The cost to reduce the nitrogen St. Mary’s County residents discharge into the Chesapeake Bay could reach $143.6 million, a work group told the county commissioners Tuesday.

That was the group’s estimated price for an upgrade of all 26,071 septic tanks in the county.

But upgrading all of those septic tanks wouldn’t be enough to meet federal standards set to improve the health of the Chesapeake Bay. Some neighborhoods on septic tanks would have to bear the cost of connecting to public sewer, the work group said.

The group said 21 percent of the county’s nitrogen load into tidal waters comes from septic tanks. Statewide, the total is 6 percent.

But some of the commissioners questioned that data, which the state is using as a benchmark.

Commissioner Cindy Jones (R) said she has seen data that says the state’s nitrogen load from septics is only 1 percent, based on a presentation by George Frigon, a water and wastewater engineer and partner with NewFields China, at a forum called “PlanMaryland: At the Crossroads.”

“A lot of other elected officials are also aware the nitrogen load is significantly less than 6 percent,” Jones said.

“There’s obviously some disparity in the numbers presented,” said Commissioner Todd Morgan (R).

“We need an objective look at the parameters,” Jones said.

Daryl Calvano, director of the environmental health division of the St. Mary’s County Health Department said it is “pretty alarming the math [Frigon’s] come up with.”

He said Wednesday, “I think the only reason it’s coming into question is the number has fluctuated” between 4 and 6 percent. “We’re kind of at the mercy of what the Maryland Department of the Environment and the Maryland Department of Planning provides us,” he said.

It doesn’t matter where the nitrogen comes from, said Sue Veith, environmental planner with the St. Mary’s County Department of Land Use and Growth Management. What is known is the damage nitrogen is doing to the Chesapeake Bay and that it has to be reduced.

Asked Wednesday if there is any reason to question the state’s data on nitrogen loads, Veith said, “I do not believe so.” The damage that nutrient loads do “is well established in science.”

Nitrogen and phosphorus feed algae blooms. When the algae dies, it is consumed by bacteria, which rob the water of dissolved oxygen, killing aquatic life.

The federal Watershed Implementation Plan calls for an annual cap in St. Mary’s of 152,641 pounds of nitrogen from septic tanks discharged by 2025, which means the existing load today needs to be reduced by 85,882 pounds, not including any new growth.

The Maryland Department of the Environment says one household on a septic system produces 30 pounds of nitrogen a year.

Commissioner Dan Morris (R) questioned that number. He said some households have very few people living in them, and others have large families. “How do we know [nitrogen entering the bay] came from St. Mary’s County? How do these numbers come about?”

The data is an average, said Jacquelyn Meiser, director of the St. Mary’s County Metropolitan Commission, which runs public sewer and water systems.

Of the 37,064 households in St. Mary’s, 26,071 are on septic tanks — 70 percent.

The regulations of the Watershed Implementation Plan “are going to have a huge impact on this county,” Jones said. “This is an issue that really comes into sharp definition for the people of St. Mary’s County.”

“In my career, this is the largest environmental change being contemplated,” Calvano said.

Septic systems remove pathogens from the waste, Veith said, but do little to remove nitrogen. The ground naturally removes some nitrogen from the discharge.

In what is labeled the Critical Area, within 1,000 feet of tidal waters and where state law has for decades limited development, there is physically little room for the ground to treat the septic discharge before it runs out to tidal waters.

Maryland charges $30 a year to both those on septic systems and public sewer to fund upgrades to wastewater treatment plants and to replace failing septic systems.

There is now a proposal to hike that to $90 a year by 2015 to adequately fund the improvements.

One alternative to reduce the nitrogen load is to bring waterfront neighborhoods onto public sewer, like Town Creek, Esperanza Farms, Scotch Neck, Blackistone Farm and the Callaway area. However, the cost per household to connect to public sewer would range from $5,897 to $11,494.

The largest sewage treatment plant in St. Mary’s County is Marlay-Taylor in Lexington Park. Plans are already under way to upgrade its treatment to reduce the nitrogen load. The upgrade is budgeted at $35.5 million.

Even without the Watershed Implementation Plan, the sewer plant would need to expand by 2020, Meiser said. By connecting some existing neighborhoods, “it would pretty much put a moratorium on new development,” she said. “Marlay-Taylor would have no capacity.”

St. Mary’s County needs to submit its local plan for the Watershed Implementation Plan by June. All states within the Chesapeake Bay watershed are involved in the plan.

jbabcock@somdnews.com