GROTON -- If all goes well, caretakers of Lost Lake said that the largest body of water in town may soon be returned to the pristine condition that many long-time residents remember.

If so, the change will have come thanks to a herbicide called Sonar being used at the lake over the past several weeks. Designed to kill unwanted plant species, the chemical leaves other plants unharmed and the water safe for swimmers.

Funding for the herbicide program was approved by residents at town meeting earlier in the year when voters appropriated $95,000 for the purpose.

Although the issue of invasive plant species such as milfoil, combomba, water chestnut and free-floating filamentous infesting the town's lakes and ponds has been around for many years, only recently has the problem reached crisis proportions with forests of plants carpeting the bottom of bodies of water such as Lost Lake.

So thick have some stands of weeds become that they clog outboard motors and present a hazard to swimmers. Some lakeshore residents even have had to cut paths through the weeds to get their boats away from docks into deeper water near the center of the lake.

With the failure of such traditional methods of weed control as mowing with a weed harvester, frustrated supporters of the lakes turned to the use of herbicides to solve the problem once and for all.

The Great Ponds Advisory Committee took the lead in the anti-weed campaign, which over the years, had spearheaded the losing battle to control the various weeds. Finally, committee members decided that herbicides were the only solution to a problem that threatened to turn Lost Pond/Knops Pond into swampland.

Arguing in favor of a chemical solution to the weed problem, committee members let it be known to selectmen and the Conservation Commission that 27 other towns in Massachusetts including Ayer and Littleton had used chemicals such as Sonar to good effect in their own ponds.

Sonar succeeds by interfering with the weeds' chloroform process.

Convincing their fellow residents that everything else had been done to solve the problem, the committee was given the money it needed to pay for Sonar to be used in Lost Lake.

That program began last April with a first dosage, which was later deemed not strong enough. A second dose was made on May 22 with tests indicating that there had been good concentrations of the chemical in the water.

After that, it was noted that the numbers of Combomba in the lake "took a dive" with milfoil also proving highly susceptible to the chemical.

Last but not least, a plant known as "variable milfoil" began to show discoloration. Once that plant is killed off, the lake should be finally rid of the invasive species that had ruled its depths for so long.

However, just as the full effects of the chemical are coming to bear, committee members now fear that heavy rains forecast could dilute the Sonar in the water and retard the eradication process.

As a result, further doses of the herbicide will await water testing needed to determine its effectiveness.

Meanwhile, at a meeting held June 26, the committee reported that already, as a result of the weeds being killed off, there is a marked increase in boat traffic on the lake as well as fishing.

To raise awareness about the lakes, the advisory committee began conducting free boat tours for local residents. Tours given June 8 proved quite popular with more being planned for coming weeks.