Arsenic in Well Water in North Carolina

Where does arsenic in NC well water come from?

In North Carolina, arsenic in well water mostly comes out of the bedrock itself. The NC DEQ arsenic brochure explains that rock born of ancient volcanic activity is richer in arsenic-bearing minerals, and groundwater moving through fractures in that rock picks the arsenic up. DEQ's own investigations concluded that arsenic in the state's groundwater is "most often naturally occurring."

The amount varies well to well. DEQ notes "there are many instances where one well contains dissolved arsenic and another well nearby is arsenic free." Orange County's review adds that deeper wells tend to have higher arsenic, and UNC researchers found that wells over the limit were more likely to have alkaline, higher-pH water. So a neighbor's clean result says little about yours.

Arsenic can also come from old pesticide use, mining and coal-fired power plants, NC DHHS says, but in the Piedmont the rock is the usual source. Iron-sulfide minerals in fractured bedrock are one of the carriers DEQ pictures on its brochure.

What is the limit for arsenic in well water?

The benchmark is 10 micrograms per liter, shown on lab reports as 0.010 mg/L or 10 parts per billion. EPA adopted that number for public water in 2001 with a 2006 compliance date, and NC DEQ took the same level in 2001 and reaffirmed it in 2011. EPA's health-based goal sits at zero.

The limit is a public-water standard, so nothing forces a private well owner to meet it, but NC DHHS uses it as the benchmark for private well results. Current DHHS guidance uses 10 µg/L, and that is the number to compare against. DHHS also notes that arsenic 3 is more toxic and harder to remove than arsenic 5, and a routine test does not tell them apart.

Which counties near Durham sit on the Slate Belt?

Of the five counties this site covers, Orange and Person lie on the Carolina Slate Belt. Orange County's health department says "most of Orange County lies within the former Carolina Slate belt" and places the county 5th of 16 Slate Belt counties for groundwater arsenic above 1 µg/L, while UNC's Superfund Research Program traces the belt "from Union County to Person County."

Rank is not the same as danger. The same Orange County review found "nearly all" of its arsenic results below the 10 µg/L limit. NC DHHS lists Alexander, Anson, Dare, Montgomery, Stanly and Union counties as the places with more frequent detections, and UNC found especially high levels in the younger Albemarle Arc farther southwest.

Parts of Durham, Chatham, Granville and Orange lie instead on Triassic Basin sedimentary rock, which USGS found often carries arsenic above 1 µg/L in its lake-bed layers. County-by-county arsenic rates for Durham, Granville and Person were not found in a primary source, so this page does not give one. The private wells overview and Triassic Basin page describe those rocks.

Which lab panel picks up arsenic in well water?

Arsenic shows up on a certified lab's inorganic (metals) panel, and in NC that panel includes it without a special request. Every new well must be tested for it under 15A NCAC 18A .3803, and Orange County's published inorganic panel names it as well. As DHHS puts it, "you can't see, smell, or taste arsenic," so the lab number is the whole answer.

For an existing well, the NC DHHS private well FAQ suggests metals every two years. Because arsenic tracks with pH in some Slate Belt rock, read the pH line on the same report. The well water testing page compares county panels and fees.

How is arsenic removed from well water?

Removal is a two-stage job in NC DHHS's guidance: oxidize the arsenic to the arsenic 5 form, then send the water through adsorptive media, ion exchange, distillation or reverse osmosis. The first stage is there because a routine test cannot say which form is present, so an oxidizing unit such as an iron oxidation or manganese dioxide filter converts any arsenic 3 before the main treatment.

Each method has a trade-off in the DHHS list. Adsorptive media makes little wastewater. Ion exchange works much like a softener. Distillation is point-of-use with low output and high electric use. Reverse osmosis gives a small output and a large amount of wastewater. NC DEQ adds that a point-of-use granular ferric oxide filter "reliably removes most arsenic forms to below detectable levels," and that whole-house versions exist at higher cost.

Other options in the DEQ brochure are bottled water, a public water connection where lines are near, or a new well, with the caveat that a new well may also contain arsenic. NSF cautions that a certification covers only the contaminants a unit was tested for, so look for arsenic by name on the listing. A softener sized for hardness, an iron filter or a sediment filter is not an arsenic treatment on its own. Retest after any system goes in, and keep up the maintenance; DEQ warns that a neglected system can expose you again.

Does boiling remove arsenic from well water?

Boiling leaves the arsenic in the pot; it does not take it out. Washington State's health department settles it in one line: "No. Boiling water does not remove arsenic" (Washington DOH). A rolling boil kills germs, which is why boil-water notices exist, but arsenic is a dissolved element and stays put.

What matters is what you drink and cook with. NC DEQ says the greatest exposure is from drinking and cooking, and that skin contact from bathing is "much less of a risk for most people." That is why a single treated kitchen tap is a common answer for arsenic, unlike radon, which needs whole-house treatment; the radon and uranium page explains the difference.

Questions worth asking

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