Matías Mudarra

Modeling spatiotemporal impacts of hydroclimatic extremes on groundwater recharge at a Mediterranean karst aquifer

Karst aquifers provide large parts of the water supply for Mediterranean countries, though climate change is expected to have a significant negative impact on water availability. Recharge is therefore a key variable that has to be known for sustainable groundwater use. In this study, we present a new approach that combines two independent methods for karst recharge estimation. The first method derives spatially distributed information of mean annual recharge patterns through GIS analysis. The second is a process-based karst model that provides spatially lumped but temporally distributed information about recharge. By combining both methods, we add a spatial reference to the lumped simulations of the process-based model. In this way, we are able to provide spatiotemporal information of recharge and subsurface flow dynamics also during varying hydroclimatic conditions. We find that there is a nonlinear relationship between precipitation and recharge rates resulting in strong decreases of recharge following even moderate decreases of precipitation. This is primarily due to almost constant actual evapotranspiration amounts despite varying hydroclimatic conditions. During the driest year in the record, almost the entire precipitation was consumed as actual evapotranspiration and only little diffuse recharge took place at the high altitudes of our study site. During wettest year, recharge constituted a much larger fraction of precipitation and occurred at the entire study site. Our new method and our findings are significant for decision makers in similar regions that want to prepare for possible changes of hydroclimatic conditions in the future.

Hydrochemical dynamics of TOC and NO 3 − contents as natural tracers of infiltration in karst aquifers

Analysis of Total Organic Carbon (TOC) and NO3− contents in the water at six springs in the province of Malaga (Southern Spain), sampled under different hydrologic conditions, revealed two different hydrochemical behaviour patterns for these natural tracers of infiltration. TOC content increased during every recharge period, following the rapid arrival of water infiltrating through the soil. On the other hand, NO3− content only rose during the first flood episodes (normally in autumn), and fell during the winter and spring ones. This difference is consequence of the distinctive biogeochemical kinetics of nitrogen with respect to organic carbon, both in the soil and within the aquifer. Unlike the mineralisation undergone by TOC from the surface to the spring, the NO3− ion remains in the aquifer almost unaffected, due to the oxidizing conditions prevailing within the karst medium, which do not allow its denitrification and favour its preservation within the saturated zone. In non-polluted aquifers, TOC and NO3− have a common origin in the soil and can be used to determine infiltration processes and the hydrogeological functioning of karst aquifers. Their different hydrochemical evolution provides information about mineralization and degradation processes of organic matter within karst aquifers, which can be used to validate the vulnerability to contamination in this type of medium.

Vulnerability mapping and protection zoning of karst springs. Validation by multitracer tests.

Protection zoning of karst springs and wells used for water supply is a key aspect in many countries, calling for specific methodologies adapted to the particular characteristics of karst media. This work presents a new approach, in view of the present state of the art and based on experiences with contamination vulnerability mapping at the pilot site of the Villanueva del Rosario karst system (southern Spain). Source (intrinsic) vulnerability maps were prepared and compared using three European procedures for karst aquifers. The vulnerability maps were then tested using dye tracers. The COP+K method and Slovene Approach appear to provide reliable results in terms of intrinsic vulnerability mapping. Nevertheless, all the methods have a margin of error. The COP+K map is adopted as the baseline to delineate the protection zones, through the conversion from vulnerability classes to degrees of protection.

Monitoring groundwater in the discharge area of a complex karst aquifer to assess the role of the saturated and unsaturated zones

The hydrochemical response at springs in the drainage area of the Sierra del Rey—Los Tajos carbonate aquifer (province of Málaga, southern Spain) was monitored in order to determine the hydrogeological functioning of this aquifer. Analysis of the most important chemical parameters, using methodologies such as the temporal evolution of chemical components, principal component analysis and discriminant factorial analysis revealed that the high level of hydrochemical heterogeneity to be found in this discharge zone, in addition to particular spatial and temporal factors, is responsible for the mineralisation of the spring water. Sampling in karst systems where discharge occurs by several springs should take into account the hydrochemical variability of them; otherwise conclusions about the hydrological functioning of aquifers deduced from mixture of spring waters can be inaccurate.

