One of the many wonderful aspects of the Arctic Circle Trail is that drinking water is plentiful and generally safe to consume without treatment. Most hikers only carry a 1L – 1.5L water bottle and refill it along the way – a relief given how heavy your pack starts out with all the food!
Flowing streams are the best sources of water to draw from, but are more common towards Sisimiut than Kangerlussuaq. For most of the hike, you will be refilling directly from the many lakes that line the route. Be sure to choose larger water bodies rather than smaller ones.
Note also that many of the lakes between Kellyville and Katiffik are brackish. Choose the larger ones to draw water from.