there are things that could lead one to believe it won't be a departure. One of the townspeople does mention fracking as a cause of the earthquakes in the first episode. There is that mysterious 'scientist' at the lake that Evie talked to and I think another she makes eyes at. Michael suggested that she might be at the lake when no one had any idea where to look the night Evie did not come home.
Then there are things that point to departure. Kevin's suicide attempt while unconscious, Evie's epileptic fit the same night, the car being Locked and running, Erika's wishing her daughter away more or less, the naked running scene not looking like an escape, and the prehistoric scene with the earthquake indicates quakes have gone on in this area for a long time (not fracking).
Lots of animal symbology going on that I don't fully understand. Tons of depth to the show.
I think the girls return, most people will believe it was not a departure, but a few (Kevin, Michael) will. The viewer will be left to decide.