An interface between saltwater and groundwater was evaluated by monitoring the vertical groundwater temperature profiles. The temperature gradients were found to be constant below certain depths in boreholes, however the gradients decreased above the depths and the temperature profiles were convex in a coastal region. The depth to change in the vertical thermal gradient indicates a hydrological boundary, where upward groundwater flow exists above the boundary and no water flow exists below the boundary. Estimated depths to the interface between saltwater and fresh water using groundwater temperature agreed well with the depths calculated by traditional methods using groundwater potentials and with the results of the groundwater salinity. Results suggest that temperature may be an economical and reliable alternative to traditional methods. ¿ 2000 American Geophysical Union