# Bioassays with _Allium cepa_ for the Monitoring of Toxicity in the Groundwater of Yucatan, Mexico
## 3 Results and Discussion
### Pesticides
\begin{table}
\begin{tabular}{l l l l l l l l l} \hline \hline \multirow{2}{*}{**Compound**} & \multirow{2}{*}{**Month**} & \multicolumn{8}{c}{**Site**} \\ \cline{3-10}  & & **P1** & **P2** & **P3** & **P4** & **P5** & **P6** & **P7** \\ \hline \multirow{2}{*}{\(\delta\)-HCH} & September & ND & 141.44 & 80.70 & ND & ND & ND & ND \\ \cline{2-10}  & October & ND & ND & ND & ND & 33.81 & ND & 54.69 \\ \hline \multirow{2}{*}{Heptachlor} & September & ND & ND & ND & ND & ND & ND & ND \\ \cline{2-10}  & October & ND & ND & ND & 17.03 & ND & ND & ND \\ \hline \multirow{2}{*}{Aldrin} & September & 15.34 & ND & ND & ND & ND & ND & ND \\ \cline{2-10}  & October & ND & ND & ND & ND & ND & ND & ND \\ \hline \multirow{2}{*}{Chloryprifos} & September & ND & ND & 175.92 & ND & ND & ND & ND \\ \cline{2-10}  & October & ND & ND & ND & ND & 90.83 & 129.80 & 98.26 \\ \hline \multirow{2}{*}{p\(\cdot\)p\({}^{\prime}\)-DDD} & September & ND & ND & ND & ND & ND & ND & ND \\ \cline{2-10}  & October & ND & ND & & ND & ND & ND & 41.75 \\ \hline \multirow{2}{*}{p\(\cdot\)p\({}^{\prime}\)-DDT} & September & 65.12 & 52.21 & 65.06 & 59.54 & 56.95 & ND & ND \\ \cline{2-10}  & October & 57.20 & 57.80 & 86.57 & ND & ND & ND & ND \\ \hline \multirow{2}{*}{Total pesticides} & September & 170.82 & 291.68 & 467.90 & 113.04 & 141.00 & 108.82 & 152.08 \\ \cline{2-10}  & October & 145.91 & 151.07 & 235.90 & 89.96 & 150.20 & 205.00 & 258.93 \\ \hline \multirow{2}{*}{Caffeine} & September & 6.16 & 5.92 & 8.12 & 10.21 & 10.10 & 5.41 & ND \\ \cline{2-10}  & October & 8.38 & 9.13 & ND & ND & ND & ND & ND \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._\(\alpha\)-HCH is one of the eight stereoisomers of hexachlororocyclohexane (HCH), and the one found in the highest proportion (53-70%) in what is known as technical HCH (mixture of isomers), in addition to \(\beta\)-HCH (3-14%), \(\gamma\)-HCH (11-18%), and \(\delta\)-HCH (6-10%). \(\gamma\)-HCH is the only isomer with strong insecticidal properties, refined from technical HCH and known as lindane. Lindane is an insecticide for fruits, vegetables, and forest crops. In the environment, it can be converted into \(\alpha\)-HCH through photochemical processes. \(\alpha\)-HCH and \(\gamma\)-HCH can be biologically transformed into \(\beta\)-HCH, which is the most persistent isomer and is most resistant to biodegradation compared to the others. This could explain why it was the third-most abundant isomer [36]. The alpha/gamma HCH ratio can be used to identify the source of HCH in water. A ratio of 0.2-1 suggests an origin from lindane, and a range between 4-15 indicates the use of technical HCH [37]. The alpha/gamma ratio obtained for the analyzed wells was between 0.42-1.29, so its origin could be attributed to using lindane. Previous works measured DDT and lindane in water from 29 wells between Merida and Progreso in Yucatan [38]. They reported values well above what was found in the present study; for example, \(\alpha\)-HCH was found in a range of 20-830 ng/L, and \(\beta\)-HCH was found in a range of 30-13,200 ng/L. Both pesticides were present in higher concentrations in the \"Nortes\" season (from November to February), where some wells exceeded the established limits in NOM-127-SSA1-2021. DDT was found in 57.14% of the samples, and its metabolite p-p\({}^{\prime}\)-DDD was only found in one sample. The highest concentration was found in site P3 oct (86.57 ng/L). This result contrasts with previous reports [38], where 65% of the wells analyzed in the \"Nortes\" season were above the limit permitted by NOM-127-SSA1-2021, 1000 ng/L. Chloryprifos was found in four samples, three of them in October, and was the pesticide found at the highest concentration (175.92 ng/L in site P3 in September). Chloryprifos is an organophosphate pesticide that controls mosquitoes, flies, and domestic pests [39]. Chloryprifos was added to the NOM-124-SSA1-2021 with a permitted value of 30,000 ng/L. This threshold contrasts with the value allowed in the European Union of 100 ng/L.