Kirkland Signature water is safe to drink. Both the purified and alkaline varieties are produced by Niagara Bottling, one of the largest bottled water manufacturers in the United States, using standard industry purification methods that meet FDA requirements for bottled water. There’s nothing in the available testing data or production process that raises safety concerns.
How Kirkland Water Is Purified
Kirkland Signature purified water goes through a multi-step process before it reaches the bottle. Niagara Bottling uses microfiltration and reverse osmosis, which together remove the vast majority of dissolved solids, bacteria, and chemical contaminants from the source water. Reverse osmosis works by forcing water through a membrane with pores so small that most impurities can’t pass through. After purification, Niagara adds a small amount of minerals back into the water to improve taste, since fully stripped water can taste flat.
Costco also sells a Kirkland Signature Alkaline Water with a pH of 9.5. This version is ionized to raise its pH above the neutral level of 7. The ingredient list simply reads “water,” so the specific electrolytes or minerals used in the process aren’t disclosed on the label. Alkaline water won’t harm you, though the claimed health benefits beyond basic hydration remain unproven.
What’s in the Water
Bottled water in the U.S. is regulated by the FDA, which sets maximum allowable levels for contaminants like arsenic, lead, and mercury. These limits mirror or exceed the EPA standards applied to municipal tap water. Kirkland water, as a product of Niagara Bottling, must meet these federal standards before it can be sold.
Available water quality data for Kirkland-associated testing shows arsenic at 0.650 parts per billion, which is well below the FDA’s maximum of 10 ppb. Mercury was tested for but not detected at all. These are the kinds of numbers you’d expect from a properly purified bottled water product. Niagara Bottling conducts its own internal quality testing throughout the production process, as required by FDA regulations for bottled water facilities.
Is the Plastic Bottle Safe?
Kirkland water bottles are made from PET (polyethylene terephthalate), the same type of plastic used by most major bottled water brands. PET does not contain BPA or phthalates, two chemicals that have drawn concern for their potential to disrupt hormones. This makes PET one of the safer plastics for food and beverage contact.
That said, PET bottles can degrade when exposed to heat. Leaving bottled water in a hot car or in direct sunlight for extended periods may cause trace amounts of chemicals from the plastic to leach into the water. This isn’t unique to Kirkland. It applies to virtually all bottled water sold in PET containers. Store your water in a cool place and you won’t have an issue.
How It Compares to Tap Water
For most people in the U.S., both Kirkland bottled water and local tap water are safe to drink. Municipal water systems are tested far more frequently than bottled water, sometimes hundreds of times per month, and are held to strict EPA standards. Bottled water isn’t necessarily cleaner or safer than what comes out of your faucet.
Where bottled water like Kirkland’s has an edge is consistency. If you live in an area with aging pipes, high mineral content, or occasional boil-water advisories, a purified bottled water gives you a reliable baseline. The reverse osmosis process removes contaminants that some older municipal systems struggle to fully eliminate. But if your local water report looks clean, you’re paying a significant premium for convenience rather than safety.
The Bottom Line on Kirkland Water
Kirkland water is a mainstream, mass-produced bottled water that goes through legitimate purification, is packaged in BPA-free plastic, and tests well within federal safety limits for contaminants. It’s comparable in quality to other major bottled water brands and, in many cases, costs less per bottle thanks to Costco’s bulk pricing. If you’re buying it because you like the taste or the convenience, there’s no safety reason to switch.