11.1 Introduction

In this section, we use the term “mass” to mean anything in the water that isn’t water. Mass exists in several phases within a groundwater system. Mass exists in a solid phase, consisting of either the minerals that make up the geologic units or suspended particles moving through the aquifer. Mass also exists as a separate liquid phase; for example, many organic contaminants do not mix with water very well and stay in a liquid state in the subsurface. Mass also occurs as a gas phase; most soils contain [latex]{CO}_{2}[/latex] in their pores, and bubbles of gas can occur in the saturated zone of aquifers. Finally, mass exists as a dissolved phase. All these phases are significant parts of aqueous geochemical systems; however, hydrogeologists are mostly concerned with the dissolved phase, and they generally lump the other three phases together as sources for dissolved species in the water.

There are two aspects of water chemistry that interest hydrogeologists. The first aspect deals with the natural chemistry of groundwater systems and the interaction of groundwater with solid mineral phases and organic matter. Water is known as the universal solvent because it reacts with and/or dissolves nearly anything it encounters. The rocks and sediments that make up an aquifer—primarily inorganic compounds—impart a chemical signature to the water, in the form of a specific set of dissolved ions. This is useful to us because we can use the groundwater chemistry to infer where the water is coming from and to link the groundwater to specific geologic units.

The second is the problem of groundwater contamination and the spread of contamination in the subsurface. This is discussed in Section 12.

In this section, we focus on mass as dissolved phases in groundwater, and consider any solid, liquid, or gas phases together as a separate phase that reacts with the groundwater. Groundwater is constantly reacting with the other masses in the subsurface. These reactions are responsible for the redistribution of mass in the subsurface and are what control the chemistry of the groundwater.