This report presents a map showing the change in the potentiometric surface of the Lower Patapsco aquifer in the Lower Patapsco Formation of Cretaceous age in Southern Maryland for September 1990 and September 2003. The map, based on water level measurements in 45 wells, shows that the change of the potentiometric surface during the 13- year period ranged from rises of 17 feet at Indian Head and 9 feet near the outcrop area in Glen Burnie, to declines of 40 feet at Arnold, 44 feet at Severndale, 48 feet at Waldorf, 69 feet at LaPlata, and 31 feet at the Morgantown powerplant.