Salinity and Temperature Variations near the Freshwater-Saltwater Interface in Coastal Aquifers Induced by Ocean Tides and Changes in Recharge

The temperature distribution of shallow sectors of coastal aquifers are highly influenced by the atmospheric temperature and recharge. However, geothermal heat or vertical fluxes due to the presence of the saline wedge have more influence at deeper locations. In this study, using numerical models that account for variable density, periodic oscillations of temperature have been detected, and their origin has been attributed to the influence exerted by recharge and tides. The combined analysis of field data and numerical models showed that the alternation of dry and wet periods modifies heat distribution in deep zones (>100 m) of the aquifer. Oscillations with diurnal and semidiurnal frequencies have been detected for groundwater temperature, but they show differences in terms of amplitudes and delay with electrical conductivity (EC). The main driver of the temperature oscillations is the forward and backward displacement of the freshwater–saltwater interface, and the associated thermal plume generated by the upward flow from the aquifer basement. These oscillations are amplified at the interfaces between layers with different hydraulic conductivity, where thermal contours are affected by refraction.Spanish Government CGL2016-77503-R IGME 10-1E-1284 IGME 15-CE3391European Regional Development Fund (ERDF) of the European Union (EU)Next-Generation EU MZSA03Junta de Andalucia RNM-36

Land uses changes effect over Motril-Salobreña aquifer recharge

Human activity over Motril-Salobreña aquifer (south of Spain) is changing traditional land uses during the last 50 years. In this work we aim to evaluate and quantify the main changes analyzing the aerial photos of different years. The results show a dramatic decrease in the crops of sugarcane and elevated increases of the greenhouses and subtropical trees agriculture extent. Also the surface of the urban areas has been extended noticeably. The direct consequences calculated in this work are a decrease of the recharge of the aquifer by irrigation of crops near to 75 %. The velocity of these changes and the magnitude of them point out the serious risk that involves the Motril-Salobreña aquifer resource

Flow patterns detection with temperature measurements in Motril-Salobreña aquifer

Se utilizaron medidas de temperatura en el acuífero Motril-Salobreña para detectar patrones de flujo. La interacción entre río el Guadalfeo y el acuífero y la presencia de flujos verticales ascendentes en las áreas cercanas a la desembocadura del río fueron caracterizadas a partir de perfiles de temperatura del agua subterránea. Los perfiles de temperatura mostraron una forma característica en algunos sondeos debido a la influencia del río sobre la temperatura del agua subterránea cuando el caudal del río fue elevado. Sin embargo no todos los sondeos siguieron esta tendencia. El estudio con envolventes de perfiles de temperatura, que normalmente son utilizados para definir si el río es ganador o perdedor, puede ser utilizado en este caso para detectar flujos verticales ascendentes en áreas donde nunca se habían definido anteriormente mediante otros medios. Algunos pozos surgentes se han detectado en este sector, pero la extensión donde estos flujos podrían ser observados ha sido ampliada hacia el norte y al esteTemperature measurements were used in Motril-Salobreña aquifer for detecting flow patterns. The interaction Guadalfeo River-aquifer and the presence of vertical upward flows in the areas close to the river mouth were characterized based on temperature groundwater logs. The temperature logs showed a characteristic shape due to the river influence in the groundwater temperature when the river flow is high in several boreholes. Nevertheless not all the boreholes follow this pattern. The study with envelope temperature profiles, that usually are used for defining if the river is losing or gaining, can be applied in this case for detecting vertical upward fluxes in areas where never were defined before by other means. Artesian wells have been detected in this area but the location where these types of fluxes could be noticed has been extended to the north and eas

Nuevos retos en el estudio de los acuíferos costeros desde la modelación numérica. El caso del acuífero Motril-Salobreña

Coastal aquifers are frequently complex systems with highly heterogeneous geological charac teristics, specific flow patterns, presence of fluids with different densities, high risk of contamina tion both by salinization and by other pollutants, and highly modified by anthropic activity. Given this situation, numerical modeling becomes the most appropriate tool to determine the potential impact of global change on marine intrusion in this kind of aquifers. The mathematical models traditionally used in coastal aquifers have been those of flow and mass transport with variable density, which allow obtaining a distribution of salinities in the aquifer and reproducing the flow pattern in the area of discharge to the sea. In addition to these models, another type of numerical modeling can be applied that could also provide information on specific aspects of this type of aquifers. The study of the Motril-Salobreña coastal aquifer shows, for example, how heat trans port models allow quantifying the recharge that occurs from rivers that frequently interact with coastal aquifers; as well as the age transport models provide data on the preferential groundwa ter circulation paths and its residence time, very relevant information in these frequently hetero geneous and anisotropic aquifers. Future challenges are directed towards determining the ef fects of sea level rise on marine intrusion and establishing the degree of equilibrium of the freshwater-saltwater contact with the current situationProyecto CGL2016-77503-R financiado por el Ministerio de Ciencia e InnovaciónGrupo de investigación de la Junta de Andalucía RNM-36

