The water authority in St. Mary's County will spend $1.6 million to replace six wells that are extracting water with arsenic levels exceeding federal standards, Steven L. King, director of the Metropolitan Commission, said recently.

The swath of arsenic, believed to be caused by a concentration of the element in mollusk shells in the underground sediment, runs roughly from St. Andrews Church Road north to Oakville and west to Leonardtown.

The decision to replace the wells came after the Environmental Protection Agency lowered the acceptable level of arsenic in drinking water from 50 parts per billion to 10 parts per billion.

The levels found in the six wells, which serve more than 33,500 people, were about 12 to 14 parts per billion, King said. They are at Breton Bay, Fenwick Manor, Holland Forrest, Hollywood, Mulberry South and the St. Mary's Industrial Park in Lexington Park.

"This is not an acute problem that's going to make anybody sick in the short term," King said. "It's a concern from a long-term, lifetime exposure."

By the end of the month, the commission will solicit bids to construct the roughly 900-foot-deep wells that will go into the Upper Patapsco aquifer, a water source several hundred feet below where the current wells reach into the Aquia aquifer, King said.

Arsenic is a naturally occurring element sometimes found in soil, water and air. Geologists believe that the arsenic in the Aquia aquifer might have been filtered out of Chesapeake Bay by oysters and then leached from their shells into the sediment, King said.

Consuming lower levels of arsenic, ranging from about 300 to 30,000 parts per billion, may cause stomach irritation, pain or vomiting, and higher levels have been associated with cancers. The new EPA regulations will go into effect in January, but King said St. Mary's will apply for an extension until next summer to meet the regulations.

"We just can't get the wells drilled that quick," he said.

St. Mary's County Commissioner Thomas A. Mattingly Sr. (D-Leonardtown) said the county relies on MetCom to "make sure it's a safe, secure and adequate water supply."

"They know at this point what needs to be done to get aquifers that have less of a problem with arsenic," he said.

Beyond the relatively confined problem of dealing with arsenic, St. Mary's County water officials also are thinking about how to meet a growing population's demands for well water. Before last month, the county had planned to focus on drilling in the Patapsco to take pressure off the more shallow Aquia.

But a preliminary report from a study of Southern Maryland's groundwater supply by the Maryland Geological Survey suggests that the Patapsco will not be sustainable through 2030, King said. The study, which was circulated among local government offices last month, projected that future drawdowns, especially in Charles County, could allow Potomac River water to intrude into the Patapsco and could reduce the flow to streams and wetlands.

"We're disappointed," King said. "We're not going to be able, over the next 30 years, as we thought, to put all of the wells into the Patapsco because of Charles County."

Since 1975, the level of the Magothy aquifer, one of five major aquifers in the area, has declined by 90 feet under Waldorf in Charles County. In parts of St. Mary's County under Lexington Park, the Aquia aquifer has dropped to 200 feet below sea level. In the past half-century, the amount of water pumped from Southern Maryland earth has roughly tripled, to 43.4 million gallons per day.

In St. Mary's, MetCom once proposed that the U.S. Army Corps of Engineers look at the feasibility of building a reservoir so residents wouldn't have to rely solely on groundwater. But "the county's budget didn't have funds for it, and it kind of died," King said. During recent County Commission meetings, Mattingly has raised the issue of a reservoir again.

King said the county must identify potential supplemental sources of water, such as reservoirs, desalination and reusing well-treated wastewater.

"We're obviously not going to wait until 2030," King said.