This chapter discusses a wide range of water contaminants and the contaminant sources—and their impact on drinking water supplies from both surface water and groundwater sources. In addition, the point is made that when it comes to freshwater pollution, nature is not defenseless in mitigating the situation. The point is made that nature through its self-purification process in running water systems is able to fight back against pollution—to a point, at least. The chapter discusses the by-products of chlorine and flocculant additives. When a underground storage tank (UST) leak or past leak is discovered, the contaminants released to the soil and thus to groundwater would seem rather straightforward to identify: fuel oil, diesel, and gasoline. The chapter discusses one such contaminate, a by-product of gasoline—one that is not well known—to help illustrate the magnitude of leaking USTs. It discusses nature’s ability to purify and restore typical river systems to normal conditions.