Analysis of groundwater resources in densely populated urban watersheds with a complex tectonic setting: Shenzhen, southern China
- 291 Downloads
Shenzhen is the major financial and high-tech center in southern China. The megacity has grown rapidly in the last 40 years with the population increasing from about 30,000 in 1979 to 20 million in 2016. The study area (2,015 km2) is about 42% urban and 58% undeveloped land. The rapid development of the megacity has resulted in severe degradation of the groundwater and surface-water resources and has created a nearly insatiable demand for water, with an average consumption of 2000 × 106 m3/year. Groundwater is an important component of the baseflow of the many streams in the area and is used for potable water supply and irrigation in some of the rural parts of the municipality. This study develops a conceptual model and quantitative framework for assessing the groundwater resources of Shenzhen. The groundwater system consists of shallow aquifers of alluvium and weathered bedrock overlying low permeability igneous and sedimentary rocks. The complex geologic setting was conceptualized as a block structure with blocks bounded by high-angle faults. The water budget in Shenzhen was quantified. The estimated average groundwater discharge is about 12% of annual precipitation. The study provides a starting point to investigate how a megacity such as Shenzhen should manage and protect its groundwater as a strategic resource and environmental asset. It is also a basic management tool for analyzing and contributing to urban drainage concepts such as the “sponge city”.
KeywordsConceptual models Numerical modeling Groundwater recharge/water budget Urban groundwater China
Analyse des ressources en eau souterraine d’un bassin urbanisé densément peuplé dans un contexte de tectonique complexe: Shenzhen, sud de la Chine
Shenzen est le principal centre financier et technologique du Sud-Ouest de la Chine. La mégapole a grandi rapidement durant les 40 dernières années, sa population croissant d’environ 30,000 en 1979 à 20 millions en 2016. La zone d’étude (2,015 km2) est à peu près à 42% urbaine et à 58% non bâtie. Le développement rapide de la mégapole s’est traduit par une sévère dégradation des ressources en eau souterraine et en eau de surface et a créé une demande en eau à peu près insatiable, avec une consommation moyenne de 2000 × 106 m3/an. L’eau souterraine est une composante importante de l’écoulement de base de beaucoup de cours d’eau de la région et est utilisée pour l’approvisionnement en eau potable et l’irrigation dans plusieurs des secteurs ruraux de la municipalité. L’étude présente un modèle conceptuel et un schéma quantitatif pour une évaluation des ressources en eau souterraine de Shenzhen. Le système hydrogéologique est constitué par les aquifères superficiels alluviaux et un substratum surcreusé recouvrant des roches ignées et sédimentaires à faible perméabilité. Le contexte géologique complexe a été conceptualisé comme une structure de blocs avec des compartiments limités par des failles de forte inclinaison. Le bilan de l’eau de Shenzhen a été quantifié. La décharge moyenne en eau souterraine estimée est d’environ 12% des précipitations annuelles. Cette étude fournit un point de départ pour explorer comment une métropole comme Shenzhen devrait gérer et protéger son eau souterraine en tant que ressource stratégique et capital environnemental. C’est aussi un outil de gestion de base pour analyse et contribution aux concepts de drainage urbain tel. que celui de « cité-éponge ».
