Source layers
Hard Water Index uses three primary public-data layers. The U.S. Geological Survey supplies hardness observations for parameter 00900. EPA SDWIS records supply public water-system identity, source type, population served, and compliance history. Census ZIP Code Tabulation Areas and place geography provide a reproducible geographic framework for local pages.
Hardness estimates
A local hardness value is selected from qualifying nearby USGS observations, with distance, observation dates, sample count, site count, and confidence retained on the page. Searches are limited to 75 miles. If no qualifying observation is available, the value remains unavailable.
The classification follows the USGS general scale: 0–60 mg/L as calcium carbonate is soft, 61–120 is moderately hard, 121–180 is hard, and more than 180 is very hard. PPM is treated as equivalent to mg/L for these dilute water concentrations, and GPG is displayed using 17.1 mg/L per grain per US gallon.
Connecting systems to places
EPA drinking-water data is organized by public water system rather than ZIP code. A system is associated with a local profile only through a supported method that is retained in the published data: an explicit SDWIS served ZIP, a served-city and Census ZCTA match, or a narrowly guarded municipal-system and county inference.
A utility's mailing ZIP is not treated as its service area. Ambiguous property-specific or overlapping systems are not silently converted into one representative utility.
Compliance and contaminant records
SDWIS compliance history is utility-wide. “No reported active health-based violation” means no such record was found in the represented reporting window; it does not mean every contaminant was measured at zero or that every tap is contaminant-free. Historical and active records are labeled separately.
Numerical lead and copper results are displayed only when present in the source data. Missing measurements remain missing and are never converted to a pass.
Known limits
- USGS hardness is nearby environmental evidence, not a household tap sample.
- Utility service areas do not perfectly follow ZIP or Census boundaries.
- ZCTAs approximate postal delivery areas and are not the complete USPS ZIP inventory.
- City and state summaries describe published profiles and are not population weighted.
- Building plumbing and in-home treatment can change tap-water results.
Update and correction practice
The pipeline validates normalized records against a strict schema and keeps source dates and URLs with the data. Corrections should be supported by an authoritative source. To report a potential error, emailhello@hardwaterindex.com with the affected URL and evidence.