This is a question that many Departments of Transportation have struggled with as well! There are other chemicals that you can use, but all of them will have at least some impact on the environment. Even sand can cause problems in our stormwater systems. Other chlorides (magnesium chloride and calcium chloride) are popular as alternatives, but still have some of the same negative impacts as common salt (sodium chloride). There are also non-chloride products being created. Here are a couple links talking about alternative chemicals:
You can also look for de-icers marked as "Safer Choice" by the EPA: search for Deicers under Product Type at https://www.epa.gov/saferchoice/products#sector=Home
Minimizing the amount of de-icer is the most important step to make your de-icing more environmentally friendly. You achieve this with two methods:
1) Practice the method of "anti-icing" by spraying down a salt brine or other anti-icing chemical right before the snowstorm. This significantly reduces the amount of de-icer that you need to use after the storm.
2) Apply the minimum amount possible of the de-icer. If you're using salt, wet it with brine before applying it. This MNDOT guide is for street de-icing but has some interesting charts about de-icing with various chemicals beginning at page 13: http://www.mnltap.umn.edu/publications/handbooks/documents/snowice.pdf - As this says, one pound of normal road salt can melt 46 pounds of ice at 30 degrees F, but only 4 pounds of ice at 5 degrees F, so the temperature matters, but you really don't need much salt.
Unfortunately I'm having trouble finding any charts to help with hand-mixed solutions & minimum amounts needed, since cities use special machines to regulate the application of their de-icers. If anybody else can find that, please feel free to edit.
For additional reading, this webpage has a bunch of information and many interesting links: http://pprc.org/index.php/2015/p2-rapid/how-to-reduce-the-environmental-impact-of-deicing/