Typically, names ending in the letter a add an n to the end to make an adjective, while other vowels add an an.
... of 13.1 km² (5.1 mi²). 11.9 km² (4.6 mi²) of it is land and 1.2 km² (0.4 mi²) of it is water. The total area is 8.91% water. ...