Análisis de los recursos de agua subterránea en cuencas urbanas densamente pobladas con un entorno tectónico complejo: Shenzhen, sur de China
Shenzhen es el principal centro financiero y de alta tecnología en el sur de China. La megaciudad ha crecido rápidamente en los últimos 40 años y la población aumentó de aproximadamente 30,000 en 1979 a 20 millones en 2016. El área de estudio (2,015 km2) es aproximadamente 42% urbana y 58% terreno no desarrollado. El rápido desarrollo de la megaciudad ha provocado una grave degradación de los recursos de aguas subterráneas y superficiales y ha creado una demanda casi insaciable de agua, con un consumo medio de 2000 × 106 m3/año. El agua subterránea es un componente importante del flujo base de las muchas corrientes en el área y se utiliza para el suministro de agua potable y el riego en algunas partes rurales del municipio. Este estudio desarrolla un modelo conceptual y un marco cuantitativo para evaluar los recursos de aguas subterráneas de Shenzhen. El sistema de agua subterránea consiste en acuíferos poco profundos de aluvión y basamento meteorizado que cubre rocas ígneas y sedimentarias de baja permeabilidad. El entorno geológico complejo se conceptualizó como una estructura de bloques con bloques delimitados por fallas de alto ángulo. Se cuantificó el balance de agua en Shenzhen. La descarga promedio estimada de agua subterránea es de alrededor del 12% de la precipitación anual. El estudio proporciona un punto de partida para investigar cómo una megaciudad como Shenzhen debe gestionar y proteger sus aguas subterráneas como recurso estratégico y activo ambiental. También es una herramienta de gestión básica para analizar y contribuir a los conceptos de drenaje urbano, como la “ciudad esponja”.
深圳市是中国华南地区重要的金融和高科技中心,经过40年的快速发展,其居住人口从1979年的3万人增长至2016年的两千万人。 深圳市土地总面积约为2,015 km2,其中城市用地约占42%,未开发用地约占58%。城市的快速发展造成地表水和地下水资源的严重恶化,同时,也产生了极大的用水需求。据统计,深圳市的年均用水量可达2000 × 106 m3/年。地下水是深圳市河流基流的主要来源,在部分区域地下水作为饮用水和灌溉水源被直接使用。为明确深圳市的地下水存储量,本研究建立了评估深圳市地下水资源的概念模型和定量框架。深圳市的浅层含水层主要由冲积层和风化层构成,基岩主要为岩浆岩和沉积岩。为计算地下水的存储量,深圳市复杂的地质环境被概化为以高角度断层为边界的块状结构。研究结果表明,深圳市年均地下水排泄量约为降雨量的12%。地下水资源是特大城市重要的战略资源和环境要素,本研究为地下水资源的管理和保护提供了可靠依据,同时,也为“海绵城市”的研究和建设提供了有力的管理工具。
Analisi delle risorse idriche sotterranee in zone densamente popolate e con un complesso assetto geologico: Shenzhen, Cina meridionale
Shenzhen è uno dei maggiori centri finanziari e tecnologici della Cina meridionale. La metropoli è cresciuta rapidamente negli ultimi 40 anni con una popolazione che dal 1979 al 2016 è passata da 30,000 a 20 milioni di persone. L’area di studio, circa 2,015 km2, è per il 42% urbanizzata mentre il restante 58% risulta non sviluppata. La rapida crescita della metropoli ha creato una forte degradazione delle risorse idriche sotterranee e delle acque superficiali accrescendo la domanda idrica in maniera smisurata, attorno ai 2000 × 106 m3/anno. Le acque sotterranee sono una componente importante nel flusso dei corsi d’acqua ed. è usata per l’approvvigionamento idrico in alcune parti rurali della città. Questo studio sviluppa un modello concettuale e quantitativo per valutare le risorse idriche sotterranee della città di Shenzhen. L’assetto idrogeologico consiste in acquiferi superficiali, fatti di alluvioni e coperture eluvio-colluviali, posti al di sopra di rocce ignee e sedimentarie a bassa permeabilità. L’assetto geologico è stato schematizzato attraverso blocchi di substrato bordati da faglie ad alto angolo. Il bilancio idrogeologico è stato quantificato. Il flusso sotterraneo corrisponde a circa il 12% della precipitazione annuale. Lo studio fornisce un punto di partenza per capire come Shenzhen o altre metropoli possano utilizzare le acque sotterranee come una risorsa strategica e, allo stesso modo, difendere questo patrimonio naturale. Lo studio rappresenta anche uno strumento di base per analizzare e approfondire i concetti di drenaggio urbano, come per esempio i progetti di “sponge city”.
