A costly plan to divert treated effluent from beneath Ocean Park to another location in Oak Bluffs has hit a snag — and may now become even more costly — because the new location lies within a public drinking water zone.

A consultant for the Oak Bluffs water district recently sent a report to the state Department of Environmental Protection (DEP) outlining concerns about the plan by the town wastewater department to divert effluent from the failed leaching beds at Ocean Park to the site off Pennsylvania avenue referred to as the Leonardo property.

It turns out that the Leonardo property lies in the aquifer for the Farm Neck well, a municipal water source.

The report, written by Lawrence Bombara of the Douglas-based Loring Consulting Group, said the Farm Neck Well aquifer is designated as a sole source aquifer by the Environmental Protection Agency (EPA).

This means the aquifer is a principal source for drinking water in the area. The EPA has the authority to designate such aquifers under the Safe Drinking Water Act of 1974.

Contamination of the aquifer would create a significant hazard to public health.

At a special town meeting in June of 2007 voters agreed to spend $1.1 million to buy the Leonardo property for future wastewater treatment plant expansion or any other purpose approved by town meeting. The following April, voters agreed to rescind a vote to spend $200,000 in Community Preservation Act funds to expand the baseball fields at Veira Park and instead spend the money on a new baseball field at the Leonardo Property.

In his report, Mr. Bombara said the water district is not necessarily opposed to the new leaching facility at the Leonardo property, but he said the wastewater department must take extra steps to protect the aquifer.

“Please understand the district has no objections to change and improvements, and our only concern is having ample, quality potable water for the future residents of Oak Bluffs,” the consultant wrote. “With that in mind, we seek answers, solutions and guaranties that our water supply and the future water supplies for those who have not yet chosen Oak Bluffs to be their home are free from possible contamination.”

Wastewater superintendent Joe Alosso said this week that the report should not pose a problem for plans for the Leonardo property. He said the treatment plant will need to meet a higher groundwater discharge standard, which in turn will require new equipment to be installed.

Filtering will be needed to reduce organic compounds, as well as ultraviolet disinfection equipment to destroy pathogens and viruses, Mr. Alosso said. The upgrades come at a cost, currently pegged at about $750,000.

It is unclear where the money will come from. The wastewater commission has already voted to repair a set of leaching beds beneath Ocean Park at a cost of $400,000 in order to comply with a consent order from the state Department of Environmental Protection. The order was issued last fall after treated effluent was found seeping to the park surface during an inspection last summer.

Repairing the leaching beds at Ocean Park will allow the town to lift a self-imposed moratorium on new sewer hookups; the moratorium affects the Martha’s Vineyard Regional High School, Martha’s Vineyard Community Services and the new YMCA building, which all want to connect to the town sewer system.

After the repairs at Ocean Park, Mr. Alosso said the long-term plan still calls for diverting a large portion of wastewater flow to the open sand pits at the Leonardo property. The leaching beds at Ocean Park are planned for overflow and future expansion of the sewer system, he said.

Mr. Alosso said he has met with water district commissioners to discuss the latest problems.

“They have raised their concerns and I have told them I understand and agree. We have the same concerns as they do . . . we all want to provide residents with clean, safe drinking water,” he said.

He said in the long run improvements to the town wastewater treatment facility will reduce the number of septic systems and reduce nitrogen loading into Sengekontacket Pond and Lagoon Pond.

Oak Bluffs water district superintendent Thomas Degnan could not be reached for comment this week.

But selectman Kerry Scott, who was an early critic of the problems at Ocean Park, continues to question the wisdom of Mr. Alosso’s master plan.

Why did the wastewater department buy the Leonardo property two years ago if the site was within a protected sole source aquifer region, she wondered.

“Did they do their homework, or did they just ignore the fact they were planning to put a leaching field in the middle of a Zone II [sole source aquifer]? This seems like bad planning. It seems this information should have been brought up at town meeting, when voters agreed to spend the $1.1 million to purchase the property,” Ms. Scott said.