It's the lead service lines. You can imagine a piece of pipe. The utility is responsible for one side and the other side is the homeowner's responsibility. If the utility cuts their side—and because it's a continuous pipe, they will physically cut it—you can imagine that there are going to be particles and debris as a result of that cut.
There's also the potential for a dissimilar metal. If the utility chooses to use copper, there's a potential to have copper and lead, which would be almost like a battery cell, and you'd actually get more lead coming off your pipe.
Finally, there are just disturbances. The lead pipe itself is a very pliable pipe. That's why it was used by the Romans and why it was used early on. By just cutting it, you're essentially disturbing the other side. You'll create vibrations, and all kinds of other debris will come off the pipe. We looked at homes at as long as 12 months after the construction, and they still had higher lead than they did when the utility started the project.