Groundwater represents almost half of the drinking water worldwide and more than one third of water used for irrigation. Agro-industrial activities affect water resources in several manners; one of the most important is leaching of agrochemical residues. This research identifies the major contributors of changes in groundwater quality comparing two contrasting land uses in a karstic area of the Yucatan peninsula as case study. Using a multiple approach, we assess the impact of land use with physicochemical data, multivariate analyses, hydrogeochemistry and nitrate isotopic composition. We confirmed that agricultural land use has a greater impact on groundwater quality, observed in higher concentration of nitrates, ammonium, potassium and electrical conductivity. Seasonality has an influence on phosphates and the chemical composition of the groundwater, increasing the concentration of dissolved substances in the rainy season. There was a clear effect of manure application in the agricultural zone and the nitrate isotopic composition of groundwater points toward recharge in certain areas. We consider that seasonality and land use effects are intertwined and sometimes difficult to separate, likely because of land use intensity and hydrogeochemical process at a local scale. Finally, we observed poor groundwater quality in the agricultural area during the wet season; thus, it is desirable to maintain nonagricultural areas that provide groundwater of appropriate quality.

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<p><strong>Background</strong>: The state of Yucatan has a significant amount of groundwater, but it is vulnerable to pollution due to agro-industrial contamination given the characteristics of thin soil and calcareous rocks that allow rapid infiltration of water, making easy for dissolved substances to reach the aquifer. The agro-industrial activities in the northeast of the peninsula of Yucatán promote changes in the geochemistry of groundwater in the region. <strong>Objective</strong>: Hereby, we present the observed changes in the geochemistry of groundwater by comparing three years of groundwater measurements from the agroindustrial area of Yucatan. <strong>Methodology</strong>: We measured physicochemical parameters, nutrients and ions during three years (2017 - 2019) in 14 wells of the municipalities of Sucilá, Panabá and Tizimín (east Yucatán). <strong>Results</strong>: Our results of three years show that ammonium, sulfate, chloride and calcium had an increasing trend; whereas nitrate and phosphate show a slight decrease. The agronomic indices SAR, CROSS<em>_f</em> and RSC were evaluated to identify risk of sodification or alkalinization of the soil for irrigation. The water-rock interaction imparts special characteristics to groundwater that should be considered for its use in irrigation. <strong>Implications. </strong>This research updates information on the quality of water for irrigation and provides useful agronomic indices for better agro-industrial practices in the northeast region of Yucatán.<strong> Conclusion</strong>: The grouping of wells did not completely respond to the spatial location; rather, it might be a combination of seasonal effects, livestock, animal feeding operations and the use of manure and fertilizers, among others. With this research, we propose to establish the groundwater monitoring of the agro-industrial region of the northeast of Yucatan in the medium (5-10 years) and long term (&gt; 20 years).</p>

Herbicides are used to increase the yield of crops and pastures. The herbicide 2, 4-D has high leaching potential in karstic environments because it is water soluble, which causes that the herbicide has been found in groundwater and submerged caves in the Yucatan Peninsula. We detected the presence of the herbicide in groundwater in agricultural non-agricultural zones in the northeaster of the Yucatán Peninsula by using the UV spectrum obtained from HPLC-DAD as qualitative indicator. This finding supports the notion that application of 2, 4-D before and during the wet season promotes the presence of the herbicide in groundwater.