Use of Thermal Infrared to Locate and Study Submarine Groundwater Discharge in the Hawaiian Islands

Airborne thermal infrared imaging is a highly effective tool for revealing locations, surface distributions, and mixing characteristics of submarine groundwater discharge in locations where even subtle temperature differences exist between discharging ground waters and receiving estuaries and coastal areas. For regional-scale studies, the spatially and temporally variable nature of submarine groundwater discharge necessitates high-resolution rapid data acquisition techniques of which airborne thermal infrared remote sensing is uniquely qualified. In the Hawaiian Islands, we have used thermal infrared imaging to generate 0.5to 3.2 m resolution sea-surface temperature maps accurate to 0.5°C formost of the western half of Hawaii Island, most all of Oahu, and critical targets on Maui and Molokai. The groundwater released issues from these four large islands as easily identifiable and spatially quantifiable point sourcedflow, and as diffuse flow. The discharge locations and ocean mixing characteristics of these flows are discernable in exquisite detail allowing for rapid assessment of submarine groundwater discharge along large tracts of coastal areas. Accurate interpretation of these coastal sea surface temperature maps requires the caveat of properly understanding each study site. Beach sand, mud flats, and partially submerged outcrops may yield temperature characteristics similar to the local submarine groundwater discharge. Shoreline vegetation can mask the discharge and deeper, offshore seeps may be too rapidly mixed before reaching the surface, where the infrared method detects the discharge. With informed interpretation, however, these caveats are easily overcome making this mapping technique the preferred method for rapid assessment and precise identification of natural and anthropogenically introduced coastal groundwater flow, at scales both large and small, and is highly desirable for many aspects of coastal-zone planning and management, as well as a prerequisite for the best use of subsequent and time-consuming field study efforts.