Multi-scale groundwater flow modeling during temperate climate conditions for the safety assessment of the proposed high-level nuclear waste repository site at Forsmark,Sweden

Forsmark in Sweden has been proposed as the site of a geological repository for spent high-level nuclear fuel, to be located at a depth of approximately 470 m in fractured crystalline rock. The safety assessment for the repository has required a multi-disciplinary approach to evaluate the impact of hydrogeological and hydrogeochemical conditions close to the repository and in a wider regional context. Assessing the consequences of potential radionuclide releases requires quantitative site-specific information concerning the details of groundwater flow on the scale of individual waste canister locations (1–10 m) as well as details of groundwater flow and composition on the scale of groundwater pathways between the facility and the surface (500 m to 5 km). The purpose of this article is to provide an illustration of multi-scale modeling techniques and the results obtained when combining aspects of local-scale flows in fractures around a potential contaminant source with regional-scale groundwater flow and transport subject to natural evolution of the system. The approach set out is novel, as it incorporates both different scales of model and different levels of detail, combining discrete fracture network and equivalent continuous porous medium representations of fractured bedrock.     

Groundwater flow modeling of periods with periglacial and glacial climate conditions for the safety assessment of the proposed high-level nuclear waste repository site at Forsmark,Sweden

The impact of periglacial and glacial climate conditions on groundwater flow in fractured crystalline rock is studied by means of groundwater flow modeling of the Forsmark site, which was recently proposed as a repository site for the disposal of spent high-level nuclear fuel in Sweden. The employed model uses a thermal-hydraulically coupled approach for permafrost modeling and discusses changes in groundwater flow implied by the climate conditions found over northern Europe at different times during the last glacial cycle (Weichselian glaciation). It is concluded that discharge of particles released at repository depth occurs very close to the ice-sheet margin in the absence of permafrost. If permafrost is included, the greater part discharges into taliks in the periglacial area. During a glacial cycle, hydraulic gradients at repository depth reach their maximum values when the ice-sheet margin passes over the site; at this time, also, the interface between fresh and saline waters is distorted the most. The combined effect of advances and retreats during several glaciations has not been studied in the present work; however, the results indicate that hydrochemical conditions at depth in the groundwater flow model are almost restored after a single event of ice-sheet advance and retreat.     

Effects on groundwater flow of abandoned engineered structures for the safety assessment of the proposed high-level nuclear waste repository site at Forsmark,Sweden

Effects on groundwater flow of abandoned engineered structures in relation to a potential geological repository for spent high-level nuclear fuel in fractured crystalline rock at the Forsmark site, Sweden, are studied by means of numerical modeling. The effects are analyzed by means of particle tracking, and transport-related performance measures are calculated. The impacts of abandoned, partially open repository tunnels are studied for two situations with different climate conditions: a “temperate” climate case with present-day boundary conditions, and a generic future “glacial” climate case with an ice sheet covering the repository. Then, the impact of abandoned open boreholes drilled through the repository is studied for present-day climate conditions. It is found that open repository tunnels and open boreholes can act as easy pathways from repository level to the ground surface; hence, they can attract a considerable proportion of particles released in the model at deposition hole positions within the repository. The changed flow field and flow paths cause some changes in the studied performance measures, i.e., increased flux at the deposition holes and decreased transport lengths and flow-related transport resistances. However, these effects are small and the transport resistance values are still high.     

Modeling of groundwater flow at depth in crystalline rock beneath a moving ice-sheet margin, exemplified by the Fennoscandian Shield, Sweden

On-going geological disposal programs for spent nuclear fuel have generated strong demands for investigation and characterization of deep-lying groundwater systems. Because of the long time scales for which radiological safety needs to be demonstrated in safety assessment applications, an analysis of the hydrogeological performance of the geosphere system during glacial climate conditions is needed. Groundwater flow at depth in crystalline rock during the passage of an ice-sheet margin is discussed based on performed groundwater-flow modeling of two bedrock sites, Forsmark and Laxemar, in the Fennoscandian Shield, Sweden. The modeled ice sheet mimics the Weichselian ice sheet during its last major advance and retreat over northern Europe. The paper elaborates and analyzes different choices of top boundary conditions at the ice sheet–subsurface interface (e.g. ice-sheet thickness and ice-margin velocity) and in the proglacial area (presence or lack of permafrost) and relates these choices to available groundwater-flow-model hydraulic output and prevailing conceptual hydrogeochemical models of the salinity evolution at the two sites. It is concluded that the choice of boundary conditions has a strong impact on results and that the studied sites behave differently for identical boundary conditions due to differences in their structural-hydraulic properties.     