Análise dos recursos hídricos subterrâneos em bacias urbanas densamente povoadas com uma complexa configuração tectônica: Shenzhen, sul da China
Shenzhen é o principal centro financeiro e de alta tecnologia no sul da China. A megacidade tem crescido rapidamente nos últimos 40 anos com crescimento populacional de aproximadamente 30,000 em 1979 para 20 milhões em 2016. A área de estudo (2,015 km2) tem cerca de 42% de área urbana e 58% terras não desenvolvidas. O rapido desenvolvimento da megacidade resultou em severa degradação dos recuros hídricos subterrâneos e superficiais e criou uma demanda por água quase insaciável, com um consumo médio de 2000 × 106 m3/ano. A água subterrânea é um importate componente do fluxo de base de muitos rios na área e é usada para o abastecimento de água e irriação em algumas áreas rurais do município. Este estudo desenvolveu um modelo conceitual e uma abordaem quantitativa para analisar os recursos hídricos subterrâneos de Shenzhen. O sistema de águas subterrêneas consiste de aquíferos superficiais de aluvião e subtrato rochoso intemperizado suprajacente à rochas ígneas e sedimentares de baixa permeabilidade. A configuração do complexo geológico foi conceitualizada como um estrutura em blocos limitados por falhas com angulação elevada. O balanço hidrico em Shenzhen foi calculado. A descarga média estimada da água subterrânea é cerca de 12% da precipitação anual. O estudo proporciona um ponto inicial para investigar como um megacidade como Shenzhen pode manejar e proteger seus recursos hídricos subterrâneos estratégicos e patrimônio ambiental. É tambem uma ferramenta básica para analisar e contribuir com o conceito de drenagem urbana, como o de “cidade esponja”.
The earliest available compilation of hydrogeological information was GBGP (Geological Bureau of Guangdong Province, China, Unpublished report on regional hydrogeological survey, scale 1:200.000, 1980) which compiled many borehole logs from throughout Shenzhen along with recovery tests and hydrogeochemical data. A decade later, Yang et al. (1991) described the geology of the Shenzhen area, attributing the current structural setting to strike-slip tectonics with a main element oriented NE–SW. Campbell and Sewell (1997, 1998) reconstructed the different plutonic stages and their relationship with the strike-slip tectonics. Xu et al. (2015) investigated the structural setting from a geophysical point of view. The most comprehensive hydrogeological description of Shenzhen is Yang et al. (2007).
Constructing the three-dimensional (3D) distribution of the aquifers and defining a conceptual circulation model
Evaluating the recharge and permeability of the hydrogeological units
Identifying and quantifying the groundwater resources of Shenzhen
Hydrogeological setting and conceptual model
The distribution of the Paleozoic and Mesozoic rocks is controlled by the different tectonic stages that also influence the trend of the river valleys. The Lower Paleozoic tectonics is characterized by contractional faults and thrust planes, NE–SW oriented and NW verging, created during the phases of the Caledonian orogenesis (Pang et al. 2016). During the early Mesozoic, the orogen experienced a distension associated with high rate of subsidence, due to roll back of the paleo-Pacific plate. The study area was intersected by strike-slip tectonics (Allen et al. 1997) with the Meixian-Shenzhen fault as important tectonic line (Sun et al. 2007). The latter divides the Bao’An area, characterized by an Upper Paleozoic hiatus, from the Paleozoic sequence of the Longgang-Pingshan area. These faults were active up to the Quaternary and in some cases up to the Middle part of the Upper Pleistocene (Lu and Sun 1991).
The Quaternary clastic unit (legend 1 in Fig. 2) consists of a multi-layered sequence with the hydraulic conductivity strongly related to the predominant size of the clasts. Deposits are heterogeneous; layers consist of coarse to silty sands, with intercalations of sandy gravels along the river valleys or silty clays in coastal areas as the result of marine transgressions and/or temporary lacustrine events. In addition, layers of detritus and talus, derived from the weathered bedrock, have been observed. This hydrogeological unit has a thickness varying between 30 and 60 m along the main valleys and in the coastal sectors.
The Tertiary reworked tuff unit consists of loose ash and tuffs (legend 2 in Fig. 2) which belong to the upper portion of the volcanic sequence. This hydrogeological unit also includes reworked deposits with intercalations of coarse gravel observed in outcrop in the Pingshan area.
