ROLE OF SEDIMENTS IN TRANSPORTING AGRICULTURALLY DERIVED NUTRIENTS IN STREAM WATER

Eighteen (18) sites within the Cedar River watershed of Iowa have been sampled for temporal and spatial distribution of sediments and agricultural nutrients. The study is funded by the Iowa Nutrient Research Center. Data from this study will become a part of Iowa State’s long term nutrient reduction initiative. The Cedar River has a drainage area of 7,815 mi²extending from southern Minnesota all the way to Conesville, Iowa where it joins the Mississippi River. About 81% of all land use in the watershed is agriculture. Iowa farm fields are known sources of nutrients that are responsible for developing hypoxia in the Gulf of Mexico. The sites were sampled once a week for stream water and sediments from the beginning of April to the end of October, 2014.

Loads of total suspended sediments (TSS) during the growing season are considerably higher than the baseline. Most sites show initial high loads of TSS in April and early May and then it appears to peak again in late June and July. The first rush of TSS can be attributed to the snow melt episodes causing soil loss from the agricultural fields that are not adequately covered by crops. The second peaks are attributed to the rain events during mid-summer causing excess moisture in the fields to mobilize these nutrients. The area had 5.02 inches of rain that fell from June 16^th through the 19^th. The data collected on June 21^st showed a jump in TSS concentration from 30 mg/L to 71 mg/L in the 9 sites toward the northern part of the study area. In the other 9 sites toward the southern part of the area the average jumped from 23 mg/L to 122 mg/L. The TSS loads in response to intense rain appear to be higher in the downstream areas, probably because of the larger drainage areas that impact these sites. Temporal and spatial loads of total phosphorus (TP) directly correlate with the transport of TSS in the watershed until the end of July. This reiterates the importance of soil conservation practices to achieve the ultimate goals of nutrient reduction, especially P. Because nitrogen is highly soluble in water, all sites showed very uniform relative concentrations over the early, middle and late seasons. The mid-season showed considerably higher concentrations compared to the other times. Most sites that are characterized by agricultural lands recorded over 100 tons/day of nitrate-N loads during peak flow.