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The antidote for arsenic

July 19, 2022

Rotary mud drilling prevents mineralized arsenic contamination in Wisconsin wells.
Ingestion of mineralized arsenic in drinking water can be harmful even in small amounts if consumed routinely over time. Although treatment systems can make contaminated water sources safe to drink, it is more desirable to prevent contamination. The state of Wisconsin in the U.S. requires water well drillers to use rotary techniques with drilling fluid to bore through the arsenic-producing stratum. Drilling fluid prevents air from stimulating arsenic formation. Casing the well permanently isolates deep, uncontaminated aquifers from the arsenic-producing zone. It’s working, as this contractor and its fleet of TH60 water well rigs prove.

The Leo Van De Yacht Well Drilling company of Green Bay had been keeping up with booming residential and municipal water well needs during a decades-long population surge in the picturesque real estate of eastern Wisconsin. The company quickly became masters of a state-required technique for getting past the poison to the clean drinking water below.

The results have been restoring confidence to landowners and community residents and setting the example for other states, which initiated similar surveys that showed the problem is more widespread than had been thought.


The highest concentration of arsenic-rich mineralization in eastern Wisconsin is at the top of the St. Peter Sandstone layer. The farther down below, the less contaminated the water. Researchers determined that the water was within acceptable standards when drillers got past this aquifer into a lower aquifer above the Cambrian sandstone bedrock. This is now the water well drillers in this region target both public and private wells.

Epiroc TH60 water well drill rig

In 2004 the WDNR (Wisconsin Department of Natural Resources) took several steps to mitigate the problem. They published land charts marked with casing and grouting depth minimums that drillers must comply with. They made it mandatory to report well geography for each well drilled and placed restrictions on drilling technique: Drilling must be done using rotary mud technique only. Since arsenic is released by the oxygenation of sulfide minerals embedded in the layer, experts believe that introducing high volumes of air into this formation during drilling dramatically exacerbates the problem. Furthermore, they think once initial oxidation occurs, the process is self-sustaining. Once triggered, this constant release of arsenic will inevitably find its way to the groundwater.


The object of drilling in strata containing embedded arsenic deposits is to do so without unnecessary disturbance, then quickly seal them so that they are not exposed to air and never come in contact with the well’s water. Bentonite mud helps a bit, too, as its clay helps seal the walls of the bore from air as it is opened up.

The new guidelines tended to slow water well drilling rates. Before 2004 a six-man Van De Yacht crew was drilling more than 500 wells a year per rig, routinely drilling two 90-meter (300 ft) wells daily. Owner Troy Van De Yacht said their personal best was 300 meters (1,000 ft) in one day, using both rotary mud and down-the-hole hammer techniques.


Switching to using rotary mud only meant that a single well required up to three days, first drilling and casing off past the St. Peter Sandstone and then drilling to the pink Cambrian sandstone target. The well shown is an example of the company’s work in the arsenic advisory area. It was drilled to replace the residential well at that site, which tested 36 parts per billion (0.04 mg/L) — more than three and a half times the federal limit. 


It was an easy task. At 36 to 73 meters (120 to 240 ft), they were into the pink cuttings that identified the arsenic-free Cambrian sandstone, advancing 6 meters (20 ft) every 15 minutes. The well produced 151 liters per minute (40 gpm) with an arsenic level test finding of “None Detected.”

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