There is hope for Lake Thonotosassa despite its poor health from years of pollution, environmental scientists say.
Now that a yearlong consultant's study has determined where those sources of pollution are, environmental officials are beginning to develop a cleanup plan to return the ailing lake back to a healthy state.
"There's no reason why the lake can't be restored to the condition it was in before it became" such a problem, said Gerold Morrison, an environmental scientist with the Southwest Florida Water Management District, known as Swiftmud.
The agency, under its Surface Water Improvement and Management (SWIM) Plan, is heading the cleanup effort.
However, environmental officials say recovery of Hillsborough's largest lake will be slow.
"We're optimistic that it's improving," Morrison said. "But people need to realize that it may take some time. It took a number of decades to get into that condition."
One of the reasons for Morrison's optimism is because he says the lake has the ability to flush itself out very rapidly _ a sort of natural cleansing. Water in the lake is replaced two to three times a year.
But for decades wastewater carrying pollutants _ mainly nitrogen and phosphorus _ has been dumped into the lake and its surrounding watershed, a 55-square-mile area of streams and ditches that drain into the lake.
Large quantities of pollutants essentially have overfertilized the 824-acre lake, causing excessive algae growth that cuts off light and kills plants and fish.
As a result, the lake turns a bright green.
The pollution reached its height in 1969, when an estimated 26.5-million fish died, the worst documented fish kill in the nation that year.
In the mid-1980s, the amount of phosphorus and algae in the lake began increasing. But scientists began to see slight improvements in 1990, probably due to improvements at the Plant City wastewater treatment plant, one of two major dischargers into the lake, Morrison said.
State Department of Environmental Regulation officials have told Plant City that it must stop discharging into the lake within five years, said Ed Snipes, manager of the DER's domestic wastewater program. He added that Plant City and DER officials are still negotiating that time frame.
The other major contributor, Treasure Isle seafood company in Dover, is closing and will stop putting wastewater into the lake by Feb. 1, said DER industrial wastewater compliance supervisor Henry Dominick.
An estimated 16 to 40 percent of the phosphorus in the lake is coming from the seafood company and the treatment plant combined, the consultant's study showed.
The remainder is coming from runoff from residential, commercial and industrial activities.
The culprits range from lawn fertilizer and pesticides to cattle manure reaching streams that flow into the lake. Pavement from commercial areas makes it easier for runoff, such as grease and oils from cars, to reach water areas.
To control these problems, environmental officials will improve stormwater treatment systems, mainly by building retention ponds that help filter pollutants from runoff before it reaches the lake.
Swiftmud will look to local governments to provide the land for the retention ponds, while the agency itself will handle design, permitting and construction, Morrison said.
He said another possibility is restoring the many wetlands in the watershed that have lost their natural filtering ability because drainage ditches have been built through them.
"Wetlands have quite a bit of potential" for cleaning pollutants, Morrison said. "They can serve an important function in the lake's restoration."