Lest they cut down on overall polution. I'm thinking that a Geothermal plant should have little to no negative impact.
I remember reading that they have to be a distance from the park so as not to sap the strength of the volcanic activity in the park. While this seems silly for Hawaii (maybe), it was a bigger concern for potential Yellowstone hydrothermal; both effects on the features and the effect of having an industrial site near the park.