The short answer
Arsenic is common in Union County well water. Three separate data sets found that roughly 18% to 21% of tested wells were above EPA's 10 µg/L arsenic standard, linked mainly to the county's slate belt bedrock. A certified-lab test is the only reliable way to know about your own well.
How common it is: three data sets, three answers
Three different sources have looked at arsenic in Union County private wells. They cover different years and different sets of wells, so their percentages differ a little. All three describe wells that were tested, not every well in the county, and none of them can tell you about yours.
| Study | Wells and period | Above 10 µg/L | How the wells were chosen |
|---|---|---|---|
| NC Division of Water Resources, 2015 groundwater assessment | 2,993 private well samples, Jan 2000–Jan 2011 | 21.0% (630 samples)[source] | State private-well sampling data; the report notes earlier samples may lean toward problem wells |
| UNC Superfund Research Program, Well Water Contaminants table (2022) | 5,154 private wells, Oct 1998–May 2019 | 19.46% (1,003 wells)[source] | NC DHHS private-well test data[source] |
| UNC Superfund Research Program with Union County Environmental Health (2023) | 224 wells | 18% (41 samples)[source] | Volunteer homeowners, not a random sample |
The takeaway isn't the exact percentage. It's that every way of looking at it lands near one in five tested wells. Union County Environmental Health says about 25 percent of the county's population relies on groundwater for drinking water,[source] so this affects a lot of households.
Why Union County: the slate belt
Most of Union County sits in a geologic unit called the Carolina Terrane, made mostly of lightly metamorphosed sedimentary rocks with a slaty look, which is why the area has long been called the "slate belt."[source] NC DWR's mapping found arsenic above the 10 µg/L standard mainly in wells drilled into a rock type called meta-mudstone, with generally lower levels in meta-volcanic rock.[source]
The same report says arsenic appears to be released from sulfide minerals in the bedrock, and that some may come from historic arsenical pesticide use. It notes that shallow dug wells, or wells cased only into the upper weathered zone, are particularly prone to arsenic from those shallow sources.[source] Its advice is direct: in an area underlain by the Carolina Terrane, untreated well water should be consumed only after it has been tested for arsenic.[source]
Step 1: Test
Arsenic is one of the required tests for new private wells in North Carolina, so if your well was drilled after mid-2008 you may already have a result.[source] Find that report and look for the arsenic line. Our guide to reading your NC new-well test walks through it.
If you don't have a result, or it's more than a couple of years old, get a new one from a state-certified laboratory. NC DHHS recommends testing private wells for heavy metals, which includes arsenic, every two years.[source] Our water testing service sets up a certified-lab panel that also covers iron, manganese, pH and lead, since those often travel together here: NC DWR found iron and manganese exceedances in about one in five and one in four Union County samples.[source]
Step 2: Treat, if the result calls for it
EPA's drinking water standard for arsenic is 0.010 mg/L, the same as 10 µg/L or 10 ppb.[source] If your result is above it, the usual approach is point-of-use treatment for drinking and cooking water, because that's where exposure happens. Two main categories:
- Reverse osmosis at the kitchen tap. Look for a system certified to NSF/ANSI 58 with an arsenic reduction claim on its listing. Our arsenic treatment page and reverse osmosis page explain how these work.
- Adsorptive media filters. Some filters use media designed to hold arsenic. Again, check the exact model's certification listing for an arsenic claim rather than relying on the box.
Which one fits depends on your full lab panel. Iron, manganese and low pH can all affect how arsenic treatment performs, so we design around the whole result, not just one number. Very high results, or arsenic alongside other problems, may call for whole-home treatment; that's a conversation after the lab report, not before.
Point-of-use and whole-home systems connect to your plumbing. Clear Bridge Water supplies and sizes the equipment; here's how licensed installation works.
Step 3: Retest
A certified system is tested under standard conditions. Your water isn't standard, so the only proof it's working in your home is a test of the treated water. We retest after setup, then on a regular schedule, because cartridges and membranes wear out. A Care Plan bundles cartridge changes with that retest so it doesn't slip.