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Montserrat Bonvehi Rosich and Seth Denizen, "Thinking Through Soil: Wastewater Agriculture in the Mezquital Valley" (Harvard UP, 2025)
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Dr. Seth Dennison
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Dr. Miranda Melcher
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Dr. Seth Dennison
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Dr. Seth Dennison
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Dr. Seth Dennison
Welcome to the New Books Network.
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Dr. Miranda Melcher
Hello and welcome to another episode on the New Books Network. I'm one of your hosts, Dr. Miranda Melcher, and I'm very pleased today because we're going to be talking about quite an interesting book on a lot of different layers and levels, sometimes literally published by Harvard University Press in 2025, titled Thinking through Wastewater Agriculture in the Mezquital Valley. Now, this book was written by two authors and I have the pleasure of one of them being able to join me here today. So the book was written by Montserrat Bonvilly and Dr. Seth Dennison and goes into, well, obviously a specific place we're going to talk about that, but some bigger questions as well around what it is to think through through soil and what it is to ask questions that might start in one place but involve lots of different people, lots of different places, lots of different ways of thinking across time as well. It turns out, as I said, there are a lot of layers to get into. So, Seth, thank you so much for joining me on the podcast.
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Dr. Seth Dennison
Oh, thanks, Miranda. I really am very pleased to be here.
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Dr. Miranda Melcher
I'm very pleased to have you. Could you start us off by introducing yourself a little bit? Maybe your co author as well and tell us why you two decided to write this book and do it together.
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Dr. Seth Dennison
Yeah. So my background is in landscape architecture and Montserrat's background is in architecture. But we both have kind of been gravitating towards the same set of questions. We both now teach in landscape architecture departments. And, you know, I think we really started to think about this when we co taught a studio at the Harvard Graduate School of Design on the Mezcital Valley. And we were going to go there and, and do a kind of studio on this particular topic, but then kind of COVID hit and we weren't able to go. And we started kind of really researching this site from afar and kind of also developing our own kind of methods for representing the knowledge that we were researching. And those methods were kind of visual methods as well as scholarly methods. I think that's also one of the things that kind of distinguishes this book is that there's really a bunch of different ways of representing the knowledge that we're kind of encountering in our process. And part of that is it was initially due to the distance that we were at from the valley.
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Dr. Miranda Melcher
That makes sense. Why then this valley to investigate, given, for example, those logistical challenges.
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Dr. Seth Dennison
The Mezquito Valley is part of Mexico City's wastewater drainage system. And it's really the largest wastewater agriculture system on the planet. So it's both the kind of geographically largest place where water is reused for agriculture, but it's also the longest running wastewater agriculture system in the world. So it's been going since, you know, around 1901. And the combination of those two facts makes this system incredibly important, not just for Mexico, but really for the rest of the world. In trying to think through the complexities of what it means to reuse urban water to produce food and, you know, because of the scale and the size of Mexico City also. Right. So over the course of the 20th century, it goes from, you know, a city under 200,000 people to now we have a city of 22 or 23 or, depending on where you draw the line in the periphery, 25 million people. That's a lot of wastewater. And so as Mexico City has grown, so has the Mezcatel Valley. And the soils that have formed in this valley have kind of grown, you know, reciprocally with the growth of Mexico City. And that kind of reciprocal growth is not again, just in geographic extent or, but it's also through the kind of material complexity of how those soils are forming in urban wastewater. And today we call it in the book, a Kind of portrait of Mexico City, the properties of this soil, offering a kind of portrait of 20th century urbanization, a kind of portrait of everything that.
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We might call it the good, the bad and the ugly in the wastewater of Mexico City. And that's what kind of also drew us to it, is that we're thinking about the soil in the Mezcita Valley in order to address really pressing and I think, global questions about how it might actually be possible to reuse urban water. But we're also fascinated by the way in which this particular process of soil formation really reflects, in sort of intimate chemical detail, the kind of parameters of a global urbanization, a planetary urbanization.
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That is mobilized. You know, supply chains that sweep the globe, pharmaceutical regimes that come from a kind of global medical knowledge. You know, this place is intensely local. You know, it's the world's largest. That means it's unique. But in, in its kind of uniqueness, it's also incredibly general and sort of stands in for a set of kind of material practices and processes that we are all sort of implicated in and tied up in and entangled in really interesting ways.
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Dr. Miranda Melcher
That's definitely a number of reasons why the valley would be interesting to focus on for this book. And also kind of not surprising, therefore, that other people have also been interested in researching the valley. Is it because of these factors, the local and the global, the long running? Are there any other kinds of key reasons why this system in this valley has had so much research conducted on it? Especially some really technical stuff from what I understood in the book.
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Dr. Seth Dennison
Yeah, I mean, most of the research on the Mesquitel Valley is technical. You know, I. I call it the wastewater. Agriculture in the Mesquitel Valley is kind of the laboratory animal of, you know, environmental research. When you're trying to figure out how contaminants move through a kind of geological and pedalogical system. You want to go to the Mesquite Valley, right? If you're trying to understand how pharmaceuticals are moving into aquifers, you want to go to the Mezcatel Valley. If you want to understand antibiotic resistance in the soil, you want to go to the Mezquito Valley. It's a place where a lot of different kinds of environmental research can be done.
