One tenet of libertarianism is self-ownership. If I own myself then it follows I have a right to refuse others the right to put toxic waste into my air or water. So based on that interpretation polluters would be required to obtain consent FROM EVERYBODY, not a majority of everybody, EVERYBODY. If libertarianism was done based on this understanding the environment would be much better off.
Less government doesn't mean no government. As a Libertarian, All Americans needs is the most efficient, effective and economical form of government possible that coincides with our Constitution and our Bill of Rights. Freedoms of life, liberties and their pursuit of happiness. No taxation without representation. Holding public officials accountable. Free markets.
Although libertarianism itself would likely have no direct impact on the environment, most libertarians are more aware of environmental issues and more apt to act or not act in favor of the environment. For that reason, libertarianism would likely be indirectly good for the environment and, over a long period of time, would be good for it.
Libertarianism would be bad for the environment because it would primarily consist of the selling off of public land to private interests, who are likely to over-exploit those resources. It's all well and good to theoretically claim that people treat things they own better, however the record of corporations in this regard is poor when it comes to the environment. The government, which is already fairly corrupt due to the money-based nature of the electoral system, can already barely rein in the forces of unbridled economic and environmental exploitation.