ATLANTA – With drought tightening its grip on the Southeast, the Atlanta area’s reservoirs are almost down to the dregs – the dirtier, more bacteria-laden water close to the bottom – and it’s going to require more aggressive and more expensive purification.

Some communities are buying stronger water-treatment chemicals and looking into other measures to make the water drinkable.

The problem is that the water levels on Lakes Lanier and Allatoona, the main sources of water for metropolitan Atlanta’s 5 million residents, have descended almost to the “dead zone,” a layer low in oxygen and high in organic material – that is, dead and decaying plants and animals.

Even with standard treatment, the water at that level can have a strong odor, taste and color. State officials consider the water “suspect” at best.

“Is there water there that could be used? Yes,” said Carol Couch, Georgia’s top environmental official. “But it’s not exactly high quality.”

The dropping of the water levels into the dead zone won’t have a major effect on the city of Atlanta, because it does not draw its water straight from Lake Lanier, but from the Chattahoochee River, which is fed by Lanier.

By the time Atlanta takes its water from the Chattahoochee, it has been circulating for a while and has been exposed to more oxygen, and is thus not as dirty.

But the booming suburbs to the north draw their water directly from Lanier and Allatoona.

The water utility in Cumming, a northern Atlanta suburb on the banks of Lanier, has brokered a deal to buy a new chemical – potassium permanganate – to treat the water pumped from the depths, and is planning to dredge silt and sediment from the bottom of the lake in search of cleaner water.

“The city will go after the water no matter how far the lake recedes,” said Jonathon Heard, the city’s director of utilities. “We intend to provide water for the citizens of this area, whatever it takes.”

Gwinnett County, the state’s second most populous county, has an intake pipe that is about 15 feet down into the dead pool. It plans to spend more money on ozonation, or the infusing of ozone gas into the water to destroy bacteria and other microorganisms.