Water treatment research

Hardness. USGS classifies water at 0–60 mg/L as calcium carbonate as soft, 61–120 as moderately hard, 121–180 as hard and above 180 as very hard. Regional hardness maps built on that classification are useful for understanding why softeners are near-universal in some metros and rare in others. They are not a substitute for a measurement at your tap.

Contaminant limits. EPA's NPDWRs are the enforceable standards for public systems: nitrate at 10 mg/L, arsenic at 0.010 mg/L, lead handled as a 0.010 mg/L action level with a treatment technique, total coliform at no more than 5% of monthly samples positive, and PFAS limits including 0.0000040 mg/L for PFOA and PFOS and 0.00001 mg/L for GenX, plus a hazard index for four PFAS combined. Any prevalence claim you read should be traceable back to those definitions.

Your own system's data. The Consumer Confidence Report is the most specific dataset that exists about your water, and your supplier is required to deliver it every year by July 1. EPA hosts a search tool for finding it. For private wells there is no equivalent — EPA recommends annual testing through a state-certified lab, and the results are the only data that exist about your water.

Cost benchmarks. Two independent 2026 datasets: HomeGuide puts salt-based softeners at $1,200–$3,800 installed and whole-house RO at $4,800–$8,000; Fixr puts the national average softener installation near $2,500 within a $1,100–$3,000 typical range. Where two reputable sources disagree, the honest conclusion is that the true range is wide, not that one is right.

How to choose

Prefer primary sources over vendor whitepapers

USGS, EPA and NSF/ANSI standards documents are checkable. A manufacturer's internal study is not.

Check whether a statistic is about your system

National prevalence data says nothing about your tap. Your CCR does.

Read cost indexes as ranges

The gap between HomeGuide and Fixr on the same product is itself the finding: local labor and sales channel dominate.

Be careful with lead service line claims

EPA treats lead through an action level and treatment technique rather than a simple limit. Service line material questions belong with your utility.

Date every figure you rely on

Standards get revised — NSF/ANSI 44, 55 and 60 all carry 2024 revision years, and 42, 53, 58, 62 and 401 carry 2023.