Characterization of the shallow groundwater system in an alpine watershed: Handcart Gulch,Colorado, USA

Water-table elevation measurements and aquifer parameter estimates are rare in alpine settings because few wells exist in these environments. Alpine groundwater systems may be a primary source of recharge to regional groundwater flow systems. Handcart Gulch is an alpine watershed in Colorado, USA comprised of highly fractured Proterozoic metamorphic and igneous rocks with wells completed to various depths. Primary study objectives include determining hydrologic properties of shallow bedrock and surficial materials, developing a watershed water budget, and testing the consistency of measured hydrologic properties and water budget by constructing a simple model incorporating groundwater and surface water for water year 2005. Water enters the study area as precipitation and exits as discharge in the trunk stream or potential recharge for the deeper aquifer. Surficial infiltration rates ranged from 0.1–6.2×10^?5 m/s. Discharge was estimated at 1.28×10^?3 km³. Numerical modeling analysis of single-well aquifer tests predicted lower specific storage in crystalline bedrock than in ferricrete and colluvial material (6.7×10^?5–2.0×10^?3 l/m). Hydraulic conductivity in crystalline bedrock was significantly lower than in colluvial and alluvial material (4.3×10^?9–2.0×10^?4 m/s). Water budget results suggest that during normal precipitation and temperatures water is available to recharge the deeper groundwater flow system.     

Using helicopter electromagnetic data to predict groundwater quality in fractured crystalline bedrock in a semi-arid region, Northeast Brazil

Geostatistical modeling, using airborne and borehole electromagnetic data, was used to estimate electrical conductivity in groundwater within fractured paragneisses and migmatites in a semi-arid climate in northeastern Brazil. Despite the geologic heterogeneity of crystalline aquifers, the use of high resolution helicopter electromagnetic (HEM) data enabled the characterization of groundwater electrical conductivity where data from drilled wells were insufficient. The tacit assumption is that HEM measurements can be used to relate rock electrical conductivity to groundwater electrical conductivity. In this study, the HEM data were used as an external drift variable in non-stationary estimation and stochastic simulation to identify the variability of groundwater electrical conductivity. Validation tests, comparing predicted values for groundwater conductivity with measurements in new wells, confirmed the success of these models in locating fresh groundwater sources in crystalline bedrock.     

Comparison of the behaviour of rare earth elements in surface waters,overburden groundwaters and bedrock groundwaters in two granitoidic settings,Eastern Sweden

This work, which was done within the Swedish nuclear waste management program, was carried out in order to increase the understanding of the mobility and fate of rare earth elements (REEs) in natural boreal waters in granitoidic terrain. Two areas were studied, Forsmark and Simpevarp, one of which will be selected as a site for spent nuclear fuel. The highest REE concentrations were found in the overburden groundwaters, in Simpevarp in particular (median ∑REE 52 μg/L), but also in Forsmark (median ∑REE 6.7 μg/L). The fractionation patterns in these waters were characterised by light REE (LREE) enrichment and negative Ce and Eu anomalies. In contrast, the surface waters had relatively low REE concentrations. They were characterised either by an increase in relative concentrations throughout the lanthanide series (Forsmark which has a carbonate-rich till) or flat patterns (Simpevarp with carbonate-poor till), and had negative Ce and Eu anomalies. In the bedrock groundwaters, the concentrations and fractionation patterns of REEs were entirely different from those in the overburden groundwaters. The median La concentrations were low (just above 0.1 μg/L in both areas), only in a few samples were the concentrations of several REEs (and in a couple of rare cases all REEs) above the detection limit, and there was an increase in the relative concentrations throughout the lanthanide series. In contrast to these large spatial variations, the temporal trends were characterised by small (or non existent) variations in REE-fractionation patterns but rather large variations in concentrations. The Visual MINTEQ speciation calculations predicted that all REEs in all waters were closely associated with dissolved organic matter, and not with carbonate. In the hydrochemical data for the overburden groundwater in particular, there was however a strong indication of association with inorganic colloids, which were not included in the speciation model. Overall the results showed that within a typical boreal granitoidic setting, overburden groundwaters are enriched in REEs, organic complexes are much more important than carbonate complexes, there is little evidence of significant mixing of REEs between different water types (surface, overburden, bedrock) and spatial variations are more extensive than temporal ones.     

