perceptible understanding that natural resources may someday limit the development of our well-being and of our growing human communities. Consequently, win-win strategies must be developed for preserving environmental sustainability together with producing efficiency gains to balance the imposed growth from the demographic issue (Ringler et al. 2013). This problem is particularly true for freshwater (i.e., natural continental water with a limited ion content from brackish water and seawater) that is indeed essential for each of us.

1.1Freshwater resources and groundwater resources

The global stock of water on Earth is currently estimated at approximately 1,387 million km3. In fact, the amount is not what is important. More than 96.5% is seawater. Other saline waters are found at depth or in salty lakes for approximately 0.96%. These numbers mean that freshwater is found only in the remaining 2.54% of water on Earth. If we subtract water contained in ice caps and glaciers (1.75%), vapor in the atmosphere, soil moisture and permafrost (0.02%), the remaining “easy to use” freshwater is only 0.77% of water on Earth. In addition, the most amazing fact for the general public and the media is that out of this 0.77%, the share of rivers and lakes is less than 0.01%; rivers and lakes provide less than 1.3% of the directly valuable water. In other words, the ratio of fresh groundwater to fresh surface water is approximately 77 to 1. Data at the global scale are scarce or often result from inadequate upscaling techniques; therefore, these global estimates can be relatively inaccurate. Nevertheless, these figures clearly show the particular importance of groundwater for future generations in a world with a growing population. An additional critical question arises with the current climate change: Is the distribution of water changing between the different reservoirs (de Marsily 2009)?

Freshwater is quite unevenly distributed, inducing many local water availability issues. In arid zones, groundwater takes a critical importance while surface water may be very limited if they are not fed by seasonal glacier melt from mountainous regions. The renewability of groundwater reserves must also be considered. Again, in arid zones, water production from very old groundwater reserves, referred to as fossil groundwater (i.e., not renewed for thousands of years), automatically brings up the question of sustainable development. For example, in northern Mauritania, in the Sahara Desert, a water recharge of a few mm/year (i.e., on a multiannual basis and over the entire territory) would be enough to balance groundwater pumping needed for the current development of the region. However, determining whether these few mm/year of recharge actually occur under current climatic conditions is not a simple problem. This question already requires very extended and detailed hydrological study involving long periods of data measurement and accurate interpretations.

1.2Anthropocentric vision

As mentioned above, research about water resources and especially about groundwater is most often motivated and supported by economic and human water needs. As noted by Hornberger et al. (2014), water sciences “have both pure and applied roots that stretch back to antiquity. The importance of water as a resource remains one of the central reasons for studying hydrological processes.” Thus, hydrogeology refers not only to natural science but also to engineering. Hydrogeology is subject to the laws and uncertainties of the natural sciences while most often being used to ensure conservation of ecosystems and various needs of humankind. Engineering is involved because the social and economic importance of groundwater supplies requires not only quantified answers but also the uncertainty estimate of the answers. However, “groundwater is not visible in policy, governance and management matters as it needs to be” (Grabert and Kaback 2016), compared with its relative importance, for example, in the production of drinking water.

In a global DPSIR (Driver Pressure State Impact Response) approach, the drivers are clearly the growing population and climate-societal changes. Global and local water management systems must be efficient to provide a “water security” (Foster and MacDonald 2014). They are dealing with quantity and quality issues and renewable water resources. There is only a finite amount of water and there are no substitutes (Cosgrove and Loucks 2015) as desalinization of seawater is not yet based on sufficiently sustainable techniques to be used on a large scale. Thus, managing water, and especially water scarcity, is among the most pressing challenges humanity is facing today (Wheeling 2016).

When assessing water, especially groundwater needs, statements are often misleading and confusion exists between “used water,” “consumed water” and “produced water,” or “withdrawn water” (Simmons 2015). Water can be used many times, ensuring different successive functions or services with (recycled water) or without water treatment (reused water). Consumed water refer to water that is not (at least locally) recycled (i.e., evaporated, transpired, or transformed into food). Produced or withdrawn water is extracted from a source, and a part of it can actually be reinjected (recycled) or reused, while the other part is consumed (see above).

Large debates recur about privatization and marketing of water. This issue is not the scope here but indeed, theoretically, a cost assigned to water would ensure that water providers would be financially healthy and improve quality in their methods of managing water resources on behalf of all (Gautier 2008). At the same time, a minimal access to water should be guaranteed to everybody, and consequently, the role of local authorities is to determine the delicate equilibrium to be found.

Facing sustainability (i.e., defined as the ability to meet present needs without compromising those of future generations), the most difficult problem is that this concept seems quite opposite to the population growth, at least at current levels of resource consumption. Lower levels of water resources consumption (i.e., by fostering reused and recycled water) should be attained as soon as possible by technological progress.

1.3Hydrogeology within hydrology

Hydrology is the scientific study of water (in general). Thus, it involves the study of movement, distribution, and quality of water on continents, including the water cycle, water resources, and environmental sustainability aspects. This study incorporates the properties of water, and the relationships between water and the biotic components of the environment. Hydrogeology is the study of groundwater hydrology, specifically taking geological conditions into account. Thus, hydrogeologists should master methods and scientific techniques from different sciences and engineering specialties: geology, physics, chemistry, biology, surface hydrology, hydraulics, fluid mechanics, and heat transfer as well as geostatistics, risk analysis, and geotechnical, numerical, and computational engineering. For fruitful interactions with other water and environment specialists and researchers, notions and understandings in civil and environmental engineering, and in water management, can also be essential.

1.4Basics about groundwater: partially and fully saturated zones

Groundwater is water underground in pore spaces and fractures of the geological media. The part of the lithosphere in which each void (i.e., pores, fissures, caverns) is totally filled with water is referred to as the saturated zone. Above the saturated zone, a partially saturated zone is found where the void space contains water and air. Many terms are used, such as unsaturated zone, variably saturated zone, or vadose zone. Upwards, the uppermost layer is usually the soil moisture zone or root zone whose thickness varies according to the local combination of climatic, lithological, and topographical conditions. A capillary fringe can be distinguished overlying the saturated zone (Figure 1.1). In this fringe, water saturation is nearly reached but the capillary forces are greater than that of gravity. Groundwater filling the pores and fissures remains preferably attached to the solid matrix by capillary action. The thickness of this capillary fringe increases as the pore sizes and fissure apertures are small and uniform. Very high capillary fringes can give rise to significant sensitivity to any additional water infiltration: in this case, flood risk from groundwater is very high if groundwater levels rise rapidly up above the natural surface.

Figure 1.1Above the water table, the capillary fringe is a zone of variable thickness where the medium is nearly saturated, but the capillary forces are greater than that ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Contents
• Foreword
• Preface
• Author
• 1. General introduction
• 2. Hydrologic balance and groundwater
• 3. Groundwater terminology and examples of occurrences
• 4. Saturated groundwater flow
• 5. Hydraulic conductivity measurements
• 6. Land subsidence induced by pumping and drainage
• 7. Introduction to groundwater quality and hydrochemistry
• 8. Contaminant transport, residence times, prevention, and remediation
• 9. Groundwater flow and transport under partially saturated conditions
• 10. Salinization and density dependent groundwater flow and transport
• 11. Heat transfer in aquifers and shallow geothermy
• 12. Methodology for groundwater flow and solute transport modeling
• 13. Main principles of numerical techniques used in groundwater modeling
• Index