What happened to my water?

One of the hot button topics again splashing across the media is the looming global water crisis. The article that prompted this post was gleaned from the CNN website on 25 February 2024, but has since spread across the political spectrum. (Isn’t it amazing how what should be such a non-political issue can so quickly become weaponized?)

This specific doomsday projection takes place at an elevation of 7100 feet in Mexico City and predicts that the city is going to run out of water real soon. All would (hopefully) agree that the disruption of adequate water supplies has the potential to become a major blow to the livelihood of the city’s twenty-two million residents; all of whom expect that something wet, and hopefully potable, will guzzle — or at least drip — out of their tap when they turn on the faucet. Unfortunately, that may not happen for very much longer.

The forecast is not pretty: in a worst-case scenario, significant portions of Mexico’s capital may run dry in a matter of months. It may be important to note that not all areas of the city are projected to be distressed to the same extent, or at the same time. (These variations are generally blamed on economic considerations, and I’ll let you decide for yourself which areas will be hit earliest and hardest… the poor, or the most affluent.)

There is definitely an overabundance of factors contributing to this impending crisis, and while I have no intention, or desire, to delve into all of them in this post, a brief overview touching on a few of the main players might be in order.

Let’s start with the obvious. Climate Change is always available to be called out as the perpetrator in any crisis, and anything to do with our global water supplies would certainly seem to qualify. There is no doubt that central Mexico has been abnormally dry in recent years, and that Mexico City itself is suffering through a long-term drought. (Click here for an earlier post that may help explain why certain portions of the globe are drier than others, even without the complexities of climate change.)

Beyond an uncooperative climate, it is clear that demand in Mexico City is rapidly increasing. Those 22,0000,000 local residents need water for their lives, and many of them are also being fruitful and multiplying (it’s a biological imperative that no individual or species can buck). It’s a simple equation: more people = the need for more water.

This inevitable population increase requires an intricate interplay of infrastructure — reservoirs and tanks and pipes and such — to get the water from where it starts and is stored, to where it’s used. Unfortunately, the complex network has grown with the population, and much of it is old, outdated, and failing (an infirmity not limited to Mexico City). It is estimated that leaking groundworks dribbles out 40% of the available fluids during transmission from the source to the faucet.

The physical reality of the city doesn’t help either. Mexico City is built upon a drained lake bed and is surrounded by mountains (including a dozen or so volcanoes). Yes, they get some runoff from the encircling heights but not enough, so they have historically augmented their meager supply with groundwater to make up the shortfall. Groundwater currently provides an estimated 70% of the city’s water needs.

Unfortunately, there isn’t sufficient surface flow to recharge the once-extensive aquifer that is (was) below the original lake bed (click here for a brief introduction to groundwater recharge).

There are several reasons for the lack of recharge, and the inevitable drawdown of the water table. Along with the diversion of surface runoff before it even reaches the city, the area where the lake used to be has in large measure been paved over. The resulting impermeable surface blocks much of the percolation that the groundwater system needs to refill itself.

Yet they continue to withdraw water from the aquifer, as they have been forced to do for hundreds of years. But seriously, what other choice do they have? This is a Hobson’s Choice faced by all too many of the planet’s clusters of humanity.

And, in addition to the loss of water there is a related consequence: the city is also sinking. According to an article in National Geographic (15 June 2023), within the last century the Mexican capital has sunk over thirty feet, “warping buildings and disrupting infrastructure.” The article lays the blame “largely on groundwater extraction.”

Let’s unpack this a bit and try to figure out what’s going on.

In an attempt to tie the loss of water and a lowering land surface together (and link them to natural earth processes), I’d like to focus on a little appreciated result of over-pumping: the lowering of the land as fluids are withdrawn from the subsurface and its effect on groundwater supplies. The geologic term is subsidence.

Here’s the short version: any fluid in the ground is contained within voids in the subsurface materials, whether it’s unconsolidated alluvium or has been lithified into rock. These open spaces are called pores, and the physical measurement of the percentage of pore space in a rock is called porosity (click here for more on this most useful of geological characteristics).

