Not all groundwater flows downhill

Happy Sunday! To start off the new week, I have some delicious tidbits on my research for you. This past Monday, I went with my advisor and another grad student to Worcester Polytechnic Institute for the NSF-sponsored Water Workshop. I presented a poster on my work, so I figured I’d talk about the details of my research focus a bit more.

Standing next to my poster for the Water Workshop at WPI!

Everyone agrees that water flows downhill. But does groundwater always flow downhill? That is, does groundwater flow always follow the topography? Not necessarily. Many people before me have proved that the groundwater table does not reflect the topography for a lot of aquifers. This depends on a lot of things, including recharge, depth of the groundwater table, the height of the aquifer, and the extent of the watershed. Strangely enough, not a single one of these factors dominate whether groundwater in an unconfined aquifer flows contrary to topography. They rather work together at different intervals to create this counter-topography behavior. And someone (i.e. me?) could spend a whole career investigating how all those factors affect one another to produce this affect.

It turns out that this behavior, which we call recharge-controlled flow, happens in a lot of places around the world, including parts of Massachusetts. More commonly, you see recharge-controlled flow in arid regions like the southwestern United States and my current study area, the Atacama!

Corenthal et al. (2016)

What’s going on in the Atacama, the world’s driest nonpolar desert, is really fascinating. Figure A is a conceptual illustration that shows how the groundwater table flows under all these high peaks to reach the salar, which is a salt flat. Based on what my research team and I know, the factors controlling groundwater flow in the Atacama include recharge (or rather, lack thereof) and the depth of the groundwater table from the surface.

The lack of recharge in Salar de Atacama as the world’s driest nonpolar desert means that its groundwater needs to come from somewhere else. That somewhere else is the relatively wetter, higher elevation peaks that we call the Altiplano (i.e. “high plains” in Spanish). This difference in recharge over time creates a difference in hydrologic head that causes the groundwater to defy all the topographical peaks in the Altiplano to flow towards Salar de Atacama.

Since Atacama is so dry, this groundwater flow creates a negative water balance equation as it continues to flow from areas with little precipitation to areas with almost no precipitation at all. In other words, more water is leaving the system than coming in. Because of this imbalance, the groundwater table probably continues to lower. As a groundwater table lowers, it becomes less dependent on the topographic variations.

This behavior has a lot of interesting and concerning implications. Atacama’s groundwater, which is the area’s only source of water, is nowhere near sustainable. This point is really important for the people and businesses that depend on this water. Plus, since groundwater takes a long time to travel, the distance that the Atacama’s water has travelled means that it is incredibly old. It’s probably on the order of thousands to tens of thousands of years old!

Well, here’s a quick taste of what I’ve been focusing on this semester. I promise I’ll talk about it more soon!