It binds tightly to soil particles, so its leaching potential is low. Some formulations are more toxic to aquatic species than others, but in general, risk to fish is low. It's also nontoxic to honeybees and virtually nontoxic to earthworms. See the back of this chart for tips on growing a vigorous lawn that will crowd out weed species. The lower the LD50, the more toxic the pesticide.
Labels vary based on formulations, so read the instructions carefully.