The pond, which contains mercury-contaminated water and sediment, is on the northeast slope of Mount Diablo near Morgan Territory Road in unincorporated Contra Costa County.
Dunn Creek is eroding the pond's containment wall, a problem that could quickly become a disaster once winter rains begin, according to the order.
If the wall collapses, the contaminated water and sediment could flow downstream to Marsh Creek and the Marsh Creek Reservoir, a flood control basin near Brentwood that leads to the San Joaquin River and eventually to the San Francisco Bay, according to the EPA.
In October, the EPA sampled water and sediment from the containment pond and confirmed that potentially harmful levels of mercury remained at the site.
Short-term exposure to mercury vapors can cause lung damage, nausea, vomiting, diarrhea, increased blood pressure and heart rate, skin rashes and eye irritation.
Long-term exposure to mercury can cause birth defects as well as brain and kidney damage and nervous system disorders, according to the EPA. Sunoco spokesman Thomas Golembeski said the company only became aware of its alleged liability with regard to the mercury mine when the EPA sent the company a notice of responsibility in October.
Cordero Mining Company, a former subsidiary of Sun Oil Company, which later became Sunoco, was involved in operations at the mine in the 1950s, Golembeski said.
According to the EPA order, the mine, also known as the Mount Diablo Quicksilver Mine, has been operated on and off by different companies since 1863. Mining at the site ceased in the 1970s.
Golembeski said Sunoco was still investigating whether Cordero Mining Company ever mined any ore at the site, which would determine whether the company was responsible for cleaning up the contamination.
Earlier estimates have put cleanup costs at the mine site at several million dollars, with the potential for assuming long-term liability for impacts downstream.
Significant levels of mercury have been found in Marsh Creek Reservoir and signs posted in the area warn people not to swim in or eat fish from the reservoir.
In the meantime, Golembeski said Sunoco was cooperating with the EPA's order and intends to sure up the dam within the EPA's deadline.
EPA spokesman Brian Higuchi said the stability of the dam needs to be addressed immediately in order to prevent a major public health crisis, but that the EPA would be continuing to look for long-term cleanup solutions.