12.1 Introduction
Beyond the natural geochemistry of groundwater discussed in Section 11, the other aspect of water chemistry that interests hydrogeologists is groundwater contamination and the spread of contamination in the subsurface. These contaminants include road salt; agricultural herbicides, pesticides, and fertilizers; manufacturing byproducts; petroleum products; wastes from mines and industrial processes; and pathogens (i.e., viruses, bacteria, fungi, protozoa, and worms). These materials may be introduced over a large area such as agricultural fields (non-point source) or in a discrete area like a salt storage facility. Many of the contaminants are organic compounds—chemical compounds that contain carbon—that are associated with petroleum-based fuels, solvents, and other industrial chemicals. Organic compounds often reach groundwater in liquid form, as lighter than water liquids (LNAPLs – light non-aqueous phase liquids) or denser than water liquids (DNAPLs – dense non-aqueous phase liquids). These liquids are often spilled on or near the surface then migrate downward to the groundwater where the LNAPLs float on the water table while the DNAPLs sink through the flow system. The pool of NAPL dissolves slowly into the groundwater and thus becomes a source of contamination. Sites that have experienced releases of contamination to the groundwater generally have a zone of dissolved contaminants around and down-gradient of the source that is called a contaminant plume. Groundwater contamination is a problem that our society has been aware of for much of the last century, and a lot of effort has been done to understand the processes involved, characterize and delineate contaminant plumes, and clean up contaminated sites. It is an extremely complicated problem because there are numerous processes that control the movement of contaminants and many possible reactions that create, change, and break down contaminants as they move in groundwater flow systems.
In Section 12.2, we develop a general overview of the processes that move mass (i.e., suspended particles and dissolved chemicals) through groundwater systems. In Section 12.3, we discuss reactions that affect the concentration of both contaminants and naturally occurring dissolved material. We do not discuss specific contaminants nor the reactions that those contaminants experience in groundwater systems.
The Groundwater Project offers many books in the contamination category↗ that can be downloaded free of charge. Information on organic contaminants in groundwater can be found in Properties of Organic Contaminants↗ by Mackay and others (2024), Petroleum Hydrocarbons in Groundwater↗ by Brown and others (2017), Dissolved Organic Carbon in Groundwater Systems↗ by Chapelle (2022), and Septic System Impacts on Groundwater Quality↗ by Robertson (2021). Each of these books is available as a free download from The Groundwater Project. Additional books related to mass transport and groundwater contamination are currently in development for the Groundwater Project as of the publication of this book, so readers are encouraged to regularly review the offerings at the Groundwater Project for related publications↗.