Historical Note/ David Deming, History Editor William ‘‘Bill’’ Back: An Incisive Geochemist and a Great Mentor by Mary Jo Baedecker 1 and Warren W. Wood 2,3 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 Han- shaw, 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 draw- ing on the concept of geological facies (units of similar lithology and depositional history). Bill defined hydro- chemcial facies as ‘‘the diagnostic chemical aspect of water solutions occurring in hydrologic systems,’’ which ‘‘reflect the response of chemical processes in the litho- logic 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 recog- nized the value of dating water and solutes in aquifer systems. He was among the first to investigate redox pro- cesses 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 anci- ent cultures. Although his numerous publications reflect his many scientific accomplishments, he is perhaps most endearingly remembered as a mentor to young stu- dents and professionals, for encouraging and supporting women in hydrogeology, and for his contributions to pro- fessional societies and international hydrogeological activities. Early Career Bill was born on August 9, 1925, in East St. Louis, Illinois, and he died on January 31, 2008, in Honolulu, Hawaii, where he had lived for the past 2 years. Bill grad- uated in geology from the University of Illinois in 1948, and he attended the University of Colorado and Univer- sity of California, Berkeley, where he received an MS degree in geology in 1955. He received a Masters in Pub- lic Administration from Harvard in 1956, and PhD from the University of Nevada in 1969 under George Burke Maxey. Bill retired from the USGS in 1996 after 47 years of service. Bill met and married, Connie, his lifelong partner for 57 years, while he was in California, and together they raised a family of three sons and one daughter. Bill was proud of his children and was pleased that two of his sons received degrees in geology, one of whom (David) works as a hydrogeologist in Virginia. Bill started his career with the USGS in the summer of 1946 in the Alaskan Section of the USGS in Seattle, Washington, as a data recorder and then in Sacramento, California, as a hydrologic field assistant, including work in Yakataga, Alaska. He developed and honed his profes- sional skills, publishing a series of USGS reports on local investigations in California. In 1954, he transferred to the USGS Regional office in Arlington, Virginia, to join a new program in ground water geochemistry for the East- ern Region. (This was 10 years before the organization of the Surface-Water, Ground-Water, and Water-Quality Branches in the current Water Resources Discipline.) His 1 Corresponding author: Scientist Emeritus, U.S. Geological Survey, Reston, VA 20192; (703)-648-5858; fax (703)-648-5484; mjbaedec@usgs.gov 2 Scientist Emeritus, U.S. Geological Survey, Reston, VA 20192. 3 Department of Geological Sciences, Michigan State Univer- sity, East Lansing, MI 48824. Journal compilation ª 2008 National Ground Water Association. No claim to original US government works. doi: 10.1111/j.1745-6584.2008.00534.x 314 Vol. 47, No. 2—GROUND WATER—March–April 2009 (pages 314–318) NGWA.org