Groundwater-Surface Water Interactions in “La Charca de Suárez” Wetlands, Spain

La Charca de Suárez (LCS) is a Protected Nature Reserve encompassing 4 lagoons located 300 m from the Mediterranean coast in southern Spain. LCS is a highly anthropized area, and its conservation is closely linked to the human use of water resources in its surroundings and within the reserve. Different methodologies were applied to determine the hydrodynamics of the lagoons and their connection to the Motril-Salobreña aquifer. Fieldwork was carried out to estimate the water balance of the lagoon complex, the groundwater flow directions, the lagoons-aquifer exchange flow and the hydrochemical characteristics of the water. The study focussed on the changes that take place during dry-wet periods that were detected in a 7-month period when measurements were collected. The lagoons were connected to the aquifer with a flow-through functioning under normal conditions. However, the predominant inlet to the system was the anthropic supply of surface water which fed one of the lagoons and produced changes in its flow pattern. Sea wave storms also altered the hydrodynamic of the lagoon complex and manifested a future threat to the conservation status of the wetland according to predicted climate change scenarios. This research presents the first study on this wetland and reveals the complex hydrological functioning of the system with high spatially and temporally variability controlled by climate conditions and human activity, setting a corner stone for future studies.This study was supported by grant CGL2016-77503-R from the Ministry of Economy and Competitiveness (MINECO), cofounded by the European Regional Development Fund (ERDF) of the European Union (EU), and the RNM-369 research group of the regional government of Andalusia

Validity of applying Theis and Cooper-Jacob analytical solutions in pumping tests carried out in coastal aquifers

Two pumping tests were performed in the unconfined detrital Motril-Salobreña aquifer in a 250 metre-deep artesian well 300 m from the coastline containing both freshwater and saltwater. The two observation wells where the drawdowns are measured record the influence of tidal fluctuations and a high vertical heterogeneity in the aquifer. The Theis and Cooper-Jacob approaches give average T and S values of 1,460 m2/d and 0.027, respectively. Numerical modelling in a synthetic model was applied to analyse the sensitivity of the Theis and Cooper-Jacob approaches to the usual boundary conditions in coastal aquifers. The T and S values calculated from the numerical modelling drawdowns indicate that the regional flow, variable pumping flows, and tidal effect produce an error of under 10% in results obtained with classic methods. Fluids of different density (freshwater and saltwater) cause an error of 20% in estimating T and of over 100% in calculating S. The factor most affecting T and S results in the pumping test interpretation is vertical heterogeneity in sediments, which can produce errors of over 100% in both parametersEn el acuífero costero Motril-Salobreña se realizaron dos ensayos de bombeo en un sondeo surgente de 250 m de profundidad localizado a 300 m del borde costero en el que se detectó la presencia de agua dulce y agua salada y una fuerte heterogeneidad vertical. En los dos piezómetros donde se miden los descensos, se registra la influencia de las oscilaciones mareales. La aplicación de las soluciones analíticas de Theis y Cooper-Jacob da valores medios de T y S de 1.460 m2/d y 0,027, respectivamente. Para analizar la sensibilidad de las aproximaciones de Theis y Cooper-Jacob ante condiciones de contorno que suelen aparecer en acuíferos costeros se ha aplicado un modelo matemático sintético. Los valores de T y S calculados a partir de los descensos resultantes del modelo indican que la existencia de un flujo regional, un caudal de bombeo variable y el efecto de las mareas producirá un error inferior al 10% en los resultados obtenidos con los métodos clásicos. La presencia de fluidos de diferente densidad provocará un error del 20% para T y mayor del 100% para S. El factor que más afecta a los resultados de T y S en la interpretación de ensayos de bombeo es la heterogeneidad vertical de los sedimentos que pueden dar errores superiores al 100% para ambos parámetro

