OUTLOOK, Sask. — Farmers need to properly decommission abandoned wells, says Saskatchewan’s Water Security Agency.
“It’s the right thing to do,” Rob Walcer, senior technologist with the agency, told the South Saskatchewan River Watershed Stewards annual meeting.
“Wells are a hazard and (they are) a direct conduit to our aquifers.”
Some farmers fail to realize that surface contamination of water in an abandoned well pollutes the water drawn up in the new well because they are connected to the same aquifer.
He recalls visiting one farm where an abandoned well was capped with a coffee tin. There were wet rings near the top of that well, so the water could have been easily contaminated.
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“It made for an interesting discussion about where (the farmer) thought the water went. It went straight down the old well into his other one,” said Walcer.
The decommissioning procedure is different for drilled wells than it is for bored wells.
Drilled wells are typically eight to 46 centimetres in diameter. The old ones were made out of steel, while newer ones are PVC or fibreglass.
Decommissioning a drilled well requires specialized equipment and a trained professional.
“It’s work that should be done by a water well driller,” said Walcer.
Professionals should also be used for flowing artesian wells, contaminated wells and wells located in high risk areas such as corrals or near surface water or septic tanks.
A drilled well is decommissioned by filling the casing with a neat cement grout, which is a mixture containing 96 percent portland cement and four percent powdered bentonite clay.
Another option is to fill it with a high solids bentonite grout, which is a mixture of 20 percent bentonite clay and water.
The neat cement is heavy and harder to pump but provides a better seal. The high solids bentonite grout can be applied with a portable pump, but it can’t be used when water quality is poor.
Bored wells are 46 to 122 cm in diameter and can be made out of wood, brick, galvanized steel, porous concrete or fibreglass, which has been the material of choice since the late 1980s.
Grouting is too costly for these wide wells. Instead, the strategic placement of granular bentonite is more economical.
“It’s basically three-quarter inch clay chips,” said Walcer.
The chips swell up 10 to 12 times their normal size when contacted by water.
The well is disinfected with chlorine and then enough bags of bentonite are added to seal the bottom.
Each bentonite layer needs to be 30 cm thick, which requires about eight, 23 kilogram bags of the product for a 76 cm diameter well.
It takes about two hours for the chips to hydrate and fully swell. Once that happens, it forms an impermeable layer that is also flexible so it can accommodate shifting ground.
A similar bentonite layer should be created every six metres with coarse sand or fine gravel placed in between the layers. The filler should be free of silt or other fine material.
The top of the well requires a special bentonite seal for both bored and drilled wells.
“In order to know the location of the top, you do need to get a copy of the water well driller report. We do have an online database for that,” said Walcer.
The top layer of bentonite needs to extend 30 cm beyond the casing of the well to cover the annular space, which is the area between the bore hole and the casing, which is usually filled with gravel.
“This is one of the most important steps in decommissioning a bored well because it’s the only place during the decommissioning we can actually seal off the annular space,” he said.
“Without putting in a proper top seal, you’re compromising all the other steps.”
It takes a lot of bentonite to do a proper top seal. A 76 cm diameter well would require about 23, 23 kg bags of the product, while a 91 cm diameter well needs about 28 bags.
The excavation site should be backfilled with clay compacted at 30 cm intervals.
Qualifying farms can receive funding from the Saskatchewan government through the Farm and Ranch Water Infrastructure Program for 90 percent of the eligible costs of a decommissioning project up to a maximum of $10,000 per well.
Walcer said special care needs to be taken when decommissioning a well that was built in a well pit because of he confined spaces that can contain dangerous gases and low oxygen levels.
