Multi-scale thermal mapping of submarine groundwater discharge in coastal ecosystems of volcanic islands

Abstract

Submarine groundwater discharge (SGD), or the flow of groundwater from land to sea, transports different constituents to the coast, such as nutrients, chemicals, and dissolved metals. In volcanic regimes, groundwater might have especially high concentrations of these constituents when, it is hydrothermal in origin. The Verde Island Passage, which lies within the Coral Triangle in the western Pacific, has been designated a marine protected area, as it is home to over thousands of different species of fish and coral reefs. Interactions of this constituent-rich groundwater with coastal waters through fractured volcanic rock can either pose a threat to or promote coastal ecosystem productivity and health. Because of this setting, it is known that SGD is a prevalent source of constituents, but how much and specifically where it is occurring is unknown. Thermal Infrared (TIR) remote sensing has gained popularity as a method of detecting SGD as it helps to resolve spatial variability issues that are inevitably accompanied by this phenomenon.

Here, we investigate the use of TIR satellite and airborne remote sensing, coupled with ground-based field measurements including thermistor rods and fiber-optic distributed temperature sensing to identify and quantify submarine groundwater discharge in a coastal area of the Verde Island Passage. This study utilizes USGS Landsat 8 TIR sensor to regionally denote SGD plumes via temperature measurements of ocean water, as well as local airborne TIR remote sensing which provides a finer spatial resolution than its satellite counterpart. This study also employs a 1 km-long fiber optic cable to continuously measure temperature along the coast for the very first time in this region, and thermistor rods buried in the seabed under 20-30 feet of water to measure vertical temperature flux. Results from these approaches reveal multiple SGD signals in both the intertidal and subtidal zones, and tens of meters offshore, with temperatures reaching up to 80°C and estimated fluxes reaching as high as 1.5 m/d. From these approaches, we conclude that airborne TIR remote sensing and vertical thermal profiling are the most feasible thermal sensing methods employed in the study area, as they capture regional to local and point scaled SGD, which yield plume location, morphology, and flux. This study serves as a foundation for mapped SGD locations which can be used in conjunction with flux estimations in future temperature and flow modeling of SGD in the region. This study also has implications on potential outflow points of natural hydrothermal constituents and/or polluted groundwater, which can be of benefit to ecosystem and resource management.