You may want to refine exactly what you mean by hardness. Hardness can mean different things to different audiences. Some may think of hardness as the literal dissolved calcium or magnesium definition or some think of hardness as all dissolved minerals in water, both surface and conveyed.
You may be able to create your own features from the simple USGS maps at the following locations.
Hardness in water, USGS Water Science School
USGS NAWQA: Quality of Water from Domestic Wells in the United States
Hope this helps.
Peter
