The ever-receding water levels at both Lake Allatoona and Lake Lanier are pushing drinking water suppliers toward extraordinary measures to keep metro Atlanta's drinking water flowing and fit to drink.

In September, Cobb County officials dispatched divers to the bottom of the drinking water intake tower on Lake Allatoona. The officials needed to know if the fourth and lowest intake gate, unused since the tower was built in the mid-1960s, still functioned 40 years later.

Pouya Dianat/AJC (ENLARGE)

The receding shores of Lake Lanier have exposed many discarded items, among them bottles, beer cans from brands that no longer exist and other larger objects.

Aerial lake photos: • Late November • Early November • October MORE ON THE DROUGHT: • Map: Heavy demands on our water • Photos: April showers bring ... • Photos: Lake rises dramatically in January RELATED: State lifts strict outdoor watering restrictions  Ban lifted on new Lake Lanier boat docks  Drought's over and Lake Lanier gets lively  Water wars judge stays patient, with Atlanta's future in his hands  Judge: Lanier use should have been decided long ago  • More on coping with the drought

They won't know until the U.S. Corps of Army Engineers lets them dredge away the muck and sediment that's blocking the lower gate.

Cobb officials have another contingency plan: running temporary pipes to a deeper part of the lake about a mile away.

"We have another month to two months to decide if we need to take that step," said Glenn Page, general manager of the Cobb County-Marietta Water Authority.

The drought is challenging the ability of some treatment plants to draw water, as levels dip close to intake gates, and its myriad consequences are driving up the amount of chemical manipulation needed to make the water potable.

The additional measures don't come cheap. The Cobb County-Marietta Water Authority, supplying drinking water to most of Atlanta's northwestern suburbs, spent an additional $500,000 on chemicals this summer.

They estimate it will cost $600,000 to run temporary pipes to deeper water in Lake Allatoona, should that be necessary, plus monthly operating expenses of $250,000.

Last week, Cumming water officials in Forsyth County began dredging around their intake point on Lake Lanier to access deeper water.

The two- to four-week project will cost about $1 million, officials say.

Besides creating worries about sufficient quantity, the low water levels have complicated the purification process.

Water treatment managers have been coping with swings in water quality, requiring different strategies to eliminate odors, make the water clear and ensure that it's safe to drink.

Under ideal conditions, intake points for treatment plants are well below the surface of the water and well above the bottom.

As the lake levels go down, the intake points are closer to the surface, where concentrations of organic matter are highest. And as the point approaches the lake bottom, the treatment plant begins to pull in more sediment.

Over the summer, record algae blooms at Allatoona and dense algae growth on Lanier, coinciding with the record-setting drought, increased the concentrations of organic carbon in the water.

High concentrations of organic carbons can present a health hazard because they react adversely with chlorine, one of the chemicals used to disinfect the water.

Over the past nine years, the federal Environmental Protection Agency has issued limits on the chemical byproducts produced during chlorination after finding an association between them and cancer in mice.

This summer, organic carbons increased from about 3 parts per million to as high as 13 parts per million on Allatoona, Page said.

That required the authority to increase the chlorination and add it earlier in the treatment process. The chemical byproducts produced during the chlorination, however, remained below EPA thresholds, Page said.

The situation on both Allatoona and Lanier has improved over the past two months, as the lakes have undergone their biannual inversions. During this process, lowered air temperatures cool the surface water and cause it to sink, mixing with stagnant water at the bottom.

On Lanier, water officials are struggling with yet another problem: higher concentrations of sediment and some metals that affect water clarity.

In Forsyth County, the dredging undertaken by the city of Cumming to access deeper water has stirred up so much sediment that the county water plant has been overwhelmed and shut down on some days.

On those days, the county runs the plant at night, when the water is more settled, said Tim Perkins, the county's water director.

"We think we'll be able to handle whatever the lake throws at us," Perkins said. "But there is an unknown of what will actually happen."

The intake point for the Gwinnett County treatment plant is already in one of the deepest parts of the lake, but water production director Neal Spivey said he also expects to have to deal with more sediment.

Sediment is removed from drinking water by adding aluminum sulfate, a coagulant that causes the particles to settle. They can then be captured in filters.

"At some point, I expect the coagulant dose to go up," Spivey said.

With all of the exposed shoreline around Lanier because of the drought, water quality may be adversely affected in the event of heavy rain. A hard rain could cause the banks to erode and turn the water muddy, requiring higher doses of coagulants to clean it up.

Most facilities have seen higher levels of manganese, a metal typically found in lake water that can turn water black or brown. Manganese is controlled by the addition of coagulants to the water during treatment.

This summer, however, some Cobb drinking water coming out of Allatoona retained a greenish hue from excess manganese that couldn't be eliminated.

"There was no health issue," Page said. "It was a color issue."