The purpose of this paper was to show that the hydrochemistry of brackish waters is closely connected with the hydrographic conditions of these waters and that chemical transformations in estuaries may exert an influence on open ocean waters. It has also been shown that many of the processes involved need a much more detailed study. Obviously we know far too little about the transformations of organic matter in estuaries and we can only guess about the changes of many inorganic chemical species. Studies of the estuarine environment have a great urgency, since conditions are rapidly modified by pollution. In many rivers the input of organic matter from sewage and of inorganic materials from industrial processes has greatly increased. Even now already it is very doubtful whether a study of the river Rhine, for example, is still useful from a geochemical point of view. It may of course, be important as a basis for the prevention of further pollution, but for geochemical processes studies in unpolluted rivers will be much more significant. It is also necessary to study many brackish water environments because of their great variability and since one environment provides an insight that the other does not.