Any interstitial fluids that might be filling the pore spaces — such as oil or gas, or water — exert pressure on the surrounding grains of rock material and help keep them from collapsing. Collapse is bad: it can (and will) pack the grains closer together; reducing the porosity and lowering everything above the depleted aquifer… the whole way to the surface.

Confused? There was a situation along the Gulf Coast (prior to the early 1970s when I was completing my undergraduate studies) that may help clarify the process. While this may fall into the “urban myth” category, I tend to believe everything Dr. Saint taught us in Groundwater Hydrology. Whatever, if it isn’t true it oughta be.

There is (was) a lot of petroleum in the sedimentary rocks beneath the Gulf of Mexico, as well as hiding below the states that border it. One of the oil companies — we ALL use oil all the time, so don’t even think about getting your back up — had developed a major field on an island in the Gulf. With time, a town built up around the wells, complete with moms and kids and schools and cheesy strip malls.

As expected, the field ultimately ran dry and the oil company moved on. But… what didn’t move was the town. The populace was fully invested in secondary industries by this time and didn’t need the oil jobs to survive. So they stayed.

Unfortunately, all the pumping had reduced the internal pore pressure under the island, the interstitial support was reduced, and the rocks began to compress. In a very short span of years the land began to subside, and the island started to sink into the Gulf. “Would you like a bucket of seawater with your fries?”

The residents — recognizing the realities and anticipating what was coming for them — sued the oil company for sufficient funds to relocate them and their town to dry land. The corporation refused so they ended up in court, in what was a true David vs. Goliath contest. The company sent in their geologists who proved — beyond a shadow of a doubt — that subsidence due to oil extraction was a myth, the flooding of the island was surely an Act of God, and they were not responsible in any way. Needless to say, they won.

Well, some clever citizen did his or her due diligence and discovered the following: oil beneath land belonged to the land owner; oil beneath the seafloor belonged to whoever held the lease; but oil in the intertidal zone was the property of the state… and subject to the payment of royalties into the state’s coffers. The legal extension was obvious: since the subsidence had been proven to have been a natural event, the island was actually part of the tidewaters… and the oil company owed the state past royalties (including interest, of course — TFG is learning all he needs to know about being on the wrong side interest payments as we speak).

Well, this cost was quickly determined to exceed the amount that the town was initially suing for, so the oil company drug everyone back into court where their geologists proved — again beyond a shadow of a doubt — that their inadvertent over-pumping had caused the porosity to collapse and the land to subside, they were very sorry and would love to relocate the town, and who do we make out the check to? (Please note: I was NOT one of the corporate geologists who was required to castrate themselves…)

Whether this narrative is true or not remains a mystery to me, but this is what I was told by Dr. Saint… who never lied and was always right.

It works like this with water, too. We do a groundwater lab in G102 that tracks the rate of subsidence in the southern portion of San Francisco Bay around San Jose during the 1900s (a bit north of the image, above, but the process is the same). When the class gets finished with their analysis the interpretation is clear: pumping of groundwater led to collapse of the aquifer and subsidence in the unconsolidated sediments south of the bay, while leaving the portions of the city built on solid rock essentially untouched. (It was interesting to follow how the rate of subsidence slowed dramatically during the WWII, when the local farmers went to war and reduced their need to irrigate, and therefore stopped pumping as much water out of the ground.)

Anyway, land subsidence due to over-pumping is now accepted as real, and can cause some major impacts on the surface. It is also generally accepted that subsidence is irreversible at human time scales. Irreversible is such a sad term: it means that there is absolutely nothing we can do to fix this.

So yeah, Mexico City is both sinking and running out of water, and while there are several contributing factors, over pumping leading to a reduction in subsurface porosity and the holding capacity of the aquifer below the city is surely one of them. And as a result of the over-pumping and collapse of the aquifer, even if surface flow from the mountains could make it to the city and penetrate the concrete, there is no longer anywhere for it to go.

Unfortunately, while many of the other contributing factors mentioned above can be fixed in the short term, this loss of groundwater capacity is not — I’m pretty sure I’ve already mentioned that subsidence is irreversible in human time scales.

The good news is that the earth will surely fix this over the implacable span of Earthtime: come back with Jarmo and Rosie — and Xoix of course — in 300,000,000 years and there will be plenty of groundwater again.

The bad news is that humans don’t have that long…