Water treatment has been implemented for decades to treat water supplies as well as “wastewater” from a variety of sources. Noteworthy are successes treating challenging contaminated waters, including industrial sources, mining influenced waters, and oil and gas produced waters. Passive water treatment is a process of simultaneously or sequentially mitigating contaminants and/or acidity and physicochemical properties in a man-made system. This is achieved by capitalizing on biological, geochemical, and coupled biogeochemical reactions, followed by the physical removal and sequestration of constituents. In its truest form, a passive water treatment system (PWTS) does not require power or chemicals after construction and can be designed as a sustainable system lasting for decades or longer with minimal intervention or maintenance. For waters that contain constituents of concern that are not amenable to treatment by naturally occurring biological, physical, or chemical pathways (e.g. sodium, chloride), hybrid or semipassive systems can be developed that incorporate energy driven processes, such as reverse osmosis coupled with passive water treatment.