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And, you know, but what we found really interesting in our research is that one of the things that we think are slightly, you know, less common to find in the Mezcuital is really serious kind of social research. It's not typically studied as a kind of social system. It's really studied as a kind of environmental system. And that is a kind of dimension of the research that we were hoping to also kind of contribute to with our work.
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Dr. Miranda Melcher
Okay, that's definitely helpful to lay out and makes sense given the way that you choose to open the book. So you do not open the book with a really technical study of soil composition. For instance. The first thing that we hear about is a protest, like actual humans protesting with. It sounds like sort of slogans and banners and that sort of thing that took place in November 2018. What was this protest and why start here?
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Dr. Seth Dennison
Yeah, so the protest was really significant to us because.
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It represents like a really difficult paradox that we want to sort of confront and confront the reader with, which is that the protests are farmers in the Mezquito Valley who are advocating for the closure of a very large wastewater treatment plant that was built.
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Partially through private funding and partially through government money. So a kind of PPP sort of project, extremely large, billion dollar wastewater treatment plant. It's now the largest wastewater treatment plant in Latin America, and it opened in 2017. And so in 2018, the farmers show up and they start saying, you need to close this giant, you know, beautiful shining new wastewater treatment plant that is a, you know, a sort of a paragon of health and public policy in Latin America. All of these great things about it. No, actually, we don't want it. You need to shut it down and please do so immediately. And, and this is kind of, you know, a very difficult thing to. To hear. I mean, what. How is it possible, in a way that, you know, in a country with very little public health and sanitation infrastructure, that the kind of, you know, shining example of public investment in public sanitation would be, would cause kind of protest, would be the subject of protest. And that's the kind of question that we wanted to open the book with. Because in order to answer that question, you have to sort of think through soil in a way. It's not actually possible to really understand that question. At the time, I have to say, the news media in, in Mexico really didn't understand the question. I mean, it didn't help that at the time the major news headlines in Mexico were about oil theft. So, you know, sort of cartel groups were tapping oil pipelines in Mexico. And of course, as you know, Mexico has nationalized its oil production. So these kinds of illicit groups are siphoning oil out of the kind of public treasury in a way. And it, you know, it was at an enormous scale. And so everybody is worried about the theft of Oil. And then these farmers show up at this wastewater treatment plant and basically what they say is that the government is stealing the organic matter out of the wastewater that we need to fertilize our fields. So the wastewater used to be full of all of this nice organic matter that came from the sewage, right? So nitrogen, phosphor, phosphorus, potassium, that's coming from municipal sewage, from 22 million people all arriving to their farm fields, and that sewage is fertilizing their crops. And now the wastewater treatment plant is removing a unknown quantity. Some say more, some say less. And, and it's, it's a little opaque exactly how much they're removing. But the farmers are reporting dramatic decreases in their yields. So what they're saying is we. You're not telling us how much you're removing, but we can tell that it's a lot because we can see it in our crops. And, and so effectively what they're accusing the government of is stealing the shit out of the water at the same time that the whole country is worried about the theft of oil out of the kind of national pipelines. So you can imagine how they didn't really get a lot of traction in the mainstream media in Mexico with that particular claim. And we actually, in our research found that what the farmers were actually saying in that moment wasn't weird or strange or wrong at all. We actually find ourselves deeply sympathetic to what they were trying to tell us and those protests. And in order to kind of explain how that particular logic works, we really had to write something book length, because it's a long story and you need to understand some history, but you also need to understand a lot of soil chemistry, which is not something a lot of people know much about. So, you know, I think one of our, our kind of goals was to try to use that protest as a kind of way of posing a question that we hoped we could actually offer an answer to. You know, a kind of book length answer to.
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Dr. Miranda Melcher
So what are some of those questions then that you both decided to ask and answer?
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Dr. Seth Dennison
So, I mean, one question was, you know, how is it that this system is actually reusing organic matter? And to what extent.
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And also to what extent is the system sustainable? So to what extent if the farmers say that we need to shut down the wastewater treatment plant and continue business as usual, the kind of normal way in which things have been working since 1901, to what extent is that scenario sustainable? And the kind of, you know, tragic sort of paradox of this wastewater system is that the same wastewater that brings organic Matter from the sewage to fertilize the farm also bring a bunch of contaminants. And those contaminants are serious, and we wanted to understand how serious they were in order to understand how sustainable the system was. So we had to look at each of them individually. We had to look at the surfactants that create these big, giant foam clouds whenever the irrigation canals encounter turbulence. We needed to research the antibiotics that were coming out of the wastewater from a city of 25 million people. We needed to study the heavy metals that are accumulate in the soils and have been accumulating in the soils for over a century. There are things like caffeine and deet.
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Some of the wastewater canals wouldn't pass a federal drug test for cocaine.
