THE IMPACTS OF ANTHROPOGENIC ACTIVITIES AND SEASONALITY ON URBAN STREAM WATER QUALITY

Water quality is critical for the health of aquatic life and human safety, as it impacts both the environment and our ability to use water resources. The strong impacts of climate change and human activities on water quality necessitate a better understanding of how water quality changes over time and space. To this end, four key parameters - Escherichia coli (E. coli), nitrate (NO₃^-), sulfate (SO₄^2-), chloride (Cl^-) - were collected at 12 sites along Fall Creek and Pleasant Run streams in Indianapolis, Indiana USA from 2003 to 2021 on a seasonal basis: March, July, October every year. The Mann-Kendall test was used to determine annual trends for these parameters while Kruskal-Wallis test was used for testing seasonal differences. Redundancy analysis (RDA) forward selection was used to determine main contributors of explanatory drivers. Multivariate regression was used to better explain each key parameters.

Strong seasonal trends were found for the four variables, but no significant annual trends emerged. March had much higher levels of NO₃^-, SO₄^2- and Cl^- than other months. July had the highest E. coli concentrations compared to March and October. Based on RDA results, six other environmental variables—water temperature, pH, stream discharge, total dissolved solid (TDS), dissolved oxygen (DO), active human activities (i.e., snow melts and fertilizer application)—can together explain 43% and 84% of variances for Fall Creek and Pleasant Run, respectively. Multivariate regression showed that the above six variables can explain 22%, 43%, 64%, and 75% of variances for E. coli, NO₃^-, SO₄^2-, Cl^-, respectively. Overall, E. coli is more strongly controlled by higher TDS concentration which could be related to more CSO (combined sewer overflow) events in July, and Cl^- peaking in March indicates salt runoff during snow melt events. NO₃^- and SO₄^2- increase are likely due to fertilizer input from residential lawns near streams. Compared to Pleasant Run, Fall Creek had much lower nutrients levels (NO₃^-, SO₄^2-), which fits in the C-Q (concentration-discharge) dilution relationship. This suggests that Indianapolis water quality changes are influenced by both human activities and seasonal cycles.