An engineering firm hired by an advocacy group has concluded that the city of Alexandria ought to be able to replace the biggest of its combined sewer systems that send overflow waste into the Potomac River by 2024.

But the city’s top environmental official said the city needs more time than that, arguing that the firm failed to consider all the issues.

The sparring comes while city government officials await a hoped-for amendment to a new state law from Gov. Terry McAuliffe (D) that would give them more time to rebuild four sewers that periodically overflow waste and storm water into the river and a tributary.

The General Assembly last month ordered the city to finish all construction by July 1, 2025. City officials say 2029 is the earliest deadline they can meet.

NewFields, an Atlanta-based environmental engineering group, said in a March 10 letter to the Potomac Riverkeeper Network that the city’s schedule to study, plan and build just one of the sewer projects in North Old Town is “inordinately long.”

The group argued that several planning studies should be able to go on simultaneously.

Yon Lambert, Alexandria’s director of transportation and environmental services, said that conclusion overlooks the fact that Alexandria is dealing with multiple sites that will require planning, study and highly disruptive construction.

“The city is very committed to addressing our [combined sewer] problem, but we just need to do so in a reasonable time frame,” Lambert said. “We don’t think it’s unreasonable.”

“Other communities have taken 15 to 20 years” on these types of projects, Lambert said. “We’re saying give us nine to 12 years.”

The governor has until March 23 to amend the General Assembly’s legislation on the matter. If he does, it would be up to legislators to react during their reconvened session in April.