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This is also kind of what we mean by the soils becoming a kind of portrait of urban life in Mexico City. But each one of these contaminants has its own kind of trajectory through environmental systems. So, you know, oftentimes when I list all of these contaminants, you know, the kind of reaction to this is this kind of revulsion like, oh, my God, this is. This is a disaster. You know, please save us from this wastewater agriculture system. Whoever thought that this was a good idea. But actually, when we kind of go through each of them individually, we start to organize them sort of according to their environmental path through the system. And some have, you know, greater half lives than others. So a lot of this stuff breaks down relatively quickly, and a lot of this stuff doesn't. So, you know, one of the things that we were interested in was that the wastewater treatment plant was really built to address biological pathogens. So the E. Coli, the salmonella.
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The helminth worms, you know, biological pathogens like that.
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And the farmers are actually least interested in those. They. They've kind of have a century of experience of keeping themselves safe. Excuse me. They have a century of experience keeping themselves safe from those pathogens. They don't. They're not worried about those pathogens as much.
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What they're really much more worried about are the heavy metals, which have, as I said, been accumulating for a century in the soils. And this is something the wastewater treatment plant doesn't actually fix. So a lot of the heavy metals are still passing through the system and still accumulating in their farm fields. But the kind of the great paradox of this system is that the same organic matter that brings sort of soil nutrients and soil fertility to the fields also increases the soil's capacity to bind heavy metals. So you can imagine the Wastewater has heavy metals and organic matter from the sewage in it. The more sewage and organic matter you deposit on the farm fields, the more organic matter there is in the soil, the more the soil actually has the ability to bind and chelate sort of heavy metals into organometallic compounds. Which means that any kind of decrease in the amount of heavy metals in the soil could result in the kind of release of a stored legacy of heavy metals from the, you know, 20th century. So in, in, in that sense, our kind of, you know, shock or surprise about this system is that there really is no turning back. I mean, the soil is full of heavy metals, and right now those heavy metals are being bound by the organic matter and the very high cation exchange capacity in the soil. Any kind of changes to that soil chemistry, their precarious soil chemistry, could result in the release or the solubilization of a kind of legacy load of heavy metals into the soil. So there's this kind of red queen, like I, you know, kind of, in evolutionary terms, a kind of like red queen scenario where you have to keep running in order to stay in the same place. And, and that means that the potential changes to soil chemistry in the body of the mesquitel that the wastewater treatment plant would produce pose a pretty serious risk to the kind of long term sustainability of the system. But we could also say that so does returning to a business as usual scenario, because in a business as usual scenario, the heavy metals are still accumulating.
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So the farmers could continue to farm as they've always farmed. But the question is, how long will this last? Because ultimately the soil will kind of run out of.
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You could imagine like parking spaces for the heavy metals in a kind of way. That's kind of what, that's how you can kind of imagine cation exchange capacity as a kind of, you know, set of parking spaces that the heavy metals fit into. And eventually we're going to run out of places for those heavy metals to bind in the soil and then they become soluble and then they go into the plants, and then the plants go into our food. So this is the kind of complex set of soil chemistry, you know, the conditions of soil chemistry that we were trying to understand in order to kind of evaluate the claims of the farmers.
- [20:58]
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Dr. Miranda Melcher
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- [22:12]
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- [22:19]
Dr. Seth Dennison
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Dr. Miranda Melcher
Marvel Studios Thunderbolts the New Avengers rated PG13 now streaming on, you guessed it, Disney. Yeah, there's a lot going on there, right? This is the sort of thing where I was like, yeah, the layers of meaning the layers of soil, the layers of structures, and also the layers of sort of time and history because as you mentioned, right, this system has been in place for over a hundred years. So there's kind of that element as well, given how much has changed politically, economically, socially, technologically in that time period, which is really interesting to sort of understand better, especially given what you've just described a moment ago, which focuses I think quite a lot on kind of where we're at now and obviously outlining some very key questions to resolve going forward that definitely haven't been figured out yet. So I want to also kind of add the going back in time element as well to our conversation because these conversations happening more recently around heavy metals, around the impact in the food system aren't necessarily things that have always been talked about in the system, if I'm understanding it correctly. So, you know, let's add more complexity. Let's add more things that are being talked about here. So can you tell us about some of the discourses, some of the histories around the valley being talked about as quote, unquote, empty, which sounds kind of ridiculous given the farmers and their commitment to the land that they go protest. But you talk in the book it's been considered that way. It's been considered problematic, it's been considered poverty stricken. So where are these narratives sort of coming from and what are they trying to do?
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Dr. Seth Dennison
Yeah, I mean, in many ways, the. The kind of set of, you know, politics and sort of conditions of soil chemistry that I was just describing, I think were pretty well known among a certain class of researchers that were investigating the Mezquital.
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But what I was kind of shocked by is that none of these researchers really understood the deeper history of the valley, which I think adds another really important layer to understanding how these farmers got so angry. Be, you know, angry enough to protest and blockade and, you know, and threaten and, you know, there hasn't been any loss of human life yet, but there have been threats. I mean, how. How is it possible that people are at this point?
