Livestock-derived organic carbon drives redox gradients shaping nitrate and trace-metal behavior across interacting waters in Querétaro, Mexico

Redox processes within the Earth’s critical zone tightly couple organic carbon turnover with nutrient transformations and the mobility of redox-sensitive trace metals in groundwater. In livestock-dominated settings, manure and wash-water can impose substantial dissolved organic carbon (DOC) loads on shallow water resources, yet the coupled relationships among DOC, redox conditions, nitrate occurrence, and trace-metal behavior across interacting water compartments remain insufficiently constrained. This study evaluates these linkages at an animal husbandry site in Querétaro, Mexico, using a multi-proxy dataset (n = 6) spanning lagoon water, well water, irrigation water derived from stored groundwater, on-site tap water, and two university campus end-use waters included as external context. DOC concentrations were elevated across all waters (48.87–190.50 mg L⁻¹), with lagoon water defining a strong organic-loading endmember. Redox conditions ranged from reducing in lagoon water (ORP −17 mV; DO 2.31 mg L⁻¹) to oxidizing in other compartments (ORP 218–291 mV; DO 3.63–5.20 mg L⁻¹). DOC showed a strong inverse relationship with ORP (r = −0.93), while nitrate increased with ORP (r = 0.80) and decreased with DOC (r = −0.93), consistent with carbon-fueled oxygen demand and diminished nitrate persistence under lower redox potential. Trace metals exhibited element-specific responses: Zn and Cr increased with DOC (r = 0.97 and 0.83) and decreased with ORP (r = −0.98 and −0.96), indicating enhanced metal mobility under DOC-rich, reducing conditions. Lagoon water also displayed the highest electrical conductivity and markedly elevated Zn and Cr, supporting its role as a concentrated source reservoir. In contrast, Cu concentrations did not scale with DOC across compartments, suggesting additional source controls such as distribution-system influences. Overall, the findings identify livestock-derived DOC as a first-order driver of redox gradients that structure nitrate patterns and trace-metal behavior across both natural and engineered water systems.