A SPATIAL AND TEMPORAL COMPARISON OF ION CHEMISTRY WITHIN THE CEDAR RIVER AND WEST OKOBOJI LAKE

This study focused on sampling 4 sites from the Cedar River in Black Hawk County, and 6 sites around West Okoboji Lake. Samples in the Cedar River were taken ~10-14 days apart, and 1 per week for 3 weeks in West Okoboji. In the field and lab, each site’s temperature, pH, dissolved oxygen (DO), biochemical oxygen demand (BOD), conductivity, turbidity, total dissolved solids (TDS), and amount of NO₃^-, Cl^-, and SO₄⁼ were tested with an Ion Chromatograph. The parameters included in this study are NO₃^-, Cl^-, and SO₄⁼. These data were used to observe how hydrologic chemistry changes over time and space, depending on the water’s flow, or lack thereof.

The Cedar River and West Okoboji lake are characterized by unique water chemistry. The average ion values for the Cedar River are 37.25 mg/L NO₃^-, 19.29 mg/L Cl^-, and 21.69 mg/L SO₄⁼. Average West Okoboji values are 6.47 mg/L NO₃^-, 30.59 mg/L Cl^-, and 23.76 mg/L SO₄⁼. When compared spatially and temporally, NO₃^- in the Cedar River consistently increases and undergoes a large drop before rising again. NO₃^- levels in West Okoboji are not significant as there were only two samples that detected presence of the ion. Cl^- levels are much higher in West Okoboji lake than the Cedar River while the Cedar River undergoes greater fluctuation. The Cedar River exhibits constantly increasing SO₄⁼ levels toward the end of the summer while West Okoboji lake is constant. Ion abundances generally vary within up and downstream sampling locations. West Okoboji lake is a homogenous body of water which is reflected in constant ion values; however, there must be a reason for the elevated Cl^- levels that originates from the nature of the water body. West Okoboji lake is enclosed so evaporative enrichment of Cl^- in lake water and sediments is likely. The Cedar River is primarily surrounded by agricultural land which is reflected in the elevated NO₃^- and SO₄⁼ levels. Variations likely arise due to changes in agricultural productivity.