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Mono considers option to reduce reliance on water sources with trace levels of arsenic

November 20, 2025   ·   0 Comments

Tests show levels of arsenic in Mono’s water system remain safe, not exceeding the maximum acceptable concentration

By JAMES MATTHEWS, LOCAL JOURNALISM INITIATIVE REPORTER

Mono is re-evaluating its water supply to reduce its reliance on raw water sources with arsenic concentrations exceeding half of what’s permitted.

Matthew Paznar, a senior engineer and vice-president at R.J. Burnside and Associates, updated Mono council when it met Nov. 18 about the town’s water environmental assessment.

Mono is supplied with drinking water from the Cardinal Woods and Island Lake systems, which operate independently.

An issue of concern is the removal of arsenic. The maximum acceptable concentration (MAC) is 0.01 milligrams per litre, and all wells in both systems were comfortably within that level.

Four of five Island Lake drinking water system wells are in compliance, but all five wells have raw water arsenic concentrations reported above half the MAC at 0.005 mg/L.

“Additional care and monitoring are required when arsenic levels exceed the half MAC,” Paznar said.

Arsenic levels tend to fluctuate and may increase over time, posing a threat to the Island Lake system and a major concern for the municipality.

One of the wells has, in the past, had arsenic concentrations exceeding 0.01 mg/L. That well, the Coles Well PW1, was taken offline in late 2017, resulting in a loss of raw water supply to the system and reducing the firm capacity of the Island Lake system.

“The Ministry of the Environment, Conservation, and Parks just requires the town to do a little bit more monitoring,” Paznar said. “So that’s what we have been doing.”

There are three approaches to do that.

They can remove arsenic from the raw water using a water treatment plant.  

Hydrogeological solutions can be employed, such as drawing water from different aquifers that contain less arsenic levels or addressing the potential for fluctuations and/or arsenic concentration increases in existing wells.

Water from another municipality’s supply can also be obtained.

“The preferred alternative might have a combination of these solutions and likely have to be implemented in a staged approach,” Paznar said.

Alternatives include providing arsenic treatment, drilling new wells, connecting both existing water systems, or connecting the Island Lake system to a nearby non-Mono system.

“Why is the do nothing not an option here?” Councillor Melinda Davie asked.

“The do nothing is not an option because we’ve taken raw water supply offline,” Paznar said. “The town is in need of it to be able to handle its long-term commitments.”

As part of the commitments in the Island Lake area, the municipality has to provide firm capacity, he said. That’s the maximum day demand. And the Island Lake system is the larger of the two.

An additional water supply would be needed for alternatives that include filtration or another arsenic-removal process. This would mean that an additional raw water supply would need to be available to maintain the production of the current potable water supply for the existing users served by the Island Lake system.

An additional raw water supply with lower arsenic concentrations is being evaluated. Paznar said field investigations have determined that there is a viable water supply with low arsenic concentrations in the Cardinal Woods area.

Deputy Mayor Fred Nix spoke about past musings regarding a water quantity threat. And now there’s a test well being drilled, and there’s talk about joining the Cardinal Woods system with the Island Lake supply.

“It just surprises me,” he said. “But that’s great if we can do it.”

Mono Mayor John Creelman said he supposes a new well would be cheaper than constructing a water treatment plant.

“That’s what we are thinking in the long run here,” Paznar said.

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