基岩岛屿地下水数值模拟发展研究现状

基岩岛屿占我国岛屿的90%以上,且岛屿上的淡水资源十分珍贵。目前,岛屿上的供水(船运、雨水收集、海水淡化等)存在成本高、设备易坏等缺点。因此,岛屿的地下水资源无疑是优质、可靠、稳定的供水源。为了更好地开发基岩岛屿地下水资源,认为针对基岩岛屿地下水模拟,从地下水理论、地下水模拟的国内外研究现状和模拟模型等方面进行了综述。总结认为: 基岩岛屿的地下水多以“淡水蘑菇体”的形式储存,有别于沙质岛屿; 模拟模型应概化为横向各项同性的达西流或非达西流,或二者的耦合模型,并归纳了相应的地下水流运动方程; 基岩岛屿地下水模拟模型应分为孔隙型、裂隙型和孔隙-裂隙型3种类型; 在岛屿边界设定上,基岩海岸应设为隔水边界,沙质海岸应设为定水头边界。综述研究认为: 基岩岛屿地下水模拟模型应尽可能精细、完整、精准地刻画基岩岛屿的地质地貌,设置完整的岛屿补径排项; 模拟时,应准确对应基岩岛屿的地下水类型,选准模拟方法与模拟软件; 模型验证工作也是十分重要,需多次与岛屿水量均衡,且与实测水量和水位作对照,不断地修善模型。以上综述成果为我国今后开展基岩岛屿地下水数据模拟提供了理论依据。     

基岩岛屿地下水数值模拟发展研究现状

基岩岛屿占我国岛屿的90%以上,且岛屿上的淡水资源十分珍贵。目前,岛屿上的供水(船运、雨水收集、海水淡化等)存在成本高、设备易坏等缺点。因此,岛屿的地下水资源无疑是优质、可靠、稳定的供水源。为了更好地开发基岩岛屿地下水资源,认为针对基岩岛屿地下水模拟,从地下水理论、地下水模拟的国内外研究现状和模拟模型等方面进行了综述。总结认为: 基岩岛屿的地下水多以“淡水蘑菇体”的形式储存,有别于沙质岛屿; 模拟模型应概化为横向各项同性的达西流或非达西流,或二者的耦合模型,并归纳了相应的地下水流运动方程; 基岩岛屿地下水模拟模型应分为孔隙型、裂隙型和孔隙-裂隙型3种类型; 在岛屿边界设定上,基岩海岸应设为隔水边界,沙质海岸应设为定水头边界。综述研究认为: 基岩岛屿地下水模拟模型应尽可能精细、完整、精准地刻画基岩岛屿的地质地貌,设置完整的岛屿补径排项; 模拟时,应准确对应基岩岛屿的地下水类型,选准模拟方法与模拟软件; 模型验证工作也是十分重要,需多次与岛屿水量均衡,且与实测水量和水位作对照,不断地修善模型。以上综述成果为我国今后开展基岩岛屿地下水数据模拟提供了理论依据。     

Evidence for deep groundwater flow and convective heat transport in mountainous terrain, Delta County, Colorado, USA

The Tongue Creek watershed lies on the south flank of Grand Mesa in western Colorado, USA and is a site with 1.5 km of topographic relief, heat flow of 100 mW/m², thermal conductivity of 3.3 W m^–1 °C^–1, hydraulic conductivity of 10^-8 m/s, a water table that closely follows surface topography, and groundwater temperatures 3–15°C above mean surface temperatures. These data suggest that convective heat transport by groundwater flow has modified the thermal regime of the site. Steady state three-dimensional numerical simulations of heat flow, groundwater flow, and convective transport were used to model these thermal and hydrological data. The simulations provided estimates for the scale of hydraulic conductivity and bedrock base flow discharge within the watershed. The numerical models show that (1) complex three-dimensional flow systems develop with a range of scales from tens of meters to tens of kilometers; (2) mapped springs are frequently found at locations where contours of hydraulic head indicate strong vertical flow at the water table, and; (3) the distribution of groundwater temperatures in water wells as a function of surface elevation is predicted by the model.     

