If you have read an O'Fallon water report that lists arsenic, or you own a private well and your lab sheet includes an arsenic line, the first question is usually what the result means for your household. This guide explains what arsenic is, where it shows up locally, and how to read a result before deciding on any next step.
What arsenic is and where it comes from
Arsenic is a naturally occurring element found in rock and soil. It can move into groundwater as water passes through certain geologic layers, and it can also reach water from some industrial and agricultural sources. Because it occurs naturally in parts of the region, it is one of the contaminants that local water reports mention alongside hardness, disinfection byproducts, and radium.
O'Fallon public water is a mix of river water and groundwater. Public suppliers test for regulated contaminants and publish the results. Private wells draw only from the ground, so their water depends on the aquifer the well reaches, and it can differ from one property to the next, even on the same street.
Reading a public water report
Annual public reports list each detected contaminant, the level found, and a reference value set by regulators. Find the arsenic line in the table of detected substances. If it is listed, read the notes beside it, since they explain whether the result came from a single sample or an average and how it compares with the reference value. If arsenic is not listed, check how the report describes substances that were not detected, because the wording varies.
A public report describes water at sample points in the supply system. It does not tell you what is in your own faucet if plumbing fittings, a home treatment unit, or an older fixture affects the water before it reaches you. Keep that limit in mind as you read.
Reading a private well result
A lab report for a well usually lists the arsenic result in the same units as other contaminants, next to a reference value. A result below the reference value is recorded as meeting that standard. A result above it is a reason to gather more information and pay attention, not a reason to panic. Ask the lab whether the sample came from raw well water or from a tap after a treatment device, because that changes what the number tells you.
Questions to answer before you test again
- Did the sample come from a tap downstream of a filter or softener?
- Did you follow the lab's collection instructions, including cleaning the faucet and running the water first?
- Is your well shallow or deep, and do you know what layer it reaches?
- Has anything changed in the well, pump, or pressure tank since the last test?
How to test for arsenic at home
Home test kits are generally screening tools. For a result you can rely on, use an accredited laboratory. The MO State Public Health Lab offers free or low-cost testing for coliform, E. coli, and iron bacteria, so check with them about what else they can run, and compare commercial lab options as well. Ask each lab which method it uses and what its reporting limit is, so you can compare the result with the reference value you care about.
If the result is above the reference value
Start by confirming the result with a second sample collected the same way, and confirm that the water comes from the source you expect. Then talk with a qualified water professional about treatment options. Ask what each option removes, what testing it requires afterward, and how its performance is checked over time. Bring the lab report so the conversation starts from the actual results.
Keeping records
Save every report and lab sheet in one place. Note the date, the sample location, and whether the sample was taken before or after any treatment device. Over time, that record shows whether a result is a one-time reading or a pattern, and it helps any future testing or repair make sense.