Identifying recharge from tropical cyclonic storms, Baja California Sur, Mexico

Hydrochemistry and stable isotopes revealed focused and diffuse recharge processes in the Sonora River basin, Mexico

A hydro-geochemical characterization was conducted in the northern part of the Sonora River basin, covering an area of 9400 km². Equipotential lines indicated that groundwater circulation coincided with the surface water flow direction. Based on the groundwater temperature measured (on average ∼21 °C), only one spring exhibited thermalism (51 °C). Electrical conductivity (160-1750 μS/cm), chloride and nitrate concentrations (>10 and >45 mg/L) imply highly ionized water and anthropogenic pollution. In the river network, δ¹⁸O values revealed a clear modern meteoric origin. Focused recharge occurred mainly from the riverbeds during the rainy season. During the dry season, diffuse recharge was characterized by complex return flows from irrigation, urban, agricultural, mining, and livestock. Drilled wells (>50 m) exhibited a strong meteoric origin from higher elevations during the rainy season with minimal hydrochemical anomalies. Our results contribute to the knowledge of mountain-front and mountain-block recharge processes in a semi-arid and human-altered landscape in northern Mexico, historically characterized by limited hydrogeological data.

Geochemistry and Provenance of Springs in a Baja California Sur Mountain Catchment

Fractured rock aquifers cover much of Earth's surface and are important mountain sites for groundwater recharge but are poorly understood. To investigate groundwater systematics of a fractured-dominated aquifer in Baja California Sur, Mexico, we examined the spatial patterns of aquifer recharge and connectivity using the geochemistry of springs. We evaluate a range of geochemical data within the context of two endmember hypotheses describing spatial recharge patterns and fracture connectivity. Hypothesis 1 is that the aquifer system is segmented, and springs are fed by local recharge. Hypothesis 2 is that the aquifer system is well connected, with dominant recharge occurring in the higher elevations. The study site is a small <15 km² catchment. Thirty-four distinct springs and two wells were identified in the study area, and 24 of these sites were sampled for geochemical analyses along an elevation gradient and canyon transect. These analyses included major ion composition, trace element and strontium isotopes, δ¹⁸ O and δ² H isotopes, radiocarbon, and tritium. δ¹⁸ O and δ² H isotopes suggest that the precipitation feeding the groundwater system has at least two distinct sources. Carbon isotopes showed a change along the canyon transect, suggesting that shorter flowpaths feed springs in the top of the transect, and longer flowpaths discharge near the bottom. Geochemical interpretations support a combination of the two proposed hypotheses. Understanding of the connectivity and provenance of these springs is significant as they are the primary source of water for the communities that inhabit this region and may be impacted by changes in recharge and use.

Groundwater Isotopes in the Sonoyta River Watershed, USA-Mexico: Implications for Recharge Sources and Management of the Quitobaquito Springs

Groundwater resources in the southwestern United States are finite and riparian and wetland areas are vulnerable to aquifer overdraft and unregulated groundwater use. Environmental isotopes and water chemistry were used to distinguish water types, recharge mechanisms, and residence time along several reaches of the Sonoyta River and Quitobaquito Springs located near the U.S.-Mexico border. Areas located upgradient from the Sonoyta River, such as the Puerto Blanco Mountains and La Abra Plain, are supported by local recharge which corresponds to water from the largest 30% of rain events mainly occurring during winter. For Quitobaquito Springs, the δ18O and δ2H values are too low to be derived from local recharge. Stable isotope data and Cl/SO4 mass ratios indicate that the Sonoyta River supplied Quitobaquito Springs through flow along a suggested fault system. Based on these results, Quitobaquito Springs flow could be diminished by any activity resulting in increased groundwater extraction and lowering of water elevations in the Sonoyta River regional aquifer.

Dynamics of major and trace elements during seawater intrusion in a coastal sedimentary aquifer impacted by anthropogenic activities

This study analyzed the dynamics of major ions and trace elements along the groundwater flow path of the coastal sedimentary Todos Santos aquifer in Baja California Sur, Mexico, moderately impacted by anthropogenic activities. The results indicate that the elements Ca²⁺, Mg²⁺, Ba²⁺, Sr²⁺ and Li⁺ are mobilized from the aquifer matrix during seawater intrusion, whereas the alkali-elements Na⁺, K⁺ and Rb⁺ are removed from solution, possibly due to cationic exchange process. The anions HCO^3- and SO₄^2- and the elements I and B are mobilized due to carbonate mineral weathering, whereas dissolved silica and the halides Br^- and F^- behave conservatively during salinization. Groundwater NO₃^- is provided by sewage infiltration. Regarding trace elements behavior, we identify three groups: i) elements that are mobilized during saline intrusion (Fe, Co, V, Se, Re), ii) elements revealing low or no mobilization (Mo, Ni, Cr, Ta, W) and iii) elements that show an undefined tendency (U, As, Ge, Sb, Cu, Mn). The U and NO₃^- levels in groundwater should be considered carefully because several wells have concentrations close to the permissible levels. This study may be useful as reference for knowing the possible effect of salinization in coastal aquifers under sea level rise scenarios driven by climate change.