Cooling Tower Loop Hole
A critical gap in Marana's current ordinance structure
Marana’s Data Center Ordinance related to water restrictions covers only potable water supplied by the Town. Based on this specific callout, the alternative for wastewater use was analyzed to determine what that gap means for Legionella risk, specifically at the Arizona Veterans Memorial Cemetery.
Marana’s data center ordinance, Section 17-6-13(H), prohibits the Town’s water department from supplying potable water for data center cooling. It’s a reasonable provision; hyperscale data centers consume water at an industrial scale, and in an arid climate with constrained groundwater, protecting that resource makes sense.
But the restriction is narrowly written. It applies only to potable water supplied by Marana Water. It says nothing about reclaimed wastewater, Class A+ treated effluent, agricultural canal water, or any other non-potable source. The Luckett Road North Specific Plan has already identified the adjacent Cortaro-Marana Irrigation District canal as the facility’s industrial water supply, with confirmed availability of up to 7.8 million gallons per day.
That canal connection is the precondition for a future transition to evaporative cooling, and nothing in the current ordinance would prohibit it. This analyzes what that means for public health, and proposes a targeted amendment to close the gap.
Why Cooling Technology Choice Matters
The companion Thermal EIS report documented that the Luckett Road North facility, as currently proposed with dry-air cooling, produces a mean summer temperature increase of +3.2 to +3.5°C (+5.7 to +6.3°F) at the Arizona Veterans Memorial Cemetery, a receptor located approximately 24 meters (~80 feet) from the facility’s eastern boundary.
That finding creates a plausible regulatory dynamic: if thermal performance standards are adopted, the developer’s most cost-effective response would be to transition to evaporative or hybrid wet cooling. Evaporative cooling transfers heat partially into water vapor rather than entirely into the ambient air, thereby reducing the sensible heat increment at the cemetery.
That appears to be an improvement. The problem is that evaporative cooling introduces a different and, in some respects, more serious public health risk to the same receptor location: Legionella pneumophila aerosolization.
A regulatory framework that addresses only thermal impact could inadvertently create the conditions for a Legionella exposure event at the same cemetery. Both pathways need to be closed.
The Pathogen
Legionella pneumophila is the bacterium that causes Legionnaires’ disease, a severe pneumonia. It was first characterized following the 1976 outbreak at an American Legion convention in Philadelphia, where the hotel’s air-conditioning system was identified as the source of the outbreak, resulting in 34 people dying, and 182 were hospitalized. That event established the epidemiological pattern that has held in the literature ever since: cooling towers are the primary environmental amplification and dispersal system for community-acquired Legionnaires’ disease.
The CDC reports that Legionnaires’ disease cases in the United States increased by more than 500% between 2000 and 2019, reaching approximately 10,000 to 10,500 cases annually. Cooling towers are implicated as the source in the majority of outbreak-associated cases where an environmental source is identified. The case fatality rate ranges from 5–10% for community-acquired cases to 25–50% for immunocompromised patients and healthcare-associated cases.
How Cooling Towers Amplify and Disperse the Pathogen
Legionella grows in water between 25°C (77°F) and 45°C (113°F), with optimal growth at 35–37°C (95°F to 98.6°F). Temperatures above 60°C (140°F) are lethal; below 20°C (68°F), growth is negligible. A cooling tower rejecting heat from a data center in the Sonoran Desert operates continuously within that growth window during the summer months.
The physical design of cooling tower fill media, warm water cascading over large surface areas in a forced-air stream, provides ideal conditions for biofilm formation. Murga et al. (2001) found that biofilm-associated Legionella concentrations in cooling towers were consistently 100 to 1,000 times higher than concentrations in the surrounding water. Within those biofilms, Legionella is substantially protected from chlorine and other disinfectants: achieving a 3-log reduction of biofilm-associated bacteria requires biocide concentrations 10 to 100 times higher than for free-floating cells.