- [24:44]
And what we realized was that there's this very interesting and pretty shocking history in the. In the bay, the Mezquita, which nobody talks about, which is that right now this whole conversation is oriented around, you know, the contaminate, the contamination, the contaminants. And, you know, when the farmers say that they don't want the wastewater treatment plant, you know, people look at them like, you. You know, why are you so. You know, how could you. How could you tolerate such dirty conditions? You know, you're farming in wastewater. Wastewater is dirty. This is like a dirty place in the Mezquital Valley. And what we realized is that if you look at the history, wastewater, agriculture and irrigation was brought to the Mezquital specifically in order to clean it up. So there was this kind of very sort of racist and eugenic discourse around implementing irrigation in the Mezquital Valley in order to kind of assimilate the Otomie indigenous people of the valley into modern agricultural practices. And the Otomi are this sort of famously poor and marginal group in Mexico.
- [26:02]
And, you know, they. Anthropologists would describe them as never taking a bath because they lived in this extremely arid environment of the Valle de Mesquitel. They were, you know, eating non traditional foods. At one point, some nutritionists came from the Rockefeller center and from Harvard, really hoping to study malnutrition in the Mesquite valley in the 1950s and kind of were very surprised by.
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The fact that none of these indigenous people were actually malnourished despite eating no foods that a nutritionist, a Western, you know, nutritionist, would classically think of as food. So they're eating a lot of, you know, think things like berdulaga, or what we call in English purslane. You know, now we think of this as a kind of superfood. But in the 1950s it was considered a weed and like a kind of a very marginal waste plant. And, you know, how is it possible that they're. They're not malnourished, eating these. These weeds off of the ground, you know, and of course, we now know that their. Their particular diet didn't lack any kind of nutritional quality whatsoever. And so this kind of image of the mezquital is. This kind of. It really served in the urban imaginary, particularly in Mexico City, as the kind of paradigm of rural immiseration. This kind of image of rural poverty and death and. And disease that really pulled at the conscience of Mexico City and Mexico City's elite. And this kind of feeling that Mexico City owed a debt to the mezcital, you know, that it had. It had failed to develop, it had. It had. It had been a. It had not benefited from the. The land ownership changes of the Mexican Revolution and the way that other parts of Mexico had, that it was continuing to suffer kind of poverty and immiseration in the kind of scales and the metrics that a Mexican urban elite would. Would use in order to assess those things.
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Building a giant irrigation dam and a kind of enormous network of irrigation canals were considered the solution to this, because not only would it provide the valley with, you know, economy, but it would also teach the indigenous Otomui people how to farm in the right way, because, of course, they were kind of primitive indigenous people who didn't know how to farm. Of course, that was not right. And we. And we show in the book the kind of sophistication of otomi farming techniques. But. But the great irony is that this entire irrigation system was really implemented to kind of civilize and assimilate and really to clean up the otomie. And of course, that entire history is really lost when we talk about the farmers now and we talk about how dirty they are and how. And how could they be opposing a wastewater treatment plant where really this is the. This is the system that was supposed to make them clean. This is the system that was supposed to solve their problems. Everyone's kind of forgotten this particular history. So this is something we start out with in the book, and we really spent a lot of time looking at the book because I think it adds a really critical historical context to the conflict that we find today.
- [29:45]
Dr. Miranda Melcher
Yeah, it definitely is, as you're explaining it, kind of immediately relevant to those events in 2018, rather than seeming like, oh, that was so long ago, that doesn't have anything to do with now, obviously. It really does. But can we go Back even further in time to think about kind of when and why this all started. Right. You've mentioned 1901 a few times. But like when did the idea even begin to make irrigation that's connected to the massive city? As you said, we can define the population in many ways. When did the Mesquitel Valley start being part of these ideas of irrigation and farming? Like, was it for the benefit of the valley Valley for the. At first or the city? What, what was the kind of original idea of this entanglement?
- [30:27]
Dr. Seth Dennison
Yeah, so the orig. The original idea was, was really to drain the lakes. So Mexico City, you know, is a high altitude basin. When the Spanish arrived in the 16th century, they, they described it as a kind of Venice, you know, a kind of lacustrine city. And the kind of, you know, European horror at living in wetlands really resulted in a 500 year project to drain those lakes. And the only way to drain lakes from a basin is to kind of punch a hole in the wall and the, in the walls of the bowl. And so, you know, this kind of 500 year project to punch a hole in the basin walls and drain Mexico City's lakes once and for all.
- [31:18]
Resulted in the need to sort of.
- [31:23]
Well, resulted in the creation of the Mezcatel Valley. Because basically the, the kind of the hole that they punched in the basin walls is where all of the wastewater now drains to. So it's the, it's the northern part of the basin. At first the, the kind of.
