St. Thomas Water Is Moderate — Here's What It Costs You Each Year

Why Is St. Thomas Water So Moderate?

St. Thomas water hardness is 6.9 grains per gallon (GPG), which is 118 parts per million (ppm). That puts it just under the USGS threshold for “hard” water, which begins at 6.9 GPG, so you’re sitting right on the line. The reason has everything to do with where the water comes from, and it’s not a local groundwater story. The city draws from the Elgin Area Primary Water Supply System, which pulls surface water straight from Lake Huron. That’s a big, deep, cold lake, and the water in it has had a long time to settle and clarify. Unlike groundwater that seeps through limestone and picks up calcium and magnesium along the way, Lake Huron’s water is mostly rain and snowmelt that flowed into the lake over granite and other hard, crystalline rock in the Canadian Shield to the north. Those rocks don’t dissolve easily, so they contribute very little hardness to the water.

The water then travels through the Huron Elgin Water system, which treats and pipes it south to the St. Thomas area. The treatment process itself doesn’t remove hardness minerals, but it doesn’t add them either. What you’re left with is water that reflects the geology of the Great Lakes basin rather than the local bedrock under St. Thomas. The limestone that does exist in the region has less contact time with this surface water than it would with a well drawing directly from an aquifer. So the moderate reading you see at the tap is essentially Lake Huron’s natural chemistry, slightly influenced by the journey south but not dramatically altered by it. That’s the whole story: surface water from a hard-rock watershed, not groundwater percolating through soluble rock.

How St. Thomas Compares to Nearby Cities

Because St. Thomas, London, and the surrounding communities all pull from the same Elgin Area Primary Water Supply System, you’re going to see very similar hardness numbers across the region. If you’ve ever filled a glass in London and thought it tasted about the same as what comes out of the tap in St. Thomas, you’re not imagining it. They’re drawing from the same Lake Huron source, treated through the same regional infrastructure. That consistency is actually one of the advantages of a shared surface-water system. You don’t get the wild swings you’d see in areas where every neighborhood has its own well tapping into a different layer of rock.

The more interesting comparison is between this region and places that rely on groundwater. Towns that draw from deep wells in the limestone-rich bedrock of southwestern Ontario routinely see hardness levels well above 10 GPG, sometimes pushing into the “very hard” category. Your 6.9 GPG reading looks modest next to those numbers. The difference isn’t that St. Thomas does something special to soften the water; it’s that the source is fundamentally different. Lake Huron water starts softer and stays that way through treatment and distribution.

That said, 6.9 GPG is still high enough to matter in your home. According to the WQA and Battelle Memorial Institute’s 2010 detergent study, soft water can cut laundry detergent use by up to 50% and dish soap by up to 70%. At your level, you’re not dealing with the severe scale problems of a 15 GPG well, but you’re also not in the “no softener needed” zone. The USGS classifies anything above 7 GPG as hard, and you’re a hair below that, so you’re in a gray area where some households notice spotting and film and others don’t. The EPA doesn’t regulate hardness as a health hazard; it’s purely an aesthetic and efficiency concern under the Secondary Maximum Contaminant Level. But the efficiency part is real. Battelle Memorial Institute and the WQRF found in 2009 that gas water heaters lose about 4% efficiency per 5 GPG of hardness, and the DOE notes that just a quarter inch of scale can cut water-heater efficiency by up to 40%. At 6.9 GPG, you’re on the milder end of that spectrum, but scale will still build up over time, and it will shorten appliance life. Hard water typically reduces water-heater service life by roughly 30-50%, according to industry estimates. So while your water is moderate compared to the rest of the province, it’s not soft, and treating it would still pay off in detergent savings and appliance longevity.

How does St. Thomas stack up against the rest of Ontario? It's moderate at 6.9 GPG — Woodstock, Guelph, and Ingersoll sit nearby with different numbers. See the full hardest-water ranking for context.

See where St. Thomas sits on the Ontario water hardness map →

Want the broader explainer on what hardness actually is and how it behaves? Read hard water vs. soft water, explained.