Overview
Adrian Lechel, hydrogeologist at SRK Consulting, has come up with a new method for measuring transmissivity and hydraulic conductivity that’s virtually free, gives answers quickly that compare well to pumping tests, and uses the Rugged TROLL 100 Data Logger to make measurements at the end of the airlifting (clean-out) that happens when all new wells are drilled.
Challenge
Finding out the hydraulic properties of sites being considered for construction, mining, or other excavations is important and can make or break the decision to proceed. The properties are fundamental to the ability of hydrogeologists to predict production rates, dewatering, and environmental impact. But obtaining that information takes time and can be extremely expensive, requiring drilling additional production wells and conducting pumping tests to determine the site's transmissivity and hydraulic conductivity. These parameters determine how quickly or slowly water moves in the aquifer.
Adrian Lechel, a hydrogeologist with SRK Consulting in Perth, Australia, knew that finding a faster and easier way to measure a site’s transmissivity and hydraulic conductivity would be a game-changer.
Solution
Lechel realised that clients were already carrying out a routine operation called “airlift development” on new monitoring wells, and that it could do double duty to also measure transmissivity.
Lechel explained the airlift development. “When the well is drilled, they have to clean out the debris and drilling fluids that fall in during drilling. To do that, they use a long tube and a compressor to push air into the well. It causes the water to gush upward and spill out, bringing the debris with it. It’s like blowing on a straw in a long water bottle. The water is going to come up and over the sides of the bottle.”
The airlift development usually takes a couple of hours, and measurements of electrical conductivity, temperature, and pH tell the hydrogeologist when the flushing is complete, and the water coming out is the pristine groundwater.
To Lechel, it seemed that this process provides the opportunity to do more than just flush the monitoring well. The airlift development is obviously “causing some trouble,” Lechel said. He realised that it causes a disequilibrium that is just like a slug test or a pumping test. “So I said, if we are pumping for the purpose of cleaning the hole, why don’t we measure how fast the water responds to the airlift? Because the hole doesn't know if it's being pumped because it's for a pumping test, or if it's being pumped because it's getting cleaned.”
Lechel’s method relied on some low-tech (a bucket and a stopwatch to measure the flow rate during the airlift) and some high-tech for measuring pressure (the In-Situ Rugged TROLL 100 Data Logger). At the end of the airlift, he stopped the compressor, pulled the tube, inserted the logger into the well, and took pressure measurements for about 20 to 30 minutes as the water recovered from the airlift stress.
Lechel gathered data from the airlift development of every new monitoring well from mine sites he was working on, about 30 in total. He could use hydraulic equations to calculate transmissivity and hydraulic conductivity of their screened aquifers. But he knew that he would need to prove his new method. He wanted to compare his results to what he calls the "gold standard of hydraulic methods," the pumping tests.
For that, additional production wells still needed to be drilled next to the monitoring wells. Because large production wells are so expensive, only a small number would be selected. The decision on where to drill them had not yet been made. And so, he waited.
Results
Several months later, the moment of truth came to compare Lechel’s new airlift method with the pumping tests. Six production wells were drilled next to monitoring wells, where he had his numbers from the airlift tests.
“I was hoping my method would point in the right direction. Most of the time, it perfectly predicted the pumping test. I had the evidence, the proof,” said Lechel.
Not only did it compare well with the pumping tests, but in further comparisons, he showed that the new method actually outperformed the slug tests. Until Lechel’s work, the slug test has been the only hydraulic testing method for a single monitoring well that can be done without the need for an adjacent production well.
The lower expense of the transmissivity measurements using the airlift method makes it possible to get hydraulic data on every new monitoring well drilled on the mine site. This increases the number of data points, which enables the hydrogeologist to understand better the heterogeneity of the aquifer across the project area. As Lechel points out, “That’s often more important than the quality of the transmissivity data gathered by the gold standard pumping test at only a few locations.”
Lechel noted that the success of his idea makes sense, because fundamentally all of the methods are measuring the same thing—the recovery (“drawdown”) after a disturbance to the water. These shared fundamentals are what originally led Lechel to his realisation that if opportunistic recovery data were gathered during the airlift, that’s done routinely during the construction of all new monitoring wells, he could determine the hydraulic parameters much more reliably than slug tests and without the delay and expense of pumping tests.
And as he also found out, that leads to happy clients!
For more information about this work: Transmissivity from Airlift Development of New Monitoring Bores | SRK Presentation
This article was first published on the In-Situ website.
