Inferences about point source loadings from upstream/downstream river monitoring data

Areas of Concern such as the Niagara and Detroit Rivers contain a myriad of point and non-point sources. These are difficult and expensive to monitor and often present unique analytical problems and so are subject to analytical error. The Niagara River Toxics Committee (NRTC) and the Upper Great Lakes Connecting Channels Study Modeling Committee designed programs to indirectly monitor these sources by upstream/downstream sampling at the head and mouth of the rivers. These two studies are compared and the data analysis problems that were encountered are discussed. Upstream/downstream samples were paired to obtain differential loadings wherever possible, but when some results were reported as below the detection limit, a maximum likelihood estimate was used. The resulting differential loading, adjusted for non-point sources, is the total point source load to the river minus any losses due to volatilization, settling or degradation.