Invasive: Non native, but able to survive and reproduce in the wild, competing with the native species and a vector for diseases. If you can't keep them, take them to a shelter or rescue, do NOT just dump them at the local park.
And...really...it's hard to say this without it sounding very harsh, but please do NOT buy any animal without researching it's needs first. Turtles live a loooong time, if kept healthy, and have very specific needs. If you can't meet those needs, it's not a good choice for a pet. The little ones are adorable, but that's not a good reason to doom an animal to a short life.