When an area of Mexico is developed, it needs to be connected to the stormwater system and the city’s stormwater plan needs to be updated.

City Engineer Drew Williford shared with the Mexico City Council what the city does to work with developers and business owners for the system tie-ins at a council work session Monday as part of regular updates to keep the council informed on city operations.

“Tonight, we’re going to go over what we call the MS4 system. MS4 stands for Municipal Separate Storm Sewers System,” he said. “Think of all the stormwater items that are involved in piping water from point A to point B within the city.”

Stormwater system updates are necessary when a manufacturer or other businesses expand, like in the case of Spartan Light Metal Products or Soft Surroundings Distribution Center, Williford said. The upgrades are needed for increased runoff from what once was patches of unused ground and to prevent any possible hazards flowing into waterways.

The MS4 system is set up through a permit from the Environmental Protection Agency as administered by the Missouri Department of Natural Resources. Those government entities created the guidelines the city uses in implementing the stormwater system.

“You can think of it from the standpoint of a certain threshold kicks you into automatically being an MS4 community. For us, that threshold is (a population of) 10,000,” Williford said. “So, we’re barely right on that threshold … and therefore we’ve gone through the process of setting up the stormwater management plan.”

The plan outlines water-quality concerns throughout Mexico and how they’re addressed, along with establishing ordinances so the city can carry out the stormwater management plan. Williford walked the council through steps the city takes in regards to construction and other items the city uses on a routine basis.

Stormwater management is considered in two phases, pre-construction and post-construction. The city keeps track of which facility is in which phase through a graphic information systems map of Mexico. Sites in construction are considered in the land disturbance phase, while post-construction are now part of stormwater management.

There are roughly 10 sites in Mexico considered to be in the second phase of stormwater management, Williford said, while four are still classified as under construction — Spartan, Homebank, Teal Lake Village and the Audrain County Jail expansion.

There are different regulations for when a facility enters construction, during construction and after construction. During construction, the city will perform monthly inspections of the site to combat any potential water-quality issues, Williford said. Prior to construction, the city will review plans, and work with the contractors to ensure stormwater systems are built through MS4 regulations.

“So when does a site classify for being regulated under the MS4 ordinances? That threshold is one acre. So if your construction disturbs an acre or greater, that kicks you into the stormwater requirements that are enforced through the city and the MS4 permit we operate under,” he said.

Construction larger than an acre is considered a permanently registered location for stormwater quality. The business owner will sign a maintenance agreement with the city and as long as the operation is in existence, the business owner has to operate stormwater facilities as designed, Williford said.

“The way we end up inspecting those is through both an annual inspection but then we also work with the owner to explain the requirements of that (design),” he said.

The designs address not only stormwater quality, but also stormwater quantity. Weirs or other specialized detention ponds are used to address stormwater quality — making sure sediment settles, for example — and to slow down the inflow of stormwater into the pipe system so it isn’t overloaded.

New warehouses or other structures will increase stormwater runoff and so the management design has to be updated and included as part of construction so it does not put undue stress on the city’s stormwater system.

“So, you’re trying to take some proactive measures, so that you don’t have to take reactive measures later on. … The intent behind this program is to minimize (reactive measures) and to put into place some countermeasures which can address some of those water-quality concerns,” Williford.