Groundwater Issues: Mining Deep Sand
When I think of mining sand, I think about a quarry where heavy machinery digs and dumps huge quantities of sand into dump trucks, not unlike the photo below (click here for reference).
Aggregates (sand and gravel, for example) have become important resources in our lives and are used in construction of roads, asphalt and concrete.
Earlier this month, I was talking with a recently-retired hydrogeologist friend of mine and he told me about a proposed sand mine in south-eastern Manitoba. After doing some digging, I learned that this proposed mine is not taking sand from the surface, like in the photo above. Rather, sand is extracted by drilling into the ground through up to 35 metres till (unconsolidated silt, clay, sand, gravel, cobbles), followed by sedimentary rock, which consists mainly of limestone followed by a deeper shale unit. The reports indicate that the limestone is between 0 to about 40 metres in thickness, while the shale is between 0 and 5 metres. Beneath the shale is a sandstone aquifer that is 20 to 23 metres thick (click here for reference).
Once the drill hole has entered into the sandstone, compressed air will be injected into the aquifer. The sand and water then returns to the ground surface as a slurry where sand is separated from water. The water is treated and returned to the aquifer underground. The method is called “air-lifting” and it’s a process that has been used to pump water out of the ground. In this case, however, air-lifting is being used to remove water AND sand.
The image below is a screen capture from a YouTube video from the mining company.
This process of removing sand from the sandstone aquifer results in a large cavity underground, as shown below (from the mining company’s presentation).
The mining company has proposed to drill thousands (1000s!) of these types of holes, spaced apart such that there’s no collapse of the ground by creating these cavities in the sandstone. This method, called “room and pillar mining”, has been used in other geologic materials, and there are design criteria that geotechnical engineers use to ensure safety. In the photo below (reference here), a mine in Slovakia shows a room and pillar method.
In the above photo example, mining is completed by physical processes (heavy machinery), and the properties of the rocks can be observed and measured directly. Resources are mined with human activity in the mine itself.
In the proposed study in Manitoba, huge cavities will be created from instruments at the ground surface, and geologic materials will only be inspected via rock core from drilling.
I’ve only done a cursory review of the documents for this proposed sand-mining project in Manitoba. A lot of consultants have been retained by the mining company as well as by the government of Manitoba and other interested parties.
I have a few concerns that may already be addressed in some of the reports, but I’ve noticed that in some reporting, the “competent limestone” is described as a “limestone aquifer”. The limestone layer is even referred to as the “Red River Carbonate Aquifer” (reference here) and provides potable water to different areas in Manitoba. With limestone units like this, the top of the limestone is typically more fractured than the rest of the unit due to erosional activity during geologic deposition millions of years ago. This particular limestone has been interpreted to be fractured near the bottom of the limestone unit as well (see slide 11 here). When this type of geologic unit is fractured, it can provide preferential pathways for movement and storage of groundwater.
The photo below is from a trip I took years ago to the Bruce Peninsula in Ontario. It shows carbonate rocks of the Niagara Escarpment. You can see lots of fractures (cracks) in the rock but at the top of the photo, there are fewer fractures. There are places where fractures are even rarer and there can be massive blocks of rock with few, if any, fractures.
An aquifer, by definition, can provide water, which, in the case of limestone, implies that the limestone likely has voids and fractures to provide the water. This begs the question: is the limestone a productive aquifer, or is it a competent limestone? From my current perspective, “productive aquifer” and “competent” are opposites.
If the limestone is fractured, will it be able to support the till that lies above the limestone if a large cavity is created in the sandstone beneath it? Additionally, when the large cavity is created by removing sand during the mining process, there will be a change in the underground pressure. Such a change in pressure will like cause a change in the geologic materials surrounding the cavity. These changes could include an increase in fractures in the limestone, thereby reducing the structural integrity of the limestone, thereby increasing the possibility of land subsidence.
According to some reports, there are two aquifers here: the deeper sandstone aquifer (the Winnipeg Sandstone) that the mining company wants to mine for sand, and the shallower limestone aquifer (the Red River Carbonate Aquifer). In between the two aquifers, there is a shale unit that is present across most of the study area. It seems quite possible that mining the sand using the methodology proposed by the mining company might cause the shale unit to be compromised, thereby connecting both aquifers. One consultant noted that there is a distinct difference in the groundwater between these two aquifers. While this might not be an issue if both aquifers are clean freshwater, what happens if there is contamination in one aquifer and the shale unit in between is compromised? At the very least, compromising the shale will provide a potential pathway between the two aquifers. A consultant has provided this schematic as a possible scenario (see slide 27 here):
Consultants for the mining company have tested the process by drilling through the till and limestone and into the sandstone. They then used their air-lifting process to remove sand. They observed the collapse of the shale unit and the lower part of the limestone. These types of failures are irreversible and because it’s happening underground there will be other immeasurable side effects.
I should mention that the sandstone aquifer (the Winnipeg Sandstone) is a productive source of drinking water for locals. CBC has covered this topic with articles here and here and has communicated with concerned citizens. The Red River Carbonate Aquifer (limestone) also provides potable water to many people in Manitoba.
As I’ve mentioned, I have not read all the reports and presentations that are available for this project (many of which are provided here). The questions I have raised might be addressed in some of the reports but my purpose here is not to state whether this project is good or not; rather, it’s to provide information about a potential project that has a significant groundwater component.
The sand at this site (outside of Vivian, Manitoba) is of high-quality, which means it has a high percentage of silica. There are a lot of uses for silica: glass, electronics. But another use for silica sand is for fracking. Fracking is another article, but in the fracking process, a combination of silica sand and water and other chemicals are injected into a deep well, with the purpose of opening fractures in a geologic material, such that oil and/or gas become more mobile underground and therefore easier to extract. Fracking is a controversial method for extraction of oil and gas.
The company’s website mentions different uses for the silica they’ve proposed to mine (renewable energy products, glass, electronics) but makes no mention of using the silica sand for fracking, but the COO has years of experience in the oil and gas industry, particularly with fracking companies.
For further reading, this link provides a good set of reference material for those who might be more interested. And the mining company has a lot of information here.
I have no affiliation with the mining company, the government of Manitoba or any of the consultants, although I have previously worked with Matrix Solutions, one of the companies who conducted an independent review on the hydrogeology and geochemistry of the site and proposed mining activity. All of the research I conducted for this article was from publicly available documents online.