High Quality Content by WIKIPEDIA articles Rheinfelden District lies in the northwest of the canton of Aargau in Switzerland, in the Fricktal region. Its capital is Rheinfelden. Around 88% of the population live in the conurbation of Basel. There are 14 municipalities, with a population of 43,889 (as of 31 December 2010) living in an area of 112.09 km . The population density is around 355 persons per square kilometre.