Beyond the Usual Suspects: A Comprehensive Look at Agricultural Stream Contaminants

Agriculture is crucial for our U.S. economy and food supply, so it's important to enhance crop and livestock production while minimizing harmful contaminants being released into the environment. While nutrients and pesticides are commonly understood contaminants in agricultural streams, a host of less understood pollutants derived from various urban and rural activities could also threaten stream health.  

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Research has shown that many activities and products we use to improve our daily lives can unintentionally pollute the environment. These pollutants include tiny pieces of plastic (microplastics), chemicals (such as per- and polyfluoroalkyl substances (PFAS), pharmaceuticals, personal care products, and pesticides), and bacteria that can cause disease or resist antibiotics. Studies have found that it’s quite common for different types of these pollutants to be found together in our waterbodies. Even when these substances seem to be at safe levels on their own, being present together can make them more harmful than expected. As a result, the mixture of these pollutants may affect aquatic life, plants, and humans that use waters for drinking and recreation.  

In agricultural settings, there is limited understanding of the complex interactions among water, sediment, living organisms, and the different pollutants that coexist. Understanding these interactions is crucial for effectively managing water quality and promoting healthier agricultural environments. To address this knowledge gap, the USGS conducted a comprehensive statewide study in Iowa, looking at 15 stream sites that represented different land use types. Six sites were primarily used for a single livestock type, including cattle, poultry, and swine, and three sites had a mix of livestock types. Additionally, three sites were influenced by the application of municipal biosolids on nearby farmland, two sites were affected by wastewater treatment plants discharging directly upstream, and one site served as a reference location, characterized by forested land and minimal agricultural impact. With more than 85% of Iowa's landscape dedicated to farming and the state being a leader in corn, soybean, pork, and egg production, this study provided an opportunity to gather baseline data on the prevalence of contaminants in agricultural waterways. 

 The study found that microplastics, PFAS, antibiotic-resistant genes, pharmaceuticals and personal care products, and pesticides were nearly ubiquitous in the water of wadable streams across Iowa. Additionally, microplastics and PFAS were commonly detected not only in the water but also in the sediment and tissues of fish. The presence of these contaminants in the environment highlights the need to consider their effects on aquatic ecosystems and human health. 

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USGS researchers calculated the potential biological risks associated with exposure to various contaminants, including pesticides, pharmaceuticals, and PFAS in water samples from each site. None of the study sites in Iowa were found to be at a level where biological effects might occur, such as chronic diseases, reproductive issues, or growth disorders. However, 13 sites were found to be at levels of potential concern. It's important to note that this analysis may not fully capture the risks involved; only 40 of the 128 chemicals detected during the study had the necessary toxicity data to assess these risks.  

A unique aspect of this study was the inclusion of microplastics in the risk assessment using established thresholds related to fish health. This study specifically focused on two main concerns: the risk of fish not getting enough food due to consuming microplastics (known as food dilution) and the risk of microplastics entering fish tissues (known as tissue translocation). According to these guidelines, all sites had microplastic levels that exceeded the moderate and high-risk thresholds for food dilution, indicating potential negative impacts on fish in both the short and long term. Additionally, every site exceeded the moderate risk threshold for chronic tissue translocation, and two sites surpassed the threshold for acute tissue translocation, highlighting the significant risks associated with microplastics. 

The findings from this study underscore the need for continued research and monitoring of contaminants in agricultural streams. Understanding how various pollutants interact and affect aquatic ecosystems is critical for developing effective management strategies. USGS science continues to help maintain agricultural productivity and ensure the health of our waterways to protect the environment and public health.   

This study was supported by the U.S. Geological Survey Ecosystems Mission Area, through the Environmental Health Program (Contaminant Biology and Toxic Substances Hydrology), Water Mission Area through the Central Midwest Water Science Center, and the University of Iowa.