Submarine groundwater discharge detection through remote sensing: An application of Landsat 7 and 8 in Hawaiʻi and Ireland

Abstract

Submarine groundwater discharge (SGD) represents an important component of the global water cycle. Improved assessment of SGD occurrences provides information to constrain current estimates of total freshwater discharges, which are crucial for evaluation of changes in the global water cycle.This study leverages SGD linkages to patterns in sea surface temperature (SST) and color. Typically, groundwater temperatures exhibit less variability than SST and tend to have temperature distinct from sea water. Consequently, thermally anomalous plumes often occur in coastal areas with SGD. Nutrient-enriched groundwater flows have been linked to changes in ocean color as a consequence of algae growth and of the presence of colored dissolved organic matter. Under such conditions, SGD presence may also manifest as plumes of different colors. Changes in SST and color can be captured through satellite imagery for the thermal infrared and visible bands, which are used here to detect potential SGD (PSGD) occurrences. A method based on clustering and spectroscopy techniques is applied to automatically detect PSGD plumes. SST data are clustered to map SST anomalies. Derivative analysis and angular distance are applied to identify a color signature linked to SGD. In a novel approach, SST and color are combined to identify PSGD through remote sensing.The methods presented in this study were applied to Landsat 7 and 8 imagery captured over southern Ireland and the west coast of the Island of Hawaiʻi, where past field surveys of radon isotopes activities and salinity have documented SGD occurrences. These selected sites are used as validation points. Derivative analysis and angular distance resulted in PSGD plumes in the vicinity of sampled locations, mainly locations with samples that are strong indicators of SGD. In general, the PSGD plumes derived from each method are compared as independent, parallel lines of evidence of PSGD occurrences. Results also show that PSGD pixels from derivative analysis and angular distance alone, without considering SST, can be obfuscated by confounding factors, such as low tides and unrelated chlorophyll enrichment, and that combining SST and color data reduces those effects. These results provide further evidence for PSGD occurrences along the south coast of Ireland and the west coast of the Island of Hawaiʻi, and showcase that satellite data can be used to better identify the location of PSGD occurrences over large areas.