Revisiting nitrate concentrations in the Des Moines River: 1945 and 1976-2001.

Recent compilations of historical and contemporary riverine nitrate (NO3) concentrations indicate that concentrations in many rivers in the north-central USA increased during the second half of the 20th century. The Des Moines River near Des Moines, Iowa, however, was reported to have had similar NO3 concentrations in 1945 and the 1980s, in spite of substantially greater N input to the watershed during the latter period. The objective of this study was to reconsider the comparison of historical and contemporary NO3 concentrations in the Des Moines River near Des Moines in light of the following: (i) possible errors in the historical data used, (ii) variations in methods of sample collection, (iii) variations in location of sampling, and (iv) additional data collected since 1990. We discovered that an earlier study had compared the flow-weighted average concentration in 1945 to arithmetic annual average concentrations in the 1980s. The intertemporal comparison also appeared to be influenced by differences in sample collection methods and locations used at different times. Depending on the model used and the estimated effects of composite sample collection, the 1945 arithmetic average NO3 concentration was between 44 and 57% of the expected mean value at a similar water yield during 1976-2001. The flow-weighted average NO3 concentration for 1945 was between 54 and 73% of the expected mean value at a similar water yield during 1976-2001. The difference between NO3 concentrations in 1945 and the contemporary period are larger than previously reported for the Des Moines River.