Assessment of trace metals in irrigation water, soil, and vegetables along the US-Mexico border and their health risks

The Rio Grande, an important river that borders the USA and Mexico is recharged with untreated and treated wastewaters from binational sources and therefore is known to contain potentially toxic elements (PTEs) like As, Cd, Co, Cr, Pb, and Sb. These elements are nonbiodegradable and can accumulate in soil and crops when contaminated surface water is used for irrigation. This study was designed to assess the contamination levels of these elements in water, soil, and crops sampled from three farms (onion, cilantro, and Swiss Chard in Hidalgo Co., TX) and use the data to perform pollution assessment (bioaccumulation factor - BAF, enrichment factor - EF, and metal pollution index - MPI) and health hazard assessment (health index-HI, estimated daily intake–EDI, and cancer risk - CR). Matrixes were digested and analyzed using ICP-OES. The mean PTE concentrations in water (0.73–81.74 ug/L) were all within acceptable levels for irrigation water. Although the mean concentration of PTEs (mg/kg) in soil Cd (0.19), Co (5.04), Cr (10.04), and Pb (18.36) were within acceptable limits, all three had above normal levels of Sb (11.64–18.62) with elevated As levels detected in the onion and Swiss chard fields (7.35 and 8.20, respectively). The mean dry weight of As and Cr in onion, cilantro, and Swiss chard tissues were either above normal or excessive (As 2.90 mg/kg – 5.85 mg/kg; Cr 0.71 mg/kg – 15.79 mg/kg), but all crop tissues had normal or below normal levels of Sb (mean 9.50 mg/kg). Swiss chard tissues also had elevated levels of Cd, Co, and Pb (0.25 mg/kg, 7.04 mg/kg, and 19.11 mg/kg, respectively). Health risk assessment showed that the EDI of all PTEs studied in the crops were within the US FDA daily acceptable intake limits; the HI for cilantro was safe for cilantro, but high for onion and Swiss chard. CR assessment for cilantro was acceptable across all the metals studied, whereas CR for onion and Swiss chard were above the threshold and were considered unacceptable. These results indicate that chronic consumption of crops that bioaccumulate PTEs can present systemic toxicological risks within the water-energy-food nexus.