Testing of four private wells near the site of an oil spill at a Gorham roundabout June 11 shows that no oil has penetrated into drinking water there, state officials reported Monday.

Cleanup, however, continues and motorists who use the Gorham Bypass should expect delays Tuesday.

The state will continue to monitor the wells “for as long as necessary,” to allay residents’ anxieties about the spill and to make certain that no oil finds its way into residential or town water and storm sewer systems, said Jessamine Logan, director of communications for the Maine Department of Environmental Protection.

An estimated 9,000 gallons of fuel oil – about 6,000 gallons of diesel fuel and 3,000 gallons of kerosene – spilled in the South Street roundabout at the intersection of routes 114 and 112 when the driver of a tanker truck, owned by the J.P. Noonan Transportation Co. of Gorham, veered out of control and landed in a ditch, Logan said.

One of the truck’s four storage tanks ruptured, and the truck’s internal valve system failed, causing all but 600 gallons of the truck’s oil load to spill onto the ground, DEP officials said.

The DEP was informed immediately when the crash occurred, and a rapid response by a number of agencies and police and fire departments succeeded in cleaning up an estimated 8,400 gallons within a day, Logan said. No one was injured.

No citations were issued in connection with the spill, but Logan would not rule out that possibility because the investigation of the incident has not been completed.

The roundabout will be closed to northbound traffic Tuesday morning but is expected to reopen in time for the afternoon commute, as the cleanup approaches its final phases, said Gorham Police Chief Ron Shepard.

Traffic coming from Gorham center will be detoured west to Route 112 north and traffic originating south of the center will be routed to Day Road, then to Brackett Road to Saco Street and onto Route 22.

Cleanup will be suspended over the July Fourth weekend but could resume next week if any work remains unfinished, Gorham police and state officials said Monday.

“The closure (Tuesday) will affect traffic somewhat,” Shepard said. The roundabout has remained open since 10 p.m. the night of the oil spill, and the closure Tuesday should be wrapped up by 2 or 3 p.m., causing minimal disruption to motorists’ travel, he said.

The stretch of road affected Tuesday morning is undergoing test boring, “right into the pavement,” to look for lingering evidence of oil, he said.

Other work will focus on removing oil that drained into soil around the rotary, Logan said.

J.P. Noonan is cooperating with Clean Harbors Environmental of South Portland, which is handling the cleanup operation, Logan said.

After the initial removal and vacuuming of the oil, the cleanup effort turned to preventing the spread of the remaining oil through the soil and into water, Logan said. Underflow dams were built to contain any oil that posed a problem with storm drains, she said.

A small but undetermined amount reached nearby wetlands, spreading into Strout Brook, a tributary of the Stroudwater River, Logan said. It briefly created what Logan described as “a sheen on the water,” but recent heavy rains brought such rapid water flow that the evidence of any oil dissipated almost immediately, she said.

No residue is visible now, and officials have concluded that whatever spillage had reached the water now has evaporated.

State officials have concluded that there has been no damage to wetlands, Logan said.

In addition to test boring and soil work, the remaining cleanup Tuesday will concentrate on making sure that the underflow dams, first installed within a few days of the spill, are still restricting seepage of any oil into the stormwater system, Logan said. They are designed to work with the established catch basins to ensure, too, that no oil left will spread into wetlands or the nearby bodies of water, she said.

The DEP continues to oversee the cleanup, Logan said.

Police still are investigating how and why the crash happened and are seeking the driver of a car that the truck driver and witnesses said may have contributed to the accident.