US Water Hardness Dataset (16,561 Cities)

Measured drinking-water hardness for 16,561 US cities — mg/L as calcium carbonate and grains per gallon, each value carrying its source, tier, and sample date. Built from public primary sources — utility Consumer Confidence Reports (CCRs), the EPA/USGS Water Quality Portal, and USGS classification bands — population-weighted per city and verified against a hand-checked golden sample. Files: water-hardness.csv and water-hardness.json (same 17 fields: slug, city, state, stateCode, lat, lng, population, hardness_mg_l, hardness_gpg, category, tier, source, sourceDate, nSamples, disputed, range_min, range_max). Canonical, continuously-updated source: tapwaterdata.com/water-hardness · GitHub: Tapwaterdata/water-hardness-dataset License: CC BY 4.0.