BELFAST, Maine — Over the course of a year, cleanup crews with the Maine Department of Environmental Protection respond to one spill every day from a home heating oil tank.

Those spills not only threaten the safety of nearby drinking water sources, they also cost the state — and therefore taxpayers — several million dollars annually to remediate.

But beginning July 1, some homeowners installing new or replacement heating oil tanks in specially designated “wellhead protection zones” will have to take extra steps — and pay extra money — to prevent contamination of drinking water sources.

A law passed by the Legislature last year will now require any new or replacement tanks installed within select areas of the state be either double walled or feature a secondary method for containing spills.

Specifically, the law will apply to replacement or new heating oil tanks installed within 1,000 feet of a community drinking water well or within the designated protective zone around that wellhead. A “community drinking water well” is defined as any water system that serves at least 25 people or that has at least 15 connections.

According to the DEP, there are more than 400 community water systems in the state, ranging from municipal water districts to mobile home parks or nursing homes with their own water systems.

The new law will not affect homeowners living outside a designated wellhead protective zone or those with their own personal wells. The law also would apply only when an affected homeowner must replace an oil tank; it does not mandate removal of functional existing tanks.

David McCaskill, an engineer with the DEP’s Bureau of Remediation and Waste Management, said cleaning up after home heating oil spills is one of his division’s busiest jobs.

Roughly 80 percent of Maine’s homes rely on oil for heat — the highest percentage in the nation.

“We spend $2 million a year cleaning up home heating oil tank spills,” McCaskill said. “There is an average of one leak a day.”

The highest percentage of those spills result from tanks that corroded away.

“It’s the long, slow leaks that are catastrophic,” McCaskill added, “because they saturate the soil underneath the house.”

Homeowners affected by the new law will have several choices in tank designs, all of which are likely to cost more than traditional tanks.

The first, less expensive option — coming in at around $200 more than a standard tank, according to McCaskill — are known as double-bottomed tanks and feature an enclosed reservoir at the bottom of the tank to capture any spillage. The tanks have a float mechanism to allow for visual inspection.

Tanks that are double-walled all the way around can set a homeowner back, on average, an additional $1,000 or more, but these tanks are more protective of the environment. For outdoor tanks, the DEP recommends double-walled, reinforced fiberglass tanks.

Finally, homeowners with outdoor tanks also can invest in a containment system that resembles a small shed for a tank. Such an enclosure captures spills while protecting the tank from the elements.

More information about the new regulation is available on the DEP’s Web site at www.mainedep.com. Information about and maps of public drinking water supplies in Maine are available online at www.maine.gov/dep/gis/datamaps/index.htm#dwp.