- [31:42]
Hole looked like a little canal, like a surface canal that was drained through this enormous cut that was carved into the basin walls. That surface canal then became a drainage pipe. And today we have three pipes and they're enormous. They're called the denaje profundo or the deep drainage of the city.
- [32:06]
And they basically start about 300ft under the city and travel all the way downhill to the, by the meskial, to the Meskial Valley. And these pipes are like you know, 30ft in diameter, you know, 10, 10 meters. So they're just enormous. And the kind of origin of those pipes that brings the wastewater of Mexico City to the Mezaw Valley was really originally about flood control for, for Mexico City.
- [32:38]
And it was only kind of later actually the first ideas in the Mezcal Valley for how to use this drainage were actually through for hydroelectric power. Was the kind of first idea is like, let's use this torrent of wastewater to produce electricity. But those ideas.
- [32:57]
Faded and folded. And then really people in the Mezcatel Valley started to use the wastewater for agriculture in a kind of ad hoc way. And it wasn't until the kind of early 1950s that there was an effort to say, wow, you know, people are farming this wastewater. You know, why don't we actually build a system that could actually support these farming efforts on a massive scale? So, yeah, I mean, initially it was really not about farming. It was really not about wastewater agriculture at all. It was really just about evacuating the city of its own wastewater and its own lakes, which had also become the kind of repository of Mexico City's waste for.
- [33:51]
The 18th and 19th centuries.
- [33:54]
Dr. Miranda Melcher
So what did these initial goals and the discourses we discussed developing later around kind of clearing out the valley and changing up what was happening there? And you mentioned the disappointment of the foreign researchers coming in and going, what are these people doing when we're talking about these kind of goals in terms of agricultural practices or cleaning out the city or things like that? What did all of this mean for the people and their interactions? For example, you talk in the book around how this is related to ideas of racial politics or kind of conceptions of identity. Can we add that thread in?
- [34:28]
Dr. Seth Dennison
Yeah. I mean, there's basically.
- [34:31]
Two main groups of. Of. Of farmers in the Mezquito Valley. There's the otomie, indigenous farmers, and there's the kind of mestizo farmers who benefited from the Mexican Revolution in 1917, when the land was expropriated from the hacienda system created by the Spanish. Right. So the Spanish would have set up these large agricultural haciendas, these kind of, you know, think of them as like, big sort of land grants to Spanish.
- [35:08]
People. And they were farmed by indigenous people. They expropriated indigenous land in order to be constructed. And they kind of turned the indigenous people of the valley into labor for the haciendas. In the Mexican Revolution in 1917, those haciendas were.
- [35:31]
Broken up and the land was kind of returned to the people in the kind of Zapatista sense. But the people that the land was returned to were largely not the indigenous people, were largely not the otomie. And what happened is that the otomie were largely kind of pushed up into the slopes of the. Of the valley and given the kind of less fertile land.
- [35:57]
And there's a kind of big difference also between the northern side of the valley and the southern side of the valley. So the southern.
- [36:07]
Side of the valley is mostly sort of mestizo farmers farming former hacienda land, because that's where Most of the water was arriving, but as the city grew, as Mexico City grew enormously.
- [36:23]
The wastewater actually started to arrive much, much farther north. And so some Otomi farmers benefited from that in the kind of latter half of the 20th century and were able to create kind of large agricultural plots.
- [36:40]
Kind of in, you know, in the 1960s and 1970s. So now you have this kind of strange geography where you have.
- [36:49]
A lot of indigenous people farming in the northern part of the valley and then mostly mestizo farmers in the southern part of the valley. And the wastewater treatment plant occupying its position in the southern part of the valley in a lot of ways affects the farmers in the south a little bit more. Although the farmers in the north are also farming the wastewater. The kind of strange chemistries of the valley mean that.
- [37:17]
The water is actually cleaner by the time it arrives to the north. So there's also kind of differences in. In the politics of kind of pathogen. Pathogens in the wastewater and kind of contaminants in the wastewater that aren't always the same between the north and the south, but I think are pretty similar.
- [37:42]
So this kind of racial geography of the valley represents this kind of, you know, ongoing sort of wound that makes sometimes organizing for the farmers difficult because their. Their sort of historical wounds run so deep. And their ability to provide a kind of united front or kind of united vision for a better environmental future in the Valley is, I think, really, really difficult in that kind of. In that kind of context. And that's. That's also, you know, what we're trying to describe in the book is how, you know, these environmental conflicts are right now defined by a very kind of reductive vision of what the sort of environmental politics of the Valley are. And the Valley finds itself at this kind of political impasse in a large way because it can't generate its own alternative environmental vision for the future, in part because the sort of contrasting interests of who would benefit from what aspects of that environmental future are not universally shared. So when we talk about the farmers, which a lot of people do in their. The way that they write about the bay, the mezquital, and the way that the environmental systems are described, what we're trying to do is to kind of nuance and unpack this kind of idea of what the farmers might be. And, in fact, that are kind of a diverse. A diverse group. And that has real implications for how we could imagine a sort of better environmental future in the Valley.
