It is well known by the Park Rangers, the doctors and the local service providers that the water at Confluencia causes many climbers diarrhoea. Some say you have a 30-40% of having stomach problems if you drink the local water.
The water supplied via hose pipes into the camp is taken from the local rivers. The high level of magnesium in the water can have you running to the toilet within hours of drinking it. For this reason some of the holiday companies transport bottled water from Penitentes for their group to drink. However this does not guarantee that you will avoid becoming ill, as the river water is used by the kitchen staff for preparation and cooking of food and for washing dishes, etc.
If you are booking your Aconcagua trip through a holiday company it is worth checking if they transport water to Confluencia. Many will not as it adds to the cost of the trip.
If you are an independent climber it's worth considering carrying extra water on the short 3-4 hour walk from the road to Confluencia, as diarrhoea early in your trip is not only a misery but it will reduce your strength significantly. Most climbers are at Confluencia for 2 days so you will need 4-6 litres of water.
Most climbers find that their diarrhoea clears up when they reach Plaza De Mulas, as the water there is taken from the glacier. It is strange that the orange poop bags are not provided until you reach Plaza De Mulas, as the walk from Confluencia to Plaza Francia and from Confluencia to Plaza De Mulas can be extremely uncomfortable if you are suffering from stomach problems.