London Water Is Moderate — Here's What It Costs You Each Year

Why Is London, Ontario Water So Moderate?

London’s water doesn’t come from a deep limestone aquifer or a mineral-rich well. It comes from the Great Lakes, and that’s the whole story behind that 6.0 GPG reading. Roughly 85% of the city’s supply is drawn from Lake Huron, with the remaining 15% coming from Lake Erie. Both are massive, cold, and relatively young bodies of water, and they simply haven’t had the time or the rock contact to pick up heavy loads of dissolved calcium and magnesium. Here’s the geology in plain terms. The Great Lakes sit in basins carved out by glaciers during the last ice age, and the water itself is mostly a mix of rainfall, snowmelt, and runoff from the surrounding Canadian Shield and sedimentary bedrock. The Shield is ancient granite and gneiss, which are notoriously stingy when it comes to dissolving minerals. Granite just doesn’t give up calcium the way limestone does. As the water moves through the lakes and into the intake pipes, it does pass over some Paleozoic sedimentary rock, including dolomite and limestone, which is where the hardness does come from. But the contact time is short and the water volume is enormous. You’re not dealing with a slow-moving well that sits in a limestone bed for months. You’re dealing with a huge, well-mixed lake system where the mineral content gets diluted to a fairly consistent, moderate level. That’s why London lands at 6.0 GPG, which puts it right in the “moderately hard” range. It’s not soft water by any stretch, and you’ll still see scale build up on faucets and showerheads over time. But it’s also nowhere near the 10.5 GPG threshold that the USGS classifies as “very hard.” The lakes do the heavy lifting here. They give London a water supply that’s mineralized enough to be noticeable, but not so loaded that it’s wreaking havoc on every appliance in the house. The mix of Lake Huron and Lake Erie water is the reason you get that middle-of-the-road reading, and it’s remarkably stable year-round because the lakes don’t swing wildly in mineral content the way shallow wells or rivers can. ## How London Compares to Nearby Cities

The honest answer is that London’s water profile is very much a function of where the city sits relative to the Great Lakes, and most of the surrounding communities are drinking from the same family of sources. If you live in London, you’re getting that Lake Huron and Lake Erie blend. Neighboring cities like St. Thomas, Woodstock, and Stratford don’t all draw from the exact same intake pipes, but they’re all pulling from the same regional bedrock and glacial history. That means you’re not going to see a dramatic hardness cliff when you cross the city line. What you will see is variation based on whether a town relies on groundwater or surface water. Some of the smaller municipalities in Middlesex and Oxford counties tap into shallow wells that sit in glacial till and limestone deposits, and those wells can run harder than London’s lake supply. A well that draws from a limestone-rich aquifer can easily push past 10 GPG, which would put it in the “very hard” category. But that’s not a knock on those towns. It’s just the difference between lake water and groundwater. The lakes give you consistency; the wells give you mineral punch. If you’re comparing London to a city like Sarnia, which also draws from Lake Huron, you’re looking at a very similar hardness profile. Both cities are pulling from the same lake, so the water chemistry is going to be close. The same logic applies to cities along the Lake Erie shoreline. They’re all drinking from the same diluted, moderate mineral soup. The real contrast in this region isn’t between London and its neighbors. It’s between the Great Lakes cities and the inland communities that rely on groundwater. That’s where you’ll find the harder water, and that’s where a water softener starts to make a lot more sense. For London, the moderate reading means you’re in a good spot, but you’re not immune to scale. It’s just a matter of degree.

How does London stack up against the rest of Ontario? It's moderate at 6 GPG — Woodstock, Guelph, and Ingersoll sit nearby with different numbers. See the full hardest-water ranking for context.

See where London 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.