- [39:25]
Dr. Miranda Melcher
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Dr. Seth Dennison
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Dr. Miranda Melcher
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- [40:40]
Before we get into imagining the future, we will get there because I've got some questions. There's some interesting ideas about that in the book. Can we unpack a little bit more about sort of the present or recent past in terms of adding this nuance to the groups here? So if we go, for example, back to those protests over the wastewater treatment plant, we talked a little bit, we talked about it at the beginning, but now that we have kind of more of this context, was the plant always controversial? How do we get to controversial? Is it sort of united of like all farmers are all against this one treatment plant? Like, let's add some more nuance now that we have a better understanding of some of the other aspects.
- [41:18]
Dr. Seth Dennison
Yeah, well, I mean, I think what the the promise of the treatment plant was always that it was going to provide.
- [41:27]
Better economic opportunities for the farmers. So the way that the treatment plant was sold was that the farmers would be able to grow lettuce, you know, and right now they can't grow lettuce. Right. Because you can't grow lettuce in wastewater. You really can't grow anything in wastewater that.
- [41:46]
Where the actual food crop touches, comes into contact, physical contact with the water. So most of what people grow in the wastewater are plants that kind of rise above the sort of wastewater. So they flood, irrigate the fields. Right. You don't spray or any. Or anything like that, you flood, irrigate the field so the water is staying on the surface. And then you grow something like corn or alfalfa that kind of grows up and out of the wastewater, and that the actual leaves of the alfalfa or the ear of corn would never come into contact. Contact with the actual wastewater that's, you know, of course, full of E. Coli and salmonella from the kind of urban sewage system.
- [42:29]
So the promise of the wastewater treatment plant was that.
- [42:35]
The. The wastewater treatment plant would make the water clean enough to grow lettuce. And this is a important economic crop for people. You can make more money growing lettuce than you can make growing corn or alfalfa. So, but the problem with that was that this, like, lettuce turns out to be a hyper accumulator of certain heavy metals that are at, you know, fairly high levels in the soil. So this is the other nuance, right? It's like not every plant absorbs metals at the same rate, but the legacy load of heavy metals in the soil means that you can't actually grow lettuce in the. In the Mesquite Valley. So the economic opportunities for, you know, using the wastewater, the treated wastewater to grow alternative crops didn't really pan out. But that was one of the kind of alternative environmental imaginaries of what. How the wastewater treatment plant would affect the valley. And that, of course, has a lot to do with also what you want to eat. So, you know, if you're like a mestizo farmer trying to feed, like, Mexico City, lettuce is really lucrative and really.
- [44:00]
Interesting as a crop potentially. But if you're an otomie farmer and you're not interested in growing lettuce because it's not the kind of food that you want to eat, then you're not that interested in lettuce in the beginning in the first place. So part of also what we were trying to draw out is this sense that in a lot of ways, this entire wastewater agriculture system was imagined as necessary in order to grow crops for a certain kind of diet. And that diet is also related to culture and, like, who. Who eats that diet?
- [44:42]
So, you know, there's kind of like also layers to the way in which we would understand the kind of, kind of colonial project of. Of the wastewater irrigation system as also implementing a kind of Western diet in order to grow crops that are not native to the valley that would require a lot more water. It's essentially kind of importing a foreign climate that's what the irrigation is kind of doing, it's kind of creating an alternative climate, literally displacing a geography and imagining the Mezquita Valley as if it existed in some other part of the earth where these plants would actually be able to survive and to grow. And I think the kind of otomie way of farming relies on native plants that actually are all quite well adapted to arid conditions. And we go into detail about what those plants are, and we even talk about the different kinds of photosynthesis that they rely on. You know, the fundamental diff. Fundamentally different kind of machinery of. Of life that it underpins. The otomie diet, or what we call the dry land diet in the Mesquite Valley. So, you know, there's also this kind of way in which this entire problem is framed around how we can grow a particular kind of food that an entire segment of the population was never that interested in growing in the first place. And, you know, the otomie also, they. They had their own native varieties of corn that were adapted to the kind of 400 millimeters of rain a year that the valley gets. They had, you know, their own kind of agricultural system which we. Which we describe in the book as the kind of zero phytic triad, which is the sort of dry, arid version of the Three Sisters, which many people will already be familiar with. The. The corn, the squash and the beans that are so famous from Mesoamerica. Mesoamerican agriculture. There's actually not enough rain for. For corn, squash and beans, the three Sisters, to grow in the Mezcita Valley. But there's a kind of xerophytic triad of amaranth and agave and conical maize that form their own kind of triad in the Mezcuit Valley as a kind of agricultural system that produces its own soil fertility through plant soil interactions, which is what we see as the fundamental sort of shift or the fundamental innovation in. In. In the Three Sisters or the Milpa system. Right. Is thinking about agriculture as not always needing to be extractive. The pla. You know, we think of agriculture as fundamentally extractive, that it will always and inevitably reduce soil fertility rather than improve it. But a lot of Mesoamerican agricultural practices were designed specifically to improve soil over time. So the agriculture itself was. It's like improving the soil, not depleting the soil. And that kind of conceptual shift is. Is, you know, fundamentally different epistemologically, materially, practically, economically different. So when we see the kind of clash between otomie indigenous farming practices and the wastewater agriculture system, you know, it's. It's. It's not just two different ways of doing things. It's not as if the same plants will now just get extra water. It's like a completely different way of thinking about the world. A completely way. A different way of living and eating. And it's really to become a different person. And in that way, I think the eugenicists were probably onto something, right? Like, you really can change people by changing the way that they live and the food that they eat and the way in which they think about themselves as subjects. So neoliberalization really does actually change you. And of course, the Mexican eugenicists were. Were. Were very different from the kind of Western European eugenicists in the way that they thought about eugenics. The. The kind of very famous basconcellos eugenics of. Of the kind of mestizo being, the. The cosmic race, the rasa cosmica, the kind of perfect Hegelian synthesis between the European and the indigenous person was the mestizo. So there's this kind of, you know, again, this. This sort of. The racial politics of. Of diet and of culture and of. And of identity and Mexican identity really all converged in the Mezcita Valley in order to set up this kind of political condition that we find ourselves in today.
