MAPPING LOCATIONS OF GROUND-WATER DISCHARGE IN REHOBOTH AND INDIAN RIVER BAYS, DELAWARE USING THERMAL IMAGERY

Ground-Water discharge locations were mapped in Rehoboth and Indian River bays in coastal Delaware using ground-, aerial- and satellite-based thermal-infrared imagery. Warmer ground water (~15 degrees C) discharging into cooler bay water (~5 degrees C) was identified in images from aerial and ground surveys conducted in March 1999 and in a LANDSAT 7 image from January 2000. The aerial survey provides sharp images of the coastline of the bays and tidal creeks (pixel size of about one meter). The signal for diffuse seepage is a positive thermal anomaly oriented parallel to the coastline with a broad transition zone between the anomaly and ambient-temperature water. Focused seepage along the coastline is identified as an anomaly with a sharper transition to the ambient water temperature. Many parts of the bays' margins do not have higher-temperature water, but some of these cannot be ruled out as significant discharge sites because strong tidal currents may mask the signal. One tidal creek contains a zone of warmer water across its entire width with cooler water upstream and downstream, indicating significant ground-water discharge to this portion of the creek. Ground-based measurements, taken at a height of ~20 meters (from a bucket truck), provide both vertical and oblique views and provide useful "eyeball" calibration for determining the water/land interface in the thermal images. The LANDSAT 7 thermal band gives regional coverage over the open water but has a pixel size (60 meters) that is much larger than the aerial imagery. Analysis of the LANDSAT image indicates some focused discharge in the open waters of Rehoboth Bay. Field measurements of water temperature and salinity and seepage are being used to verify the interpretations of the images. This work is part of an EPA-funded CISNet (Coastal Intensive Sites Network) project entitled "CISNet: Nutrient Inputs as a Stressor and Net Nutrient Flux as an Indicator of Stress Response in Delaware's Inland Bays Ecosystem."