Hydrogeochemical tools applied to the study of carbonate aquifers: examples from some karst systems of Southern Spain

Karst aquifers present hydrogeological characteristics that distinguish them from others such as fractured and granular aquifers. In particular, they are characterized by highly heterogeneous functioning (large temporal and spatial variability), which makes difficult the application of traditional research methods to karst hydrogeology. For this reason, during the last decades, hydrogeochemical tools have been developed for carbonate aquifers characterization by means of spatial and temporal hydrochemical variability, mixing processes (recharge vs. stored water in the system), groundwater origin (saturated/unsaturated zones, epikarst, etc.), karstification degree and, in general, the hydrogeological functioning of karst aquifers. In the present work, some of the most used techniques to study this type of media are applied to pilot sites from South Spain, in order to show hydrogeochemical processes related to karst aquifers functioning: groundwater origin, distribution of its chemical composition, aquifer behavior, water residence time, substratum influence and geochemical modelling. The results display that the combination of different hydrogeochemical tools provides a comprehensive understanding of the functioning of karst systems, which is useful for groundwater management and protection as well as for environmental applications (groundwater ecodependent systems, wetlands, coastal aquifers, etc).

Hydrochemical Heterogeneity in the Discharge Zone of a Karstic Aquifer

The hydrochemical response at springs in the drainage area of the Sierra del Rey – Los Tajos carbonate aquifer (province of Malaga, southern Spain) was monitored to determine the hydrogeological functioning of this aquifer. Analysis of the most important chemical parameters, using various methodologies, such as the time variation of electrical conductivity (EC), Principal Components Analysis (PCA) and Discriminant Factorial Analysis (DFA), revealed that the high level of hydrochemical heterogeneity existing in the zone of discharge, in addition to particular spatial and temporal factors, is responsible for the mineralization of the spring water. This is an example of how karstic heterogeneity may be transmitted to the discharge area of the aquifer, a fact that should be taken into account when investigating karst systems.

Hypothesis on the hydrogeological context of wetland areas and springs related to evaporitic karst aquifers (Málaga, Córdoba and Jaén provinces, Southern Spain)

The northern sector of the External Zone of the Betic Cordillera (S Spain) is formed by an olistostrome unit known as the Chaotic Subbetic Complexes (CSC). This megabreccia is basically made of clays and evaporite rocks (gypsum and salt) of Upper Triassic (Keuper) age as well as other lithologies (dolostones, limestones, marls and calcareous sandstones) belonging to different ages (Jurassic, Cretaceous and Tertiary). Despite of low permeability has been traditionally assumed for these materials, water flow and storage through them is possible due to the aquitard behavior of clays and sandstones and the presence of conduits generated by dissolution/karstification processes within the evaporite rocks. The lithological complexity of the CSC determines its hydrogeological heterogeneity, with groundwater flows of different length and various scales from recharge areas to discharge zones. Thus, wetlands and springs placed at lower altitudes are associated with large (regional) groundwater flows, of greater residence time within the aquifer, and they normally drain high salinity waters mostly connected with ascending flow. This work provides an overview of the main geological and hydrogeological aspects related to groundwater flow within the CSC on the basis of several case-studies. Conceptual models are proposed for these cases, since they are the base to carry out a genetic-functional classification of wetlands potentially useful for their management.

Improved Assessment of Groundwater Recharge in a Mediterranean Karst Region: Andalusia, Spain

Karst aquifers provide a significant contribution to the drinking water supplies of many countries in Europe. Estimating their recharge rate, i.e. the fraction of precipitation that is turned into groundwater recharge, is an essential tool to assess usable groundwater water volumes. In this study, we extended a previously developed GIS-based recharge estimation method (APLIS) to take into account climate variability by adding a simple soil moisture routine whose parameters were a priori estimated with globally available FAO soil property data. We applied the new approach to a karst system in southern Spain and evaluated our results with spring discharge observations. To exemplify the prediction skills of the new method, we applied the five climate models from the Inter-Sectoral Impact Model Intercomparison Project to assess changes in recharge rates of the study site until the end of this century.

Regional-scale analysis of karst underground flow deduced from tracing experiments: examples from carbonate aquifers in Malaga province, southern Spain

