There are very nice sand dunes at Knolls, UT, just off of I-80. However, according to Wikipedia, Knolls is also a site of a hazardous dump, at Grassy Mountain.
Are the sand dunes in Knolls safe to visit?
According to the Knolls Recreation Area Page: Yes as long as you don't enter the bombing range:
If you travel south you will come to a fence, if you cross this you will be entering the Dugway bombing range, not a good place to be. So stay in bounds and watch for the fences, which are clearly marked, and you'll be ok.