BANGOR, Maine — The Maine Department of Environmental Protection says it did not approve a series of recent upgrades along the Bangor Waterfront, meaning the city could have to retroactively request the required approvals or make significant changes to the site at the end of each concert season.

Under DEP rules, “any changes or new construction at [Waterfront Park] requires the review and approval of the Department prior to implementation,” states a mid-October letter from James Beyer, regional licensing and compliance manager for the DEP, to Bangor Code Enforcement Director Jeremy Martin. “None of the improvements … were reviewed or approved by the Department, but need to be.”

Bangor officials are working with the DEP to resolve its concerns, according to Tanya Emery, the city’s director of economic and community development.

The letter, sent to the city in early October, stemmed from a Sept. 20 meeting between DEP and city officials in which they discussed the construction that took place this past spring along the waterfront and at its concert venue. It lists several improvements to the venue that required approval but didn’t get it, including:

• A new, “semi-permanent” stage, which was repositioned for this concert season and is about twice the size of the previous one.

• A retaining wall built to increase the concert seating area, as well as a set of stairs and gravel parking area for performers and their crews.

• The relocation of art pieces, including the “Continuity of Community” sculpture and a monument recognizing the sister-city relationship between Bangor and Harbin, China, to the waterfront.

• Three “permanent” buildings used to sell beverages.

• A small building used to house electrical components and connections for lighting and sound systems.

• Pea stone that replaced grass and some porous concrete walkways after a rainy start to this year’s concert season prompted the city to lay down new materials to resolve odor and mud problems at the venue.

Most, such as the art relocation and retaining wall, are minor issues and likely could be resolved through permit revisions, but site alterations “raise concerns as to whether or not they will meet Department standards,” according to the DEP letter.

Specifically, Beyer says the gravel parking area drains onto an existing pervious pavement area, which could cause gravel to clog pores in the pervious pavement, “rendering it ineffective as a stormwater management measure.”

The waterfront site already is permitted, but some changes at the site, such as replacing damaged sod with pea stone and moving monuments, seemed relatively minor, so city staff didn’t immediately seek approval for revisions to the permit, according to Emery.

“This is a very large and evolving site,” Emery said.

Continued public use for events and demand for public space likely will mean the city will continue to improve the site.

The DEP letter outlines two options for the city, which owns the waterfront land and rents the concert venue to Waterfront Concerts: Alter the stage and buildings so they are “more temporary in nature,” or apply for an “after-the-fact” permit to keep the park and concert venue the way it is.

Both options present a challenge for the city. If it makes structures “more temporary” by putting the beverage structures onto trailers and hauling them off or disassembling the stage after concert season ends, it will cost substantially more to set up and tear down the venue each year.

“If they said pull it all down and go back to temporary, I don’t know if we could exist,” Waterfront Concerts promoter Alex Gray said Monday.

If it were to seek to adjust the permit to keep the area as is, then the site might have to comply with the DEP’s noise rules, which set a limit of 75 a-weighted decibels, or dBA, at any “development” property line — in this case, the performance venue fence. The level of 75 dBA is roughly equivalent to a car passing 25 feet away.

Gray said any concert — even an indoor performance overheard outside a place such as the Cross Insurance Center — would be hard-pressed to fall in line with that restriction.

Those DEP noise regulations often apply to commercial and industrial sites with sustained sound, such as a quarry operations. “Occasional cultural events,” such as Waterfront Concerts shows and the American Folk Festival, are exempt from those rules. The DEP letter argues that “the more the Waterfront Park contains permanent concert venue structures and features, the less it can be viewed as holding occasional cultural events.”

Both Gray and Emery likened applying the DEP noise rules to a cultural center like the waterfront to “fitting a square peg into a round hole.”

Because of the challenges presented by both of those options, the city has been working with DEP officials since receiving the letter to come to some sort of middle ground, according to Emery.

The state defines “permanent structure” as any structure designed to stand in a fixed location for more than seven months within a given year. Gray, the Waterfront Concerts promoter, said he viewed a permanen” structure as something that would need to be torn down with an excavator rather than disassembled. Gray said the whole site could be considered “temporary” to some degree, as each building could be moved and the stage disassembled, but that would carry an unnecessary cost and inconvenience.

Waterfront Concerts opted for a large-scale permanent stage in the wake of a 2011 stage collapse at the Indiana State Fair, which left 58 people injured and seven dead, according to Gray. Emery agrees with the DEP that the stage can be considered permanent, but argued that there’s a good reason it’s there — additional safety for concertgoers. Waterfront Concerts hopes to keep the stage’s frame up year round, as disassembling it would require heavy equipment and about 16 tractor-trailers to haul away the pieces for storage.

The city has until the middle of this month to work out a solution with the DEP. Emery said the city is ironing out a draft response with recommendations for how it would address the agency’s concerns.

“The need to collaborate continues to be important,” Emery said.