Some pollution problems are daunting; others less so. This chapter begins with a brief overview as to how we can separate the “trivial” from the serious, whether the risk of a chemical or of a broader environmental problem. The chapter emphasis is on preventing or controlling pollution. Also introduced are the tools that assist us in moving toward sustainable societies. Section I introduces risk assessment concepts, how to evaluate the magnitude of a given risk, chemical risk assessment, and comparative risk assessment. Section II asks, how do we reduce, even eliminate, pollution's risks? Legislation is important. Many laws stress trapping pollutants already produced. The pollution prevention (P2) paradigm, also called source reduction, is more effective. P2 involves changing a process so that it produces less pollution or, sometimes, none at all. Section III introduces industrial ecology (IE), which goes beyond P2. IE examines pollution holistically, searching for means to mesh the human enterprise into the natural environment. Some tools of IE are briefly examined including life-cycle assessment and design for the environment.

SECTION I

Risk assessment

For many years, society has struggled with the problem of how to evaluate environmental risks. Risk assessment tools have been developed over decades, and continue to be refined. ▪ Chemical risk assessment examines the risk of individual chemicals.

Whether animal, plant, or microbe, water is essential to life. Fish and other water-dwelling creatures are vulnerable to polluted water and there are waters in the world so polluted that life has disappeared. In other locales fish and shellfish survive, but are not safe to eat because their flesh is contaminated. Humans enjoy being around water, but contamination with infectious organisms often makes swimming unsafe, and being near water with noxious odors or scum is not pleasant. Clean water – and enough of it – is vital. ▪ Laws governing water quality existed in the United States before 1972, but no uniform national law existed. Water pollution was not well controlled and some states, eager to keep or attract industry, were negligent. The Clean Water Act (CWA) of 1972 and the Safe Drinking Water Act (SDWA) of 1974 were laws mandating that water pollution be treated uniformly nationwide; they have been updated over the years. ▪ In developed countries, water bodies are generally cleaner. In the United States, some are much cleaner than in 1972, the year that Congress passed the CWA: in 1972, only 30% of US waters were judged fishable and swimmable. By 1994, it was greater than 60%.

Before discussing the formation of the major geological units of the solid Earth, we should review the internal structure of our planet as described by seismic wave studies (Fig. 11.1).

The most important discontinuities observed by seismologists are:

• The base of the crust (called the Mohorovičić discontinuity or Moho), 40 km below the continents, but only 5–7 km beneath the oceans.

• The base of the lithosphere, on average 80 km below the oceans, and deeper still beneath the continents. Rather than a discontinuity, this is a rather diffuse transition. This is the lower boundary of the rigid tectonic plates. The softer part of the upper mantle underneath the lithosphere is called the asthenosphere. The 410 km discontinuity corresponding to a change in olivine structure (spinel or ringwoodite phase). The 660 km discontinuity corresponding to the transformation of all minerals into perovskite and minor Fe–Mg oxide (magnesio-wüstite). This is the base of the upper mantle.

• The mantle–core boundary at about 2900 km. Above this boundary is a seismically abnormal layer some 200 km thick, known as the D”layer.

• The outer core–inner core boundary at about 5150 km. The core is composed mostly of metallic iron and nickel. The motion of the fluid outer core generates the Earth's magnetic field.

Plate tectonics is a powerful theory that unifies the geological expression of crustal and upper-mantle geodynamics (Fig. 11.2). The Earth's surface is covered with rigid lithospheric plates that may or may not carry continents.

Before discussing the composition of the different systems of geological interest and the exchanges of matter that make those systems evolve relative to each other, it is worth recalling the principles governing the geochemical differentiation of our planet. These seemingly simple principles conceal what are often daunting complexities. They are: the principle of conservation of mass, elementary and isotopic fractionation induced by phase changes, kinetic fractionation, and radioactivity. In the Introduction, we alluded to the contrast between mixing processes and differentiation processes. We will now look at a number of examples.

1. Partial melting of the mantle beneath mid-ocean ridges produces basaltic liquids whose chemical composition is different from the ultramafic chemical composition of the source peridotite. This chemical fractionation of elements between the molten fluid and its parent medium can be described by thermodynamic rules. The former makes up the oceanic crust while the latter forms the refractory base of the lithosphere (located in the oceanic plates beneath the crust). It must be kept in mind that there is no chemical or isotopic fractionation in the system unless at least two phases co-exist (solid/liquid, vapor/liquid, mineral A/mineral B, …) each to host a different share of the initial inventory. Conversely, when the oceanic crust and oceanic lithospheric mantle plunge at subduction zones, they begin a long journey within the mantle, where convection folds and stretches them, in much the same way as a baker kneads dough, progressively eliminating the differences that initially existed between the two constituent parts. Convective mixing, or better, stirring, therefore undoes the effect of magmatic differentiation.

2. […]

Geochemistry utilizes the principles of chemistry to explain the mechanisms regulating the workings – past and present – of the major geological systems such as the Earth's mantle, its crust, its oceans, and its atmosphere. Geochemistry only really came of age as a science in the 1950s, when it was able to provide geologists with the means to analyze chemical elements or to determine the abundances of isotopes, and more significantly still when geologists, chemists, and physicists managed to bridge the chasms of mutual ignorance that had separated their various fields of inquiry. Geochemistry has been at the forefront of advances in a number of widely differing domains. It has made important contributions to our understanding of many terrestrial and planetary processes, such as mantle convection, the formation of planets, the origin of granite and basalt, sedimentation, changes in the Earth's oceans and climates, and the origin of mineral deposits, to mention only a few important issues. And the way geochemists are perceived has also changed substantially over recent decades, from laboratory workers in their white coats providing age measurements for geologists or assays for mining engineers to today's perception of them as scientists in their own right developing their own areas of investigation, testing their own models, and making daily use of the most demanding concepts of chemistry and physics. Moreover, because geochemists generate much of their raw data in the form of chemical or isotopic analyses of rocks and fluids, the development of analytical techniques has become particularly significant within this discipline.

I am specially happy to preface this book. First, because it is always a pleasure to be able to speak well of a friend's work; and Francis Albarède is a friend of long standing! We both embarked on our academic careers at about the same time. After some solid grounding in geology at the University of Montpellier, we were fortunate enough to begin our doctoral research in geochemistry in the 1970s in Professor Claude Allègre's laboratory at the Paris Institut de Physique du Globe, at a time when the discipline was really taking off in France. We also helped set up degree courses in geochemistry at the recently founded University of Paris 7, where we were appointed Assistant Lecturers. Our work together resulted in the publication of a short book in 1976, primarily for students, which quickly sold out and curiously enough was never reprinted! Few universities in those days offered specialist courses in geochemistry.

Times have clearly changed since then! Geochemistry is now taught in most universities and it is needless to recall here the fundamental contribution that this discipline has made to all areas of Earth sciences and cosmochemistry. It is always helpful, though, for students and for non-specialist faculty to have a textbook that provides a review of the basic concepts and the most recent contributions to the discipline. And this is the second reason why I am happy to present this book; because Francis Albarède's work fulfills both these requirements.