The Chalco sub‐basin, the hydrogeological reserve of Mexico City, is located in the southern portion of Mexico City's basin, an intramontane valley located at 2200 m a.s.l. in the Trans‐Mexican Volcanic Belt (Central Mexico). Several geophysical studies (dc resistivity, seismic refraction, gravity, well‐logging) have been conducted in this area. The objectives of these independent surveys were different and of regional and local scopes. This study represents the first comprehensive effort to assimilate and interpret the data available to date from the area of the Chalco sub‐basin and to infer its main basinal characteristics. According to the topography we obtained of the basement, a northeast‐southwest structural high separates the sub‐basin into two portions. The western portion is asymmetric and deeper than the eastern one. An east‐west half graben‐like structure is located in the western portion. This structure could make possible an east‐west fluid flow at depth, and it also implies a differential subsidence. It was also possible to establish the stratigraphy of the volcano‐sedimentary infill. The gravity models give the interface: (1) between lacustrine sediments (aquitard) and the granular aquifer and (2) between the granular aquifer and the basaltic container. The resistivity boundaries correspond to the contact zone between saline and fresh water in the aquitard and semiconfined aquifer respectively, and they correlate with the existence of a vertical flow of saline fluids from the aquitard to the aquifer. The estimate of the volume of fluids contained in the granular aquifer range between 2 and [Formula: see text].

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A study of the fracture systems in the basement of the Matatlan urban waste dump of Guadalajara City, near the western edge of the Rio Grande de Santiago canyon (southwestern Mexico), is reported. The Matatlan dumpsite was developed on top of andesitic and rhyolitic rocks. Measurements at outcrops indicated that N–S (0°–15° and 165°–180°), N45–90E and N105–135E fracture systems affect the basement. Statistical analysis (micromagnetic method) of magnetic lineament azimuths indicated the presence of two lineament groups. The major one (N60–120E) has a maximum at N90–105E, and a secondary group (N120–165E). The gravity anomalies are featured by a major group (N60–120E, with maximums at N60–75E, and N105–120E) and a secondary group (N120–180E). Thus, a good to fair correlation exists between the measured fracture orientations and the lineaments featuring the gravity and magnetic anomalies. Horizontal derivative and tilt angle depicted horizontal limits of anomaly‐causative bodies. Results of the micromagnetic method are consistent with fracture orientations and with the limits established through edge‐detection techniques. Lineaments established by means of the gravity and magnetic Euler deconvolution are also consistent. Gravity and magnetic Euler deconvolution helped to estimate the depth extent of the limits of gravity and magnetic common sources. Euler deconvolution indicated the fault or contact nature of these lineaments. Furthermore, Euler deconvolution indicated slopes of the limits of gravity and magnetic sources. The fault or fracture nature of the major lineaments was finally confirmed by forward modelling (forward 2D cooperative modelling of gravity and magnetic profiles). For example, lineaments delimiting the northern depression could be characterized as normal faults. In general, this study indicates that it is possible, based on gravity and magnetic data, to characterize fractures affecting the basement of a dumpsite, a key parameter in hydrogeophysical studies. In particular, through this study, the inferred structures could be associated with regional tectonic faulting, which might provide migration paths to unhealthy lixiviate outside the dumpsite, giving rise to the contamination of the neighbouring environment. In particular, the study explains the presence of an unhealthy leachate waterfall to the northeast of the dumping site. Heavy metal contents in samples from this waterfall support that lixiviates originate at the dumpsite.

La inversión de datos gravimétricos nos permitió obtener la topografía del basamento del acuífero de Costa de Hermosillo.La profundidad del basamento es compleja y varía entre 300 y 3500 m, caracterizado por alternancias de horsts y grabens. Entrela ciudad de Hermosillo y Bahía Kino se presenta un alto con una estructura semicircular. Muchos de los grabens están interconectados,sirviendo de canales para el agua de mar que fluye hacia el continente, mientras que los horsts actúan como barreraspara este flujo. La forma que presentan las isolíneas de sólidos totales disueltos permite avalar estas hipótesis. Los datos gravimétricosnos permitieron también establecer la existencia de lineamientos que se correlacionan con la prolongación hacia el sur de la fallade Cerro Prieto y con el lineamiento del margen del Cratón. De acuerdo con la distribución de resistividades eléctricas en el área,el frente del agua de mar se extiende por más de 25 km dentro del continente y tiene una profundidad superior a los 200 m.Basándonos en estos resultados y en los datos de pozos, reconstruimos la estratigrafía del acuífero. La información geofísica,geológica y geohidrológica se utilizó para inferir un modelo geológico del acuífero de Costa de Hermosillo, que posteriormentepermitió construir secciones bidimensionales para realizar los modelos numéricos de la intrusión salina. Esta modelación serealizó con base en un tratamiento adimensional de las ecuaciones de Darcy, de continuidad para el agua y para la sal, y en unarelación constitutiva entre el agua y la concentración de sales. El sistema de ecuaciones adimensionales resultante fue resuelto endos dimensiones mediante un método de diferencias finitas, en perfiles seleccionados sobre el modelo geológico de la zona.En donde no existen barreras para el flujo de agua de mar se observan, en los modelos, patrones advectivos de circulación delos fluidos. Sin embargo, en las porciones separadas por intrusivos o horsts se observan celdas convectivas. El flujo en el acuíferoes complicado debido a la diversidad de materiales en el relleno sedimentario y a la sobreexplotación del mismo. Se tomó comocondición de frontera el régimen de explotación actual y se realizaron simulaciones numéricas de circulación de fluidos hasta queel sistema alcanza un estado estacionario. Los valores de concentración de sales así obtenidos reproducen de manera aceptable losvalores observados.