Typically, with a truck-and-shovel operation, when they're extracting the bitumen from the ground, it's combined with all the sand, cobble, and gravel. Essentially the tailings are the leftover waste after the bitumen is extracted. So tailings typically are wet as a result of the extraction process, and it's a mixture of sand, larger particles, water, and other contaminants potentially that go into these holding areas.
The goal is that through a settling process, the heavier solids would settle out to the bottom and the water on top would then be available for recycling through the operation. That's typically what the tailings pond would be.