Distribution of pesticides in groundwater and surface water flood-impacted unconnected communities in South Texas following Hurricane Hanna

Pesticides are a vital farming input as they help protect crops from pests and diseases for quality yields and high productivity. However, pesticides' high production, extensive use, and persistence are a potential risk for human health and ecosystems due to their leaching or runoff into the surface water (SW) and groundwater (GW). Over the years, there has been an increasing concern regarding the water quality of SW and GW next to farming areas, specifically after spills or industrial discharges, heavy rainfall, and intense storms. In the case of hurricanes, these cause deposition of contaminants from the atmosphere, disturbance and transport of the sediments in surface water bodies, surface run-off, and penetration of floodwaters to GW. In addition, pesticide degradation in the environment due to different biotic and abiotic transformation processes produces transformation products (TPs), sometimes even more toxic than the original compound. In July 2020, Hurricane Hanna severely damaged infrastructure and agriculture in many counties across the Rio Grande Valley. This area has the largest colonias in South Texas, predominantly Hispanic low-income semirural communities near the Texas-Mexico border that mainly rely on GW as a drinking water source. As part of this study, surface water and groundwater samples were collected from colonias in South Texas communities after hurricane Hanna for one year. The analytical approach involved solid-phase extraction (SPE) followed by suspect and target screening of pesticides using ultrahigh-pressure liquid chromatography (UPLC) coupled to a high-resolution Orbitrap Fusion Tribrid mass spectrometer system (OT-FTMS). The advantages of the state-of-the-art high resolution mass spectrometry allowed the accurate identification and quantification of pesticides using an in-house compound database of 308 pesticides for target screening. This is one of the most extensive studies that has screened and quantified over 200 pesticides in a single sample in the United States and, to my knowledge, is one of the largest in Texas. The results of eight monitoring sampling campaigns revealed the occurrence of 21 pesticides with minimum instrumental limit of detection (LOD) of 0.05 ng/mL and limit of quantification (LOQ) of 0.10 ng/mL. Six herbicides were the most detected compounds, followed by insecticides (5), fungicides (4), TPs (3), and a plant growth regulator (1). The most frequently detected compounds were atrazine, clothianidin, cycluron, DEA, dinotefuran, and norflurazon and those with the highest concentrations were clothianidin (211 ng/L), DEET (13,885 ng/L), and norflurazon (952 ng/L). In general, more compounds were detected in GW than in SW, except for atrazine and DEA. Atrazine in SW showed higher detections frequency (67%) than GW (15%) as well as DEA (GW = 6%, SW = 8%). DEET was the compound detected at the highest frequency in GW (74%) while atrazine was detected at the highest frequency in SW (67%). The leaching of pesticides to GW is a complex process and depends on the pesticide's chemical properties, characteristics of the soil, volatilization, climatic conditions of the site, plant uptake, and pesticide management practices. Mainly, pesticides with high leaching potential are more likely to be transported in groundwater beyond application area. In this study, clothianidin and norflurazon with high leaching potential showed significant concentrations in GW. This is one of the few studies able to survey more than 200 pesticides in the United States in general and in Texas specifically, using high resolution mass spectrometry. This work provides a baseline of the water quality for those communities that rely on GW as the primary source of drinking water supply. The occurrence of pesticides in GW demonstrates the vulnerability of the area and the need to continue monitoring the water quality in this region.