The Closed Loop Cooling Chemistry Problem
Closed-Loop Cooling and the Upper Santa Cruz Watershed
This is the third and final installment in STT’s series on data center cooling infrastructure and its environmental implications for Southern Arizona. Part One examined thermal output and microclimate impact. Part Two covered open-loop evaporative cooling and consumptive water use. This installment addresses the chemical risk profile of closed-loop systems, the technology most frequently offered as the responsible alternative.
What “Closed Loop” Actually Means
The term implies continuous recirculation of the same working fluid, sealed off from the environment. In engineering brochures, this is presented as near-hermetic; in practice, it is not.
Every large-scale closed-loop cooling installation contains several obligatory interface points with the outside world: makeup water connections, chemical injection ports, blowdown and bleed-off discharge lines, pressure relief valves, heat exchanger interfaces, and full-system drain connections for inspection and periodic chemical refresh.
Each of these is a release pathway. The “closed” descriptor refers to the recirculation architecture, not the degree of chemical containment. The People’s Tribune put it plainly in April 2026: “There is really no such thing as a true closed-loop system.”
That is not an activist framing; it is a description of how the engineering works.
The Chemical Inventory
To understand what gets released, you first need to understand what is inside these systems. Closed-loop cooling water is heavily treated with a rotating portfolio of chemical additives: corrosion inhibitors, biocides, and scale-control agents. The principal compounds of concern in the context of the Upper Santa Cruz watershed are:
Benzotriazole and tolyltriazole (BT/TT). These are copper corrosion inhibitors found in virtually every closed-loop system that contains copper or copper-alloy components, which describes most large-scale data centers. BT is highly water-soluble, highly persistent, and poorly removed by conventional wastewater treatment. Wastewater Treatment Plan effluent concentrations ranging from 10 to 100 μg/L have been documented in European studies. In a pan-European groundwater survey, benzotriazole was the fourth most commonly detected polar organic persistent pollutant above 100 ng/L. In U.S. groundwater sampling in Minnesota, tolyltriazole was detected in 11% of samples tested, at concentrations up to 0.78 μg/L.
There is no federal Maximum Contaminant Level for either compound. There is no Arizona monitoring requirement for cooling system blowdown. Australia has established a precautionary drinking water guideline of 7 ng/L for tolyltriazole.
Nitrite, at 600 to 1,200 ppm. Steel corrosion inhibitor. Readily mobilized in blowdown and not routinely monitored in cooling system effluent permits.
Molybdate, phosphate, glutaraldehyde, DBNPA, isothiazolinones, ethylene glycol. Each carries its own risk profile in receiving waters, bioaccumulation, acute aquatic toxicity, oxygen depletion, and eutrophication at recharge-adjacent discharge points.
How the Chemistry Gets Out
Blowdown is the primary mechanism. Operators periodically bleed a fraction of system water to prevent total dissolved solids from rising to levels that cause scale formation or inhibitor breakdown. Even a modest 2% monthly bleed can allow contaminant concentrations to rise substantially, since chemical loading does not dilute in proportion to makeup water additions. When blowdown finally leaves the site, it carries nitrite, glycol, and dissolved metals at concentrations that are potentially thousands of times above limits set for surface water discharge.
Beyond scheduled blowdown, the catalog of routine maintenance release pathways includes heat exchanger cleaning flushes, chemical slug dosing overflow, filter backwash, annual system drain-and-recharge events, pipe and heat exchanger failures, and pressure relief valve actuations. None of this is hypothetical; they are expected operational events.
The monitoring structure in place, where monitoring exists at all, is quarterly sampling. The water treatment engineering literature specifies continuous real-time monitoring of pH, conductivity, and inhibitor concentration, with monthly laboratory confirmation of dissolved metals as best practice. Loop chemistry can shift within hours if oxygen ingress occurs. Quarterly sampling is structurally incapable of detecting the episodic shifts that precede or accompany significant release events.
Why This Site Is Different
The Luckett Road data center corridor sits within the Upper Santa Cruz Active Management Area, a designated groundwater management zone under Arizona’s Groundwater Management Act. This designation reflects hydrogeological conditions that amplify the risk profile of closed-loop chemical discharge.
