Merck-Saint-Liévin is a commune in the Pas-de-Calais department in the Hauts-de-France region of France. Merck-Saint-Liévin lies about 10 …Merck-Saint-Liévin is a commune in the Pas-de-Calais department in the Hauts-de-France region of France. Merck-Saint-Liévin lies about 10 miles southwest of Saint-Omer, on the D225 roads. The river Aa flows through the village.