JACKSON, Wyo. (AP) — Tests have indicated slightly elevated levels of arsenic in the drinking water supply at Old Faithful in Yellowstone National Park.

But park spokeswoman Morgan Warthin says the latest tests show the arsenic levels have improved and park officials are working on making sure the problem doesn't come up again.

The U.S. Environmental Protection Agency last week said in a notice that samples from the Old Faithful water system had found arsenic levels at 0.011 milligrams per liter. The maximum level of arsenic allowed is 0.010 milligrams per liter.

The noncompliant samples were gathered from the second quarter of 2015 through the first quarter of 2016.

An EPA official says oftentimes arsenic is a natural byproduct of geology.