The Environmental Protection Agency has reached settlement agreements with four Southcentral Alaska gas stations that violated federal law for underground tanks.

The agency says facilities that store petroleum must maintain leak detector systems on underground piping and conduct line tests to ensure that petroleum doesn't leak into groundwater.

Two Holiday stations in East Anchorage, at 5501 DeBarr Road and 285 Muldoon Road, were penalized $10,650 and $6,390 for failing to provide an adequate leak detector system by delaying required annual tests.

Tesoro stations in Girdwood and on Glacier View Avenue in Palmer were penalized $6,390 for delaying line tests.

Three Oregon and two Washington gas stations also were penalized.

The EPA says the failure to properly monitor tanks and underground piping contributed to more than 6,800 new U.S. petroleum spills in 2015.