DES-MOINES RIVER NITRATE IN RELATION TO WATERSHED AGRICULTURAL PRACTICES - 1945 VERSUS 1980S

Concern for nitrate (NO3-N) contamination of surface waters in Iowa has brought much speculation with regard to the source of NO3-N. It is frequently assumed that the large percentage increase in fertilizer N use over the last 50 yr is the primary cause of high concentrations of NO3-N in surface waters of central Iowa. We performed an analysis of data relating flow and NO3-N concentration for the Des Moines River to identify the effect of agricultural practices on NO3-N in the river. Annual and weekly stream flow values for the 1980-1990 period at a site just below the city of Des Moines were positively related to the NO3-N concentration in the river water for the 11-yr period (r2 = 0.60, P < 0.01). From 1980 to 1991 the average annual NO3-N concentration ranged from 2.0 mg L-1 in the low flow year of 1989 to 9.1 mg L-1 in 1982. Over the 11 yr, the river averaged 5.6 mg L-1 NO3-N and had an average flow of 733 580 m3 h-1. In 1945, the average flow was 723 710 M3 h-1 and NO3-N concentrations were 5.0 mg L-1. These values were similar to the 11-yr average data obtained 35 to 45 yr later. We infer from the data that the intensive agricultural activities in 1945 and 1980 to 1990 are the major source of the NO3-N to the river rather than solely N fertilizer. Changes in land management, cropping patterns, and land use are therefore required to markedly lower the NO3-N levels of surface water in this basin.