ter dissolves salts from soil, carries them to the ocean, and then evaporates, leaving the collected residue to accumulate. Moving through the atmosphere, water arrives again over the land surface to replenish the puddles, rivulets, lakes with fresh water. But within the past few decades Americans have realized that increased concentrations of toxic residues are contaminating fresh water prematurely—long before it can naturally process the material—largely because of the actions of humans.
Traditionally, waste products have been allowed to flow into the nearest body of water. As a consequence, streams, estuaries, and large parts of the coastal environment have become contaminated. Coastal zones, including estuaries, are areas of active sedimentation, and deposited materials often contain toxic wastes. These wastes may seep to the surface or landward into aquifers. They may be exhumed—either naturally or through channel dredging and widening—and redistributed by erosion. Solid wastes placed in landfill sites and other terrestrial depositories are gradually dissolved by moving groundwater; in many cases they contaminate the groundwater supply (Figure 4.5). Because streamflow is often nourished by groundwater, dissolved contaminants can eventually make their way into streams and other water courses.
Groundwater contamination studies have produced many surprises. Volatile organic compounds are mobilized through the atmosphere and may become components of the soil, where they remain for long periods of time. Transport through the soil and groundwater is generally slow, and long periods are required to move contaminants significant distances. Hydrologists agree that once contaminated, groundwater will remain contaminated unless remedial action is taken. Much of the water contamination identified today is the result of waste disposal practices 20, 30, 40, or more years ago.
There is a significant difference between pollution by toxic compounds and pollution by toxic elements. Compounds may spontaneously decompose, through inherent instability, or may be decomposed by heat, biological action, or catalytic properties of earth materials. Either course may result in the loss of toxic character. Heavy metals are elements and remain toxic unless they are immobilized into an insoluble state or detoxified by chelation. Chelation constrains metal atoms within an innocuous but stable chemical species; the heavy metals are tamed or caged by an organic compound that establishes bonds with the metallic ions and eliminates the potential for the metal to react with any other compound.
It is possible that ill-conceived detoxification tactics might accidentally exacerbate the predicament. For example, the native element mercury is toxic but only very slightly soluble. Conversion by industry produces the compound methyl mercury (Hg(CH3)2), which is both soluble and toxic.