Both eastern and western trails up Cerro Chato to the lagoon are currently closed by the Government for "scientific research".
On the east, The GreenLagoon resort will still charge $5 for a walk across their grounds - you could continue up into the National Park trails "at your own risk".
As you walk up, there is a sign obstructing the path stating that the trail is closed. Continue up the path a few hundred metres into the forest, the path is further obstructed with caution tape, barbed wire and another sign.
It's probably best to avoid at present, or call ahead and check whether the trails are open again.