René P. Schwarzenbach: four decades of stimulating cooperation and friendship.

One day in the year 1974 at Eawag, nearly four decades ago, an energetic young postdoc stormed into the room of a slightly older colleague, asking, “are you Dieter Imboden?” and, before I had a chance to breathe, sat down at my table. “I am the new organic chemist at Eawag”, he proudly said, “ we should work together, you should apply your lake models to organic chemicals, that is much more interesting than modeling lake eutrophication. And by the way, my new colleagues at Eawag told me that you are the right person to work with in order to quickly advance in my academic career!” I do not recall what I thought about my new self-confident colleague, but I certainly remembered how at the university the theoretical physicist had always tried to keep some distance from chemistry, especially organic chemistry, that for him consisted of endless lists of complicated names difficult to remember and to pronounce properly. But destiny, fate or just chance had a different plan. Sitting in my Eawag office and listening to enthusiastic Rene ́ Schwarzenbach, it neither did occur to me that I just had met that person with whom, over the years to come, I would develop the longest, closest, and most fruitful scientific collaboration of my career nor did I anticipate the beginning of a dear friendship. In fact, during these many years our collaboration extended well beyond proper research into various other areas of academia. But that comes later. At the moment, we are still back in the seventies when mathematical modeling of environmental systems was more limited by the available computer capacities than by ideas. We tend to forget how radically things have changed. When I began to develop models at Eawag, the only “computing machine” at the institute was a mechanical monster, suitable perhaps to calculate the salaries of the collaborators at the end of the month, but not to solve systems of differential equations. Although the computational limits changed quickly in the eighties, they nonetheless strongly shaped research, particularly the emerging collaboration between Rene ́ and me. In our common search for the narrow path between the adequate and the manageable complexity of models, it became a necessity to mutually engage in the area of the partner. I quickly realized that Rene ́ by no means was the mathematical novice that fitted my preconception of a chemist. I learnt about his doctoral dissertation in which he had pioneered the use of data banks for the classification of organic compounds. In turn, my travel from physics into chemistry has certainly been more difficult and slow, but finally I got used to the strange names chemists like to give their compounds, could even remember a few, and began to understand why chemists invent tongue twisters like 2,2′,4,4′,5 pentachlorobiphenyl. Our collaboration underwent different phases. For a long time we did not really have common projects, but our interaction shaped and changed our way to look at problems. For the understanding of environmental systems dualism became the normal approach: Transformation (chemistry) versus transport (physics), substance versus system properties, spatial versus temporal structure, etc. Looking back, I was amazed to realize that, in fact, our first common peer-reviewed publication only appeared in 1984 (Schwarzenbach, R. P.; Imboden, D. M. Modeling concepts for hydrophobic organic pollutants in lakes. Ecol. Model. 1984, 22, 171−212), followed by a book chapter (Schwarzenbach, R. P.; Imboden, D. M. Spatial and temporal distribution of chemical substances in lakes: Modeling concepts. In Chemical Processes in Lakes; W. Stumm, Ed.; Wiley-Interscience: New York, 1985; pp 1−30). For a mathematical modeler of today who is used to shear unlimited computational capacities, the concepts presented in these publications may look primitive. But times were different. The dynamic equations had to be kept, if ever possible, linear (or one had to linearize them) and their number small. Often the results were presented in terms of analytical solutions. But simplicity was and is not always a disadvantage. To the contrary, like a cartoonist, one has to sharpen the eye for the essential. By analyzing the analytical solutions one is able to

Field of study: Environmental science