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.
Utility-reported hardness
We separately transcribe selected official utility reports, retaining the water-system ID, source URL, reporting period, source table, and date we checked the source. The utility source directory lists the current coverage. A utility’s reported range stays a range; we do not invent an average from its midpoint. An average is shown only when the utility publishes one.
Utility-reported results appear before environmental context, but they are still system-wide records. Confirm the provider on your bill. When no verified utility hardness is available, the page explicitly says so and recommends asking the provider or testing the tap.
Regional hardness evidence
A regional hardness value is calculated only from source-type-aligned USGS observations within 25 miles of the ZIP centroid. Groundwater systems can use only well or spring observations; surface-water systems can use only stream, river, lake, or reservoir observations. Sites explicitly classified as coastal or estuarine and all source-type fallbacks are excluded. If no qualifying observation exists, the value remains unavailable.
Every page retains monitoring distance, dates, site count, confidence, and evidence tier. Regional environmental evidence is never labeled as a utility-treated-water value or a household tap result. Research comparisons apply a stricter rule: at least three recent sites, non-low confidence, a nearest site within 10 miles, and a median no greater than 1,000 PPM. More extreme values stay visible locally but are not ranked because saline or otherwise unusual source conditions may not be comparable.
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. Each numerical result retains its water-system ID. Local pages let you filter by provider; measurements are never pooled into a single result for all homes. The imported history retains all qualifying records, including active records older than the historical lookback.
Numerical lead and copper results are displayed only for completed monitoring periods present in the source data. Action levels are labeled as regulatory triggers rather than pass/fail health boundaries. Missing measurements remain missing and are never converted to a favorable result.
Known limits
- USGS hardness is regional environmental evidence, not utility-treated water or 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.