Mar 22, 2019

Reassessing Groundwater

Q&A investigates the depth of groundwater

Jennifer McIntosh
Jennifer McIntosh

In rural areas of the U.S., water drawn from wells is vital for both human consumption and agricultural use. In the past, studies have posited that the amount of fresh groundwater available in the U.S. is relatively high and that the point at which freshwater turns brackish is fairly deep, but a recent study published in Environmental Research Letters suggests that this is not exactly the case. WQP Associate Editor Michael Meyer asked study co-author Jennifer McIntosh—a professor in the Department of Hydrology & Atmospheric Sciences at the University of Arizona—what she and her colleagues found and what it could mean.

Michael Meyer: Please tell me about the study. 

Jennifer McIntosh: We looked at the depth of fresh and more saline groundwater, which is called brackish groundwater, throughout basins in the U.S., and we discovered that the depth of fresh groundwater was less than previous studies had estimated—approximately half of the amount of water.

The second thing that we looked at [was] how that depth of fresh groundwater, which is the water that we can drink—we call it potable water supply—corresponds to existing water well depth, so it gives you an idea of locations in the U.S. where existing wells are approaching the limit of fresh groundwater, and in other areas of the U.S. where you could potentially drill deeper wells to access more water supply.

The third thing that we looked at was how the depth of freshwater corresponds to the depth of oil and gas activities, and to find areas where oil and gas activities are relatively close to freshwater resources that might be used for drinking water supply in the future.

Meyer: What were the results? 

McIntosh: We found places in the U.S. where the zone of fresh groundwater is relatively shallow and existing water wells are approaching the limit of that fresh groundwater at depth. For example, in the state of Michigan, that zone of freshwater is pretty shallow, and deeper than that, you find more saline water that’s not potable, so your zone of freshwater is thin. So if, for example, you are approaching your limit of freshwater supply in Michigan, you can’t drill deeper wells necessarily, because the water will be too saline to drink.

In other places, like in the western U.S. in an area that’s called the Basin and Range Province—which extends from Arizona up through Nevada up to Oregon—there are places where fresh groundwater extends much deeper than existing water wells, and so there are some locations where, if you’re running out of water supply, you could drill deeper wells to find fresh groundwater.

Meyer: What are the next steps we can take?

McIntosh: We looked at the depth of this window of potable groundwater supply, and we looked at it on a continental scale, so the whole U.S., and if you were to look in one specific area in a lot more detail, then I think that would provide really useful information, because our study was pretty broad-brush. If you say you wanted to know the depth of groundwater supplies within a city or a county, then you would need to look in that area in much more detail, because it does vary spatially.

Meyer: How do the oil and gas activities factor into this?

McIntosh: There’s evidence that in order to extract oil and gas, often the oil and gas industry will bring up water with that oil and gas—it’s called produced waters—and there’s evidence in several different locations that they’re producing brackish groundwater, so it’s slightly saline groundwater. That’s an example where there’s an industry that’s already using groundwater that we might be able to use in the future. 

The second example is that, with that produced water production, the oil and gas industry will often reinject those waters underground, and they’re injected into what’s called exempt aquifers—aquifers that they’ve gotten a permit to inject into—and sometimes those aquifers also represent this fresh-to-brackish groundwater. You can think of it as this water supply is already being used, in some cases by the oil and gas industry. It means that you couldn’t use it for potable water supply. 

About the author

Jennifer McIntosh, Ph.D., is professor in the Department of Hydrology & Atmospheric Sciences at the University of Arizona. McIntosh can be reached at [email protected]