Estimation of hydrogeological parameters by tidal influence at different depths in the discharge zone of the Motril-Salobreña Coastal Aquifer (Granada)

La influencia de la oscilación de mareas en las medidas de nivel piezométrico se detectó de forma clara en puntos profundos de la zona de descarga del acuífero costero Motril-Salobreña, obteniendo diferentes valores de difusividad (T/S) en función de la profundidad, de la distancia del punto de medida al borde costero y del método de cálculo utilizado. Se ha registrado el nivel piezométrico en diferentes puntos someros y profundos del borde costero con el fin de comparar los resultados con los obtenidos previamente a mayor profundidad, concluyendo que los valores de difusividad aumentan a mayores profundidades. No obstante, considerando un medio homogéneo, los valores de transmisividad se presentan diferentes en un sector tan reducido del acuífero cuando deberían ser similares. Se deduce de ello que el factor variable debe ser el coeficiente de almacenamiento, en función de si se produce o no saturación-desaturación (variaciones de nivel cerca de la superficie)The tidal oscillation influence on the water head was clearly detected at deep points at the discharge zone of Motril-Salobreña coastal aquifer, obtaining different diffusivity (T/S) values according to depth, distance of point to the shoreline and depending on the applied method.Water table was also monitored at different shallow and deep locations close to the coastal border in order to compare the results with the previously obtained at deeper points, finding that the diffusivity values increased with depth. However, if we considered a homogeneous medium, the transmissivity values are different when they should be similar for this reduced sector of the aquifer, which would imply that the variable factor should be the storage coefficient (S), depending whether or not there is saturation-desaturation (water table variations close to the surface

Groundwater recharge-discharge estimation with differential flow gaugings in the final stretch of the Guadalfeo river (Granada)

Las interacciones entre las aguas subterráneas y las superficiales son muy complejas y dependen de múltiples factores. La dependencia mutua de ambos cuerpos de agua crea la necesidad de tratarlos como un único sistema. El río Guadalfeo constituye la principal recarga del acuífero Motril-Salobreña, y su caudal depende de la actividad antrópica y de un clima variable. Este estudio se centra en un año completo, de noviembre de 2017 a octubre de 2018, y presenta por primera vez el comportamiento del río en detalle. Para ello, se realizó una campaña mensual de aforos diferenciales en 5 puntos del río, y se analizó el carácter influente o efluente del río por tramos. Además, las medidas de nivel freático sirvieron para relacionar las pérdidas o ganancias del río con el comportamiento del acuíferoGroundwater-surface water interactions are very complex and dependent on multiple factors. The mutual dependence of both water bodies create the need for treating them as a unique system. The Guadalfeo river is the main entrance of water to the Motril-Salobreña aquifer, and it relies on the anthropogenic activity and on the variable climate of the area. This study presents the detailed behavior of the river during an entire year, since November 2017 to October 2018. To that end, differential flow gaugings were made at 5 points of the river to finally analyze its effluent-influent behaviour by sections. Furthermore, the groundwater level measurements allowed to relate the gains or losses of the river flow with the aquifer behavio

Study of the freshwater-saltwater interface position en the Guadalfeo River mouth sector (Motril-Salobreña aquifer) with geophysical techniques and salinity logs

Se realizaron campañas geofísicas (Sondeos electromagnéticos en el dominio del tiempo y tomografía eléctrica) en el acuífero Motril-Salobreña para detectar la cuña salina que revelaron incertidumbres acerca de la posición exacta y la forma que presenta esta. En 2009, se perforó un sondeo de 250 m de profundidad cerca de la línea de costa. Los registros de salinidad en el sondeo mostraron un cambio de agua dulce a agua salobre que es considerado como el inicio el inicio de la interfaz agua dulce-agua salada. La información obtenida a través de los registros de salinidad y los métodos geofísicos se combinó para presentar una primera aproximación de la morfología del límite superior de la transición desde agua dulce a agua saladaSeveral geophysical surveys (time domain electromagnetic soundings and electrical resistivity tomography) have been carried out in Motril-Salobreña aquifer for detecting the saline wedge but there are uncertainties about the exact position and shape. In 2009 a 250 metres depth borehole was drilled near the coastline. Electrical water conductivity logs showed a fresh water-brackish water change that is considered as the beginning of the saltwater-freshwater interface. The information obtained with the logs and the geophysical methods has been combined and an approximation to the upper limit of the saltwater-freshwater interface shape is presente