A map was prepared that shows the potentiometric surface of the Aquia aquifer in southern Maryland in September 1982. The map is based on measurements from a network of 78 observation wells. The highest measured level of the potentiometric surface, 34 ft above sea level, was near the outcrop of the aquifer in a topographically high area of Anne Arundel County. The potentiometric surface slopes downward to the extensive cone of depression--much of it more than 40 ft below sea level--has developed around the well fields in the vicinity of Lexington Park. The network of observation wells was developed as part of the cooperative program between the U.S. Geological Survey, the Maryland Geological Survey, and the Maryland Energy Administration.