Depth to groundwater in portions of the Marana agricultural corridor ranges from approximately 30 to 60 feet. That is a shallow, unsaturated zone with limited capacity to absorb and slow the downward migration of mobile compounds like benzotriazole. CAP recharge infrastructure operates nearby; chemical contamination of recharge water bypasses the surface treatment step and enters the regional aquifer supply directly. The Santa Cruz River’s effluent-dependent and ephemeral reaches can receive direct surface drainage from industrial sites during storm events. Agricultural irrigation infrastructure in the Marana Farming Area provides a conduit for shallow groundwater movement. Additionally, there is currently no liner requirement for retention basins that receive cooling system discharge in the applicable Marana code.
The Regulatory Gap
ADEQ’s Multi-Sector General Permit covers stormwater discharges from industrial facilities, but routine cooling system blowdown discharged to on-site retention basins is typically classified as process wastewater, not stormwater, and may fall entirely outside MSGP monitoring requirements. Where analytical requirements do apply, they routinely exclude benzotriazole, tolyltriazole, molybdate, and biocide residuals.
Percolation of cooling system blowdown through an unlined retention basin to groundwater could constitute underground injection under EPA UIC definitions, but is not consistently regulated as such by ADEQ for industrial retention basins not constructed with injection intent. The result is effective groundwater injection of chemically treated cooling water without the permitting, well construction standards, or monitoring that formal injection wells require.
The Marana UDC, as currently written, contains no provisions for secondary containment of chemical storage tanks, engineered liner and leachate monitoring for basins receiving cooling system discharge, chemical inventory disclosure as a condition of site plan approval, or analytical monitoring of blowdown prior to discharge.
The Risk Comparison the Industry Doesn’t Make
A complete environmental risk assessment of data center cooling has to compare open-loop and closed-loop systems across multiple dimensions, not just water consumption volume.
Closed-loop systems consume less water. That is true, and it matters. They also concentrate chemical additives over time rather than diluting them continuously, produce periodic high-concentration blowdown events rather than lower-concentration continuous discharge, carry higher catastrophic release potential due to pressurized high-inventory systems, receive less regulatory scrutiny because the infrastructure is buried and less visible, and present a groundwater pathway through retention basin percolation that open systems typically do not.
What Is The Recommendation?
Five specific additions to the Marana regulatory framework, detailed with full ordinance language:
Secondary containment for all chemical storage tanks serving closed-loop systems at facilities exceeding 5 MW of installed IT load, consistent with EPA SPCC standards.
Engineered liner meeting RCRA Subtitle D industrial lagoon standards, plus a minimum of two groundwater monitoring wells, for any on-site basin receiving cooling system blowdown.
A Chemical Management Plan is a condition of site plan approval that identifies all compounds used, volumes on-site, approved discharge pathways, and spill response protocols, and is filed with both the Town of Marana and ADEQ.
Quarterly laboratory analysis of cooling system blowdown prior to any discharge, with results submitted to ADEQ and the Town of Marana. Full system drain events, defined as any discharge exceeding 10% of the total system volume, require 30 days' advance notice to ADEQ and pre- and post-sampling.
Real-time continuous monitoring of pH, conductivity, and dissolved oxygen in closed-loop systems at covered facilities, with automated alarm thresholds and data available to regulators upon request.
The full technical brief on closed-loop cooling systems:
Closed Loop Cooling System Brief
Closed Loop Cooling System Ordinance Update
Sonoran Think Tank is a civic research organization based in Marana, Arizona, focused on environmental policy, land use, and infrastructure governance in Southern Arizona.
Colin, much thanks for your series on DC cooling. Very informative, very helpful.
Please go a little further in re the specific chemicals you cite in this third installment: let folks know how/where to find specific information on toxcities, environmental fate, etc on these compounds including basic research tools like CAS numbers, MSDSs when available, URLs for standard user-friendly databases, etc.
A lot of this fight, not just in Marana, is going to depend on citizen science and you are making a strong contribution to that. Thanks. Onward!
Michael Gregory
Thank you Colin for your research on this topic. Once again we can see how we the people in the community are shielded from any public scrutiny into the the potential unintended consequences of the relentless building of data centers in Arizona and the the whole country for that matter.
I wonder what kind of health conditions/concerns might result from the chemicals described in your article.?