Updated
Two sources, two treatment trains: Cincinnati's tap water explained
Greater Cincinnati Water Works (GCWW) draws from two very different sources. Most of the water comes from the Ohio River, a busy industrial river. The rest comes from wells in a buried glacial aquifer. The two are treated differently, and the report shows they reach your tap with slightly different chemistry.
The river plant: built for a working river
The Richard Miller Treatment Plant, on the east side of Hamilton County, treats Ohio River water and supplies about 88% of what GCWW customers drink. A river that carries barge traffic and upstream industry needs extra protection, so after coagulation, sedimentation and sand filtration, Miller sends water through granular activated carbon (GAC) and then ultraviolet disinfection, followed by chlorine. GCWW says it's one of only a few plants in the country to use all of these together, and it reactivates its spent carbon in an on-site furnace. The carbon guards against spills and cuts taste and odor, and because it lets the plant use less chlorine, it also means fewer disinfection byproducts.
Upstream monitoring is the other half of the plan. ORSANCO, the interstate river commission, runs 17 monitoring stations along the Ohio. GCWW is one of them, testing river water several times a day all year. The utility updated its Miller source water protection plan in December 2025.
The aquifer plant: softened groundwater
The Charles M. Bolton Treatment Plant in southern Butler County pumps from 13 wells in the Great Miami Buried Valley Aquifer and supplies the other 12% or so. Its treatment starts with lime softening, which removes some hardness and adjusts pH. GCWW also belongs to a local groundwater consortium of public and industrial well owners that protects the aquifer.
Side by side
| 2025 result | Miller (Ohio River) | Bolton (aquifer) |
|---|---|---|
| Average hardness | 126 mg/L (7 gpg) | 140 mg/L (8 gpg) |
| PFOA / PFOS average | not detected | about 5.0 / 5.2 ppt (maximums about 10.7 / 10.6) |
| GenX (HFPO-DA) | average 0.7 ppt, up to 3.5 | not detected |
For context, EPA's new 4 ppt limits for PFOA and PFOS don't take effect until 2029, and initial compliance monitoring is due by April 2027. GCWW says it meets all current regulations and is studying ways to meet the new limits. The Bolton averages above are higher than 4 ppt, so this is the issue to watch for customers on aquifer water. A filter certified to NSF/ANSI 53 or 58 for PFOA/PFOS works at the kitchen tap. See our filter selector.
Other Miller figures: fluoride 0.88 ppm (added under a 1969 Ohio law), the highest TTHM compliance level 49.6 ppb (limit 80) and HAA5 12.6 ppb (limit 60). Turbidity peaked at 0.16 NTU and met the standard 100% of the time.
Lead and copper in 2025
GCWW says no lead is detectable in water leaving its plants. Corrosion control limits how much lead the water picks up from older home plumbing. Tap sampling at high-risk homes was low in both 2025 rounds:
- January–June: 90th percentile 1.32 ppb, 0 of 113 homes above the 15 ppb action level (highest 13.5).
- July–December: 90th percentile 1.92 ppb, 0 of 108 homes above (highest 11.9).
The water is moderately hard. To see whether a softener pays off at 7 to 8 grains per gallon, try the softener salt cost calculator.
Cincinnati water FAQ
How does Cincinnati deal with spills on the Ohio River?
GCWW analyzes Ohio River water several times a day, every day of the year, as one of the monitoring stations in ORSANCO's early-warning network for spills. When a spill is coming, the utility can stop pumping from the river and use stored water until it passes. Granular activated carbon at the Miller plant adds another barrier against chemical spills.
Why does my water's hardness depend on where I live?
Ohio River water from the Richard Miller plant averages about 126 mg/L (7 gpg). Groundwater from the Charles M. Bolton plant averages about 140 mg/L (8 gpg), even after lime softening. Which plant supplies your tap depends on your location. Most customers get mainly Miller water, since that plant provides about 88% of the supply.
Can I get my water tested for lead for free?
Yes. GCWW tests customers' water for lead at no cost. You can request a kit and look up your service line on the lead map at lead.myGCWW.org or by calling 513-651-LEAD.
Sources
- Greater Cincinnati Water Works, 2025 Water Quality Report
- Ohio River Valley Water Sanitation Commission (ORSANCO)
- EPA PFAS National Primary Drinking Water Regulation
Reviewed November 1, 2026 against the sources above. Public water system ID OH3102612 (Greater Cincinnati Water Works). Figures describe the utility's system, not your home's plumbing; a certified lab test tells you what comes out of your tap. Spot an error? Tell us.