Antidote to Arsenic Is Rotary Mud Drilling
November 28, 2012
A surging rural population brought boom times for residential water well drilling in eastern Wisconsin. For companies like Leo Van De Yacht Well Drilling of Green Bay, it long has been its bread and butter – even after the discovery of arsenic in the Fox River Valley’s aquifers in 1987. They are well-skilled in the techniques required to get past the poison to the clean drinking water below.
Poison in the Well
The discovery of inorganic arsenic in this area’s ground water came about almost by accident, resulting from a routine feasibility study for a proposed landfill northwest of Oshkosh. The mineral content survey showed arsenic in five of the eight wells to be above federally accepted minimums. That prompted the Wisconsin Department of Resources (WDNR) to conduct studies of the cause and extent of the arsenic problem, which continued into the year 2000 and beyond.
The U.S. Environmental Protection Agency (EPA) classifies arsenic as a carcinogen. Long-term exposure also may cause medical issues, such as cardiovascular disease, diabetes and immunological disorders. Drinking water with high arsenic levels also may cause more immediate symptoms, such as stomach pain, nausea, vomiting and diarrhea. The federal government limit for arsenic in potable water sources is 0.010 mg/L, or 10 parts per billion.
Though there are treatment systems for arsenic-contaminated water, it’s much more desirable to find water sources that do not require treatment. In the Fox River Valley, the solution wasn’t farther away, just further down.
Department of Natural Resources (DNR) researchers established an arsenic advisory area, with a buffer zone over a buried geology called St. Peter Sandstone, which runs from southwest of Oshkosh to just west of Green Bay. Although arsenic has been found in wells throughout the state, the principal zone of concern lies over this particular formation.
Fanning the Flames
The highest concentration of arsenic-rich mineralization is present at the top of the St. Peter Sandstone layer. The farther down one samples below it, the less contaminated the water. Researchers determined that when drillers got past this aquifer into a lower aquifer above the Cambrian sandstone bedrock, the water was within acceptable standards. This is the target of water well drillers in this region today – for both public and private wells.
In 2004, the DNR took several steps to mitigate the problem. It published land charts marked with casing and grouting depth minimums with which drillers must comply. They made it mandatory to report well geography for each drilling. And they placed restrictions on drilling technique – drilling must be done by only rotary mud.
Because arsenic is released by the oxygenation of sulfide minerals imbedded in the layer, experts believe that the introduction of high volumes of air into this formation during drilling greatly exacerbates the problem. Furthermore, they believe once initial oxidation occurs, the process is self-sustaining, and this constant release of arsenic, once triggered, inevitably will find its way into the ground water.
Equipping for Success
Always a forward-looking company, Van De Yacht has a demonstrated history of staying ahead of the game – investing in new tooling, and remaining diversified to sustain its profitability.
Troy Van De Yacht, who succeeded his father, Leo, as owner in 2000, had been drilling with him since 1986, when he was 14 years old. Leo started the company in 1963. He was 17 years old when his parents – who were intent on finding each of their 12 children a secure job – saw an advertisement for an old cable drill.
He worked that rig hard enough to afford a kelly-drive-rig in 1965. Upgrading to an Ingersoll Rand (Atlas Copco, today) rotary top-head in 1967 established the company as leading-edge pioneers in the industry. Van De Yacht remains an avid customer – its latest acquisition is the newest version of the T3W.
One thing Troy has noticed about the new T3W is the longevity of the cables. As a rule of thumb, Troy brings his equipment into the shop when temperatures fall below zero to spare his crew, and to make use of the time for maintenance. They rebuild mud pumps and tophead swivels, replace hydraulic and compressor filters, and re-cable the rigs. But three years into service, the new T3W’s cables show no signs of wear. He attributes it to this version’s larger sheaves and its cable tensioner.
The company can handle just about any drilling application within its operating area, including municipal blended water wells, elevator ram shafts, larger-diameter pre-construction piling, and vertical geothermal installations. And they are skilled experts in arsenic preventive well drilling techniques, having adjusted to the new regulations without missing a beat.
The new guidelines have slowed drilling rates down some. Prior to 2004, Troy and his six-man crew were drilling more than 500 wells a year per rig, routinely drilling two 300-foot wells per day. Their personal best, Troy says, was 1,000 feet in one day, using both rotary mud and down-the-hole hammer techniques.
Switching to rotary-mud-only now means that a single well takes up to three days to first drill and case off past the St. Peter Sandstone, and then finish drilling to the pink Cambrian sandstone target. However, these wells pay more. So financially, the change has been a wash for them, and the new regulations have not significantly impacted the company’s bottom line.
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.
Case in Point
The well in the accompanying photos is a good example of the company’s work in the arsenic advisory area. It was drilled to replace the residential well at that site, which tested at 36 parts per billion – more than three-and-a-half-times the federal limit.
The DNR map specifies that Van De Yacht case the hole to 168 feet at this site, and then drill the well to a total depth of 240 feet.
Using a 9-inch bit on 4 -inch pipe, Troy says they drilled, cased and grouted the upper hole one day, then completed the 6-inch lower hole to total depth the following day after a 24-hour cure time for the grout.
As they drill, he explains, they also are required to sample and report the geology of the hole to the DNR’s well log. At this hole, the first 50 feet were clay. From 50 feet to 120 feet, they encountered limestone with a penetration rate of 22 minutes per 20-foot pipe. At 120 feet through 240 feet, they were into the tell-tale pink cuttings of arsenic-free Cambrian sandstone, advancing 20 feet every 15 minutes.
The results were right on the money. The well is producing 40 gallons per minute, and the resulting arsenic level is “none detected,” proof that the technique works perfectly when it’s in the hands of master drillers.
This residential site’s existing well tested at 36 parts per billion – three times the federal limit. However, tests of this replacement well photographed as it was drilled by Leo Van De Yacht’s experienced, certified crew resulted in the finding “none detected.” ND