Microplastics in the Desert: A Growing Concern in Phoenix Soils

Located in the American southwest, Arizona is a state known for its cactus-filled Sonoran Desert landscape. The Sonoran Desert stretches across southern Arizona into California and northern Mexico. Known for its unique biodiversity, the Sonoran Desert is home to iconic flora and fauna, such as the towering saguaro cactus, which has become a symbol of the American West, and animals like the Gila monster and the roadrunner (1).

Because a significant portion of the Sonoran Desert comprises Arizona, the continued health of this landscape is essential. Researchers have dedicated their efforts to studying the soils in the Central Arizona–Phoenix Long-Term Ecological Research (CAP-LTER) area (2,3). A recent study led by Pierre Herckes at Arizona State University revealed that a concerning amount of microplastics was found in these soils, which raises questions about how these contaminants could negatively impact this desert environment (2).

Sonoran Desert at Sunset | Image Credit: © Brent Coulter - stock.adobe.com

Microplastics are becoming a pervasive issue not just in desert landscapes but all geographical landscapes. Microplastics are defined as tiny fragments of plastic less than 5 mm in size (2). Microplastic pollution poses significant challenges to conservationists and researchers. Because of their small size and resistance to traditional sampling methods, microplastics are tough to characterize, which means researchers are constantly trying out new techniques to achieve the desired results (4).

Herckes and his team used micro-Raman spectroscopy in their study to characterize microplastics found in soil samples collected during the Ecological Survey of Central Arizona (ESCA) in 2005 and 2015 to map the spatial distribution and temporal trends of microplastics in Phoenix and its surrounding areas within the Sonoran Desert (2). By doing so, they uncovered a variety of polymers, including polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC), polyamide (PA), polyester (PES), and polypropylene (PP) (2). Among these, polyethylene was the most prevalent, present in 75% of sampling sites and dominating the overall composition of microplastics in the soil (2).

The study found that microplastics were present in every soil sample analyzed, with concentrations ranging from 122 to 1,299 microplastics per kilogram of soil (2). Despite this widespread distribution, the researchers observed no clear spatial patterns. Instead, the variability was likely influenced by environmental factors such as resuspension and redistribution caused by dust storms, which are frequent in arid regions like the Sonoran Desert (2).

Further insights were gained from microplastics deposition fluxes measured over a one-year period (October 2020 to September 2021) in Tempe, Arizona (2). Deposition rates varied widely, from 71 to 389 microplastics per square meter per day, with an average flux of 178 microplastics per square meter per day (2). The fluctuations indicate that local weather events could impact the depositions and concentrations of microplastics found in the soil.

However, a significant portion of the microplastics remained chemically unidentifiable, highlighting the need for more advanced analytical methods to fully understand the composition and origins of these pollutants (2). Microplastic pollution is sometimes difficult to uncover because of the miniscule size of the pollutants, and it makes detection in urban desert environments more challenging than usual. The findings in this study have implications for the Central Arizona region, as well as other arid and semi-arid ecosystems worldwide, because it shows the importance of improving analytical techniques to identify a broader range of microplastic types and their chemical compositions (2).

As cities like Phoenix continue to grow, addressing microplastic pollution in terrestrial ecosystems will be vital for sustainable urban development (2). This study helps shed light that microplastics are not just a problem for oceans and rivers but are also a threat to land-based ecosystems, including the deserts of Arizona.

References

1. Arizona-Sonora Desert Museum, Regional Natural History and Image Galleries. Desert Museum. Available at: https://www.desertmuseum.org/desert/sonora.php (accessed 2024-12-04).
2. Chandrakanthan, K.; Fraser, M. P.; Herckes, P. Microplastics are Ubiquitous and Increasing in Soil of a Sprawling Urban Area, Phoenix (Arizona). Sci. Total Environ. 2024, 906, 167617. DOI: 10.1016/j.scitotenv.2023.167617
3. Sikora, C. Arizona Researchers Finding Higher Concentrations of Microplastics in Soil Around the Valley. 12News. Available at: https://www.12news.com/article/news/local/valley/arizona-researchers-finding-higher-concentrations-of-microplastics-in-the-valley/75-240fba9d-1b94-4e38-8927-de4cf1318754 (accessed 2024-12-04).
4. Workman, Jr., J. New Fluorescent Raman Technique Enhances Detection of Microplastics in Seawater. Spectroscopy. Available at: https://www.spectroscopyonline.com/view/new-fluorescent-raman-technique-enhances-detection-of-microplastics-in-seawater (accessed 2024-12-04).