Out of the Missouri American Water Co.’s 450,000 customers, an estimated 28,000 — or approximately 6% — own lead service lines, which pipe water into homes from water mains.

The percentage used to be slightly higher, but since 2017, the investor-owned water utility — which serves much of St. Louis County along with other pockets of the state — has swapped in about 2,000 replacements. It’s part of an effort to gradually remove lead service lines — and the health risk posed by lead exposure, which is particularly dangerous to children — over the next decade and beyond.

That pace, though, puts things a bit behind its originally envisioned schedule.

“The plan was to replace all of them with about a 10-year window, which would be about 3,000 a year, instead of the just over 2,000 that we’re at now,” said Bruce Aiton, the company’s director of engineering, in an update on the program provided earlier this week to state utility regulators at the Missouri Public Service Commission.

The rate of progress is influenced by the fact that finding and replacing lead service lines is not a factor used by the company to prioritize its infrastructure projects, Missouri American officials said. Instead, lead lines are simply replaced as the company comes across them while updating adjacent water mains.