"Environmental issues" being the code word for pork. Specifically, most of the money from the sale of helium not going to the National Park Service is going to fund a continuation of the Secure Rural Schools Act. The SRSA itself is essentially a hand-out program for dying, rural counties that ran into budget problems after logging and other natural resource extraction activities were significantly scaled back, which had left those counties with no other significant economic activity to tax for income (and the voters, already hurting, always shoot down income/property tax hikes).
This is one of the bigs reason for why Congress has been raiding the helium reserve, despite the fact that they've cratered prices in the process, as helium is seen as one of the few natural resources and/or assets under exclusive Federal control that can be quickly sold off to raise much needed revenue. Which doesn't really solve the issue - we'll run out of helium eventually - but it at least kicks the can down the way for a while longer.