by Andrew Gelderloos, Geology Specialist, Water Resources Division, Michigan Department of Environment, Great Lakes, and Energy
Last month, I discussed some groundwater basics. This month, I will illustrate how groundwater moves in the subsurface as a preface to a discussion next month about drinking water and environmental considerations.
In the first diagram, Underground Saturated and Unsaturated Zones, water at the ground surface can enter the underground unsaturated zone. In the unsaturated zone, the pore space between sediments is filled with air and water. Water infiltrates downward through the unsaturated zone and makes its way down to the water table. The depth of the water table varies. It could be a couple of inches in one place and more than a hundred feet in another place. And it changes over time due to factors such as rainfall and vegetation. At and below the water table, in the saturated zone, all pore space is filled with water; this area is known as an aquifer. Groundwater moves primarily horizontally within an aquifer but can flow in any direction as it slowly moves from locations with a higher water table to locations with a lower water table.
In the second diagram, Aquifers and Confining Layers, groundwater moves through the pore spaces within an aquifer. The individual particles of some sediments, such as sand, are large enough that when they are packed together, the pore spaces are still connected. However, the individual particles of some sediments, such as clay, are so small that, when they are packed together, the pore spaces have only limited connections. In a confining layer, connections between pore spaces are so limited that groundwater cannot move through the layer.
In the third diagram, Shallow and Deep Wells, the groundwater pumped out of the shallower well may have only travelled a short distance during days or weeks since it infiltrated from the surface. It is possible that the water pumped from the shallow well may have even infiltrated from the ground surface adjacent to the well. The groundwater pumped out of the deeper well may have travelled many miles during the many years since it was last at the ground surface. Water drawn from the well drilled under the confining layer has traveled horizontally from a location not depicted in this diagram, where there is no confining layer between the aquifer and the ground surface.
When we start putting the pieces together, groundwater flow can become complex very quickly – often due to varying amounts of connectivity. For the final hypothetical diagram, Groundwater Flow Paths, consider:
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How might the groundwater have gotten to any point on the diagram? Where did it originally infiltrate from the ground surface?
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Where do confining layers separate aquifers? How many aquifers might a well driller encounter from any given point on the ground surface?
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This diagram only illustrates one “slice of the cake”. What might another slice look like? What if we sliced through in a different direction? What if we zoom out further or deeper?
The illustrations and the terms that I’ve used here simplify a very complex process down to some basic groundwater concepts. As you engage with groundwater topics, you will find that trained hydrogeologists evaluate many nuances that can become incredibly important in particular situations.