- [49:38]
Dr. Miranda Melcher
What sort of political condition could we find ourselves in in the future? What are some of the possibilities of what the Mezquitel Valley could look like going forward?
- [49:47]
Dr. Seth Dennison
So one of the things that we suggest is that the amount of water that Mexico City can continue to send to the Mezcata Valley is really unsustainable. Mexico City is running out of water, which is kind of paradoxical because, of course, it still floods all the time, but it, you know, very nearly arrived at day zero or kind of completely running out of water.
- [50:15]
Last year, and it may again this year. So as Mexico City kind of suffers its own sort of crisis of water, it needs to rethink the way that it uses water in Mexico City. And if Mexico City becomes more sustainable in its use of water, that actually means less wastewater for the Mezquita Valley. And that's. That's a kind of hard, you know, pill to swallow for a lot of people in the Mezcat Valley who already feel like they need all the water that they can get. But. But we imagine this is a kind of inevitable reality of the environmental system that these. That has been kind of coupled here. And so one of the things that we imagine is that if Mexico City Needs to send less wastewater to the Mesquitel valley. Could we start to kind of reclaim and revalue the kind of otomi dry land farming techniques that had been marginalized and kind of pushed up onto the slopes of the valley? You know, is this a moment where we could start to remember the kind of intelligence and the utility of those farming practices? And, you know, of course, Mexican food is a world heritage cuisine. There's an enormous amount of interest in how people eat and what kind of Mexican food is, and this kind of attention to traditional ingredients and traditional ways of preparing those ingredients. We think that the sort of otomie ingredients, the kind of. The mesquite pods, right. The Mezquita valley is named after the mesquite tree, which has these sugary bean pods that make an incredible flower that also create resource islands as mesquite trees fix nitrogen. There's the agave, the pulque that that is produced from the agave plants along with the agua miel that is harvested from them as well. There's just a tremendous amount of sort of interesting things to grow and to eat in sort of traditional otomie culinary culture. We think that this is a great opportunity to start remembering those things because the valley needs to become drier. We also have a another set of proposals around.
- [52:46]
Thinking about farming in the Mezcita valley as a kind of ecosystem service. Now, I have never been a huge fan of the ecosystem service paradigm, and I think that there's a lot, a lot of it that's really problematic. But in this particular condition in the Mezcatel valley, I think it would make a lot of sense for people to start thinking about the wastewater agriculture itself as a service to clean the water from Mexico City, because that's, of course, what the soil does, right? So all the pathogens, all the biological pathogens die in the first 5cm. Farmers with the proper kind of education and PPE can farm in the wastewater safely with these pathogens. There's ways of dealing with the pathogens that the wastewater that's in the wastewater and that particular. In that particular condition. Instead of paying a billion dollars for a wastewater treatment plant that doesn't actually make the system sustainable, we could actually just start thinking about paying the farmers to farm as a way of really, you know, not only cleaning up the water, but efficiently cycling nitrogen out of the system. Because if the farmers don't farm this wastewater, it goes straight into the Gulf of Mexico, which is what we do in the United States. Right. You know, and we see the kind of the dead zones, the. The massive anaerobic dead zones that form off of the coast of Louisiana because of where the Mississippi empties into the Gulf. That dead zone is basically from agricultural fertilizers, from high nitrogen levels and high phosphorus levels that come from agricultural land in the American Midwest. Mexico would have its own dead zone where all of this wastewater enters the Gulf if the farmers don't farm this. So they're providing an enormous service of cycling nutrients, of turning nutrients from the sewage into biomass. And it actually, they do so at rates of efficiency that are far, far higher than farms that use industrial fertilizer. So, you know, when you use industrial fertilizer, about 30% winds up in your crops, 70% washes away into the aquifers or into the surface water. In the Mesquite Valley, they're able to cycle nitrogen at an efficiency of 70%. So 70% of the nitrogen is going into biomass, 70% of the phosphorus is going into biomass.
