This set contains data on the regional aquifers of the United States. Hardness is usually reported in a graphic in the section on the water quality of each aquifer, however, it is not reported in all cases.
This web site features hardness data for some aquifers but has a challenging interface. Select "Water Quality" under Data Category. Choose the Samples dataset. Keep moving through the retrieval options until you are able to select a particular station or aquifer.