Methodology

How the risk profile is built.

Approach last reviewed · 28 May 2026

The report answers one question: for a private well at this address, which contaminants are worth testing for, and in what order? Here's exactly how it gets there, and where the limits of a regional model lie.

1. We locate the well in its setting

From your address we place the well on a map and pull the two things that drive well-water risk: the aquifer type it likely draws from, and the surrounding land use. Arsenic and radon are geological, they come from certain bedrock. Nitrate is mostly agricultural and septic. Bacteria come from surface water reaching the well. Location tells us which of these are plausible here.

2. We read what turns up in nearby wells

The USGS domestic-wells program has sampled tens of thousands of private wells across the country and published which contaminants exceed health benchmarks, broken down by aquifer and region. We use that to rank the contaminants most reported in wells like yours, rather than guessing. This is population-level evidence: what has actually been found in wells in comparable settings.

3. We compare each risk to the EPA limit

For every contaminant we surface, we show the EPA's Maximum Contaminant Level (for regulated contaminants) or health advisory level (for others), so "a risk here" always comes with the number that defines "too much." These are the same thresholds a public water system must meet, applied to your well.

4. We turn it into a prioritized panel

We combine how likely a contaminant is in your area with how serious it is to health, and sort the list. Bacteria and nitrate are near-universal baseline tests. Arsenic, lead, radon and others move up or down based on your local geology and land use. The result is a short "test this first" list instead of an intimidating catalog.

What this model can and cannot tell you

This is the most important section, so we'll be blunt.

• It is a regional estimate, not a measurement. It describes risk for your area based on nearby wells and geology. It cannot tell you the concentration in your specific well. Two wells on the same road can differ.
• Only a lab test reveals your actual water. The report's purpose is to make sure you order the right lab test. It is a guide to testing, not a substitute for it.
• Local factors we can't see still matter. Well depth, casing condition, a nearby spill, or a failing septic tank can change your risk in ways no regional model captures. When in doubt, test.

We think a regional model is genuinely useful precisely because it's honest about this. It gets you from "I have no idea what to check" to a specific, affordable test plan, which is where most well owners are stuck.

Keeping it current

EPA drinking water limits change over time as standards are updated, and USGS periodically publishes new well-water assessments. Because the report leans on figures that can be revised, we re-check the EPA limits and the USGS datasets we rely on before each update and note the review date above. If a limit changes, the number in the report changes with it.

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