- [55:18]
And that means that the system is really providing an enormous service to the environmental health of all of the downstream communities that exist along the Tula river, which is where the wastewater would flow to the Gulf of Mexico. What if, instead of trying to build this incredibly expensive machine, which doesn't solve our problems, what if we started to really pay the farmers for this ecological service that they're. That they're producing? And what if that payment could also kind of subsidize and catalyze a series of farming practices and infrastructures that made farming much safer? So thinking about the Mezcata Valley as like, this kind of massive ecological service to Mexico City, which Mexico City should recognize, support, and subsidize as a part of their sort of municipal infrastructure.
- [56:14]
The other thing that we're really trying to think of is, you know, in order for this system to become really sustainable, we need to address the heavy metal contamination at the source. So the wastewater treatment plant doesn't remove the heavy metals enough to make a difference. And without the wastewater treatment plant, the heavy metals are still continuing to flow to the Mesquite Valley. So the only option is to really think about reducing the amount of heavy metals in the urban environment in Mexico City. And, of course, you know, cleaning up the urban environment in Mexico City would have a bunch of really important health benefits for the people who actually live in Mexico City. So we kind of are hoping to make this argument that, you know, if you really think about the Mezquita Valley as a place that is connected and as A part of Mexico City, not some distant place far away that we can forget that we can imagine as a sacrifice zone for our unwanted waste. If we think of the Mezcita Valley as really just another neighborhood in Mexico City, then what we really need to do is to clean up the urban environment to reduce the amount of heavy metals that we're sending in order to imagine a city that's capable of producing a wastewater that's of a quality that would make farming in the Mezcita Valley sustainable. And that would have both enormous benefits for the people in the Mezcita Valley and for the people in Mexico City in terms of their, their actual health.
- [57:35]
Dr. Miranda Melcher
Well, we can certainly hope that we would get to something that would improve things for everyone, really. Is there anything further you want to say to conclude our discussion about the book? Any final best case scenarios or things you hope readers take away from this?
- [57:51]
Dr. Seth Dennison
I mean, I really hope that part of what readers can take away from this is a sense that in order to think through soil, we need to start thinking about human, environmental, or sort of eco social processes together.
- [58:10]
We live in a world right now where we have a tremendous amount of environmental problems. I mean, obviously the paradigmatic example is climate change. And one of our biggest challenges for solving climate change or to kind of imagining more equitable futures is our kind of, you know, difficulty in understanding this sort of eco social dimension to environmental problems. The way in which we can't just think about environmental problems as separate from politics, as separate from culture, as separate from kind of human thriving and non human thriving. So, I mean, I think part of what I hope that this case study contributes to is also a kind of a look at what it would mean to really think about a complex environmental system rigorously from both a scientific, material, empirical perspective and from a social, cultural and historical perspective. And to then imagine better environmental futures at the intersection of both of those kinds of knowledge. We're really trying to think across the kind of nature culture dividend, because all of our kind of opportunities for thinking about better environmental futures will actually require us to do precisely this kind of thinking. So I'm hoping that if anyone is interested in sort of a case study that tries to think rigorously in a sort of eco social way or a kind of geosocial way that this book would be of relevance.
- [59:44]
Dr. Miranda Melcher
That's a great way, I think, to end our discussion about the book, leaving me with just the final question of what you might be working on now that it's done. And if you know of your Co authors, authors, upcoming or current projects. Anything you want to give us a sneak preview of?
- [59:58]
Dr. Seth Dennison
Yeah. So I mean, we're going to be publishing another book with a large group of partners in Mexico City. We've convened a symposium of scientists, of politicians, of people who work in NGOs, of healthcare workers, of transportation experts in policy.
- [60:21]
Experts, to really try to follow up on the question that I just mentioned of how we could imagine a Mexico City that sent the Mezcal Valley a higher quality of wastewater. And we called that symposium Destino Mezquital. And through this kind of rich interdisciplinary conversation about how the city actually works and what's actually possible in terms of next steps, we got some incredible ideas. And we included a large team of both architectural design students and environmental science students from Mexico's National University unam, to help us transform those conversations into design proposals. So we're going to be publishing a kind of collective, like multi authored work from a.
- [61:13]
A rich interdisciplinary conversation in Mexico that offers specific proposals for how Mexico City could, could change. Think of it as a kind of like visual policy memo or architectural policy memo, urban design policy memo that will hopefully create an image of the kind of environmental future we're imagining that can be more easily grasped and more sort of accessible to a broader conversation. So look out for that because in that sense, the book is really, I think, just the first step.
- [61:52]
Dr. Miranda Melcher
Well, if anyone wants to get in on that first step, they can of course read the book titled Thinking through Wastewater Agriculture in the Mesquitel Valley, published by Harvard University Press in 2025. Seth, thank you so much for telling us about your research.
- [62:07]
Dr. Seth Dennison
Well, thanks, Brenda. I really appreciate your time here today.
- [62:25]
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