Tracer concentration data from field experiments conducted in several carbonate aquifers (Malaga province, southern Spain) were analyzed following a dual approach based on the graphical evaluation method (GEM) and solute transport modeling to decipher flow mechanisms in karst systems at regional scale. The results show that conduit system geometry and flow conditions are the principal factors influencing tracer migration through the examined karst flow routes. Solute transport is mainly controlled by longitudinal advection and dispersion throughout the conduit length, but also by flow partitioning between mobile and immobile fluid phases, while the matrix diffusion process appears to be less relevant. The simulation of tracer breakthrough curves (BTCs) suggests that diffuse and concentrated flow through the unsaturated zone can have equivalent transport properties under extreme recharge, with high flow velocities and efficient mixing due to the high hydraulic gradients generated. Tracer mobilization within the saturated zone under low flow conditions mainly depends on the hydrodynamics (rather than on the karst conduit development), which promote a lower longitudinal advection and retardation in the tracer migration, resulting in a marked tailing effect of BTCs. The analytical advection-dispersion equation better approximates the effective flow velocity and longitudinal dispersion estimations provided by the GEM, while the non-equilibrium transport model achieves a better adjustment of most asymmetric and long-tailed BTCs. The assessment of karst underground flow properties from tracing tests at regional scale can aid design of groundwater management and protection strategies, particularly in large hydrogeological systems (i.e. transboundary carbonate aquifers) and/or in poorly investigated ones.RésuméLes données de concentration de traceurs provenant d’expériences de terrain menées dans plusieurs aquifères carbonatés (province de Malaga, sud de l’Espagne) ont été analysées en suivant une approche duale basée sur la méthode d’évaluation graphique (MEG) et la modélisation du transport de soluté pour déchiffrer les mécanismes d’écoulement dans les systèmes karstiques à l’échelle régionale. Les résultats montrent que la géométrie du système de conduits et les conditions d’écoulement sont les principaux facteurs qui influencent la migration du traceur par les voies d’écoulement karstiques examinées. Le transport de soluté est. principalement contrôlé par l’advection longitudinale et la dispersion sur toute la longueur du conduit, mais aussi par le partage d’écoulement entre les phases fluides mobiles et immobiles, alors que le processus de diffusion matricielle semble moins pertinent. La simulation des courbes de percée du traceur (CPT) suggère que l’écoulement diffus et concentré à travers la zone non saturée peut avoir des propriétés de transport équivalentes pour une recharge extrême, avec des vitesses d’écoulement élevées et un mélange efficace en raison des gradients hydrauliques élevés générés. La mobilisation des traceurs dans la zone saturée dans des conditions d’écoulement faible dépend principalement de l’hydrodynamique (plutôt que du développement du conduit karstique), ce qui favorise une advection longitudinale inférieure et un retard dans la migration du traceur, entrainant un effet de traine marqué des CPT. L’équation analytique de l’advection-dispersion permet une meilleure approximation de la vitesse d’écoulement effective et des estimations de dispersion longitudinale fournies par le MEG, alors que le modèle de transport sans équilibre réalise un meilleur ajustement de la plupart des CPT asymétriques et à longue traine. L’évaluation des propriétés de l’écoulement souterrain karstique à partir des essais de traçage à l’échelle régionale peut aider à concevoir des stratégies de gestion et de protection des eaux souterraines, en particulier dans les grands systèmes hydrogéologiques (c-à-d. les aquifères carbonatés transfrontaliers) et/ou dans des systèmes peu étudiés.ResumenSe analizaron los datos de concentración de trazadores artificiales procedentes de varios ensayos realizados en algunos acuíferos carbonáticos de la provincia de Málaga (sur de España), mediante una aproximación dual basada en el método de evaluación gráfica (GEM) y transporte de solutos, para descifrar los mecanismos de flujo en sistemas kársticos a escala regional. Los resultados muestran que la geometría de los sistemas de conductos y las condiciones de flujo son los principales factores que influyen en la movilización del trazador a través de las líneas de flujo kárstico estudiadas. El transporte de solutos está controlado principalmente por procesos de advección y dispersión longitudinal a lo largo de los conductos, pero también por la partición del flujo entre fases móviles e inmóviles, mientras que el proceso de difusión desde la matriz de la roca parece ser menos relevante. La simulación de las