A new parameter estimation procedure has been invented for the direct field determination of the natural reaeration coefficient by frequency response analysis. By the use of a mechanical instream aerator device, which is operated in a periodic fashion with a variable amplitude, it is possible to determine quantitatively the value of the natural reaeration coefficient from field measurements of dissolved oxygen concentration. The method makes use of a detailed one-dimensional, unsteady-state Streeter-Phelps type model of a polluted river. Also, nonlinear optimal control theory has been applied to the distributed BOD problem in polluted rivers. Included in the control theory analyses are dual water quality standards and economic factors for pipeline distributors and waste treatment plants. (WRSIC abstract)