Investigating the Impact of Land Use Composition on Water Quality of the Ala Wai Watershed

Abstract

Abstract Polluted water is a significant source of waterborne disease, specifically in urban areas where land use composition and runoff can drastically change water quality. This study investigates the relationship between land-use and water quality in the Ala Wai watershed of Honolulu, HI, focusing on the Mānoa stream during a heavy rainfall event in May 2024. Water quality parameters such as Enterococcus concentrations and turbidity were sampled from Mānoa Valley down through the Ala Wai Canal and out into coastal marine waters. Our analysis shows that during heavy rain events, water quality worsens in urban areas of Oʻahu with ANOVA results showing statistically significant results (p = 0.00139). Further, a post hoc Tukey test identified two statistically different groups, sites in dense, urban Honolulu (a), and sites furthest in Mānoa Valley less affected by urbanization (b). Poor water quality was most common around the McCully area along the Ala Wai Canal, taking approximately five days to return to baseline levels, two days after rain stopped. These findings highlight potential impacts of urban land-use on water quality and suggest that further studies with larger sample sizes are needed to support these trends in Hawaiʻi.