30 Year historical trends in Coralville Reservoir water quality

Iowa’s water resources are perhaps its single most valuable natural asset, yet the number of water bodies and river miles impaired by nutrients and siltation has increased in the last years. With agriculture as the leading industry in Iowa, yet a major contributor to water resources degradation, it is difficult to protect our water resources without negative economic and social impacts to farmers. In watersheds that are primarily agricultural, it has been estimated that over 80% of the total nitrogen added in to the watershed comes from commercial fertilizers use related to agriculture. Many times, the amounts of in-stream nutrients can be correlated to the land-use of the watershed. In this study a 30 year historical water quality record for the Coralville reservoir has been used in order to determine the water quality trends of the reservoir and the river that flows into it. Preliminary analysis of the data set shows that the amount of nitrate-nitrogen in the reservoir waters has a positive trend over the 30 year time span. The expansion of suburban developments has increased the impervious land cover and has affected not only the runoff patterns within the watershed, but has also increased the amount of anthropogenic nitrogen contribution due to the changes in land use, installation of industrial enterprises and increases in population waste load. The amount of total suspended solids in the water appears to decrease over 30 years and there appears to be greater variability in the amount of sediment in the water prior to 1980. Another nutrient of concern in the Coralville watershed is phosphorus. The waters of the reservoir and the incoming river usually contain less than 1 mg/L of orthophosphate (as phosphorus) but much of the time, 4-5 times more than the recommended 100 ppb. Examination of the land use changes, the urbanization patterns and evaluating the efficiency of BMP’s will help elucidate the factors contributing to the long-term data trends.