SPATIAL AND TEMPORAL DISTRIBUTION OF PHOSPHORUS IN THE CEDAR RIVER WATERSHED OF IOWA, USA

The Cedar River Watershed, which covers a drainage area of 7,815 mi²extends its headwater from southern Minnesota and flows together with Iowa River in Conesville, Iowa before it reaches the Mississippi River (IDNR, 2006). About 81% of all land use in this watershed is predominantly agriculture, because of which the impact of agriculturally derived nutrients on the quality of water in Iowa is a matter of great concern. Phosphorus is one of the nutrients that primarily cause eutrophication, and its significant increase in Iowa’s River is also responsible for developing a zone of hypoxia as far away as the Gulf of Mexico. A comprehensive study of nutrient distribution, transport, and its dynamic relationship between the watershed and surrounding environment is an urgent need at the moment.

Eighteen sampling sites were selected that are located along the main channel Cedar River and six of its major tributaries. Weekly sampling started from April 5, 2014, to be continued till the end of August. Total phosphorus in stream water ranged from 62.4 µgP/L at Clarksville-Shell Rock River on May 26 to 464.1 µgP/L at Little Cedar River near Ionia on April 25 with an average value of 175.1 µgP/L ± 4.3 µgP/L. Most of the water samples (i.e., 91%) tested showed phosphorus concentrations greater than the USEPA recommended level of 100 µgP/L for stream water. The study showed high concentration with an average value of 268.2 µgP/L on April 5, which gradually declined to an average value of 83.1 µgP/L on May 26. On June 15, the average concentration of phosphorus was 136.9 µgP/L, which increased dramatically to an average value of 325.4 µgP/L. The episodic increase of phosphorus is attributed to a storm event that not only led to high discharge but also transported sediments from surrounding fields to the stream. In such situation, high load of phosphorus could be lost from the watershed. The average transport of phosphorus observed on June 15 was 3.22 x 10² kg/day, which then increased significantly to 1.22 x 10⁴ kg/day on June 21. Since Cedar River watershed is considered as one of the largest sources of nutrients from eastern Iowa going into the Mississippi River, such study plays a vital role to make useful recommendations on nutrient reduction practices in Iowa.