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DNR flags water‑quality blending issue; Fairfax staff to study new raw‑water main and blending options
Summary
A DNR inspector alerted staff that treated water from a proposed treatment plant may not blend uniformly with water from an existing well that contains ammonia; staff will study options including a raw‑water conveyance to the new plant (preliminary cost estimates range into the low millions).
Fairfax — State and city staff flagged a water‑quality blending issue that could require additional piping or operational changes if the new treatment plant treats ammonia out of one source while other wells remain untreated.
City staff told the council that the treatment plant planned for Well 4 would remove ammonia (which is associated with nitrite/nitrate cycling after chlorination) while Well 3 currently contains ammonia. A DNR official advised the city that dissimilar water qualities can cause distribution system issues and recommended exploring ways to ensure uniform residuals across the system.
Brian (water/wastewater supervisor) explained the operational consequence: the city currently flushes large volumes of water at dead‑end portions of the system (estimated at roughly 2 million gallons per month, or about 24 million gallons per year) to manage aging and nitrification in low‑use branches. He described a possible engineering solution: piping raw water from the existing well to the new treatment plant so that all water receives consistent treatment before distribution. Preliminary contractor feedback suggested the raw‑water main could cost in the order of magnitude of a million dollars (staff called this a broad, initial ballpark), depending on alignment and right‑of‑way complexity.
"If we take all the ammonia out of the new well and go to free chlorine, we will cut back on our chlorine usage by 50 to 75%," Brian told the council, noting multiple operational and cost benefits if a blending/transfer solution can be engineered.
Council directed staff to work with the city engineer (ISG) and the DOT/consultant team to scope a study and cost estimate for options, and to return with a recommendation.
Why it matters: The issue affects drinking‑water stability, distribution chlorine strategy, and long‑term operational costs and could require capital investment. Further study will quantify options and costs.
Next steps: Staff will scope engineering study options with ISG to evaluate a conveyance solution or other blending alternatives and report back to council with cost and operational impacts.