Cooling towers then aerosolize that water. The infectious aerosol is droplets between 1 and 10 micrometers in aerodynamic diameter that penetrate to the alveolar level of the lung. Modern drift eliminators reduce this by 99.9%, but cannot eliminate it entirely and are subject to fouling and mechanical degradation during continuous high-load operation.
Dispersal modeling studies establish the relevant distances. Prussin et al. (2017) found viable Legionella concentrations at potentially infectious levels at 200 to 600 meters (~650 to 2000 feet) downwind under typical urban meteorological conditions. Nguyen et al. (2006) found that ground-level receptors within 500 meters (~1600 feet) of a source were consistently exposed to the highest concentrations, and that low wind speeds, common during morning hours in the Sonoran Desert, produce the highest ground-level accumulation because aerosols concentrate rather than dilute. The cemetery at 24 meters (~80 feet) from the facility boundary is within the near-field dispersal zone by any measure in the literature.
Why Non-Potable Water Makes This Worse
The distinction between potable and non-potable water in a cooling tower is not merely regulatory; it reflects real differences in microbial content and nutrient availability that directly affect Legionella growth.
Hamilton et al. (2018) found that reclaimed water systems consistently supported Legionella concentrations 10 to 100 times higher than comparable potable water systems. Three factors drive this:
First, reclaimed water and agricultural canal water carry higher concentrations of assimilable organic carbon, the primary growth substrate for Legionella and the protozoan species that serve as its intracellular amplification hosts. Second, non-potable distribution systems in Arizona are not required to maintain a chlorine residual, the primary control mechanism for waterborne pathogens in potable systems. Third, reclaimed effluent and irrigation water contain elevated concentrations of iron and zinc, which are micronutrients that accelerate Legionella growth.
Arizona Class A+ reclaimed water, the highest quality tier under ADEQ’s framework, meets turbidity and fecal coliform standards appropriate for irrigation and non-potable uses. It has no Legionella-specific treatment requirements, monitoring obligations, or concentration limits. That is an entirely appropriate standard for its intended uses. It is not an appropriate standard for the inlet water of a hyperscale industrial cooling tower near an outdoor public gathering space.
The CMID canal water, the Luckett Road Specific Plan identifies as the facility’s water source, is in a different category still: it is unclassified, unmonitored, open-channel irrigation water subject to waterfowl contamination, algal growth, and summer temperatures within the Legionella growth window. No Arizona regulatory requirement would trigger Legionella monitoring or water management obligations for its use in a cooling system.
The Impact
Legionnaires’ disease risk is heavily concentrated in a specific demographic: adults over 50 with underlying respiratory or systemic conditions. The CDC identifies age over 50 as the strongest demographic predictor of disease severity. Case fatality rates for those over 65 with underlying conditions exceed 25–30% in outbreak settings.
The Arizona Veterans Memorial Cemetery serves primarily veterans from the World War II, Korean War, and Vietnam War cohorts. The median age of veterans using Arizona’s state veterans cemeteries is approximately 72 years. Veterans have substantially higher lifetime smoking rates than the general population. Current smoking increases Legionnaires’ disease risk by approximately 3.5-fold; former smoking by 1.9-fold. Chronic obstructive pulmonary disease, prevalent in this cohort, is an independent risk factor for both infection and fatal outcome.
Funeral service attendance compounds the exposure. Attendees stand stationary outdoors for 30 to 60 minutes, maximizing aerosol inhalation dose. Morning service hours, 9 to 11 AM, correspond to the low-wind, temperature-inversion conditions under which ground-level aerosol accumulation is highest. The exposure is not discretionary.
The Regulatory Gap
Under current Arizona law and the Marana Town Code, a cooling tower operating on CMID canal water at the Luckett Road site would face no mandatory Legionella water management plan requirement, no culture monitoring obligation, no drift eliminator specification or performance verification requirement, no notification requirement to ADEQ or the Arizona Department of Veterans’ Services if Legionella is detected, and no specific setback or location requirement from the cemetery or any other outdoor gathering space related to this particular issue.
