TEMPORAL DYNAMICS OF NUTRIENT FLUX ACROSS HYDROLOGIC UNIT BOUNDARIES

This study was conducted in a small suburban watershed in Cedar Falls, Iowa. Hydrologic units like prairie, wetland, and stream in a watershed interact with each other under the influence of urban and agricultural activities. Exchange between hydrologic units is a key factor that determines the level of pollution within a watershed. To understand the dynamics of their interactions soil and water samples were collected from prairie, wetland, stream, urban areas and agricultural lands over a period of four weeks. Soil and water samples were analyzed for nitrate, chloride and sulfate along with the percentages of organic matter. In the first set of soil samples, about 67 % of those collected from stream banks had nitrate concentration below detection level with the highest value of 109 mg/kg. The overall nitrate concentration increased in the second set of samples. Soil samples from active agricultural lands had maximum nitrate concentration of 437 mg/kg, which showed a considerable increase in the second set. Similarly, though nitrate was not detected in the first set of prairie soils, the median value during the second set was recorded as 32 mg/kg. About 92 % of the first set of stream bank sediments had Chloride concentration below detection level. In the second set, 50% of them had chloride detected with the highest value of 97 mg/kg. Initial chloride concentrations in agricultural lands ranged between 8 and 61 mg/kg while most of them (72%) didn’t show any chloride during the next sampling. In prairie samples, although chloride showed temporal increase in concentration sulfate showed an opposite trend in all sites (i.e., down from a range of 12 - 96 mg/kg to below detection). Urban soil showed an increasing trend of organic matter percentage whereas the other units showed a decreasing trend. The median value of dissolved nitrate in the stream decreased gradually from 45 mg/L to 6 mg/L. Groundwater and surface water from other sources didn’t show any significant amount of nitrate. The seasonal trends of the two sets of samples demonstrate that all hydrologic units within the watershed are actively interacting with one another.