Groundwater availability, management and protection are great challenges for the sustainability of groundwater resources in the scattered rural areas of the Atlantic regions of Europe where groundwater is the only option for water supply. This report presents a hydrogeological study of the coastal granitic area of Oia in northwestern Spain, which has unique geomorphological and hydrogeological features with steep slopes favoring the erosion of the weathered granite. The hydrogeological conceptual model of the study area includes: (1) the regolith layer, which is present only in the flat summit of the mountains; (2) the slope debris and the colluvial deposits, which are present in the intermediate and lowest parts of the hillside; (3) the marine terrace; and (4) the underlying fractured granite. Groundwater recharge from rainfall infiltration varies spatially due to variations in terrain slope, geology and land use. The mean annual recharge estimated with a hydrological water balance model ranges from 75 mm in the steepest zone to 135 mm in the lowest flat areas. Groundwater flows mostly through the regolith and the detrital formations, which have the largest hydraulic conductivities. Groundwater discharges in seepage areas, springs, along the main creeks and into the sea. The conceptual hydrogeological model has been implemented in a groundwater flow model, which later has been used to select the best pumping scenario. Model results show that the future water needs for domestic and tourist water supply can be safely provided with eight pumping wells with a maximum pumping rate of 700 m³/day.

[Abstract] Groundwater availability, management and protection are great challenges for the sustainability of groundwater resources in the scattered rural areas of the Atlantic regions of Europe where groundwater is the only option for water supply. This report presents a hydrogeological study of the coastal granitic area of Oia in northwestern Spain, which has unique geomorphological and hydrogeological features with steep slopes favoring the erosion of the weathered granite. The hydrogeological conceptual model of the study area includes: (1) the regolith layer, which is present only in the flat summit of the mountains; (2) the slope debris and the colluvial deposits, which are present in the intermediate and lowest parts of the hillside; (3) the marine terrace; and (4) the underlying fractured granite. Groundwater recharge from rainfall infiltration varies spatially due to variations in terrain slope, geology and land use. The mean annual recharge estimated with a hydrological water balance model ranges from 75 mm in the steepest zone to 135 mm in the lowest flat areas. Groundwater flows mostly through the regolith and the detrital formations, which have the largest hydraulic conductivities. Groundwater discharges in seepage areas, springs, along the main creeks and into the sea. The conceptual hydrogeological model has been implemented in a groundwater flow model, which later has been used to select the best pumping scenario. Model results show that the future water needs for domestic and tourist water supply can be safely provided with eight pumping wells with a maximum pumping rate of 700 m3/day. | [Résumé] La disponibilité, la gestion et la protection des eaux souterraines sont de grands défis pour la durabilité de ces ressources en eaux dans les zones rurales dispersées des régions atlantiques de l’Europe où l’eau souterraine est la seule option pour l’approvisionnement en eau. Cet article présente une étude hydrogéologique de la zone granitique côtière d’Oia dans le nord-ouest de l’Espagne, qui présente des caractéristiques géomorphologiques et hydrogéologiques uniques avec des pentes fortes favorisant l’érosion du granite altéré. Le modèle conceptuel hydrogéologique de la zone d’étude comprend: (1) la couche de régolithe présente seulement sous le sommet plat des montagnes; (2) les débris de pente et les dépôts colluviaux présents dans les parties intermédiaires et les plus basses du versant; (3) la terrasse marine; et (4) le granite fracturé sous-jacent. La recharge des eaux souterraines par l’infiltration des précipitations varie dans l’espace en raison des variations de la pente du terrain, de la géologie et de l’occupation des sols. La recharge annuelle moyenne estimée à l’aide