A Field Evaluation on the Thermal Mitigation Effect of an Iron Sand–Enhanced Bioinfiltration Basin

Cold-water streams and the associated trout habitat are at risk from warm stormwater runoff as a result of increasing developments and changes in climate. Green stormwater infrastructure (GSI), such as a bioinfiltration basin, is one potential measure to alleviate the impact by minimizing the directly connected impervious cover, reducing runoff volume, and buffering the temperature change in streams. However, the variation in water temperature along the complete path of runoff from the catchment to the GSI and the downstream waterbody has not been well characterized. Also, the variation in GSI thermal mitigation performance due to climatic and rainfall characteristics is uncertain. To address these questions, this case study monitored the water temperature into, within, and out of an iron sand–enhanced, underdrained bioinfiltration basin in the year 2024, serving as a pilot study to determine the value and practicality of implementing a longer-term monitoring program. Results confirmed the positive effect of bioinfiltration basins, demonstrating their ability to mitigate thermal pollution to streams (peak and mean temperature reductions of 2.0°C to 7.4°C and 1.0°C to 5.5°C, respectively). The basin’s effluent temperature was lower than the stream water temperature during storm events in late spring to early summer. Its thermal mitigation performance was greater during events with higher air temperatures and less runoff. This pilot field study underscores the value of bioinfiltration systems for thermal load mitigation and justifies further long-term, multiseason studies to optimize designs for cold-water stream protection.