Entendimiento y simulación del flujo del agua subterránea ante diferentes escenarios climáticos y de extracción en el sector de Pinilla, Santa Cruz, Guanacaste
Fecha
2023
Tipo
tesis de maestría
Autores
Alvarado Piedra, Paola Beatriz
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Resumen
La cuenca del río San Francisco cubre un área de 64 km2, se ubica en el distrito de Tamarindo, Santa Cruz, Guanacaste. En ella se encuentran algunos poblados rurales como San José de Pinilla, Hernández, Linderos y Cebadilla, mientras que en la sección inferior de la cuenca están localizados los terrenos de Hacienda Pinilla, donde existe un gran desarrollo turístico.
El objetivo de este estudio es simular la dinámica del flujo del agua subterránea, como base para mejorar la gestión del recurso hídrico en la zona, a partir de la elaboración de un modelo hidrogeológico conceptual, para ello se evalúan las condiciones de flujo en el año 2020 y se simulan las condiciones para los años 2030 y 2050.
El basamento de la cuenca corresponde con rocas asociadas al Complejo de Nicoya, las cuales son sobreyacidas por depósitos del Cuaternario. Si bien la cuenca del río San Francisco, y su principal afluente el río Pinilla, es una cuenca costera, el levantamiento de una antigua plataforma de abrasión ha provocado el estrechamiento del valle aluvial, lo cual limita la escorrentía superficial y subterránea generada en la estación lluviosa, hasta su desembocadura en Playa Langosta.
El acuífero Pinilla cubre un área de 23 km2, se desarrolla en sedimentos finos a medios de los depósitos aluviales y los primeros metros fracturados y meteorizados de los basaltos del Complejo de Nicoya, su espesor promedio es de 20 m, sin embargo, la morfología irregular en el Complejo de Nicoya provoca variaciones laterales en el espesor de sedimentos no consolidados lo que genera un acuífero heterogéneo. La dirección de flujo es hacia el oeste-noroeste con un gradiente promedio de 0.01, los valores de transmisividad se encuentran entre 50 m2/d y 430 m2/d, mientras que la conductividad hidráulica es de 1.4 m/d - 26 m/d, sin embargo, este presenta un bajo volumen de almacenamiento de 0.56 km3. En general, el acuífero es libre con un coeficiente de almacenamiento entre 0.02 y 0.2, aunque puede presentar localmente sectores semiconfinados a confinados.
El comportamiento de los niveles de agua en los pozos muestra fluctuaciones promedio entre 3 m y 10 m, los cuales tienen una relación directa con la precipitación, con una tendencia a disminuir entre los meses de noviembre a mayo y aumentar entre junio y octubre, lo cual afecta el caudal aprovechable de los pozos durante el año.
La recarga potencial por infiltración del agua de lluvia representa un 26% de la precipitación y ocurre entre mayo y noviembre, en concordancia con las precipitaciones. Además, existen otros mecanismos de recarga en la zona, los cuales incluyen: recarga lateral desde los cerros de la cuenca, conexión hidráulica con los ríos y a través de grietas en suelos arcillosos al inicio de la época lluviosa.
El agua subterránea de la zona de estudio se clasifica como bicarbonatada cálcica a bicarbonatada sódica, en la parte baja de la cuenca predomina la fase de lavado o endulzamiento frente a la intrusión salina, es decir, el agua dulce está expulsando el frente salino; es muy importante considerar que este es un proceso dinámico, por lo cual debe darse un constante monitoreo.
El acuífero Pinilla se modela en estado estacionario, según las condiciones de frontera y de recarga definidas en el modelo conceptual para la época lluviosa del año 2020. La correlación entre cargas hidráulicas medidas y observadas presentan un coeficiente de correlación de 0.99 y un NRMS de 4.97%, lo cual es un buen ajuste, tomando en cuenta la heterogeneidad del acuífero y la incertidumbre de los datos. La mayor parte de la descarga se da por medio de los ríos y la descarga al mar. A partir de escenarios de cambio climático simulados para el año 2030 (-2% de recarga, +10% de bombeo respecto a 2020) y 2050 (-8% de recarga, +30% de bombeo respecto a 2020), se determina que tales escenarios no generan cambios significativos en el flujo y el caudal de descarga al mar. Esta situación se da debido a la baja capacidad de almacenamiento del acuífero, en relación con el volumen de recarga que recibe o podría recibir en tales años, esto provoca que el mayor caudal de descarga se dé por medio de los ríos y quebradas.
