PORTSMOUTH — Peak demand for water to quench parched lawns and gardens in the continuing heat wave has strained the pumping capacity of the Portsmouth Water and Fire District so much that the district imposed mandatory water restrictions yesterday.

So far, Portsmouth is the only community on Aquidneck Island where water conservation measures have been put into effect.

In neighboring Massachusetts, Swansea instituted its annual, mandatory water restrictions on July 1 and Seekonk has voluntary limits.

On Aquidneck Island, the problem is not the amount of available water but the ability of the Portsmouth Water and Fire District to pump enough water to its 6,400 customers and still maintain an adequate amount of water under pressure in case of a fire emergency, according to general manager William J. McGlinn.

The Portsmouth Water and Fire District buys all its water from the Newport water division, which serves Newport, and Middletown.

McGlinn said demand for water in recent days is reaching the district’s top pumping capacity of 2.5 million gallons a day. At that rate, the district can’t keep enough water in its tanks to ensure adequate pressure in case of fire.

The water restrictions began on a voluntary basis last Friday but became mandatory yesterday, because demand continued to peak in hot, dry conditions that are likely to persist without relief for at least 10 days, McGlinn said.

He said customers with odd-numbered street addresses may water only on odd-numbered days of the month, with watering on properties with even-numbered street addresses permitted on the even-numbered days.

The water restrictions will continue indefinitely, he said.

The 10-day weather forecast shows some thunderstorms, which tend to skip over Aquidneck Island, McGlinn said. When these summer storms do strike, they bring very little rain, he said.

He said the odd-even water restrictions apply to all residential, commercial and industrial customers, but not to farmers, who depend on irrigation for their livelihood.

The restrictions also apply to areas in the southwestern section of Portsmouth, including the Melville Campgrounds and the Melville Navy housing, which are served by the Navy water system with water supplied by the Portsmouth Water and Fire District, McGlinn said.

Those who violate the ban will get a warning. The second offense will bring a $100 fine. The third violation will result in a 24-hour suspension of water service with a $120 fee for reconnection, according to a statement from the Water District.

Swansea instituted its annual, mandatory water restrictions on July 1 to conserve ground water supplies.

Residents who live north of Route 6 are restricted to watering their lawns on odd-numbered days only, between the hours of 6 a.m. to 9 a.m. and 6 p.m. to 9 p.m. Those living south of Route 6 can water their lawns only on even-numbered days. There are no restrictions for watering plants and gardens or washing cars.

As of July 14, the Swansea Water District instituted penalties for violations of the water bans. The first violation results in a warning. A second violation comes with a $50 fine, and all subsequent violations carry $100 fines.

Swansea hasn’t declared a water emergency yet, but it’s getting close, said Robert A. Marquis, the water district’s superintendent. Ground water is receding at about a foot a week, he said. Three of the town’s 10 wells are in “draw down” mode, meaning water levels are so low that the pumps have automatically shut off.

“It can be managed with enforcement as long as we keep demand down,” Marquis said of the community’s water concerns. The town usually lifts its water restrictions by September, depending on weather conditions.

In Seekonk, drinking water comes from ground water supplies.

Those who want to conserve should do their outdoor watering on even- or odd-numbered days, according to their house number, between the hours of 5 a.m. to 9 a.m. or 5 p.m. to 9 p.m.

The Seekonk Water District’s superintendent, Robert Bernardo, said the district is closely monitoring water levels to determine if mandatory restrictions should be put in place.

“We’re keeping an eye on the well levels and, particularly, the rain we’re not getting,” Bernardo said.

Newport has no immediate plans for odd-even restrictions or other conservation measures, according to Julia Forgue, the city’s director of utilities. That goes for Middletown, which is part of the Newport water system.

Newport’s seven reservoirs on Aquidneck Island, however, are now dropping down to a level where the city, as it does most summers, begins preparing to pump water from Watson Reservoir in Little Compton. The reservoir isn’t tapped throughout the year because of the costs, electricity mostly, of pumping the water to the island through a pipeline under the Sakonnet River, Forgue said.

“We only do it when we have to,” Forgue said. “Obviously we haven’t had rain…and the demand is up.”

It’s not only the quantity of water in the seven island reservoir that affects the decision to tap Watson, but the quality of the water. One or more reservoirs could be experiencing algae blooms that require more extensive treatment, Forgue said.

Watson might be relied on to provide water through a dry fall, as it was last year when it remained in service into December, she said.

Newport hasn’t resorted to odd-even restrictions since 2002. Forgue said that some critics of the measure questions its effectiveness, arguing that people will simply use more water on their assigned days in anticipation of having to go a day without.

With reports from Richard Salit and Meaghan Wims.

gmacri@projo.com