ASHRAE Standard 188, the primary voluntary consensus standard for Legionella risk management in water systems, is not mandated by Arizona law or Marana’s code. New York City, California, Nevada, and New Jersey have enacted mandatory water management requirements for cooling towers following outbreaks. Arizona has not.
A simple mechanical permit application would be sufficient for the Luckett Road applicant to transition from dry-air to wet cooling at any point in the facility’s operational life, no public notice, no Planning Commission review, no reassessment of public health impacts.
What the Proposed Amendment Does
A targeted amendment to Section 17-6-13(H) that closes the loophole without modifying the water conservation objective of the existing provision is being proposed as part of this assessment.
The core addition is a prohibition on evaporative cooling systems using any water source, including reclaimed water, Class A+ effluent, agricultural canal water, and non-potable groundwater, at principal-use data centers above 100 MW where a sensitive thermal receptor exists within 1,500 meters (~1 mile) of the facility boundary.
The 1,500-meter threshold is drawn from the dispersal literature: Walser et al. (2014) found statistically significant case clustering within 650 meters (~2100 feet) of implicated towers; Courcoul et al. (2011) modeled detectable concentrations at up to 1,000 meters (~3,300 feet) under adverse meteorology; Hoge et al. (1994) traced cases up to 2 kilometers (1.25 miles) under wind transport. The 1,500-meter (~1-mile) threshold provides a margin beyond the near-field zone while remaining proportionate and defensible.
For facilities outside the sensitive receptor zone, the amendment requires a Legionella Water Management Plan prepared in accordance with ASHRAE Standard 188 by a certified industrial hygienist or licensed professional engineer, with baseline testing before startup, quarterly culture monitoring during summer months, defined action levels, and notification to the Marana Zoning Administrator within 72 hours of any result exceeding 1,000 CFU/mL.
Conclusions
The thermal and Legionella analyses address two sides of the same regulatory gap. Neither is sufficient alone.
A facility operating dry-air cooling produces the thermal impacts documented in the companion report, a +3.2 to +3.5°C (+5.7 to +6.3°F) mean summer warming at the cemetery. A facility that transitions to wet cooling in response to thermal performance pressure would reduce that increment while introducing Legionella aerosolization risk to the same receptor. The combined risk table is straightforward: the only cooling technology that satisfies both standards simultaneously, near-zero thermal increment and no Legionella aerosolization risk, is closed-loop liquid cooling using a dielectric fluid. The proposed amendments do not mandate that technology, but they create the regulatory conditions under which it becomes the only fully compliant option in this location.
The proposed amendments to Section 17-6-13(H) and the companion Section 17-6-13(K) thermal standards are both being presented to the Marana Town Council at the upcoming council meeting.
The Full Report
The complete Legionella Public Health Risk Assessment, including the full epidemiological literature review, water quality comparative analysis, proposed ordinance text, and regulatory gap analysis, is available here:
Legionella Public Health Risk Assessment
See Report for Full List of References.
The Sonoran Think Tank is an independent civic research organization based in Southern Arizona.
"A simple mechanical permit application would be sufficient for the Luckett Road applicant to transition from dry-air to wet cooling at any point in the facility’s operational life, no public notice, no Planning Commission review, no reassessment of public health impacts."
In conjunction with everything else you've discussed and your scientifically well-researched facts, this paragraph becomes exponentially scarier. Coupling that with the naivete and ignorance Marana’s town leadership consistently demonstrates, their unwillingness to listen to experts, seek additional unbiased outside advice, and consider the potential health risks is gravely concerning. They seem content brushing off potential risks, chalking them up to "leftist" and "extremist" talking points rather than taking them seriously and investigating further in an effort to do right by the people they were elected to represent. It seems like a recipe for a serious problem, and apparently now a problem that may be airborne and aerosolized. Thank you, Colin, for looking into this. It was exceptionally well written, as usual.
Colin, thank you so much for all this great information. I am in awe of your knowledge and intelligence and I hope that the Town of Marana will start listening and paying attention to what you are reporting.
On a separate note, have you heard how the stricker bill 2873 is doing?