La zonificación hidrogeológica de la provincia de Buenos Aires incluye la denominada Costa Atlántica Bonaerense, caracterizada por la presencia casi continua de una cadena de dunas o médanos que se extiende desde Punta Rasa al norte, hasta Punta Alta al sur. A pesar de que estos depósitos de arena son excelentes almacenes de agua subterránea, constituyendo la única fuente de aprovisionamiento en la mayoría de las localidades costeras, las investigaciones sobre este vital recurso estratégico son escasas. El objetivo principal de esta tesis es reconocer los factores y procesos hidrogeológicos que condicionan y determinan la presencia y calidad del agua subterránea en los médanos costeros, en el sector comprendido entre las ciudades de Monte Hermoso y Punta Alta. Se llevaron a cabo distintos trabajos de campo y gabinete, incluyendo análisis hidrometeorológicos, levantamiento de perfiles geológicos, descripción y correlación de perforaciones, censos de captaciones, estudios geofísicos, análisis hidroquímicos, caracterización de parámetros hidráulicos e hidrodinámicos, cálculos de recarga, recurso y reservas hídricas subterráneas, caracterización y tipificación de humedales e identificación de actividades potencialmente contaminantes, entre otros. El acuífero libre limita al norte con la llanura pampeana y al sur con el mar argentino, configurando una interfaz agua dulce-salobre al norte y otra agua dulce salada al sur. Los estudios geofísicos evidencian que el hidroapoyo del sistema no es plano, sino que posee leves ondulaciones. Las arenas que conforman el acuífero suelen ser finas a medias, bien seleccionadas y compuestas principalmente por cuarzo y plagioclasas. Se constató mediante la utilización de tomografías de resistividad eléctrica la existencia de paleocauces y diversas variaciones litológicas. Mediante un ensayo de bombeo se obtuvo un coeficiente de almacenamiento de 0,21, mientras que la transmisividad se calculó en 217 m2 /día y la conductividad hidráulica en 21 m/día. La superficie piezométrica copia de manera suave el relieve topográfico. La profundidad al nivel freático no suele superar los 6 m, registrándose una oscilación máxima de 46 cm, vinculada a variaciones estacionales. En general el acuífero presenta aguas de baja salinidad, con una conductividad eléctrica promedio de 800 μS/cm. El patrón químico dominante corresponde a aguas bicarbonatadas sódicas y bicarbonatadas cálcicas/magnésicas. El ingreso de agua al acuífero depende de los excesos hídricos de las precipitaciones. El valor medio de recarga obtenido fue del 32% de la precipitación media anual (732 mm/año). El recurso hídrico potencial se calculó en 93,60 Hm3 / año, mientras que las reservas ascienden a 2.500 Hm3 . Las actividades potencialmente contaminantes se concentran en el balneario Pehuen-Có, destacándose la falta de red cloacal y la presencia de un basural a cielo abierto. La vulnerabilidad y el peligro de contaminación del acuífero clasifican como alto. Se destaca la presencia de un acuífero con importantes volúmenes de agua dulce, apta para todo uso, en una zona de clima árido a semiárido. Se pretende que la información generada en esta tesis sea de utilidad para la sociedad y los tomadores de decisiones, en pos de utilizar y conservar de manera sustentable y sostenible el importante recurso hídrico aquí expuesto. / The hydrogeological zoning of the province of Buenos Aires includes the so called Atlantic Coast of Buenos Aires, characterized by the almost continuous presence of a chain of sand dunes that extends from Punta Rasa in the north to Punta Alta in the south. Regardless of the fact that these sand deposits are excellent stores of groundwater, constituting the only source of supply in most of the coastal localities, research on this vital strategic resource is scarce. The main objective of this thesis is to recognize the hydrogeological factors and processes that condition and determine the presence and quality of groundwater in the coastal sand dunes, in the sector between the cities of Monte Hermoso and Punta Alta. Different field and lab works were carried out, including hydrometeorological analysis, geological profiling, description and correlation of drillings, census of water wells, geophysical studies, hydrochemical analysis, characterization of hydraulic and hydrodynamic parameters, recharge calculations, groundwater resources and reserves, characterization and classification of wetlands and identification of potentially contaminating activities, among others. The free aquifer is bounded to the north by the Pampean plains and to the south by the Argentinean Sea, forming a freshwater-brackish water interface to the north and a freshwater-saltwater interface to the south. Geophysical studies show that the hydro-support of the system is not flat, but has slight undulations. The sands that make up the aquifer are generally fine to medium, well-sorted and composed mainly of quartz and plagioclase. The existence of paleochannels and several lithological variations was