How HardnessCheck Measures Your County's Water Hardness

One public record

Every figure comes from the Water Quality Portal, where USGS, the EPA and state agencies publish their sampling results. We pulled every hardness result (as calcium carbonate) from 1990 to 5005: 607,842 results across 3,035 counties, joined to county by the sampling station's FIPS code.

Why the median of site medians

A single heavily-studied lake or well field can contribute thousands of results. Averaging raw results would let that one site decide the county. So each site is reduced to its own median first, and the county figure is the median across sites. In testing, one Vermont lake survey made up most of the state's results and would have made several counties look soft; the site-median approach removed that distortion.

Wells first, when there are enough

Where a county has at least 5 groundwater sites, the headline uses wells only, because that is what most private wells and many utilities draw on. That applies to 1,425 counties. The rest use every site, wells and surface water together, and each page says which basis it used.

Confidence

A county is high confidence with 10 or more sites and 30 or more samples (1,597 counties), medium with 3 sites and 5 samples (1,153), and low below that (285). Thin data lowers the confidence label; it never moves the verdict.

The extremes, and what they mean

The softest county median is Hempstead County, AR, at 0.1 gpg; the hardest is Ward County, TX, at 218.5 gpg. Readings far above 30 gpg (164 counties) usually mean mineral-heavy or brackish groundwater, which is a different problem from ordinary hard water and is not something a household softener is built to handle.

Where to go next

See the range across the country: Moore County, NC (0.8 gpg), Warren County, KY (9.7 gpg), Clay County, MO (31 gpg).

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