Using saline tracers to evaluate preferential recharge in fractured rocks,Floyd County,Virginia, USA

Potassium chloride (KCl) and potassium bromide (KBr) tracers were used to explore the role of geologic structure on groundwater recharge and flow at the Fractured Rock Research Site in Floyd County, Virginia, USA. Tracer migration was monitored through soil, saprolite, and fractured crystalline bedrock for a period of 3 months with chemical, physical, and geophysical techniques. The tracers were applied at specific locations on the ground surface to directly test flow pathways in a shallow saprolite and deep fractured-rock aquifer. Tracer monitoring was accomplished with differential electrical resistivity, chemical sampling, and physical monitoring of water levels and spring discharge. KCl, applied at a concentration of 10,000 mg/L, traveled 160 m downgradient through the thrust fault aquifer to a spring outlet in 24 days. KBr, applied at a concentration of 5,000 mg/L, traveled 90 m downgradient through the saprolite aquifer in 19 days. Tracer breakthrough curves indicate diffuse flow through the saprolite aquifer and fracture flow through the crystalline thrust fault aquifer. Monitoring saline tracer migration through soil, saprolite, and fractured rock provided data on groundwater recharge that would not have been available using other traditional hydrologic methods. Travel times and flowpaths observed during this study support preferential groundwater recharge controlled by geologic structure.     

Fracture detection and groundwater flow characterization using He and Rn in soil gases,Manitoba, Canada

Surveys of the distribution and migration of He and Rn were carried out in the well-characterized granitic terrane of the AECL Underground Research Laboratory (URL), Manitoba as part of a joint AECL Research, United Kingdom Department of the Environment, and United States Department of Energy research initiative. The investigations were designed to determine whether concentrations of He and Rn in soil gases could be used to identify locations of groundwater recharge and discharge from bedrock fractures. The results obtained indicate that subsurface transport of He and possibly Rn in this setting appears to be controlled largely by the groundwater flow system in the bedrock. Release of dissolved gases near the ground surface causes soil gas anomalies, which reflect discharge from the deeper flow system. In the recharge area of the deep groundwater flow system at the URL site, He abundances are close to the atmospheric level, but the discharge area of the deep flow system is characterized by significant He anomalies (up to 0.5 ppm above atmosphere levels). For Rn, the recharge area has broadly distributed high concentrations, probably caused by local Rn production in U-rich overburden, while the discharge area has only localized concentrations of Rn, which are not at the same location as the He anomalies. The general nature of the groundwater flow regime in both areas is reflected in the presence and distribution of the soil gas anomalies. In addition, major fractures in bedrock, which act as preferential groundwater flow paths, have been located from soil gas anomalies, even when obscured by overburden of variable thickness and character. The distribution of He in soil gas appears to be most representative of groundwater recharge and discharge conditions in the granitic rock, while Rn may be useful for locating specific channels where more rapid groundwater discharge is occurring from deep fracture zones.     

Assessing the impact of large-scale dewatering on fault-controlled aquifer systems: a case study in the Acque Albule basin (Tivoli, central Italy)

The development of large-scale bedrock quarry operations often requires high-volume and long-term groundwater extraction to maintain a sustainable working environment. These dewatering activities often influence groundwater levels and flow patterns regionally. In the present study, the influence of the dewatering of the travertine quarry operations near the city of Tivoli, Italy, are quantitatively investigated through an integrated analysis of field data and numerical modeling. Lowering of regional groundwater levels in the vicinity of the quarry has led to destructive land subsidence and alterations to the flow system sustaining a hot-spring area. The study employs a finite element numerical model (FEFLOW) to evaluate and quantify the impact of the extensive dewatering on fault-controlled regional groundwater flow in the Acque Albule basin. By incorporating the physical field data and historical hydrologic information, the numerical model was calibrated against three groundwater scenarios, reproducing the effects of different exploitation activities, coupled with natural changes over the course of the quarry operation. The results indicate that groundwater withdrawals by the mining industry and by “Terme di Roma” spa resulted in the cessation of flow from the primary thermal spring and a drop in the phreatic level in the area consequently affected by land subsidence.     

