Diversity of contaminants of emerging concern in a semi-arid groundwater system

A total of 65 groundwater sites within a semi-arid aquifer were analysed for 77 contaminants of emerging concern (CECs), including 33 pharmaceuticals and personal care products (PPCPs), 17 pesticides, and 27 industrial compounds. Only 2 sites showed no detections. Detection frequencies were 90% for PPCPs, 44% for pesticides, and 67% for industrial compounds. Common CECs included ethyl-paraben, fluconazole, sulfamethoxazole, PFOS, PFOA, and BTSA. Total CEC concentrations ranged from below detection to 1898 ng L^-1, where antimicrobials (fluconazole ≤208 ng L^-1, median: 5.3 ng L^-1), antibiotics (sulfamethoxazole ≤31.4 ng L^-1, median: 3.6 ng L^-1), and stimulants (caffeine ≤233 ng L^-1, median: 2.9 ng L^-1) were most prevalent. Industrial CECs were also frequent (e.g., BTSA ≤285.5 ng L^-1, median: 44.5 ng L^-1). Pesticides occurred at 29% of sites, mainly metolachlor (≤22 ng L^-1, median: 0.9 ng L^-1) and 2,4-D (≤3.1 ng L^-1, median: 0.7 ng L^-1). Spatial and statistical analyses linked wastewater treatment plants (WWTPs) as key CEC sources. CEC diversity was greatest within 3 km of WWTPs, and importantly, carbamazepine, caffeine, and BTSA were identified as highly persistent and mobile compounds, extending up to 18 km downgradient. This long-range transport demonstrates their role as powerful indicators of regional-scale groundwater contamination and reveals a significant risk of far-field pollution beyond immediate source areas. Spearman correlations revealed few significant links between CECs and proximity to hotspots, suggesting diffuse pollution from urban, industrial, and irrigation sources. Depth to groundwater had no effect on CEC presence, indicating aquifer-wide vulnerability. These findings highlight widespread, persistent contamination in drought-impacted semi-arid aquifers, driven largely by WWTPs and urban runoff, emphasizing the need for integrated groundwater protection strategies.