GroundwaterChemistry

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9.1 IntroductionIf H2O molecules were the only thing present in groundwater, this chapter could be veryshort. Thanks to the many other substances within or in contact with groundwater, there isa lot more to talk about. The distribution, reactions, and transport of these other substancesmake for an interesting and complex topic.

Solutes are other molecules dissolved within the sea of H2O molecules in the aqueousstate. Many solutes occur naturally, such as inorganic ions like Ca2+ or SO2−

4 . Sometimeshigh concentrations of naturally occurring solutes render the water unfit for drinking,irrigation, or other uses. Other solutes are chemicals introduced by human activities.Many of these are troublesome contaminants such as heavy metals and organic solvents.

The solid phases that make up the aquifer matrix can react with and dissolve into thegroundwater. At the same time, some solids precipitate out of water, a phenomenon thatcan lead to clogged pipes. Some solids may also exist as tiny particles suspended in thegroundwater.

In the unsaturated zone, water is in contact with pore gases and molecules will transferbetween the liquid and gas states. This mechanism can be an important way that subsur-face contaminants migrate, particularly for volatile organic compounds (VOCs).

When organic liquids like hydrocarbon fuels and solvents are spilled into the subsur-face, they dissolve sparingly into water and can persist for a long time as a separate liquidphase. The acronym NAPL, for nonaqueous-phase liquid, is often used to describe theseseparate liquid phases.

Chemical reactions can involve substances in the aqueous, gas, solid, or NAPL phases,and some reactions transfer mass from one phase to another. Some reactions occur withinthe bodies of microorganisms; they are a vital link in the attenuation of certain con-taminants. Chemical processes in the groundwater environment are both complex andfascinating. Characterizing and predicting these processes are some of the most challeng-ing problems in groundwater science. Groundwater chemistry is relevant to all users ofgroundwater resources, whether it be for drinking, irrigation, industrial, or other purposes.Chemistry is also central to understanding the fate of groundwater contamination and howto remediate contamination.

280 Groundwater Chemistry

This chapter provides an introduction to aqueous geochemistry as it relates to ground-water. Much more detailed treatment of the subject can be found in aqueous chemistrytexts like those of Drever (1988), Pankow (1991), Morel and Hering (1993), Stumm andMorgan (1996), and Langmuir (1997). Domenico and Schwartz (1998) cover aspectsof chemistry that are relevant to groundwater. The next chapter introduces groundwatercontamination, building on the fundamentals introduced in this chapter.

9.2 Molecular Properties of WaterThe geometry of a water molecule is not unlike the face of a famous cartoon mouse(Figure 9.1). The two hydrogen atoms are bonded to the oxygen atom by sharing outerelectrons, forming covalent bonds. The angle between the two bonds is about 105◦.

Water is a polar molecule because the distribution of electrical charge associated withprotons and electrons is asymmetric. The oxygen end of the molecule is somewhat nega-tively charged, while the hydrogen ends are somewhat positively charged.

The polarity of the water molecule causes electrostatic attraction to other polar molecu-les and to charged molecules. The hydrogen ends of a water molecule are attracted to theoxygen ends of other water molecules, forming weak bonds known as hydrogen bonds(Figure 9.1). Hydrogen bonding causes water molecules to bond together in clusterswithin which there is an ephemeral fixed arrangement like in a crystalline solid. Theseclusters are continually forming and breaking up, existing for only a short slice of time,on the order of 10−12 sec (Stumm and Morgan, 1996). These clusters grow as large as100 water molecules (Snoeyink and Jenkins, 1980).

Several different isotopes of both hydrogen and oxygen occur in natural waters, but themost common isotopes 1H and 16O are far more abundant than all others (see Table 9.13,Section 9.10). The different isotopes of a specific element differ only in the number ofneutrons in the atom’s nucleus and, of course, their total mass. Because various isotopesof the same element have the same number of electrons, all isotopes behave similarly inchemical reactions.

The difference in mass between different isotopes can lead to different behavior incertain physical processes. For example, water molecules containing the heavier isotopes2H, 17O, or 18O are less prone to evaporate from liquid water than the common watermolecules containing 1H and 16O. This discrepancy leads to near-surface ocean or lakewater that is enriched in the heavier isotopes compared to atmospheric water (more aboutthat in Section 9.10).

Figure 9.1 Geometryof a water molecule (left)and hydrogen bonding ofwater molecules (right).

Solute Concentration Units 281

Natural waters are water-based (aqueous) solutions, with other elements and com-pounds are dissolved as solutes within the solvent water. Most solutes in natural ground-waters carry a charge, either as cations (+) or anions (−). Ions dissolved in water aretypically surrounded by water molecules that orient themselves in accordance with thecharge of the ion, as shown in Figure 9.2. The larger the ion, the more oriented watermolecules can surround it. The orientation of water molecules extends beyond the adja-cent layer of water molecules, but the degree of orientation decreases with distance fromthe ion.

The polar nature of water molecules makes it a good solvent of ionic and polar molecu-les. The mutual attraction of ions and polar water molecules allows large numbers ofions to be accommodated in the midst of water molecules, resulting in high solubilitiesfor ionic substances. Table salt (NaCl) and other salts dissolve readily into their ioniccomponents.

On the other hand, nonpolar molecules have a relatively symmetric distribution ofcharge and little affinity for water molecules. Lacking attraction to water molecules, rel-atively few of these nonpolar molecules are accommodated within the water. Nonpolarmolecules have low solubilities; they only dissolve to low concentrations in water.

9.3 Solute Concentration UnitsThe concentration of a solute in an aqueous solution may be expressed in several differentways. In chemical calculations, it is standard to use molar concentration units, which aremoles of solute per liter of solution (mol/L), denoted M. For example, a 2.5 M Ca2+ solu-tion contains 2.5 moles of Ca2+ per liter. A mole is an amount of a substance consistingof N atoms or molecules, where N = 6.022 × 1023 is Avogadro’s number, here roundedto four significant digits. The mass of a mole of atoms is called the atomic mass andthe mass of a mole of molecules is called the formula mass (also called formula weight).For example, the atomic mass of oxygen is 16.00 g, and the formula mass of CO2 is12.01 + (2 × 16.00) = 44.01 g. For calculations involving chemical reactions, it is handyto use moles and molar concentrations, because chemicals react in direct proportion to thenumbers of molecules present (a periodic table of elements is on the back inside cover).

The concentrations measured in a laboratory water analysis are usually reported inmass/volume units like mg/L or µg/L. Conversion between molar and mass per volumeunits may be done using the formula weight of the solute. Converting from mol/L tomg/L units could be done as follows:

Figure 9.2 Orientationof water moleculesaround dissolved cation(left) and anion (right).