The Upper Mesozoic lava unit (legend 3 in Fig. 2) constitutes Wuton Mt. in the southern part of Shenzhen. An interconnected fracture system characterizes the lava flows. Individual fracture orientation is directly correlated to the movement of the flow. Intercalations of paleo-soils or weathered deposits locally reduce the hydraulic conductivity.
The Jurassic granite unit (legend 4 in Fig. 2) is an aquitard. Facture density is very low and RQD (rock quality designation) index records excellent values (90–100%), beyond 30 m from the ground surface. Towards the land surface, a blanket of more fractured and weathered deposits is present. This band typically extends to 30 m below the land surface based on the analysis of the stratigraphy of many water-wells and boreholes. Usually the shallowest 15-m-thick portion of this unit consists of a very permeable coarse quartz sand. This shallowest part represents the main aquifer, where the groundwater circulation is more developed. In the eastern sector, the granite aquitard (legend 4a in Fig. 2) is more fractured (Yang et al. 2007) but with a thinner weathered band, gravitationally mobilized because of the steep morphology. From a hydrogeological point of view, this granite (legend 4a in Fig. 2) is grouped with the Upper Mesozoic lava unit.
The Upper Paleozoic–Mesozoic unit (legend 5 in Fig. 2) consists of a sequence of quartz sandstone, siltstone, sandy siltstone and shale. Deposits are widely intersected by tectonic sediments. Sediments are lithified and the primary porosity is very low even in the coarser deposits. Fractures are abundant but the apertures were strongly reduced by dynamic and regional metamorphic processes as proven by the presence of andalusite, chlorite and other metamorphic minerals (Yang et al. 2007).
The Carboniferous carbonate unit (legend 6 in Fig. 2) is strongly karstified and it is present at the bottom of the main Quaternary depressions in the Pingshan and Longgang areas. Numerous sinkholes occur where this unit is present. Sporadic outcrops also suggest metamorphic facies of marble, dolomitic marble and crystalline limestone.
The Paleozoic unit (legend 7 in Fig. 2) consists of metamorphic sandstone, phyllite and slate with serpentine. Porosity is very low due to the huge lithostatic load that these rocks experienced. Due to the different tectonic cycles, fracture systems are interconnected and more developed than in the Upper Paleozoic–Mesozoic unit. The Basement unit (legend 8 in Fig. 2) is metamorphosed Lower Paleozoic rocks of quarzitic sandstone and muddy shale. Porosity is very low and fracture apertures were strongly reduced by metamorphic processes. From a hydrogeological point of view, this unit is an aquitard and is grouped with the Upper Paleozoic–Mesozoic unit (legend 5 in Fig. 2).
The Lower Carboniferous carbonate unit is an exception to this conceptual model; it is the most permeable unit of the study area but it is only found in the bottom of the Pingshang and Longgang River valleys, buried by the Quaternary aquifers, as shown in Fig. 3b. As a result of granite intrusions there is no continuity of the karst unit from one hydraulic basin to the next. Despite the high hydraulic conductivity and the ongoing karstification, evidenced by sinkholes (Yang et al. 2007), there is little groundwater movement in this hydrogeological unit except for local flows caused by groundwater pumping.
In this conceptual model, groundwater and surface watersheds coincide and thus groundwater circulation can be studied at the catchment scale. Bedrock is essentially an aquitard but the hydraulic conductivity can be locally altered by the numerous faults. Across the fault plane, the cataclastic bands increase the hydraulic conductivity, functioning as a conduit. On the contrary, the fault zones with a mylonite core have significantly lower permeability (Caine et al. 1996). The effects of faults on groundwater flow are small and localized, as observed by Ran et al. 2013 for normal displacements and Cilona et al. 2015 for strike-slip systems. In the Mazhou River basin (Fig. 2), a single thermal spring illustrates upward heat migration from the bedrock to the shallow zone along a fault, affecting the local groundwater path. The thermal occurrence is isolated and favored by the cataclastic band; however, there is no impact of the faults on the bedrock circulation at the regional scale.
