Hard Water: Measured and Modelled Hardness of the World's Rivers, Lakes and Groundwater, and the Rocks Behind It

Maps of water hardness by supply zone are familiar in England and Wales, but there is no open world equivalent. I assembled one from the open subset of the UNEP GEMS/Water Global Freshwater Quality Archive: 16,682 river, lake, reservoir and groundwater stations in 34 countries with 611,858 usable samples, each station summarised by its median hardness in mg/l as calcium carbonate. The median station is 202 mg/l; groundwater (276 mg/l) is harder than rivers (199) and lakes (139). I then fitted a deliberately simple mixing model that predicts hardness from the shares of seven rock groups within 10 km (from Macrostrat's generalised world geology), annual rainfall and the type of water. Under spatial cross-validation, which hides whole 5 degree squares, it places 29% of stations in the right one of six hardness classes and 64% within one class, against 38% for a constant guess; it explains 24% of the variance of log hardness. Crystalline rock is the clearest signal: stations surrounded mostly by it have a median of 34 mg/l. For Britain and Ireland, where the archive has no British stations, a sharper model using 1 km BGS and GSI bedrock and calibrated on 388 Irish stations reaches 82% within one class in Ireland and reproduces the soft north and west and the hard chalk of the south and east, while missing places such as London whose water comes from far away. The measurements are not tap water, and the modelled layer is an estimate with the errors stated. Interactive version: https://tristan-hotham.github.io/hard-water/. Data and code: https://github.com/Tristan-Hotham/hard-water. Blog post: https://blog.tristanhotham.com/2026/10/28/hard-water/. Sources: UNEP GEMS/Water Global Freshwater Quality Archive v3, open subset (CC BY 4.0); Macrostrat map tiles (CC BY 4.0); BGS Geology 625k (Open Government Licence v3.0); Geological Survey Ireland and Geological Survey of Northern Ireland 1:500k bedrock (CC BY 4.0, Open Government Licence); CRU CL 2.0 rainfall; GeoNames (CC BY 4.0); Natural Earth (public domain); ONS boundaries (Open Government Licence v3.0); AWS Terrain Tiles.