Duluth staff outline plan to cut road salt as MPCA readies chloride limits

Jake Michna, a Minnesota GreenCorps member serving the city of Duluth, told the Committee of the Whole the city is preparing a chloride-reduction plan as the Minnesota Pollution Control Agency (MPCA) completes a TMDL for local trout streams.

Michna said the city “uses about 13 and a half thousand tons of salt, per year,” an average from 2009 to 2022, and summarized how road salt dissolves and runs off as stormwater, harming fish, aquatic insects and stream vegetation. He told the council that surface waters are judged impaired for chloride under MPCA and U.S. Environmental Protection Agency criteria when concentrations reach 230 milligrams per liter (mg/L) on two occasions in a three-year period or 860 mg/L on a single occasion.

The presentation noted five trout streams in Duluth listed for chloride impairment: Tischer Creek, Miller Creek, Chester Creek, Keene Creek and Kingsbury Creek. Michna said ongoing monitoring found four of those streams tested above 230 mg/L and, on some occasions, above 860 mg/L.

Why it matters: MPCA’s TMDL development for Duluth attributes roughly 93% of the chloride causing the impairments to road salt, Michna said, with municipal street operations accounting for the majority of that share. MPCA expects to complete the TMDL process by next spring; the city’s MS4 (Municipal Separate Storm Sewer System) permit is expected to include a waste-load allocation that will limit the total salt allowed in affected watersheds per year.

City response and current practice: Michna described operational changes that Duluth has made since 2022. Every plow truck now has prewetting equipment; spreaders are calibrated; electronic controls adjust application rates based on truck speed; the city has acquired two direct-liquid-application trucks that will be operational next winter; and salt sheds are in place at four maintenance locations to keep material dry. He said smart-salting training is provided to plow operators and that staff from street maintenance and stormwater engineering are collaborating on reporting requirements in anticipation of the TMDL.

Potential reductions and costs: Michna said the city’s GPS-derived average salt application last winter was about 517 pounds per lane mile. He said direct liquid application can reduce chloride use by industry-typical ranges of 39%–76% at common application rates and that a realistic citywide reduction over time is commonly about 30%–40%. He also noted secondary savings from reduced corrosion damage to infrastructure, vehicles and street trees.

Recommendations and capital needs: Michna recommended expanded brine-production and storage capacity (he listed a preliminary need for four designated brine-production facilities and roughly a dozen 6,000–8,000 gallon tanks), four 400-ton salt storage buildings, continuing to acquire liquid-capable trucks and maintaining a replacement rate of about three plow trucks per year. He also urged adequate full-time staffing for operations and reporting, continued use of prewetted materials, use of road-weather information systems, and public communication about changes in winter maintenance practices. He suggested the city consider requiring smart-salting training for private contractors who apply salt within city limits.

Public-works comment on variability: A city public-works staff member responded to a council inquiry that annual salt use varies with winter severity; staff reported the lightest recent winter used about 4,000 tons and heavier winters have exceeded 15,000 tons.

What’s next: With the MPCA TMDL expected next spring, city staff and elected officials will need to determine how to meet a future waste-load allocation, track and report material use through the city’s GPS system, and budget for additional brine capacity and liquid-capable equipment.

The presentation concluded with a brief question-and-answer period from council members about monitoring frequency, equipment replacement and the location of maintenance facilities.