356-10 The Cost of Staying Cool: Assessing the Environmental Impacts and Community Perspectives of Reflective Pavement for Urban Heat Mitigation

Abstract:

Rapid urban development in Austin, Texas, has increased impervious surfaces, altering watershed hydrology while exacerbating the urban heat island effect. Reflective pavement sealants containing titanium dioxide (TiO₂) have been proposed to reduce pavement temperatures, but their environmental fate remains poorly understood. This study evaluates the impacts of a TiO₂-containing reflective pavement sealant on water quality in the Onion Creek watershed of Austin, Texas, where the City of Austin conducted a pilot reflective pavement project in June 2024, while also assessing community perspectives regarding its implementation.

Titanium (Ti) concentrations were measured monthly in the baseflow of the receiving waterway upstream and downstream of the reflective pavement neighborhood to evaluate temporal and spatial trends. During storm events, sheetflow and storm-drain outfall were sampled at reflective (test) and conventional (control) pavement plots to quantify Ti transport. Bioavailable Ti was measured in storm-deposited sediments to assess Ti accumulation.

Following installation, baseflow Ti concentrations increase above historical background levels before declining toward pre-installation concentrations over twelve months. Throughout the study, Ti concentrations remain below known toxicological thresholds (<3 ppb). Ti concentrations are significantly higher immediately downstream of the test plot than upstream but indistinguishable at the far downstream site, indicating localized impacts that diminish with downstream dilution. Storm runoff exhibits substantially higher Ti concentrations, with sheetflow at the test plot reaching 857 ppb and storm-drain outfall reaching 128 ppb, demonstrating that storm events are major periods of elevated Ti mobilization.

Mass-balance calculations estimating stormwater mixing with Onion Creek baseflow indicate that runoff from the existing reflective pavement could produce an in-stream Ti concentration of approximately 482 ppb during storm events, periodically exceeding sublethal effect and acute toxicity concentrations for aquatic organisms (60 ppb; 318 ppb). Simulated expansion of reflective pavement (2-100x) increases estimated concentrations to 622-850 ppb. Storm-deposited sediments from the test plot contain elevated bioavailable Ti, consistent with field observations of cracking, raveling, and sealant-coated gravel, indicating that pavement degradation mobilizes Ti-containing particles during storm events.

Survey responses (n=132) indicate limited public awareness of reflective pavement but interest in additional information before broader implementation. Together, these findings demonstrate that reflective pavement can generate substantial Ti mobilization during storm events and highlight the importance of evaluating watershed-specific dilution capacity, ecological risks, and community perspectives before expanding reflective pavement applications.

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