Evaluation of Geophysical and Thermal Methods for Detecting Submarine Groundwater Discharge (SGD) in the Suwannee River Estuary, Florida

Submarine groundwater discharge (SGD) represents a significant portion of the total discharge from coastal aquifers through diffuse seepage and point source springs, but can be difficult to locate. SGD is important as it can be a source of nutrients to estuaries and other coastal ecosystems. In an effort to evaluate geophysical and thermal methods for detecting SGD on the Florida Gulf coast, a suite of water-borne surveys were run in conjunction with aerial thermal imagery over the lower Suwannee River and estuary in March and September 2005. Thermal imagery exploits temperature differences between discharging groundwater and surface water. Thermal images were collected in March (dry season), at the end of winter, and at night to maximize the differences between warm groundwater and colder surface water. Generally pore waters in zones of concentrated SGD should be fresher, and hence more resistive than background values. Marine streaming resistivity data can detect pore water resistivity variations and were collected alongside continuous 222Rn and CH4 sampling from surface waters. Naturally occurring tracers, 222Rn and CH4, are used as the standard against which resistivity and thermal images are compared. Based on the expected properties of discharging groundwater, we hypothesize that in zones of elected tracer concentrations, increased thermal image temperatures and increased terrain resistivities will be observed. The data set as a whole supports this hypothesis. However, regional-scale correlations are clearly and significantly influenced by factors other than SGD including thermal-image noise, the presence of the fresh/salt water interface, and a large regional tracer gradient generated by a first-order spring. At local scales (tens to hundreds of meters) there are no significant correlations between thermal image temperature and tracer concentrations, due at least in part to flight-line edge effects that dominate the thermal imagery. After correcting for regional trends,significant correlations between tracer concentration and log resistivity exist only in a subset of the data that lies offshore. Because neither thermal imagery nor streaming resistivity data consistently support the hypotheses, this study suggests that neither method by itself is reliable for detecting SGD in this are