COMPARING LOSS OF PHOSPHORUS FROM FARM FIELDS WITH TRANSPORTING SUSPENDED SOLIDS 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 Cedar River Watershed, which covers a drainage area of 7,815 mi² extends its headwater from southern Minnesota and flows together with Iowa River in Conesville, Iowa where it joins the Mississippi River. About 81% of all land use in this watershed is predominantly agriculture. The impact of agriculturally derived nutrients on the quality of water in the Midwest USA is a matter of great concern. Phosphorus is one of the nutrients that cause eutrophication in Iowa’s surface water bodies. The significant build-up of phosphorus in Iowa’s stream sediments is linked to developing a zone of hypoxia as far away as the Gulf of Mexico. A comprehensive study of nutrient distribution, transport, and its dynamic relationships with the watershed and the surrounding environment is an urgent need at the moment. The study shows a considerably higher loads of total suspended sediments (TSS) during the growing season as compared to 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 peaks 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, which is attributed to the larger drainage areas impacting these sites. The direct correlation between total phosphorus and TSS in the watershed is observed until the end of July. This reiterates the importance of soil conservation practices to achieve the ultimate goals of nutrient reduction, especially P.