Recently visited several ports that export coal, cement, potash and grain. By far the most dust escapes from grain. Coal dust on the water is virtually nil, and the other commodities are somewhere in the middle. Adding the barge option just increases the handling and as a result increases the likely hood of a spill or accident. Shipping by rail to the final port is the safest option and it also has the smallest carbon footprint.
As with most things green these days, being clean is secondary to how big of a pain in the ass they can create. (Speaking of that, I have to go flush my low flow toilet for the third time to get it to clear the bowl---actually using more water than my old high flow toilet used, but the damn grandson broke my good old one.)