curvas de paso de trazadores (BTCs) sugiere que el flujo difuso y concentrado a través de la zona no saturada puede tener propiedades equivalentes de transporte en condiciones extremas de recarga, con altas velocidades de flujo y una mezcla eficiente de flujos de agua como consecuencia de los elevados gradientes hidráulicos que se generan. La movilización del trazador dentro la zona saturada en condiciones de aguas bajas depende principalmente de las características hidrodinámicas (en lugar del desarrollo de los conductos kársticos), que promueven una menor advección longitudinal y un mayor retraso en el movimiento del trazador, lo que se traduce en BTCs con colas pronunciadas. La ecuación analítica de advección-dispersión se aproxima mejor a las estimaciones de la velocidad efectiva de flujo y de la dispersión longitudinal proporcionadas por el GEM, mientras que el modelo de transporte de no-equilibrio consigue un mejor ajuste de la mayoría de las BTCs asimétricas y con colas pronunciadas. La evaluación de las propiedades de los flujos subterráneos kársticos a partir de pruebas de trazadores a escala regional puede ayudar al diseño de estrategias de gestión y protección de las aguas subterráneas, particularmente en grandes sistemas hidrogeológicos (por ejemplo, en acuíferos carbonáticos transfronterizos) y/o poco investigados.摘要遵循基于图形评估方法和溶质运移模拟的双重方法分析了(西班牙南部Malaga省)几个碳酸盐含水层野外实验获取的示踪剂浓度数据,以在区域尺度上解译岩溶系统是水流机理。结果显示,通道系统的几何结构和水流条件是影响示踪剂通过岩溶水流路径迁移的主要因素。溶质运移主要受到整个通道长度内纵向平流和离散的控制,但是也受到移动和固定相之间水流分区的控制,而基质弥散过程似乎关联性较小。示踪剂突破曲线模拟表明,由于产生的很高的水力梯度,在伴有很高流速和有效混合的极端补给条件下,通过非饱和带的离散和集中水流可能有相等的传输特性。低水流条件下饱和带内的示踪剂活动化主要取决于水动力学(而不是取决于岩溶通道的发育),水动力学促进示踪剂迁移中较低的纵向平流和延迟,导致示踪剂突破曲线有明显的拖尾现象。解析平流-离散方程可以更好地估计图像评估方法得到的有效水流速度和纵向离散估算结果,而非平衡运移模型能够更好地调整大多数不对称和长尾示踪剂突破曲线。通过区域尺度示踪实验得到的岩溶地下水流特性评价结果可支持谋划地下水管理和保护战略,特别是在大的水文地质系统中(如跨边界碳酸盐含水层)及/或欠调查的水文地质系统中尤其如此。ResumoDados de concentração de traçadores de experimentos conduzidos em diversos aquíferos carbonáticos (província de Málaga, sul da Espanha) foram analisados seguindo uma abordagem dual baseada no método de avaliação gráfica (MAG) e modelagem de transporte de soluto para decifrar os mecanismos de fluxo em sistemas cársticos em escala regional. Os resultados mostram que a geometria do sistema de conduto e as condições de fluxo são os principais fatores que influenciam a migração do traçador através das rotas de fluxo cárstico examinadas. O transporte de soluto é controlado principalmente por advecção longitudinal e dispersão ao longo do comprimento do conduto, mas também por particionamento do fluxo entre as fases de fluido móvel e imobilizado, enquanto o processo de difusão da matriz parece ser menos relevante. A simulação das curvas de identificação (CI) do traçador sugere que o fluxo difuso e concentrado através da zona não saturada pode ter propriedades de transporte equivalentes sob recarga extrema, com altas velocidades de fluxo e mistura eficiente devido aos altos gradientes hidráulicos gerados. A mobilização do traçador dentro da zona saturada em condições de baixo fluxo depende principalmente da hidrodinâmica (e não do desenvolvimento do canal cárstico), que promove uma advecção e atraso longitudinal mais baixos na migração do traçador, resultando em um efeito de cauda marcado nas CI. A equação analítica de advecção e dispersão se aproxima melhor das estimativas de velocidade de fluxo efetivas e de dispersão longitudinal fornecidas pelo MAG, enquanto o modelo de transporte sem equilíbrio alcança um melhor ajuste da maioria das CI assimétricas e de cauda longa. A avaliação das propriedades de fluxo subterrâneo cárstico a partir de testes de traçadores em escala regional pode auxiliar na elaboração de estratégias de gestão e proteção de águas subterrâneas, particularmente em sistemas hidrogeológicos grandes (isto é, aquíferos carbonáticos transfronteiriços) e/ou em meios pouco investigados.

Analysis of Natural Response and Hydrochemical Data by Statistical Approaches to Characterize the Hydrogeological Functioning of a Highly Karstified Evaporitic System in South Spain

At southern Cordoba Province (S Spain), an evaporitic karst plateau drained by brine springs is located. A periodic monitoring of discharge rate, EC, water temperature, and pH was performed in one of them (Lower Anzur spring) and water samples were collected for chemical analysis. Physicochemical data were used to perform a principal component analysis (PCA) to characterize the functioning of the system. Temporal evolution of the controlled parameters reveals a markedly karst behavior. PCA has defined two principal factors, one related to salinity and other linked to infiltration processes (NO3 −, Ca2+). The slight thermal anomaly and the high mineralization registered indicate the existence of ascending regional groundwater flows of long residence time, which would converge into the spring, mixed with recently infiltrated water that circulated through a conduit network formed in the evaporite rocks.