CHARACTERIZING THE WATER QUALITY TRENDS AND DRIVERS OF TWO URBAN STREAMS IN INDIANAPOLIS

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 landscape alteration 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 and two-way ANOVA tests were used to determine annual trends and seasonal differences for these parameters. Correlation and RDA (Redundancy analysis) forward selection were used to determine the relationships between variables and main contribution of explanatory drivers. Land cover types and NDVI (Normalized Difference Vegetation Index) were used to assess the impacts of landscape and vegetation dynamics on water qualities spatially.

Strong seasonal trends were found for the four variables. March had higher levels of NO₃^-, SO₄^2- and Cl^- than other months. July had the highest E. coli concentrations compared to March and October. 7-days antecedent snow and precipitation were found to be significantly related to Cl^- and log₁₀(E. coli) and can explain up to 53% and 31% of their variations, respectively. Spatially, more urban land in a 1000m buffer zone of the sampling site had positively impacted the log₁₀(E. coli) and more lawn cover was positively related to NO₃^- concentrations. Conversely, NDVI values were negatively related to log₁₀(E. coli), NO₃^-, SO₄^2- and Cl^-. In conclusion, E. coli is more impacted by higher precipitation and urban land coverage which could be related to more CSO (combined sewer overflow) events in July in the city. Cl^- peaking in March and its relationship with snow indicate salt runoff during snow melting events. NO₃^- and SO₄^2- increases are likely due to fertilizer input from residential lawns near streams. This suggests that Indianapolis water quality changes are influenced by both climate change and land cover types.