RESUMO
The Yaqui River Irrigation District is a region in Mexico with intensive agricultural production; thus, large quantities of fertilizers are used, and excess fertilizer can affect the quality of water bodies. The aim of this work was to estimate the water budget and nitrogen (N) and phosphorous (P) mass budgets to evaluate possible contamination of a littoral water body (Tóbari Bay) and leachates into an aquifer (Yaqui Valley aquifer). Wheat and corn crops were studied, climate information was compiled, and soil and water samples were collected for analysis. The water budget showed excess irrigation occurred due to the need for soil washing to prevent salinization. A total of 24% of all irrigation water was used for crops, 60% was discharged into the bay through runoff of the drainage system, and 16% corresponded to effective infiltration (aquifer recharge). The N budget showed that of the 100% N input, the highest percentage was used by plants (63%), and only minimal loss occurred through runoff (11%) and leachate into the aquifer (7%). The remaining N stayed in the soil (18%) or was volatilized (1%). These results indicate that the drainage system prevented large amounts of N from entering the aquifer; thus, the N concentrations in the groundwater did not exceed the regulated maximum limit for drinking water (10 mg N-NO3/L). In terms of the water pollution level in the bay, the presence of NO3- was minimal (concentrations below the quasintifiable limit). Of the 100% of P that was applied, 55% was used by the plants, and 40% remained in the soil; therefore, the P that was transported by runoff or was leached was minimal (3 and 2%, respectively). However, this minimal amount of P ranged from 0.1 to 0.2 mg/L in the bay, and these values exceeded the suggested values for the protection of aquatic life (0.01 mg/L). The administrators of the irrigation district must pay special attention to phosphate fertilizer management and implement irrigation techniques that increase water use efficiency.
Assuntos
Água Potável , Água Subterrânea , Fósforo/análise , Nitrogênio/análise , Fertilizantes/análise , México , Monitoramento Ambiental/métodos , Água Subterrânea/análise , Água Potável/análise , Produtos Agrícolas , SoloRESUMO
While arsenous acid, As(OH)3, has been the subject of a plethora of studies due to its worldwide ubiquity and its toxicity, pentavalent As in the form of arsenic acid, AsO(OH)3, has recently been found in rivers in central Mexico as the most abundant naturally occurring arsenic species. To better understand the solvation patterns of both toxic acids at the molecular level, we report the results of Born-Oppenheimer molecular dynamics simulations on the aqueous solvation of the AsO(OH)3 and As(OH)3 molecules at room temperature using the cluster microsolvation approach including 30 water molecules at the B3LYP/6-31G** level of theory. We found that the average per-molecule water binding energy is ca. 1 kcal mol-1 larger for the As(v) species as compared to the As(iii) one. To account for the asymmetry of both molecules, the hydration patterns were studied separately for a "lower" hemisphere, defined by the initially protonated oxygens, and for the opposite "upper" hemisphere. Similar lower hydration patterns were found for both As(iii) and As(v), with the same coordination number CN = 7. The upper pattern for As(iii) was found to be of a hydrophobic type, whereas that for As(v) showed the fourth oxygen to be hydrogen-bonded to the water network, yielding CN = 3.7; moreover, a proton "hopped" from the lower to the upper side, through the Grotthuss mechanism. Theoretical EXAFS spectra were obtained that showed good agreement with experimental data for As(iii) and As(v) in liquid water, albeit with somewhat longer As-O distances due to the level of theory employed. Proton transfer processes were also addressed; we found that the singly deprotonated H2AsO3- species largely dominated (99% of the simulation) for the As(iii) case, and that the deprotonated H2AsO4- and HAsO42- species were almost equally present (45% and 55%, respectively) for the As(v) case, which is in line with the experimental data pKa1 = 2.24 and pKa2 = 6.96. Through vibrational analysis the features of the Eigen and Zundel ions were found in the spectra of the microsolvated As(iii) and As(v) species, in good agreement with experimental data in aqueous solutions.