Fiber optic (FO) distributed temperature sensing (DTS) was used for long-term monitoring of a 280-m by 360-m geothermal exchange borefleld with 2596 boreholes to quantify the role of groundwater flow and thermophysical properties on subsurface heat transfer and storage. Most district-scale borefields are subject to a sustained imbalance in heating or cooling loads and experience yearly subsurface temperature increases that may, without corrective action, lead to diminished borefleld performance and unsustainable operation. This research provides a template to advance techniques to optimize district-scale borefleld performance using calibrated, long-term FO-DTS monitoring of field temperatures.