GIS based groundwater modeling study to assess the effect of artificial recharge: A case study from Kodaganar river basin,Dindigul district,Tamil Nadu

Groundwater is a dynamic and replenishable natural resource. The numerical modeling techniques serve as a tool to assess the effect of artificial recharge from the water conservation structures and its response with the aquifers under different recharge conditions. The objective of the present study is to identify the suitable sites for artificial recharge structures to augment groundwater resources and assess its performance through the integrated approach of Geographic Information System (GIS) and numerical groundwater modeling techniques using MODFLOW software for the watershed located in the Kodaganar river basin, Dindigul district, Tamil Nadu. Thematic layers such as geology, geomorphology, soil, runoff, land use and slope were integrated to prepare the groundwater prospect and recharge site map. These potential zones were categorized as good (23%), moderate (54%), and poor (23%) zones with respect to the assigned weightage of different thematic layers. The major artificial recharge structures like percolation ponds and check dams were recommended based on the drainage morphology in the watershed. Finally, a threelayer groundwater flow model was developed. The model was calibrated in two stages, which involved steady and transient state condition. The transient calibration was carried out for the time period from January 1989 to December 2008. The groundwater model was validated after model calibration. The prediction scenario was carried out after the transient calibration for the time period of year up to 2013. The results show that there is 15 to 38% increase in groundwater quantity due to artificial recharge. The present study is useful to assess the effect of artificial recharge from the proposed artificial structures by integrating GIS and groundwater model together to arrive at reasonable results.     

Hydraulic conductivity characteristics in mountains and implications for conceptualizing bedrock groundwater flow

Influences of hydraulic conductivity (K) heterogeneities on bedrock groundwater (BG) flow systems in mountainous topography are investigated using a conceptual 2D numerical modelling approach. A conceptual model for K heterogeneity in crystalline bedrock mountainous environments is developed based on a review of previous research, and represents heterogeneities due to weathering profile, bedrock fracture characteristics, and catchment-scale (～0.1–1 km) structural features. Numerical groundwater modelling of K scenarios for hypothetical mountain catchment topography indicates that general characteristics of the BG flow directions are dominated by prominent topographic features. Within the modelled saturated BG flow system, ～90 % or more of total BG flux is focussed within a fractured bedrock zone, extending to depths of ～100–200 m below the ground surface, overlying lower-K bedrock. Structural features and heterogeneities, represented as discrete zones of higher or lower K relative to surrounding bedrock, locally influence BG flow, but do not influence general BG flow patterns or general positions of BG flow divides. This result is supported by similar BG transit-time distribution shapes and statistics for systems with and without structural features. The results support the development of topography-based methods for predicting general locations of BG flow-system boundaries in mountain regions.     

Hydrogeological characterization of Gold Valley: an investigation of precipitation recharge in an intermountain basin in the Death Valley region, California, USA

Gold Valley is typical of intermountain basins in Death Valley National Park (DVNP), California (USA). Using water-balance calculations, a GIS-based analytical model has been developed to estimate precipitational infiltration rates from catchment-scale topographic data (elevation and slope). The calculations indicate that groundwater recharge mainly takes place at high elevations (>1,100?m) during winter (average 1.78?mm/yr). A resistivity survey suggests that groundwater accumulates in upstream compartmentalized reservoirs and that the groundwater flows through basin fill and fractured bedrock. This explains the relationship between the upstream precipitational infiltration in Gold Valley and the downstream spring flow in Willow Creek. To verify the ability of local recharge to support high-flux springs in DVNP, a GIS-based model was also applied to the Furnace Creek catchment. The results produced insufficient total volume of precipitational infiltration to support flow from the main high-flux springs in DVNP under current climatic conditions. This study introduces a GIS-based infiltration model that can be integrated into the Death Valley regional groundwater flow model to estimate precipitational infiltration recharge. In addition, the GIS-based model can efficiently estimate local precipitational infiltration in similar intermountain basins in arid regions provided that the validity of the model is verified.     

