TapWaterData US Water Hardness Dataset

Measured drinking-water hardness for 16,561 United States cities, in milligrams per litre as calcium carbonate (mg/L as CaCO3) and in grains per gallon (gpg). Each value carries its source, source tier and sample date. Values are derived from public primary sources — utility Consumer Confidence Reports (CCRs), the EPA/USGS Water Quality Portal, and USGS classification bands — then unit-normalised, population-weighted across each city's utilities, tier-resolved and verified against a hand-checked golden sample. Source tiers are labelled rather than blended: T1 utility-reported, T2 computed from utility-reported calcium and magnesium, T3 labelled county ambient estimate from the EPA/USGS Water Quality Portal. Cities are classified into the USGS bands soft (0–60), moderately hard (60–120), hard (120–180) and very hard (>180 mg/L). Two files are provided with identical fields: water-hardness.csv (17-column header, licence and citation in comment rows) and water-hardness.json (with a _meta block). Columns: slug, city, state, stateCode, lat, lng, population, hardness_mg_l, hardness_gpg, category, tier, source, sourceDate, nSamples, disputed, range_min, range_max. The canonical, continuously updated source is tapwaterdata.com/water-hardness; full methodology at tapwaterdata.com/water-hardness/methodology.