Invertebrates like clams and aquatic insects are a very common way to test water quality, even outside the realm of drinking water. One of my favorite biological surveys is benthic invertebrate monitoring, which is when you go to a river, dredge up some Wet Bugs, and interpret the health of the stream based on the species you find. A wider variety with pollution-sensitive species means a healthy water body, and fewer/more pollution-tolerant species means a polluted water body.