This question is a bit ahistorical, like asking what their views were on Darwinian evolution, or how they would think about automobiles. It's not as obviously ahistorical if you don't know about the history of the term "environment" (dates to the early 20th century) or the environmentalism movement (dates to the mid-20th century). In brief, environmentalism (a movement/mindset that argues that the environment should be preserved for its own sake, independent of human use of it) is a relatively recent creation. Even the "environment," with its ecological overtones, is a recent conception. Prior to environmentalism, there was conservationism (the movement/mindset that natural resources and splendor should be preserved for future generations), but even that only dates to the 19th century.

What would 18th-century thinkers have thought about these topics? I can't claim to have studied all of the Founding Fathers' views on this. But I think one can say that, in general, the 18th century view would conceive of your question as what the relationship between man and nature was, the latter being the key category for what we would consider the environment at the time. In general, the 18th century approach to nature was that of exploitation: the natural environment of the United States, for example, was not something to be just admired for its own sake but to be used. Indeed, it was taken as an article of faith that one of the reasons that the new Americans were entitled to it was because it had not been properly used by the native Americans — that they were "wasting" it in the manner of "savages." So this is a very far cry from conservationism, much less environmentalism.

If we dove into the writings of a few of the specific Founding Fathers we might have more to say. Jefferson, for example, wrote voluminously on the natural resources of Virginia ("Notes on the State of Virginia," ca. 1785), but it is primarily about the use of them (e.g., his chapter on rivers is about navigating them and using them for shipping). When he wrote out what animals and plants were in the state, he limited himself to ones that were "Medicinal, 2. Esculent [edible], 3. Ornamental, or 4. Useful for fabrication." In some other writings, Jefferson indicated prudent use of resources (rather than stripping them entirely), which can be warped into a kind of conservationism, but is more likely just the view of someone who relies on fixed land ownership for their income (farming/plantation). Jefferson's ideal citizen, it is worth noting, was a farmer: someone who works directly at the transformation of nature into human benefit. (We might contrast this view to that of say, Hamilton, who saw America's future less in an agrarian citizenship and more as an urban one, but neither of those views get us close to anything like environmentalism.) This does not sum up their views as a whole (Jefferson was Jefferson alone), but it gives you a flavor of it, from one of the few Founding Fathers who actually did write extensively about what we would today call the environment (and who regarded himself as a natural philosopher, what we would call a proto-scientist). I do not know of any writings of the Founding Fathers, for example, that touch upon pollution — an idea that was just becoming an issue in the Industrial Revolution.

Anyway — the ahistorical aspect of asking what their views on environmentalism was is that they had no views on this topic that did not exist at the time. It is like asking their views on space travel or gay marriage — they hadn't really conceived of these things, because they did not yet exist in their worldviews. Hence a lot of discussion of the history of environmentalism is about talking about why it did not really exist prior to the 1960s, and what transformations and events and thinkers were required before this radically different view of the relationship between humanity and the non-human world emerged.