After winning a gold and a silver medal at the European Junior Swimming Championships 1998 in Antwerp she made her international debut at the 1999 FINA Short Course World Championships in Hong Kong. Groot immediately won her first international medal, a silver, in the 4×100 m freestyle relay together with Thamar Henneken, Wilma van Hofwegen and Inge de Bruijn. Individually she finished 14th in the 100 m butterfly. A few months later she competed in the 1999 European Aquatics Championships in Istanbul. In the 50 m butterfly she won her first individual medal, at the age of 17, reaching third place behind Anna-Karin Kammerling and Johanna Sjöberg. Chantal also reached the final of the 50 m freestyle ending up 7th. In the 4×100 m freestyle she finished fourth with the team.