William "Bill" Back: an incisive geochemist and a great mentor.

Introduction William ‘‘Bill’’ Back (Figure 1) was one of the great ‘‘idea people’’ in low-temperature geochemistry during the blossoming of ground water hydrology in the latter half of the twentieth century. Prior to the1950s, most of the research in ground water chemistry was related to water quality and was carried out by chemists who rarely ventured into the field. Bill, Ivan Barnes, Bruce Hanshaw, Blair Jones, and Don White of the USGS were among the first to change that by using aquifer systems to test models of natural solute evolution and thus changed the way people thought about water chemistry. Bill Back was an early leader in recognizing the importance of thermodynamics in ground water systems. He related ground water chemistry to flow in aquifers, and in what is now a benchmark paper (Back 1960), he introduced the concept of hydrochemical facies by drawing on the concept of geological facies (units of similar lithology and depositional history). Bill defined hydrochemcial facies as ‘‘the diagnostic chemical aspect of water solutions occurring in hydrologic systems,’’ which ‘‘reflect the response of chemical processes in the lithologic framework and the pattern of water flow in it.’’ In later work, he used geochemical mass balance and stable isotope data to constrain possible reactions and recognized the value of dating water and solutes in aquifer systems. He was among the first to investigate redox processes in iron-reducing aquifers and later turned to extreme geochemical environments such as landfills and investigation of the role of organics in solute evolution in ground water. Later in his career, he investigated the importance of ground water in the development of ancient cultures. Although his numerous publications reflect his many scientific accomplishments, he is perhaps most endearingly remembered as a mentor to young students and professionals, for encouraging and supporting women in hydrogeology, and for his contributions to professional societies and international hydrogeological activities.