# Bioassays with _Allium cepa_ for the Monitoring of Toxicity in the Groundwater of Yucatan, Mexico
###### Abstract  
This study employed the _Allium cepa_ bioassay to evaluate the toxic effects of contaminants in the Yucatan aquifer. Seven monitoring wells were studied during September and October 2021. Nutrient concentrations showed significant variation between sites, with samples closer to the coast (P3 and P7) presenting higher ammonia and phosphate concentrations. The pesticides found at the highest concentration were 6-HCH and chlorpyrifos, with 141.44 and 175.92 ng/L, respectively. Heptachlor and aldfin were present in sites P4 oct and P2 sept. Interestingly, DDT values were highly correlated with caffeine concentrations. The PAHs acenphthylene and the sum of B(k)fluoranthene and B(b)fluoranthene presented the highest prevalence. B(k)fluoranthene and B(b)fluoranthene were the PAHs found at the highest concentration. The results of the _A. cepa_ bioassay indicated no nuclear abnormalities. The study also found no statistical differences in the mitotic index, root length, biomarkers of oxidative stress, and inhibition of B-esterases between sites and controls. In summary, the wells sampled in the present study had low concentrations of contaminants that can be used as a proxy of anthropogenic discharges; the lack of effect in the biomarkers used at organism, cellular, and biochemical levels indicated no toxic effect on _A. cepa_ roots.  
Allium cepa; biomonitoring; pesticides; PAHs; nutrients; Mexico; Yucatan; groundwater; water quality +
Footnote †: journal: Medical  
the permitted water quality limits for human use and consumption. This NOM includes many physical, chemical, and bacteriological parameters but does not evaluate toxicity. Interestingly, the evaluation of biomarkers of effect in wild fish caught in the Yucatan aquifer has led us to determine that the organisms present responses at the genetic and biochemical level that indicate exposure to toxic compounds, finding a response by exposure to endocrine disruptors, neurotoxic compounds, and oxidizing compounds [8,9].  
However, it is concerning that no evaluation of the genotoxicity of Yucatan's groundwater has been conducted. Genotoxicity, the ability to damage genetic material by physical, chemical, or biological agents, is a crucial aspect that has been overlooked. The lack of research in this area is a significant gap that needs to be addressed. [PERSON] first reported the use of _Allium cepa_ (common onion) as an indicator of genetic damage in 1938 [10].  
The characteristics of _Allium cepa_ make it an excellent genetic model for evaluating environmental contaminants and can be used to monitor physical and chemical toxic agents. The advantages of the assay are not only due to its sensitivity to detect mutagenic compounds, but also to the possibility of evaluating numerous genetic variables ranging from a point mutation to aberrations in chromosomes and nuclei [11]. The assay is an excellent in vivo test, where the roots grow directly with the substance of interest. The assay is highly efficient and correlates well with other systems; it is low-cost and very easy to implement, and the onion only has 16 chromosomes [12]. In addition, it complies with the trend in the ecotoxicology and environmental sciences to reduce the use of animals by using other types of models to perform toxicity bioassays.  
The bioassay with _Allium cepa_ has been used to evaluate the presence of various contaminants, such as heavy metals, domestic and industrial waste, and biocides [13,14], in rivers [15], sediments [16], and lakes [13]. Therefore, it is proposed as a method to evaluate genotoxicity in groundwater in Yucatan. The present study aimed to assess groundwater toxicity from seven wells in Yucatan, Mexico, using the _Allium cepa_ bioassay and its relationship with some contaminants.