My front steps after I shoveled
Today is a pretty miserable day to be outside. It started out snowing and quickly moved to freezing rain. We were running out of ice melt at my apartment and I wanted to go and get some more. We have treacherous stairs and a driveway in the best conditions, getting in and out in this weather is very tough. Of course I didn't know what the least environmentally disastrous type of ice treatment was. I went with Calcium Chloride (CaCl2) because it claimed to work at the lowest temperatures.
When I got home I did a little research. I'm satisfied with my purchase. CaCl2 is pound for pound worse for the environment than regular salt (NaCl), but it is far more effective, you don't have to use as much. However, you might have to reapply it. The most important environmental concern is proper use and application of whatever you use. Use only as much as you need to, and center it on the surface you are deicing. As it melts it will spread. If it has snowed and starts to freezing rain, you are better off waiting to shovel so that the snow acts like a buffer from the freezing rain and a hard shell of ice forms on top of the snow instead of on the sidewalk. And always remember to shovel before you apply any ice melter. You don't need to melt snow, just move it.
If you can find it, Calcium Magnesium Acetate (CMA) is the environmentally friendly solutiotion, but it costs thirty times as much as salt in bulk. You can often find it blended with CaCl2 or NaCl which is a good choice. If it has at least 20% CMA, then corrosion can be greatly reduced.
An Environmental Program Manager for the USPS sums up all the options
Road Management Journal on Deicing