Construction of the numerical model
Constant head boundary conditions (IBOUND = −1, Harbaugh 2005) were specified along the coast with a hydraulic head equal to zero. Rivers have been represented via the river boundary package (RIV 1, Harbaugh et al. 2000). River stage has been derived by DEM. Along the river beds, values of Carc (conductance for unit length, GMS 2017) vary between 0.02 and 0.0001 m2/s, as a function of the width of the rivers and assuming the hydraulic conductivity of the river sediments equal to 0.0001 m/s. The unsaturated zone flow package (UZF) was used to represent the upper model surface (Niswonger et al. 2006). The package can represent thick portions of unsaturated soils, as it is typical for rough and steep morphologies. Recharge rate is the input variable of this package and depends on the hydrogeological units that outcrop. The Brooks and Corey (1966) model has been assumed constant to describe the groundwater flow in the unsaturated soils. Epsilon (EPS) has been set equal to 3.5 and the water content of the unsaturated zone (THTS) to 0.33 (GMS 2017).
The northern boundary of the model consists of many sub-catchments that perfectly follow the watershed lines, as for the Guanlang and Pingshan river valleys. Because the groundwater does not flow beyond the watershed boundary, no-flow boundary conditions (IBOUND = 0, Harbaugh 2005) were specified (Fig. 4c). Nevertheless, the Longgang River catchment extends beyond the boundary of the studied area. Here, the limit of the studied area cuts across a granite mountain section with narrow gorges. Because of the particular hydrogeological setting across the boundary, continuous aquifers are not present. In accordance with the constructed hydrogeological conceptual model, no-flow boundary conditions (IBOUND = 0, Harbaugh 2005) have been extended along the north-west boundary (Fig. 4c).
However, the model boundary cuts across the Longgang River Valley and the groundwater level varies with a horizontal gradient. The general head boundary (GHB) package (Harbaugh et al. 2000) has been set to better define the groundwater setting of the area (Fig. 4c). The conductance term of the GHB package has been calculated, analyzing the elevation of the river and its distance from the boundary cells.
The boundary between Hong Kong and the study area follows the Shenzhen River and the Shatoujian stream (Fig. 2). The downstream portion of the Shenzhen River has been included in the River package. The upper stream portion of the Shenzhen River and the Shoutoujian Stream are less important and characterized by a seasonal discharge. This area is characterized by volcanic lavas with a shallow groundwater flow directed towards these two rivers. Hydraulic exchanges between Hong Kong and Shenzhen do not occur and the no-flow boundary condition (IBOUND = 0, Harbaugh 2005) is appropriate.
Homogeneous hydraulic conductivity and recharge were assumed for each hydrogeological unit. Analysis of recovery tests of water wells (Geological Bureau of Guangdong Province, China, Unpublished report on regional hydrogeological survey, scale 1:200.000, 1980) showed a wide range of hydraulic conductivities. For each hydrogeological unit, a starting value was set at the average of the recovery test results. Recharge in undeveloped areas was based in the first step of calibration on average recharge rates from Yang et al. (2007), with a zero value for urban recharge. Recharge and hydraulic conductivity for each block were calibrated using a trial-and-error procedure.
The manual calibration investigates how changes in model parameters vary the fit between simulated values and targets. The calibration targets were 158 water-wells and piezometers. The data were collected via a classic water level meter in March 2017, at the end of the dry season, and in July 2017, during the wet season. The two datasets typically differ by 3 m in the rural areas and less than 1 m in the flat urban areas. Calibration started by varying the hydraulic conductivity of the Quaternary aquifers that outcrop in the coastal plain and in the alluvial aquifers. Next, the conductivity of weathered soils and then the bedrock units were calibrated. Initial results were satisfying from a qualitative point of view but simulated hydraulic heads differed from the observed by up to 30 m, indicating that recharge needed to be included as a model calibration parameter.
