DEQ plans to measure Bayou Lafourche’s rate of flow

A second phase of dye studies designed to measure Bayou Lafourche’s stream flow will be conducted next week between Napoleonville and Thibodaux, state environmental regulators said.

The series of studies — the second of four such studies planned — involves injection of bright, red nontoxic dye into the bayou, state Department of Environmental Quality officials said in a news release.

DEQ officials said residents who see the dye should not alarmed. The water will return to its normal color by the end of the day.

Tim Beckstrom, DEQ spokesman, said Friday the Bayou Lafourche Freshwater District requested the dye studies to predict how the bayou will react if district pumps on the Mississippi River in Donaldsonville are turned off during major rainstorms.

“In particular, it will show how long it would take for freshwater to make its way to the various public water supply intakes in the event the bayou goes septic, as has happened in past storm events,” Beckstrom said in an email Friday.

The pumps move freshwater from the Mississippi River into Bayou Lafourche, which is the drinking water source for 300,000 people living downstream from Donaldsonville.

But the bayou is narrow in Donaldsonville and is the city’s major drainage artery. Pumping river water into the bayou during periods of heavy rain can contribute to flooding in the city and lead to calls from Donaldsonville officials to reduce pumping, freshwater district consultants reported in a 2012 master plan.

The freshwater district finished a $20 million bayou dredging project in Donaldsonville last year to improve the bayou’s flow. Last month, the district hired engineers to determine the best location in Bayou Lafourche for the next $20 million bayou dredging project.

For the first phase of DEQ stream flow studies in March, red dye was injected near the pumping station in Donaldsonville with two pumps running. Instruments measured the concentration of dye in the water between Donaldsonville and the public water supply intake in Napoleonville, DEQ’s Beckstrom said.

The next phase will involve injecting dye just downstream of the Napoleonville intake with two pumps running.

Dye concentrations will be measured downstream to the Lefort Canal south of Thibodaux, where Schriever’s water supply intake is located, Beckstrom said.

The third and fourth phases, Beckstrom said, focus on the same sections of bayou but with three of the freshwater district’s four pumps running.

The latest round of dye testing is expected to begin the week of April 15 and end the week of April 22, DEQ officials said.