Researcher of the Month

Dr. Thomas Johnson

Dr. Thomas Johnson, a CWACES affiliate, is an Associate Professor in the Geology Department here at the University of Illinois. His educational training is in the field of Geology, with degrees from Amherst College in Massachusetts (B.A., 1985), and the University of California at Berkeley (M.S., 1990 and Ph.D., 1995). He then completed a postdoctoral position with Lawrence Berkeley National Lab in Environmental Geochemistry in 1995-1996. Dr. Johnson then moved on to accept an Assistant Professor position at the U of I in January 1997.

His core area of specialization, and that of his research group, lies in the area of isotope hydrology. There has been a long history of utilizing isotope methods in Geology, but their use in hydrology has expanded strongly during the past two decades. Dr. Johnson was introduced to isotopic studies through his Ph.D. advisor (Donald DePaolo), who previously used isotopes mostly in work on igneous rocks. DePaolo and Dr. Johnson realized that some isotopic measurements are useful in understanding groundwater transport and reaction. Subsequently, Johnson’s research group has pioneered analytical methods and applications of heavy stable isotopes, developing methods for selenium and chromium isotope measurements and examining how measured isotope ratios respond to chemical reactions. This knowledge has been used for the application of these measurements in field studies addressing the mobility and bioavailability of these elements. Advanced approaches like this are becoming particularly attractive as isotope measurements are becoming cheaper to obtain. Collaborative work in this area has been undertaken with the US Geological Survey, the Chinese Academy of Science, Univ. of Leicester in the U.K and Univ. of Utrecht in the Netherlands.

Other types of isotope data have been used by the Johnson research group, including strontium isotope ratios, measured in the groundwater of the Snake River Plain Aquifer to map areas of fast groundwater flow. Nitrogen and oxygen isotope ratios in nitrate were used to assess the influence of microbial denitrification on nitrate fluxes in shallow groundwater. These projects were also collaborative efforts, with the former project completed in conjunction with Idaho National Laboratory, while the latter was performed close to home, with the Illinois State Geological Survey.

Beyond the core area of his research program, Dr. Johnson has examined nutrients, and has worked with the Illinois State Geological Survey and Mark David and Greg McIsaac (both NRES) to develop research programs because of the importance of nutrients in the Midwest. Dr. Johnson assisted in the formation of the Program on Nutrient Dynamics (POND), an informal group on campus that focused on nutrient-related research and contributed to several proposals involving CWACES groups.

During the past three years, Dr. Johnson has established an advanced mass spectrometry facility in Geology with his colleague Craig Lundstrom. This lab houses a Nu Plasma Multicollector ICP-mass spectrometer, which is designed for very high precision determination of isotope ratios of many elements. Several of the techniques used are undertaken at only a select few laboratories worldwide.

In addition to his ground-breaking research work, Dr. Johnson is an associate editor for the journal Ground Water, and has published in many journals in his field, and has advised numerous graduate students.

Moving into the future, Dr. Johnson expects to continue developing and applying stable isotope measurements with many elements in the periodic table whose isotopic vartiation is being explored for the first time. Work on mercury isotopes is very new in the geochemical community and shows great promise in revealing transformations that determine the mobility and toxicity. Cadmium, antimony, and vanadium isotope measurements are similarly exciting, but not yet developed. Dr. Johnson expects to contribute to this growing area of research. He also wishes to continue working to apply the techniques developed within the research community to national and international environmental concerns.

Dr. Johnson is strongly tied into CWACES via his work on nutrients; this is an interdisciplinary problem of interest to the State of Illinois. He provides his water chemistry experience to CWACES, and is one of the people available for consultation on this topic within the Center.