Integration of aerial infrared thermography and in situ radon-222 to investigate submarine groundwater discharge to Pearl Harbor, Hawaii, USA

The largest freshwater spring complex in the Hawaiian Islands discharges groundwater into Pearl Harbor, Oahu, which is the largest estuary in the islands. This discharge occurs as subaerial and submarine springs, as non-point-source (diffuse) seepage, and as baseflow to streams. We conducted airborne thermal infrared remote sensing surveys of sea surface temperatures and ocean-based Rn-222 surveys of surface and deep waters to evaluate the efficacy of combining these methods to locate and quantify these differing groundwater discharges to the harbor. High spatial-resolution (<= 3.2 m) sea surface temperature data were advantageous for identifying and differentiating point-source and diffuse groundwater discharge. Surveys of surface waters for Rn-222 confirmed point-source and diffuse seepage zones identified from the sea surface temperature maps. The Rn-222 mid- and deep-water surveys were crucial for locating benthic seepage not detectable by the remote sensing method. Groundwater discharge estimates from radon time-series data agreed with radon surface water survey estimates. Fresh groundwater discharge estimates were also of the same order of magnitude as previous hydrologic estimates of spring discharge. We produced linearly and highly correlated relationships between plume areas and groundwater discharge by combining the thermal infrared and radon techniques. Such relationships may provide a way to up-scale local groundwater discharge assessments to a regional area.