高放废物深地质处置地下水数值模拟应用综述

开展地下水数值模拟研究是高放废物处置场地安全评价的重要组成部分,然而深地质处置介质类型的复杂性、基岩深部资料的相对匮乏性导致模拟结果存在不确定性,如何刻画深部地下水动力场并评估可能引起的风险已成为高放废物处置安全评价中重点关注的问题。在大量文献调研的基础上,综述了世界典型国家高放废物深地质处置场地的地下水数值模拟与不确定性分析应用,并归纳总结该领域研究经验,得到以下认识:（1）深地质处置场深部构造、裂隙的发育与展布决定了地下水循环条件,探究适用于基岩裂隙地区新的水文地质试验方法是提高地下水数值模型仿真性的基础;（2）不同尺度模型融合是解决深地质处置地下水模拟的有效技术方法,区域尺度多采用等效连续介质法,场地尺度使用等效连续多孔介质和离散裂隙网络耦合模型,处置库尺度使用离散裂隙网络方法,其次需重点关注未来大时间尺度下放射性核素在地质体中的迁移转化规律,模拟预测场址区域地下水环境长期循环演变对核素迁移的潜在影响;（3）考虑到不同的处置层主岩岩性以及在多介质中发生的THMC（温度场—渗流场—应力场—化学场）过程,目前国内外常用的地下水模拟软件有:Porflow、Modflow、GMS及MT3DMS等用于模拟孔隙或等效连续介质,Connectflow、Feflow及FracMan等用于模拟地下水和核素在结晶岩、花岗岩等裂隙中的迁移,TOUGH系列软件主要应用于双重介质的水流、溶质及热运移模拟;（4）指导开展有针对性的模型和参数的不确定性分析工作,减少投入工作量,提高模型精度,并可针对处置库长期演变、废物罐失效、极端降雨等多情景预测模拟,为处置库安全评价及设计提供基础数据支撑;（5）针对我国深地质处置地下水数值模拟研究现状,下一步应加强区域地质、水文地质、裂隙测量以及现场试验等相关的调查及监测工作,多介质耦合、多场耦合模拟及不确定性分析研究将会是未来的研究重点。     

Palaeohydrogeology: A methodology based on fracture mineral studies

Two sites on the east coast of Sweden (Forsmark and Laxemar/Simpevarp) are currently being investigated as potential geologic hosts for a deep repository isolating high-level nuclear waste. In this paper, a methodology for fracture mineral studies is suggested with focus on the variation in depth of the fresh/saline water interface and location of the redox front in the bedrock. The most commonly precipitated fracture minerals in crystalline rocks are chlorite, calcite, quartz, K-feldspar, Ca–Al-silicates like epidote, prehnite and laumontite, sulphides and Fe-oxides. Of these, calcite is the mineral best suited for palaeohydrological studies since it precipitates during a wide range of conditions including low-temperature conditions during the Pleistocene and Holocene epochs. Sulphides and Fe-oxides/hydroxides provide information on the position of the redox front. In order to carry out palaeohydrological studies, a number of prerequisites are required such as; high quality drill core material, geological knowledge of the sequence of fracture mineralizations; the post-glacial (Holocene) evolution in the area; high quality groundwater chemistry, including stable isotopes; and a conceptual model of the hydrogeochemistry that is to be tested. The choice of methods used here is based on the fact that both the Forsmark and Laxemar/Simpevarp sites are situated in Palaeoproterozoic crystalline rocks with reactivation of fractures over at least 1.5 Ga, and they have been exposed to glaciations/deglaciations and transgressions/regressions of the Baltic Sea during the Quaternary. This has resulted in a palaeohydrology with a range of groundwaters of quite different chemistry and stable isotopic composition. The suggested scheme for solving the variation in depth of the fresh/saline water interface focuses on fracture calcite. It includes a step-by-step procedure with;

(1)

Initial δ¹⁸O and δ¹³C, analyses and complementary petrographic studies of thin sections and crystal morphology followed by     

Basin-scale groundwater response to precipitation variation and anthropogenic pumping in Chih-Ben watershed, Taiwan

The sustainable use of groundwater has become increasingly challenging due to extreme hydrological events and anthropogenic activity. In this study, the basin-scale groundwater response to precipitation variation was analyzed using an integrated model that comprises lumped models for land and river recharges and a distributed model for groundwater. The integrated model was applied to the Chih-Ben watershed, Taiwan, using 20?years (1988?C2007) of data. The hydrological data were analyzed for trends using statistical tests. Based on decreasing trends in precipitation and groundwater levels and an increasing trend in stream flow, the oblique-cut method was applied to precipitation and excess infiltration to assess land and streambed recharge. Distributed numerical groundwater modeling was used to simulate the basin-scale groundwater responses to precipitation variation and anthropogenic pumping. The model was calibrated using stable-isotope and groundwater-level data. The safe yields were estimated for the Chih-Ben watershed for dry, wet, and normal precipitation scenarios. The safe yield of groundwater was shown to vary with precipitation, which does not guarantee the sustainable use of groundwater resources. Instead, water resources should be assessed at a basin scale, taking into account the whole ecosystem, rather than only considering water for human consumption in the alluvium.