San Francisco watershed covers an area of 64 km2. It is located in Tamarindo’s district in Santa Cruz, Guanacaste. In this watershed, it can be found some rural towns such as San José de Pinilla, Hernández, Linderos, and Cebadilla. Meanwhile, in the lower section of the watershed, the lands of Hacienda Pinilla are located, where there is a great tourist development. This study aims to simulate the dynamics of groundwater flow, as a basis for improving water resources management in this area, based on the development of a hydrogeological conceptual model, for which the flow conditions in the year 2020 are evaluated and the conditions for the years 2030 and 2050 are simulated. The basement of the watershed corresponds with rocks of the Nicoya Complex, which are overlain by Quaternary deposits. Although San Francisco River basin and its main tributary, Pinilla River, is a coastal basin, the uplift of an old abrasion platform has caused the narrowing of the alluvial valley. This narrowing limits the surface and underground runoff generated during the rainy season until the river flows into Playa Langosta. The Pinilla aquifer covers an area of 23 km2. It develops in fine to medium sediments of alluvial deposits, and the first fractured and weathered meters of the basalts of the Nicoya Complex. This aquifer's average thickness is 20 m; nonetheless, the irregular morphology in The Nicoya Complex causes lateral variations in the thickness of unconsolidated sediments, which generates a heterogeneous aquifer. The flow direction is towards the west-northwest with an average gradient of 0.01. The transmissivity values are between 50 m2/d and 430 m2/d while the hydraulic conductivity is between 1.4 m/d and 26 m/d; nevertheless, it has a low storage volume of 0.56 km3. In brief, the Pinilla aquifer is an unconfined aquifer with a storage coefficient between 0.02 and 0.2, although it might locally present semi-confined to confined areas. The water behavior levels in the wells show average fluctuations between 3 m and 10 m. These levels have a direct relationship with precipitation, with a tendency to decrease between the months of November to May and increase between June to October, which affects the usable flow of the wells during the year. Potential recharge due to rainwater infiltration represents 26% of precipitation and occurs between May and November. In addition, there are others recharge mechanisms in the area which include: lateral recharge from the hills of the basin, hydraulic connection with the rivers, and rainfall infiltration through cracks in clay soils at the beginning of the rainy season. Groundwater in the study area is classified as calcium bicarbonate to sodium bicarbonate type, in the lower part of the watershed the freshening phase predominates against the saline intrusion, In other words, there is a regression process of the saline front. It is very important to consider that this is a dynamic process, which is why constant monitoring must be implemented. The Pinilla aquifer is modeled in a steady state. According to the boundary and recharge conditions defined in the conceptual model for the 2020 rainy season. The relation between measured and observed hydraulic heads has a weighting coefficient of 0.99 and a normalized RMS of 4.97%; therefore, it fits well considering the heterogeneity of the aquifer and the uncertainty of the data. Most of the discharge occurs through rivers and discharges into the sea. Based on simulated climate change scenarios by the year 2030 (-2% recharge, +10% pumping compared to 2020) and 2050 (-8% recharge, +30% pumping compared to 2020), it is determined that these scenarios do not generate significant changes in the flow and discharge rate to the sea. This situation occurs due to the low storage capacity of the aquifer, concerning the volume of recharge it receives or could receive in a few years; as a consequence of this, it causes the highest discharge flow to occur through rivers and streams.
San Francisco watershed covers an area of 64 km2. It is located in Tamarindo’s district in Santa Cruz, Guanacaste. In this watershed, it can be found some rural towns such as San José de Pinilla, Hernández, Linderos, and Cebadilla. Meanwhile, in the lower section of the watershed, the lands of Hacienda Pinilla are located, where there is a great tourist development. This study aims to simulate the dynamics of groundwater flow, as a basis for improving water resources management in this area, based on the development of a hydrogeological conceptual model, for which the flow conditions in the year 2020 are evaluated and the conditions for the years 2030 and 2050 are simulated. The basement of the watershed corresponds with rocks of the Nicoya Complex, which are overlain by Quaternary deposits. Although San Francisco River basin and its main tributary, Pinilla River, is a coastal basin, the uplift of an old abrasion platform has caused the narrowing of the alluvial valley. This narrowing limits the surface and underground runoff generated during the rainy season until the river flows into Playa Langosta. The Pinilla aquifer covers an area of 23 km2. It develops in fine to medium sediments of alluvial deposits, and the first fractured and weathered meters of the basalts of the Nicoya Complex. This aquifer's average thickness is 20 m; nonetheless, the irregular morphology in The Nicoya Complex causes lateral variations in the thickness of unconsolidated sediments, which generates a heterogeneous aquifer. The flow direction is towards the west-northwest with an average gradient of 0.01. The transmissivity values are between 50 m2/d and 430 m2/d while the hydraulic conductivity is between 1.4 m/d and 26 m/d; nevertheless, it has a low storage volume of 0.56 km3. In brief, the Pinilla aquifer is an unconfined aquifer with a storage coefficient between 0.02 and 0.2, although it might locally present semi-confined to confined areas. The water behavior levels in the wells show average fluctuations between 3 m and 10 m. These levels have a direct relationship with precipitation, with a tendency to decrease between the months of November to May and increase between June to October, which affects the usable flow of the wells during the year. Potential recharge due to rainwater infiltration represents 26% of precipitation and occurs between May and November. In addition, there are others recharge mechanisms in the area which include: lateral recharge from the hills of the basin, hydraulic connection with the rivers, and rainfall infiltration through cracks in clay soils at the beginning of the rainy season. Groundwater in the study area is classified as calcium bicarbonate to sodium bicarbonate type, in the lower part of the watershed the freshening phase predominates against the saline intrusion, In other words, there is a regression process of the saline front. It is very important to consider that this is a dynamic process, which is why constant monitoring must be implemented. The Pinilla aquifer is modeled in a steady state. According to the boundary and recharge conditions defined in the conceptual model for the 2020 rainy season. The relation between measured and observed hydraulic heads has a weighting coefficient of 0.99 and a normalized RMS of 4.97%; therefore, it fits well considering the heterogeneity of the aquifer and the uncertainty of the data. Most of the discharge occurs through rivers and discharges into the sea. Based on simulated climate change scenarios by the year 2030 (-2% recharge, +10% pumping compared to 2020) and 2050 (-8% recharge, +30% pumping compared to 2020), it is determined that these scenarios do not generate significant changes in the flow and discharge rate to the sea. This situation occurs due to the low storage capacity of the aquifer, concerning the volume of recharge it receives or could receive in a few years; as a consequence of this, it causes the highest discharge flow to occur through rivers and streams.
Descripción
Palabras clave
AGUA, AGUA SALADA, AGUA SUBTERRÁNEA, FLUJO, CAMBIO CLIMÁTICO, MODELO MATEMÁTICO, RECURSO HÍDRICO, ACUÍFERO, COSTA RICA