Arizona allows a public water system to use source blending for contaminant-limit compliance only under specified written approvals and continuing verification. The supplier needs an approved blending plan and downstream-compliance monitoring program, with amendments when source or flow changes alter the blend. A calculated average or household mixing practice is not that approval.
Is blending an approved compliance method here?
This question concerns regulated public-water-system sources and a drinking-water maximum contaminant level, or MCL. It is different from mixing household softened and hard water or changing a fixture’s temperature. No blend calculation in this article establishes safe water or authorizes operation.
Under R18-4-217, a public water system may use blending for MCL compliance only when the section’s conditions are met. These include written Department approval of both a blending plan and a monitoring program. A satisfactory sample or a theoretical average does not replace those approvals.
Identify the actual system, contaminant, sources and responsible drinking-water authority before asking whether blending is an acceptable option. Have the operator and appropriate technical professionals evaluate the proposal with that authority.
What evidence does the blending plan need?
The rule identifies detailed drawings and schematics showing flows, concentrations and controls. It calls for the proposed automatic or electronic devices that maintain the desired range, shut off an offending source or trigger an alarm when the blend leaves that range.
It also requires individual test results for the proposed sources, projected contaminant levels showing best-case and worst-case scenarios, and identified techniques or other requested information demonstrating that the plan will produce compliant water. A single recent result or a nominal pump flow cannot establish all of those conditions.
Prepare the actual source records and operating constraints for review. Do not improvise a valve-setting, pump-speed or dilution procedure from a concentration number. Changes and failures need evaluation in the approved technical design.
How is continued downstream compliance verified?
The separately approved monitoring program must verify continued MCL compliance at all subsequent downstream service connections. R18-4-217 specifies monitoring at least quarterly for all sources contributing to the blend and for the blended water. Follow the actual approved program and any additional applicable requirements.
The microbiological sample-siting plan serves another monitoring purpose. Its taps and schedule do not automatically constitute the approved blending-compliance program. Preserve source identification, sample location, dates and the evidence needed to explain results.
Which changes require another review?
The rule requires an amended blending plan when sources enter or leave service, or when relative source flow rates change in a way that changes the blend. A replacement pump, unavailable source or revised operating arrangement should therefore be assessed for its effect rather than assumed covered by an old approval.
Separately, R18-5-509 requires approval of a detailed plan and safeguards before modifying an existing treatment process. ADEQ’s facilities-review instructions explain applicable construction and operating approvals. The general ATC/AOC guide addresses that engineering-review process; a blending approval does not automatically replace it.
Keep the approved plans, monitoring program, changes and compliance evidence available for the operator and sanitary survey. Have the authority resolve an uncertain change before treating dilution, a repaired component or one clean result as permission to operate.