MONITORING SALTWATER INTRUSION INTO COASTAL AQUIFERS USING TIME DOMAIN ELECTROMAGNETIC SOUNDINGS: CENTRAL COASTAL PLAIN OF NORTH CAROLINA, USA

The coastal plain of North Carolina is underlain by an eastward-thickening wedge of Cretaceous and Cenozoic sediments overlying a pre-Cretaceous crystalline basement complex. The sedimentary section ranges in thickness from zero to ~3000 m, and includes six major aquifers which are the principle source of fresh water for communities in eastern North Carolina. Rapid population growth accompanied by increased demand for this limited resource has resulted in declining water levels and saltwater encroachment beneath parts of the coastal plain. The lateral extent of saltwater encroachment will vary from one aquifer to another, as well as within individual aquifers, due to variations in hydrologic properties. Because of the limited number of observation wells, the position of the freshwater-saltwater interface in the various hydrostratigraphic units beneath the coastal plain is not well-constrained. Time domain electromagnetic (TDEM) soundings are used to estimate the position of the interface in those areas where well control is lacking. TDEM methods measure the earth’s response to an induced electromagnetic field, and the resulting signal is converted to apparent resistivity values. Subsequent processing provides an apparent resistivity vs. depth profile that can be compared to borehole resistivity logs. TDEM soundings measure bulk resistivity, and thus are sensitive to the porosity, clay content, and lithology of an aquifer as well as the contained formation fluids. A thorough understanding of the regional hydrogeologic framework is thus necessary to make the most effective use of the TDEM method for monitoring groundwater resources.