Removal of Heavy Metals in Swine Wastewater Treatment and Their Contribution to Groundwater Contamination in a Karstic Area of Southeast Mexico

Irrigation with pig slurry has been employed to discharge large volumes of slurry and to recover resources. However, using swine wastewater for agricultural irrigation may cause the accumulation of heavy metals in soil and their potential leaching to groundwater. Wastewater treatment plants (WWTPs) are crucial to mitigate heavy metal contents in swine wastewater through physical, chemical, and biological processes. This study tracked the fate of eight heavy metals in industrial swine farms: arsenic (As), cadmium (Cd), copper (Cu), chromium (Cr), lead (Pb), mercury (Hg), nickel (Ni), and zinc (Zn). Zn reported the highest removal (82 to 99%) in all WWTPs and Cu the lowest (−5 to 97%). Cu (0.59–1.64 mg L−1) and Zn (0.35–1.14 mg L−1) were the metals reported in all samples for the target treatment stages (influent, after biodigester, and effluent). Comparing the heavy metal concentration in the effluents, Cu and Zn reached the highest concentrations in all WWTPs. As, Cd, and Pb reported values under the practical quantification limit. In groundwater, Cr reported the highest average concentration in farm GP19 for upstream (0.006 mg L−1) and for downstream (0.032 mg L−1) in GP1. In irrigated soil the Cu and Zn reported the highest concentrations in all farms, showing an enrichment compared to natural soil, indicating that wastewater is the main source of these metals in soil in the farm areas. Although all metals met the Mexican and international regulations, total suspended solids (TSSs) and chemical oxygen demand (COD) for effluent were above the reference limits (TSS ≤ 24 mg L−1 and COD≤ 72 mg L−1) more than ten and four times, respectively, for all WWTPs evaluated. These two parameters were positively related and significantly correlated (p < 0.05) with the presence of metals in the different water fractions, implying possible transport of metals in solids. Cd, Pb, and As, were never reported in treated wastewater and groundwater, but Cr and Hg were. This may be related to external activities such as agriculture for Cr. The enrichment of metals in irrigated soils can be related to the metal presence in groundwater due to leaching because of the karstic soil in the area.