DEVELOPING METHODS TO CALCULATE NUTRIENT CONTRIBUTIONS FROM FOUR SUBWATERSHEDS OF THE CEDAR RIVER IN IOWA

Four sub-basins of the Cedar River watershed, the Winnebago, Shell Rock, West Fork Cedar, and Little Cedar were studied for this project. Ten sites were chosen and sampled weekly from April 13 to September 21, 2010 and analyzed for total dissolved solids (TDS), total suspended solids (TSS), conductivity, temperature, dissolved oxygen (DO), pH, nitrate, sulfate, and chloride. Five of the sites were located near USGS gaging stations so discharge data was readily available. A method was developed to calculate discharge at 4 of the sites not located near USGS stations. This method included measuring the change in water level each week and then calculating the change in discharge. The stream was profiled and discharge was calculated for one week and this was used as the baseline for discharge in other weeks. The highest discharge was 21,670 ft3/sec (cfs) observed at site 1 on June 29, while the lowest discharge was 122 cfs observed at site 7 on August 24. The highest and lowest nitrate concentrations observed were 84.03 ppm at site 7 on June 26, and below detection limits at site 9 on August 24, respectively. Average DO ranged from 8.33 ppm at site 3 to 11.17 ppm at site 9. Turbidity and TSS showed a general trend of increasing from upstream to downstream.

According to the Iowa Department of Natural Resources, the nutrient contribution of eastern Iowa rivers to the Gulf of Mexico is estimated at 25%. This is very large when looking at an area to percent contribution ratio. The high load of nutrients carried by the Mississippi is known to be the primary cause of developing the Zone of Hypoxia in the Gulf water. The purpose of this study is to find a mass balance equation to better understand nutrient fluxes from 4 of the subwatersheds of the Cedar River. The work is in progress and once completed, the contribution of each watershed will be known. Understanding where the nutrient hotspots are located both spatially and temporally can aid in implementing best management practices (BMP) at appropriate locations.