A simple one-dimensional mathematic model is presented to discuss the balance and behavior of elements in estuary system.The results are as follows:(1) Besides chemical and biological factors, the removal of non-conservative elements is closely concerned with physical processes.(2) When the behavior and balance of non-conservative elements are studied, the mean residence time of these elements is a useful parameter, which can be obtained by combining the information from field-observation with mathematical model.(3) Based on different situations in estuary, the sensitivity and accuracy of the analytical method used must be considered in the study of the removal of such elements.
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