During warm weather, urban surfaces heat up in the sun. Runoff from these surfaces during rain events is much warmer than under natural conditions. Warming is enhanced in stormwater management ponds due to solar heating. Without intervention, stormwater management pond outflows often exceed temperature thresholds set for the protection of aquatic ecosystems. Several strategies and technologies exist to mitigate outflow temperatures. Using monitoring data from a recent pilot project, this article presents geothermal cooling as a new thermal mitigation option. The pilot was implemented in a stormwater management pond located in Brampton, Ontario, Canada, in 2019. The geothermal cooling system used a surface water heat exchanger to absorb excess heat energy from the warm stormwater outflows and, via a circulating hydronic fluid in a closed circuit, it rejected that heat energy to the deep ground using a vertical borehole. A system model was developed that was in good agreement with the monitoring data. The model was used to estimate the size of a full-scale system for the pond. This pilot demonstrated several key benefits of geothermal cooling. It has good performance, and it is also cost-effective, scalable, retrofittable, space-efficient, highly-engineerable, and it has minimal visual impact on the pond
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