tion of dissolved sulfate often creates odorous compounds. Most odorous gases, such as hydrogen sulfide and the sulfur-bearing mercaptans, are formed under anaerobic conditions, so providing adequate dissolved oxygen is the first step toward controlling odors. The control of odors is among the priority issues with respect to public acceptance of a project.
Adequate dissolved oxygen in surface waters is required for aerobic respiration and is needed to protect fish and other aquatic life. The presence of dissolved oxygen (DO) leads to oxidizing conditions that minimize the formation of noxious odors and prevents the solubilization of certain metals (e.g., iron, manganese); however, introduction of DO into anaerobic or reduced aquifers may oxidize sulfide minerals and increase the release of metals. The inverse situation may occur as well; MUS activities that low DO water into a previously oxidized part of the aquifer may lead to reductive dissolution of minerals and the release of metals.
The hydrogen ion concentration is an important quality parameter for all waters. The usual means of expressing hydrogen ion concentration is pH, which is defined as the negative logarithm of the hydrogen ion concentration. pH influences the surface charge on solid surfaces, the distribution of acidic and basic compounds, the form of a chemical in solution, the solubility of compounds, the physical shape of organic molecules, and the toxicity of the medium.
Eh is another critical parameter because of the effect of high Eh waters on iron-bearing minerals. Such solutions, which often contain high levels of dissolved oxygen, alter primary minerals to iron oxyhydroxides, thus changing the water chemistry as well as altering the aquifer properties. Eh and pH are also primary controls on the population of subsurface bacteria that biodegrade certain organic contaminants, as well as on those that cause illness. Some sulfate-reducing bacteria, for example, survive or thrive only in the absence of dissolved oxygen.