Chasing Monsoons

A Life of Science: A Series by New Scientists

How analyzing chemicals in stormwater runoff following desert monsoons will help cities design better rainwater harvesting systems.

  
The sun shines oppressively in the summer heat of Southern Arizona. In the distance, tall, dark, layered storm clouds dump rain—a hazy gray line connecting the sky and the land. The monsoons are rolling in.

Residents take notice and begin wrapping up their afternoon activities and making their commutes home. The clouds draw closer to the city, bringing thunder and lightning, seemingly apocalyptic downpours. Within seconds, thick drops drench those unlucky enough to remain outside.

It’s dramatic: stately saguaros against the storms that flash violently. Streets flood, closing off low points throughout the city, choking access. The normally dry washes, or arroyos, come to life—water flowing through Tucson in the veins etched into the earth long before the city came.

This is when I get to work. I like to say my life revolves around the rain during monsoon season in Tucson, although that sounds much more romantic than it actually is. My hydrology team at the University of Arizona and I meet up and we head out to collect samples from the washes.

We do so with the aid of a machine that looks like R2-D2’s analog grandfather. Rather unimaginatively called an “autosampler,” it collects samples using two arms that extend into the water. One arm measures water levels while the other actually collects the samples.

In the lab we can take hours to filter the rainwater samples so they may be used for analysis. Using a vacuum to suck the dirty stormwater through two different filters, we fill four small bottles: a glass amber vial, a clear vial, and two smaller plastic bottles. Each will shed light on a different dimension of the water’s chemistry. And that’s what we’re after—those chemicals that will shed light on our questions. We are interested in looking at chemical compounds that are not only transported physically and/or retained by soils, but also transformed by life and human activity. Our analyses of chemical compounds such as dissolved organic carbon, physical tracers such as chloride, and nitrogen “species” such as nitrate, nitrite, and ammonium will help us piece together the story of both local ecology and the movement of water.

We want to understand how rainwater harvesting basins function at collecting stormwater that runs through the city streets. These basins are randomly distributed in Tucson, each with its own peculiar design and mini-ecosystem. The streetside basins range in size, but generally they are small, containing a tree or two with plants or rocks in between. The basins are installed to address a fundamental problem of Tucson and cities everywhere: too much concrete.

Concrete does not absorb water well, leading to excessive stormwater and so, in heavy rains, choking off transportation and leading to short-term flooding. Cities that install rainwater harvesting basins are trying to combat this problem by getting the water to flow or percolate back into the ground—an idea that’s taken hold globally with the creation of the “sponge city.”

This is more than a matter of mere temporary convenience for residents when it’s raining hard. Flooding in cities is a pressing issue of climate change. As storms get stronger and more intense, where do we put the water? We’ve been witness to catastrophic flooding in cities throughout the country, the most intense floods coming on the heels of historic hurricanes like Harvey, Florence, and Maria.

It may be surprising to think of a desert city like Tucson facing flooding issues. Yet as climate change intensifies, though this portion of the Sonoran Desert is expected to get drier, it will also face more intense storms. To address these dual challenges, city planners are paying attention to stormwater.

By harvesting stormwater as it runs through the streets, Tucson can add it to its “water portfolio.” With basins distributed throughout the city, the movement of stormwater following intense monsoonal storms is both slowed down and allowed to percolate into soil. This captured water could be vital to recharging climate-stressed aquifers that humans, plants, and animals increasingly rely upon.

But how effective the basins are in totality is still unclear. And that’s where my research fits in. As a doctoral student in the University of Arizona’s Department of Hydrology and Atmospheric Sciences, I am a part of a team of hydrologists-in-training, and together we work to collect data that can shed light on how the small basins along streets can scale up to have an influence on both water quantity and quality.

And so we collect data from neighborhood washes, where water drains to a common point. In a city that has exponentially more dry days than rainy days, using the streets to route stormwater may be smarter than a costly network of underground concrete pipes.

Tucson enjoyed the majority of its growth following World War II, with much of its infrastructure planned after the 1940s. The post-WWII approach to infrastructure design has meant that the washes have been intentionally folded into the design of the city, rather than buried in an attempt to stamp them out, which is typically the case in older cities. Yet some of these washes have become frozen in time because of above-ground concrete structures, protecting nearby properties from the wandering paths of natural waterways.

I stand next to one of these concrete channels during a recent monsoon that has sent everyone else ducking for cover. I wear a rain jacket originally purchased for the torrential downpours of the Amazon rainforest, and it’s served me surprisingly well in the desert. I watch the dark, murky stormwater flood the streets and flow through the concrete channel in front of me, carrying debris along its path. A couch flows over the arms of our equipment on its path to join the shopping cart farther downstream.

My job is to check in on the autosampler, to make sure it collects samples properly and isn’t dislodged by the flowing debris.

Back in the lab, we’ll try to understand how chemicals—natural and humanmade—flow through the city in order to understand how best to design and place rainwater harvesting basins along Tucson’s streets. We combine this data with other data collected on the flow of the water to help us understand what the collective influence of the basins are on our ephemeral waterways, including whether they help manage flooding or keep chemicals out of the arroyos.

Through our research, we’ll be able to help residents plan for a future in which a hot sun is a certainty and rains a gift—one we can manage for the future.

Neha Gupta is a PhD candidate at the University of Arizona Department of Hydrology and Atmospheric Sciences pursuing research characterizing the cumulative watershed influence of green infrastructure on stormwater quality and quantity. She holds an MS in Hydrogeology and a BS in Geology from Ohio University, and worked on soil and groundwater contaminant remediation prior to attending the University of Arizona. She likes to hike and read science fiction in her free time.

Header photo of monsoon storm over Tucson by John D. Sirlin, courtesy Shutterstock.