Cross contamination or failing waterheaters

I inspected a duplex and did the usual drainage flow check by filling the tubs, sinks. The hot water at the left side smelled very putrid and was discolored. The right unit was not as bad but similar issues. Each unit has its own WH. I have smelled sulfer before usually caused by anode, but no discolored water. The left unit hot water smelled a lot worse than rotten eggs. More like sewer.
The client and I drained the heaters and grey gunk initially oozed out followed by a full stream of water.
There has been work going on at nearby units and the water supply may have been contaminated. Both heaters have been inactive for a couple of months. I know a water purity sampling is in order. Has anyone run into this situation? If so, would it be possible to sanitize the heaters? Hot water supply lines?