Influence of dissolved organic carbon on multimedia distribution and toxicity of fipronil and its transformation products in lotic waterways

Evidence from In Situ Bioassays and Suspect Analysis Revealed the Region-Specific Aquatic Risk across Socioeconomic Gradients in China

Various contaminants are present in aquatic environment and pose potential threats to pelagic and benthic organisms, calling for effective risk assessment. Traditional risk assessments based on target analysis are useful when the principal contaminants responsible for ecological risk are known; however, these approaches become challenging when dealing with chemical mixtures. In addition, the compositions of chemical mixtures often differ in regions with different levels of socioeconomic development, requiring risk assessment methods that are applicable under different pollution scenarios. Herein, in situ bioassays were conducted with two native species, Chinese rare minnows (Gobiocypris rarus) and Asian clams (Corbicula fluminea), in economically developed watersheds in China (Pearl River Basin (PRB) and Taihu Lake Basin (THB)) and agriculture-dominated Poyang Lake Basin (PYB). Significant lethal and sublethal effects (e.g., neurotoxicity, reproductive toxicity, and metabolic and oxidative stress) were observed in fish and clams irrespective of economic gradients. Notably, ecological effects differed significantly between water and sediment phases within the same region. Target (98 contaminants) and suspect screening (942 contaminants) revealed regional-specific characteristics. Ecological risk assessments using a weight of evidence approach demonstrated that both water and sediment in the PRB were at moderate to high risk, as was the sediment in the less developed PYB. However, the characteristics of mixture pollution varied greatly among regions. Suspect screening identified many pollutants that are not regularly monitored but are present at high environmental concentrations and are linked to local industrial production. These distinct mixture risk characteristics across different basins suggest that mitigating aquatic pollution requires region-specific management measures.

Quantitative differentiation of toxicity contributions and predicted global risk of fipronil and its transformation products to aquatic invertebrates

Biotransformation often alters chemical toxicity, yet its impacts on risk assessment are hardly quantified due to the challenges in acquiring internal exposure-based thresholds for chemicals that are readily metabolizable. Here, we integrated toxic unit and toxicokinetics to quantitatively assess toxicity contributions and potential risk of both parent compound and transformation products (TPs) to aquatic organisms, using fipronil (FIP) as a representative toxicant. In aquatic invertebrates Chironomus dilutus and Hyalella azteca, approximately 90 % of FIP was transformed to fipronil sulfone (SUL). FIP and SUL exhibited similar intrinsic toxicity to these organisms, which was contrary to conventional perception that SUL was more toxic than FIP. However, biotransformation was still important in risk assessment because the TP had 10-fold slower depuration rate than FIP. The amphipod H. azteca was found to be as sensitive to FIPs as the insect C. dilutus, which was previously considered ten times more sensitive based on external thresholds. This discrepancy has led to overlooking the toxicity of FIP to H. azteca in regional risk assessments. Lastly, we predicted the lethal risk of FIPs in global surface water. When using external thresholds for prediction, FIPs in 3.4 % of the water samples were lethally toxic to H. azteca, and the percentage of water samples at risk increased to 14 % when internal thresholds were used and SUF dominated the risk. This study presents an improved method for quantifying aquatic risk of readily metabolized toxicants. Our findings underscore the urgency of considering TPs in water quality assessments, especially for sensitive species that are at risk in the environment.

Occurrence and water-sediment exchange of systemic insecticides and their transformation products in an agriculture-dominated basin

Neonicotinoids (NEOs) and fipronil (FIP) are ubiquitous in aquatic environment, yet the transformation and water-sediment exchange are largely unknown for these systemic insecticides and their transformation products (TPs). Herein, occurrence, field-based partitioning coefficients, and fugacity fractions (ff) of NEOs, FIP, and their TPs were analyzed in the drainage and receiving rivers near a rice paddy field. NEOs and FIPs were frequently detected in the sediments with concentrations of TPs being often higher than the parent compounds. Average ff values of NEOs (0.944-1.00) were larger than those of FIPs (0.399-0.716), indicating NEOs had a greater tendency to diffuse from sediment into water. Similar as well-studied hydrophobic compounds, hydrophobicity was the main factor impacting the water-sediment exchange of moderately hydrophobic FIPs. Alternatively, electrostatic interactions governed the fate of hydrophilic NEOs in water-sediment system. The log Kd values of NEOs were positively correlated with their N/C ratios (p < 0.05), possibly because the negatively charged sediments (zeta potential were from -19.1 ± 0.6 to -5.84 ± 0.57 mV) generated electrostatic attraction with amino functional group. Our study highlighted the ubiquitousness of TPs and distinct water-sediment interaction for moderately hydrophobic and hydrophilic insecticides in an agriculture-dominated watershed.