Development of Remote Sensing as a Tool for Detection, Quantification and Evaluation of Submarine Groundwater Discharge (SGD) to Irish Coastal Waters
Summary: Recent advances in remote sensing provide an affordable tool with which to evaluate contamination of coastal waters from Submarine Groundwater Discharge (SGD).
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Submarine groundwater discharge (SGD) is defined broadly as any and all flow of water from land to the ocean across the seabed. Groundwater in transit from land to sea can become contaminated with a variety of substances, including nutrients, heavy metals and other dissolved substances. SGD is receiving considerable attention in the literature as a major pathway for freshwater and anthropogenic pollutants to coastal waters. SGD inputs could be a significant source of contaminants to Irish coastal waters, with implications for the type and extent of monitoring required to fulfil the terms of the Water Framework Directive. Despite acknowledgement of its potential impact on coastal ecosystem functioning, SGD remains a poorly understood and often overlooked process when implementing coastal monitoring and management programmes.
Recent advances in remote sensing provide an affordable tool with which to evaluate contamination of coastal waters from SGD. Temperature has been used successfully to study groundwater discharge by comparing the relatively constant temperature of groundwater with that of surface waters which fluctuate with season. Using freely available medium-resolution satellite imagery acquired during summer months, clearly discernible anomalous cold-water plumes were revealed to be emanating from nearshore waters at over 35 locations around the coastline of Ireland. The plumes constitute the dominant sea surface temperature feature observed from a selection of temperature maps derived from 60-m resolution Landsat Enhanced Thematic Mapper Plus thermal infrared images. Potential sites of SGD were linked to geological features on land acting as possible sources by combining within a Geographical Information System-mapped temperature anomalies with ancillary onshore spatial data sets describing bedrock geology including aquifer fault lines. Geochemical tracing undertaken to determine the spatial distribution of radon and salinity on surface waters was completed to both verify the remote sensing results as to the presence of SGD (as radon is found in significantly higher concentrations in discharging groundwater relative to the sea) and to provide qualitative and subsequent assessments of sources and flow rates.