A seasonal study of dissolved oxygen and biochemical oxygen demand in Willow Creek as an indication of water quality

Background/Question/Methods Dissolved oxygen (DO) and biochemical oxygen demand (BOD) help characterize the health of water systems and can be beneficial as indicators of pollution. Willow Creek is a channelized, urban creek roughly 500 meters in length, which receives large amounts of storm sewer discharge and surface runoff from its 3.15-square mile watershed and University of Wisconsin-Madison campus location. Inquiries into the health of the creek led to a 2006 seasonal study, taking weekly measurements of benthic macroinvertebrate frequency and richness, dissolved oxygen levels, biochemical oxygen demand, turbidity, water temperature, depth, and various physical observations. Upstream and downstream sections of the creek were used as unique regions for comparison, where upstream represents watershed impact and downstream represents watershed impact as well as University runoff. Results/Conclusions Very low DO (4.3mg/L± 1.19) was measured overall. BOD was consistently higher throughout the season in upstream (3.77mg/L± 1.05) versus downstream (2.84mg/L± 0.79) water, counter to our prediction of higher microbial activity downstream. The benthic macroinvertebrate survey indicated the creek was relatively species poor (33), with a dominance of species tolerant to oxygen deprivation. In 2007 an expanded study included carbonaceous biochemical oxygen demand (CBOD), nitrates and sterilized BOD to investigate the role of nitrifying bacteria that may be drawing down oxygen through the conversion of ammonium to nitrate. Similarities between upstream and downstream CBOD/BOD ratios and nitrates suggest that nitrification does not contribute greatly to differences in BOD in Willow Creek. In sterilized samples, upstream BOD was greater than downstream, suggesting an abiotic pollutant may be accounting for the high BOD upstream. Further investigations examining water chemistry may provide a clear idea as to additional abiotic factors contributing to low oxygen levels and eroding water quality.