TRACING NITRATE-N FROM A DRAINAGE TILE THROUGH SURFACE WATERS OF THE CANNON RIVER WATERSHED

Numerous studies have documented the link between agricultural land use and the eutrophication of natural surface waters. Fertilizer nutrients such as nitrate can give rise to algal blooms and oxygen depletion, making the water inhospitable both for human recreation and for other aquatic life. Our study monitors nitrate on two scales in the Cannon River Watershed, a 1460 sq. mi. area in southeastern Minnesota where agriculture dominates the landscape. First, we investigate the effect of a single agricultural drainage tile on Rice Creek, a tributary of the Cannon River which drains over 80% agricultural land. Second, we trace nitrate through the watershed from Rice Creek to the Cannon and Mississippi Rivers. In the early summer, tile effluent NO3-N concentrations have consistently exceeded 20 mg/L, and tile NO3-N load has been as high as 70 mg/s. This contribution of NO3-N represents as much as 28% of Rice Creek's NO3-N load downstream from the tile. Although concentrations of NO3-N generally decline in the downstream direction, they remain alarmingly high, ranging from over 16 mg/L in the upper reaches of Rice Creek to approximately 10 mg/L in the Cannon River and Mississippi River. The high NO3-N levels are particularly troubling in Rice Creek, which is home to a rare population of native brook trout.