BOHARM, Sask. – With thousands of abandoned water wells in existence across Western Canada, the risk for contaminating ground water is substantial.
At a recent well decommissioning demonstration, the Moose Jaw Creek Watershed Association used two old wells to show landowners what’s involved in capping old water wells.
“The most critical part is to seal any contaminants from going into your water-bearing aquifer. You don’t want a well being a source of contamination. Any open well can allow material into the aquifer,” said Michael Friesen, operations manager with Andrews and Sons Drilling Ltd. from Regina.
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“A well on a river flood plain can mean whatever is in river water gets into the ground water. Animal waste washing down a well is also pretty serious. So you want to seal it up to prevent future damage.”
Sealing up the right spot in the well shaft is important, said Twyla Armstrong, technical co-ordinator with the Moose Jaw Creek Watershed Association. Putting the plug below the aquifer won’t do anything to protect that aquifer.
“If you’ve got a well going through four different aquifers and you’ve got contaminants going down that well, you can contaminate any one, or all four of those aquifers,” Armstrong said.
“Then those aquifers can contaminate each other, because you’re providing a channel between them.
“If other people are pulling water off those sources, the contaminants can travel that way, as well. To minimize or eliminate the possibility that contaminants reach the aquifers, we tend to use bentonite clay to plug it off.”
Armstrong said sand particles are large, with a lot of pore spaces in between the particles that allow contaminants through. Clay particles are much smaller, and the spaces are smaller, so the bentonite clay provides a better barrier.
Concrete is an option in some cases, but when concrete cures, it can crack and shrink, which leaves spaces for contaminants to reach the water source. Depending on specific situations and local geology, various blends of cement and bentonite can also be used.
Armstrong said the basic steps of decommissioning a water well are as follows: chlorinate the well to get rid of contaminants before you start filling it; remove all debris and equipment from the well head; fill up the well with proper materials; remove the cribbing near the surface; and mound up the final hole with clean fill to allow for settling.
“The amount of chlorine depends on the depth of the well and the level of water within the well. By finding the quantity of water in the well, you can calculate how much bleach to add to reach the proper concentration to disinfect it,” she said.
Friesen, who drills and fills wells throughout southern Saskatchewan, said new wells will have either a 20 or 75 centimetre casing. But there’s a wide range of sizes for older wells.
“With large diameter wells, if you know the geology of the area and are aware of what depth the (sand lenses) are, you can backfill with clean material, use bentonite as a plug where the water-containing sand is, then more clean fill for the rest of the well,” said Friesen.
For smaller diameter wells, he said it’s best to “tremie” the fill in.
“We’ve grouted wells anywhere from (15 to 120 metres) deep. That involves running a grout or tremie line to the bottom of the well,” he said.
“If you don’t get the tremie line to the bottom of the well, air pockets might form, as the grout may not drop right to the bottom. By getting the hose directly to the bottom, you know the grout is going in at the bottom and it has nowhere else to go but up.”
At the Moose Jaw sites, Friesen used a 50:50 blend of cement and bentonite, pumped to the bottom of the wells through a four centimetre plastic hose, with a gas-powered grout pump.
“Common mixing rates are 20 gallons of water to a sack of bentonite grout, but that can vary, depending on mixes and specific situations. Those are guidelines, but water quality makes a difference and if it’s too thick, it’s not pumpable. You want to be able to pump it,” he said.
“For the mixture that we do, it does tend to harden up when dried, but when you keep it moist, the bentonite in the concrete swells and keeps it from becoming as hard as cement. It becomes stiff, but not hard, nor fluid.”
The only time Friesen uses straight concrete is with flowing wells, where the water level comes above the ground. He said in those cases, bentonite grouts by themselves can be pushed out.
After the well shaft is filled and the top casing is removed, Friesen said clean clay is the preferred fill to cap off the top of the well.
“Water goes through sand, but clay is nonpermeable. But quite often, what happens in the field is we use whatever is available.”
The cost of decommissioning a well varies, depending on each specific situation. Friesen said his company usually works on an hourly basis, plus materials.
There are government programs available to help cover part of the costs of decommissioning water wells.
“The first option is the Canada-Saskatchewan water supply expansion program. If you’re planning to develop a well, this program will support you in decommissioning a well at the same time a new well is drilled. It will cover up to a third of the costs, through the (Prairie Farm Rehabilitation Administration),” Armstrong said.
“A second source is through the environmental farm plan program. Once landowners go through a workshop and do an approved farm plan, there’s funding available to decommission a well, at half the cost, up to $6,000.”