Hydraulic conductivity and recharge datasets used for the calibration of the numerical model
Hydraulic conductivity [m/s]
Recharge rate [mm/year]
Quaternary clastic unit (legend 1 in Fig. 2)
1 × 10−4
Tertiary reworked tuff unit (legend 2 in Fig. 2)
1 × 10−3
4 × 10−7
5 × 10−6 (if weathered)
Jurassic granite unit (legend 4b in Fig. 2)
1 × 10−7
5 × 10−5 (if weathered)
Upper Paleozoic-Mesozoic sedimentary unit
and Lower Paleozoic (?) basement
(legends 5 and 8 in Fig. 2)
6 × 10−7
5 × 10−6 (if weathered)
Carboniferous carbonate unit (legend 6 in Fig. 2)
1 × 10−3
Paleozoic sedimentary unit (legend 7 in Fig. 2)
8 × 10−7
5 × 10−6 (if weathered)
However, calibration of a ground model does not usually yield a unique solution. A reduction in hydraulic conductivity will raise the simulated heads and can be counterbalanced by a reduction in recharge. Therefore, a sensitivity analysis was performed; the entire hydraulic conductivity and recharge datasets were varied by ±10%. An increase in the error was observed in both cases, without varying the groundwater flow direction. This informal sensitivity analysis gives some confidence that the numerical model is satisfactory.
Results and discussion
From this area of maximum hydraulic heads, the simulated groundwater fluxes are toward the NNW into the Maozhou River basin and NNE to the Guanlang River (Fig. 6). Shenzhen Bay receives groundwater from the granite intrusions of the Yangtai Mt. and Futian area, via fluxes directed from North to South. In the Shenzhen River basin (Fig. 6), the flatter morphology and the clastic morphology contribute to flatter gradients, with a variation of 50 m in 15 km (average hydraulic gradient of 0.3%). In the southern part of the catchment, groundwater directly discharges to the Mirs Bay. The hydraulic head variation is related to the topography, with a gradient varying from 3 to 5%. Fluxes are directly perpendicular to the coastline and are strongly influenced by the fluvial morphology. The groundwater of the Dapeng peninsula discharges into the Daya Bay in a similar way. The irregular coastline favors the discharge of the groundwater directly to the sea, via hydraulic gradients from 0.3 to 1% and hydraulic heads typically between 0 and 30 m a.s.l. Finally, the Pingshang and Longgang river basins have similar settings where groundwater discharges directly to the river valleys. Hydraulic gradients vary strongly as a function of the predominant lithology along the flow path and range from 0.1% in the alluvial aquifer to 4% in the granite. Carboniferous karstified layers, located at the bottom of the Pingshang and Longgang valleys, are fully saturated, with a groundwater flow that reflects the shallow groundwater paths.
Estimated groundwater balance from the Shenzhen numerical model
Total rainfall [m3/year]
4,031.6 × 106
Average groundwater discharge [m3/year]
485.6 × 106
Groundwater from urban recharge [m3/year]
211.6 × 106
Limits and uncertainties of the numerical analysis
The conceptual model incorporates the most plausible geological and hydrogeological settings, based on the stratigraphic and the structural analysis from borehole data. All the available information has been used for the model construction and an uncertainty analysis of alternative conceptual models (Reefsgaard et al. 2006) cannot be performed. The parameter values of the calibrated model are consistent with the literature ranges indicated by GBGP (Geological Bureau of Guangdong Province, China, Unpublished report on regional hydrogeological survey, scale 1:200.000, 1980) and Yang et al. (2007), and are in accordance with the aquifer and aquitard definition of the hydrogeological conceptual model, and have hydrosense (Hunt and Zheng 2012); however, many combinations of parameter values can give similar results in groundwater modelling. A sensitivity analysis was performed that showed that small variations in hydraulic conductivity and recharge (±10%) slightly increase the error in head calibration but do not change the groundwater flow paths, whereas, on the contrary, the hydraulic head distribution is not sensitive to the conductance specified in the river package. The study has the objective of identifying and quantifying the groundwater resources at a regional scale from the current constrained dataset. Many limitations are inherent from the regional character of this study and a formal uncertainty analysis of the numerical results (Refsgaard et al. 2007) is not possible. Parameter values are suitable to describe the regional flow paths; downscaling to analyze a single catchment, though, would require a new calibration process. Shenzhen is one of the pilot areas for the Chinese “Sponge City Initiative” (Shenzhen Committee 2016), involving the introduction of LID (low impact development) elements to store rainfall in the subsoil and ease its reuse (Chang et al. 2018). In order to assess the interaction between urban drainage and groundwater, the model would require finer discretization and the inclusion of a sewer network as a boundary condition.