d’un modèle hydrologique de bilan en eau varie de 75 mm dans la zone la plus abrupte à 135 mm dans les zones plates les plus basses. Les eaux souterraines s’écoulent principalement à travers le régolithe et les formations détritiques, qui ont les plus fortes conductivités hydrauliques. Les eaux souterraines émergent dans les zones de suintement, aux sources, le long des principaux ruisseaux et en mer. Le modèle hydrogéologique conceptuel a été mis en œuvre dans un modèle d’écoulement des eaux souterraines, qui a ensuite été utilisé pour sélectionner le meilleur scénario de pompage. Les résultats du modèle montrent que les futurs besoins en eau pour l’approvisionnement en eau domestique et touristique peuvent être fournis en toute sécurité avec huit puits de pompage d’un débit maximum de 700 m3/jour. | [Resumen] La disponibilidad, la gestión y la protección de las aguas subterráneas es un gran reto para la sostenibilidad de los recursos hídricos subterráneos en las zonas rurales dispersas de las regiones atlánticas de Europa en las que las aguas subterráneas son la única opción para el suministro de agua. En este artículo se presenta un estudio hidrogeológico de la zona granítica costera de Oia en Galicia en el noroeste de España, con unas especiales características geomorfológicas e hidrogeológicas y pendientes que favorecen la erosión del granito meteorizado. El modelo conceptual hidrogeológico de la zona de estudio incluye: (1) la capa de regolito presente en la cumbre plana de la sierra costera; (2) los derrubios de ladera y los depósitos coluviales en las partes intermedias y bajas de la ladera; (3) la terraza marina; y (4) el granito fracturado subyacente. La recarga por infiltración de la precipitación varía espacialmente debido a la variabilidad espacial de la pendiente del terreno, la geología y el uso del suelo. La recarga media anual estimada con un modelo hidrológico de balance de agua oscila entre 75 mm en las zonas de mayor pendiente y 135 mm en las zonas llanas más bajas. El flujo subterráneo discurre principalmente a través del regolito y las formaciones detríticas que presentan las mayores conductividades hidráulicas. La descarga subterránea se produce en las zonas de rezume, los manantiales, los principales arroyos y en el mar. El modelo conceptual hidrogeológico se ha implementado en un modelo numérico de flujo subterráneo que posteriormente se ha utilizado para seleccionar el escenario óptimo de bombeo. Los resultados del modelo muestran que la futura demanda de agua para el abastecimiento doméstico y turístico se puede satisfacer de forma segura mediante ocho pozos de bombeo con un caudal punta de 700 m3/día. | [摘要] 地下水的可利用性, 管理和保护是欧洲大西洋地区分散的农村地区地下水资源可持续性的重要议题, 而地下水是这些地区唯一的供水选择。本文介绍了西班牙西北部Oia沿海花岗岩区的水文地质研究, 该地区具有独特的地貌和水文地质特征, 其陡峭的斜坡有利于风化花岗岩的侵蚀。研究区的水文地质概念模型包括:(1)仅存在于扁平化山峰的风化层; (2)山坡中部和最低处存在的坡积碎屑和崩积物; (3)海成阶地; (4)下伏的破碎花岗岩。由于地形坡度,地质和土地利用的变化, 地下水的降雨入渗补给在空间上也不同。采用水文水平衡模型估算的年均补给量从最陡的75 mm到最低平地的135 mm之间变化。地下水主要沿渗透系数最大的风化层和碎屑岩层流动。地下水排泄形式包括渗出区, 泉和沿小溪流入大海。概念性水文地质模型已用于建立地下水流模型, 之后用于优化抽水方案。模型结果表明采用八口开采井可以安全地满足生活和旅游用水的未来用水需求, 最高开采量可达700 m3/day。 | [Resumo] Disponibilidade, gestão e proteção das águas subterrâneas são grandes desafios para a sustentabilidade dos recursos hídricos subterrâneos nas áreas rurais dispersas das regiões atlânticas da Europa onde as águas subterrâneas são a única opção de abastecimento. Esse artigo apresenta um estudo hidrogeológico da área granítica costeira de Oia, no noroeste da Espanha, que apresenta características geomorfologias e hidrogeológicas únicas com encostas íngremes que favorecem a erosão do granito intemperizado. O modelo conceitual hidrogeológico da área de estudo inclui: (1) a camada regolítica, que está presente apenas no cume plano das montanhas; (2) os entulhos de encosta e os depósitos coluviais, que estão presentes nas partes intermediarias e mais baixas da encosta; (3) o terraço marinho; e (4) o granito fraturado subjacente. A recarga das águas subterrâneas pela infiltração da chuva varia espacialmente devido a variações na inclinação do terreno, geologia e uso da terra. A recarga média anual estimada com um modelo de balanço hídrico varia de 75 mm na zona mais íngreme a 135 mm nas áreas planas mais baixas. A água subterrânea flui principalmente através do regolito e das formações detríticas, que possuem as maiores condutividades. As