Rosemount addresses 'gross alpha' drinking-water violation; city weighing blending, mains or treatment plant

Rosemount city and state water officials outlined options Monday for responding to a recent 'gross alpha' drinking-water violation, saying the spike reflects changing aquifer conditions and that the city has followed state steps while evaluating engineering and financial tradeoffs.

In an opening presentation, Sam Swanson of the Minnesota Department of Health said gross alpha measures total alpha radiation in water (picocuries per liter) and that Southern and Central Minnesota saw increases last year. "At the beginning of this year, a violation was issued… the average came out to be 16, which was higher than the limit of 15," Swanson said, adding that Rosemount issued the required public notice and will continue quarterly notices until the issue is resolved.

City officials described three technical pathways under review. Chris Larson, an environmental engineer with Short Elliott Hendrickson, said engineers evaluated (1) minimizing use of Well 8 (making it emergency-only), (2) blending Well 8 with nearby Well 9 via roughly 2,000 feet of new raw-water main (estimated cost about $2,000,000) to lower concentrations, and (3) building a new treatment plant (likely at Well 12) connecting several wells. "A treatment plant at Well 12 could cost in the range of $36 million to $54 million," Larson said, noting construction and new trunk mains would add years to the timeline.

Public-works director Nick Egger said the city already adjusts operations so Well 8 normally turns on last and is used mainly during hot-weather irrigation or in major incidents. Egger told attendees the city submitted a plan of action, MDH reviewed it and formalized an agreement, and the city is collecting financial data to understand utility-rate impacts before bringing a final recommendation to the City Council. "We're very much on track to achieve that point yet within 2026," Egger said, and design and construction could begin as early as 2027 depending on the solution chosen.

Agency roles and testing cadence Minnesota Department of Health staff emphasized that MDH implements the Safe Drinking Water Act standards and sets testing methods. Swanson said the radionuclide (gross alpha) test methods specified by EPA and MDH take time because of the chemistry involved and that chronic contaminants are evaluated on a running annual average. That approach is why MDH required the city's quarterly public notices and reviewed Rosemount’s plan rather than imposing an alternate rapid-testing regime.

The Minnesota Pollution Control Agency (MPCA) and Dakota County discussed complementary roles. MPCA supervisor Chris Gosenac said MPCA’s remediation work focuses on confirming releases, identifying potential point sources, and sampling private wells that may be at risk; those investigations can take years. Dakota County groundwater supervisor Valerie Nepple emphasized the county's role assisting private-well owners and noted the county has public dashboards and a 20-year sampling record available online.

What residents asked and what comes next Attendees raised several practical questions: whether the city can test gross alpha in the distribution system, whether emergency wells are sampled for chronic contaminants, and whether industrial users (a recently discussed data-center site) could affect supply. Officials said system blending that happens passively in pipes is not an allowable controlled solution; Rosemount is pursuing point-source or targeted treatment solutions. MDH noted emergency wells are not routinely sampled for chronic contaminants (they are sampled annually for acute contaminants such as nitrate).

Egger said the city will finalize the financial analysis, evaluate rate impacts, and return to the City Council with a recommendation in 2026 so work could proceed in 2027 if approved. In the meantime, MDH, MPCA, DNR and county staff remain available for follow-up and one-on-one questions at tabling after the presentation.

Ending The panel closed by directing residents to the city's water-quality web page, the MDH consumer-confidence materials, and county and MPCA online tools for maps and data; staff encouraged anyone with specific concerns to speak with agency representatives at the booths.