Despite the high number of well-distributed hydraulic head measurements, there is a lack of measured hydraulic heads in the areas with the highest heads near Yangtai Mt. and the Yantian Mts. In these areas, the simulated hydraulic head suggests a thickness of unsaturated granites from 400 to 800 m. Supplementary hydraulic heads in these areas would help the calibration process by providing more data at locations away from the boundary conditions (i.e. main rivers and coastline); nevertheless, since the granite is an aquitard, changing the hydraulic head maxima would not influence the flux directions.
The diffuse urbanization and the construction of numerous reservoirs make unreliable the calibration of the model via discharge measures. Indeed, the estimation of the base flow cannot be performed in the areas located downstream the reservoirs, as here, the flux is controlled by the water authorities and can also vary in a small lapse of time. Obviously, reliable baseflow data also cannot be inferred in the urban areas. Data can be collected just in the rural portion, where small sub-catchments close to the watersheds have not been urbanized. Discharge measures would not be sufficient to homogeneously cover the study area and provide a reliable calibration for the whole model.
Discretization also creates inaccuracies, and more cells would better define morphology as well as the geometries of the aquifers. The top layer has a thickness of 30 m and was introduced to represent the Quaternary clastic complex and the eluvium/colluvium overlying the bedrock. In reality, the thickness is not homogeneous and has an inverse relationship with the slope angle; indeed, eluvium/colluvium is usually gravitationally mobilized on slopes steeper than 30°, leaving the bedrock exposed. A better definition of the eluvium/colluvium thickness would improve the model, defining the small perched aquifers that feed the ephemeral mountain streams. The role of these aquifers has been omitted from this regional-scale study.
A hydrogeological conceptual model of the Shenzhen area (Figs. 2 and 3) and numerical analysis identified and quantified the Shenzhen groundwater resources (Fig. 6). Despite a lack of measurements of hydraulic conductivity and recharge, the model fits well to the observed head data. The Quaternary coastal plain, alluvial valleys and eluvium/colluvium blankets represent the main aquifer. Tropical humid conditions have created the eluvium-colluvium blanket, which facilitates infiltration. The dense vegetative cover, in the nonurban areas, increases the porosity of the soil which slows the runoff and also facilitates infiltration. Bedrock is a regional aquitard with relatively unimportant circulation. According to the conceptual model watersheds and groundwater basins coincide. The numerical simulation based on this conceptual model constrained the hydraulic conductivity and recharge dataset.
Over 40% of the land area of Shenzhen is densely urbanized, and in this area, infiltration from precipitation is small but leakage from water mains, sewers and irrigation excess provides a significant source of groundwater recharge. Estimated urban recharge from model calibration is approximately 250 mm/year, a value which is significant, and consistent with Lerner (2002) who found that in urban areas, recharge is equal to or higher than in natural areas. In natural areas, recharge varies as a function of the outcropping hydrogeological units.
The conceptual model and associated numerical simulation provide a comprehensive framework for understanding groundwater conditions in Shenzhen and supporting future management of the city. The shallow aquifer is important in supporting streams which are environmentally significant. A sustainable development and management of the water resources is essential for the Shenzhen area and this study represents an important first step for pollution remediation of the aquifers, and the water-table restoration. An adaptation of the model could be useful to analyze the “sponge city” concept. The same approach of a block-structured hydrogeological model and calibrated dataset can be reapplied to describe groundwater flow in other areas of Guangdong and nearby Fujian province which have similar geological and climate features, and high population density.