águas subterrâneas descarregam em áreas de infiltração, nascentes, ao longo dos riachos principais e no mar. O modelo hidrogeológico conceitual foi implementado em um modelo de fluxo de água subterrânea, que posteriormente foi usado para selecionar o melhor cenário de bombeamento. Os resultados do modelo mostram que as necessidades futuras de água para abastecimento doméstico e turístico podem ser fornecidas com segurança com oito poços de bombeamento com uma vazão máxima de 700 m3/dia. | This work has been funded by the Water Authority of the Galician Regional Government (Xunta de Galicia), the Spanish Ministry of Economy and Competitiveness (Project PID2019-109544RB-I00), FEDER funds and the Galician Regional Government, Xunta de Galicia (Grant number ED431C 2017/57) from “Consolidación e estruturación de unidades de investigación competitivas”, Grupos de referencia competitiva | Xunta de Galicia; ED431C 2017/57

Over the past decades, fractured and karst groundwater systems have been studied intensively due to their high vulnerability to nitrate (NO₃⁻) contamination, yet nitrogen (N) turnover processes within the recharge area are still poorly understood. This study investigated the role of the karstified recharge area in NO₃⁻ transfer and turnover by combining isotopic analysis of NO₃⁻ and nitrite (NO₂⁻) with time series data of hydraulic heads and specific electrical conductivity from groundwater monitoring wells and a karstic spring in Germany. A large spatial variability of groundwater NO₃⁻ concentrations (0.1–0.8 mM) was observed, which cannot be explained solely by agricultural land use. Natural-abundance N and O isotope measurements of NO₃⁻ (δ¹⁵N and δ¹⁸O) confirm that NO₃⁻ derives mainly from manure or fertilizer applications. Fractional N elimination by denitrification is indicated by relatively high δ¹⁵N- and δ¹⁸O-NO₃⁻ values, elevated NO₂⁻ concentrations (0.05–0.14 mM), and δ¹⁵N-NO₂⁻ values that were systematically lower than the corresponding values of δ¹⁵N-NO₃⁻. Hydraulic and chemical response patterns of groundwater wells suggest that rain events result in the displacement of water from transient storage compartments such as the epikarst or the fissure network of the phreatic zone. Although O₂ levels of the investigated groundwaters were close to saturation, local denitrification might be promoted in microoxic or anoxic niches formed in the ferrous iron-bearing carbonate rock formations. The results revealed that (temporarily) saturated fissure networks in the phreatic zone and the epikarst may play an important role in N turnover during the recharge of fractured aquifers.

Over the past decades, fractured and karst groundwater systems have been studied intensively due to their high vulnerability to nitrate (NO₃⁻) contamination, yet nitrogen (N) turnover processes within the recharge area are still poorly understood. This study investigated the role of the karstified recharge area in NO₃⁻ transfer and turnover by combining isotopic analysis of NO₃⁻ and nitrite (NO₂⁻) with time series data of hydraulic heads and specific electrical conductivity from groundwater monitoring wells and a karstic spring in Germany. A large spatial variability of groundwater NO₃⁻ concentrations (0.1–0.8 mM) was observed, which cannot be explained solely by agricultural land use. Natural-abundance N and O isotope measurements of NO₃⁻ (δ¹⁵N and δ¹⁸O) confirm that NO₃⁻ derives mainly from manure or fertilizer applications. Fractional N elimination by denitrification is indicated by relatively high δ¹⁵N- and δ¹⁸O-NO₃⁻ values, elevated NO₂⁻ concentrations (0.05–0.14 mM), and δ¹⁵N-NO₂⁻ values that were systematically lower than the corresponding values of δ¹⁵N-NO₃⁻. Hydraulic and chemical response patterns of groundwater wells suggest that rain events result in the displacement of water from transient storage compartments such as the epikarst or the fissure network of the phreatic zone. Although O₂ levels of the investigated groundwaters were close to saturation, local denitrification might be promoted in microoxic or anoxic niches formed in the ferrous iron-bearing carbonate rock formations. The results revealed that (temporarily) saturated fissure networks in the phreatic zone and the epikarst may play an important role in N turnover during the recharge of fractured aquifers. | Projekt DEAL