The paper reports on a study of five pupils' (13-14 years old) learning about freshwater pollution and related theoretical concepts such as drainage basin and water pollution. Much of the instruction is devoted to fieldwork conducted at a polluted lake and other practical activities designed to promote the pupils' understanding of the central concepts in the study of freshwater pollution. The results show, however, that the pupils do not seem to make the connection between the theoretical concepts and the practical context. Instead they interpret the concepts within separate practical contexts, i.e. interpretative contexts other than the theoretical contexts. The pupils' practical interpretation of the theoretical concepts limited what they were able to learn about the problem of freshwater pollution. The outcome of the study is put in relation to prior research on concept formation and indicates that contextualisation is a main factor in pupils' difficulty to understand theoretical concepts. (Contains 3 figures and 1 note.)