We would like to thank the editors Dr. Jean-Michel Lemieux and Dr. Rebecca Doble for the detailed and helpful reviews provided. The constructive comments of Dr. Tao Cui and an anonymous reviewer are acknowledged.
This work has been supported by the National Key R&D program of China, project no. 2016YFC0402806. Additional support was provided by the National Natural Science Foundation of China (grant no. 41330632).
- Brooks RH, Corey AT (1966) Properties of porous media affecting fluid flow. J Irrig Drain Div Am Soc Civ Eng 92:61–87Google Scholar
- Campbell SDG, Sewell RJ (1998) A proposed revision of the tectonic stratigraphy and related plutonic classification of Hong Kong. J Geol Soc Hong Kong 4:1–11Google Scholar
- Chang NB, Lu JW, Chui TFM, Hartschorn (2018) Global policy analysis of low impact development for stormwater management in urban areas. Land Use Policy 70:368–383Google Scholar
- GMS (2017) GMS user manual tutorial 10.4 version, groundwater modeling system. Available at http://gmsdocs.aquaveo.com/GMS_User_Manual_v10.3.pdf. Accessed September 2018
- Harbaugh AW, Banta ER, Hill MC, Mc Donald MG (2000) MODFLOW 2000, the U.S. geological surveys modular groundwater model: user guide to modularization concepts and the groundwater flow process. US Geol Surv Open-File Rep 00-92, 191 ppGoogle Scholar
- Harbaugh AW (2005) MODFLOW 2005 – The U.S. Geological Survey modular groundwater model: the groundwater flow processes. US Geol Surv Techniques Methods 6-16A, 253 ppGoogle Scholar
- Held I, Wolf L, Eiswirth M, Holtz H (2006) Impacts of sewer leakage on urban groundwater. In: Hellan JH et al. (eds) Urban groundwater management and sustainability. NATO Science Series IV, Springer, Heidelberg, Germany, pp 189–204Google Scholar
- Lu Y, Sun J (1991) An investigation on geomorphology and quaternary depositions for activity of the Shenzhen fault zone, Guangdong province. Seismol Geol 13(2):138–146Google Scholar
- Niswonger RG, Prudic DE, Regan RS (2006) Documentation of the unsaturated zone flow (UZF1 package) for modeling unsaturated flow between the land surface and the water table with MODFLOW-2005. US Geol Techniques Methods, Book 6-19, 62 ppGoogle Scholar
- Niswonger RG, Panday S, Ibaraki M (2011) MODFLOW-NWT, a Newton Formulation for MODFLOW-2005. US Geol Techniques Methods 6-A73, 44 ppGoogle Scholar
- Pang C, Li ZX, Xu YG, Wen S, Krapez B (2016) Climatic and tectonic controls on Late Triassic to Middle Jurassic sedimentation in northeastern Guangdong Province, China. Tectonophysics 677–678:68–87Google Scholar
- Shenzhen Committee (2016) The plan factors and censor for Sponge City in Shenzhen (in Chinese). https://wenku.baidu.com/view/df529c634a35eefdc8d376eeaeaad1f346931198.html. Accessed September 2018
- Sun J, Jian J, Zhan W, Yi S (2007) A study of the tectonic activity of the Shenzhen fault. Adv Earth Sc 22(3):234–240Google Scholar
- WRBSM (2016) Shenzhen Water Resources Bulletin (in Chinese). Water Resources Bureau of Shenzhen Municipality, Shenzhen, China, 18 ppGoogle Scholar
- Xu C, Wang H, Luo Z, Ning J, Liu H (2015) Multilayer stress from gravity and its tectonic implication in urban active fault zone: a case study in Shenzhen, South China. J Appl Geophys 114:174–182Google Scholar
- Yang Z, Yi S, Wang X (2007) Hydrogeological map of Shenzhen, scale 1:50.000. Shenzhen Geological Construction, Shenzhen, ChinaGoogle Scholar
- Yang C, Sun Y, Zhao X (1991) An assessment of the crustal stability of Shenzhen municipality (in Chinese). Geol. Publ. House, Beijing, ChinaGoogle Scholar
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.