confirmed by means of electrical resistivity tomography. By means of pumping tests, a storage coefficient of 0.21 was obtained, while transmissivity was calculated at 217 m 2 /day and hydraulic conductivity at 21 m/day. The piezometric surface smoothly copies the topographic relief. The depth to the water table does not usually exceed 6 m, with a maximum oscillation of 46 cm, linked to seasonal variations. In general, the aquifer has low salinity waters, with an average electrical conductivity of 800 μS/cm. The dominant chemical pattern corresponds to sodium bicarbonate and calcium/magnesium bicarbonate waters. Water inflow to the aquifer depends on excess rainfall. The average recharge value obtained was 32% of the mean annual precipitation (732 mm/year). The potential water resource was calculated at 93.60 Hm3 /year, while the reserves amount to 2,500 Hm3 . Potentially polluting activities are concentrated in the Pehuen-Có resort, especially the lack of a sewage system and the presence of an open dump. The vulnerability and danger of aquifer contamination are classified as high. The presence of an aquifer with important volumes of fresh water, suitable for all uses, in an arid to semi-arid climate zone is highlighted. It is intended that the information generated in this thesis will be useful for society and decision makers, in order to use and preserve the important water resource presented here in a sustainable way.

Modelling seawater intrusion (SWI) has evolved from a tool for understanding to a water management need. Yet, it remains a challenge. Difficulties arise from the assessment of dispersion coefficients and the complexity of natural systems that results in complicated aquifer geometries and heterogeneity in the hydraulic parameters. Addressing such difficulties is the objective of this thesis. Specifically, factors that may affect the flow and transport in coastal aquifers and produce heterogeneous salinity distributions are studied.First, a new paradigm for seawater intrusion is proposed since the current paradigm (the Henry problem) fails to properly reproduce observed SWI wedges. Mixing is represented by means of a velocity dependent dispersion tensor in the new proposed problem. Thereby, we denote it as "dispersive Henry problem". SWI is characterized in terms of the wedge penetration, width of the mixing zone and influx of seawater. We find that the width of the mixing zone depends basically on dispersion, with longitudinal and transverse dispersion controlling different parts of the mixing zone but displaying similar overall effects. The wedge penetration is mainly controlled by the horizontal permeability and by the geometric mean of the dispersivities. Transverse dispersivity and the geometric mean of the hydraulic conductivity are the leading parameters controlling the amount of salt that enters the aquifer.Second, the effect of heterogeneity was studied by incorporating heterogeneity in the hydraulic permeability into the modified Henry problem. Results show that heterogeneity causes the toe to recede while increases both the width and slope of the mixing zone. The shape of the interface and the saltwater flux depends on the distribution of the permeability in each realization. However, the toe penetration and the width of the mixing zone do not show large fluctuations. Both variables are satisfactorily reproduced, in cases of moderate heterogeneity, by homogeneous media with equivalent permeability and either local or effective dispersivities.Third, the effect of aquifer geometry in horizontally large confined aquifers was analyzed. Lateral slope turned out to be a critical factor. Lateral slopes in the seaside boundary of more than 3% cause the development of horizontal convection cells. The deepest zones act as preferential zones for seawater to enter the aquifer and preferential discharging zones are developed in the upwards lateral margins. A dimensionless number, Nby, has been defined to estimate the relative importance of this effect.All these factors can be determinant to explain the evolution of salinity in aquifers such as the Main aquifer of the Llobregat delta. Finally, a management model of this aquifer is developed to optimally design corrective measures to restore the water quality of the aquifer. The application of two different optimization methodologies, a linear and a non-linear optimization method, allowed (1) to quantify the hydraulic efficiency of two potential corrective measures: two recharge ponds and a seawater intrusion barrier; (2) to determine the water necessary to be injected in each of these measures to restore the water quality of the aquifer while minimizing changes in the pumping regime and (3) to assess the sustainable pumping regime (with and without the implementation of additional measures) once the water quality has been restored. Shadow prices obtained from linear programming become a valuable tool to quantify the hydraulic efficiency of potential corrective measures to restore water quality in the aquifer.