Groundwater Complexity in Urban Catchments: Shenzhen, Southern China

Abstract

Groundwater interactions with surface water and sewers in an urban setting are complex, and classic hydrogeological approaches must be combined with anthropogenic elements to characterize them. The level of detail needed to understand these interactions is illustrated by the analysis of an urban subcatchment in the megacity of Shenzhen in southern China that has had a drastic urban expansion in the last 40 years. The study area is characterized by the Yanshanian granite that is widespread across southern-eastern China. The urban setting is studied using multitemporal analysis of satellite images, borehole investigations and field surveys. Given the local hydrostratigraphy, a conceptual model was developed to identify the physical and anthropogenic factors that regulate the urban groundwater system. Based on the conceptual model and the data collected from the field or compiled from the literature, the average annual effective recharge is estimated to be 290 mm/year, after the urbanization process. From rural to urban conditions, it is estimated that the effective recharge increased by 170% and sewers intercept at least 23% of the effective recharge. Groundwater captured by sewers reduces river flows and increases the required capacity and costs for waste water treatment plants.

Hydrogeology of the Pearl River Delta, southern China

“…Mylonithic fault cores and metamorphism limit fissured aquifer extension. Towards the ground surface, subtropical conditions have created a continuous weathered blanket (Chen et al, 2017;Lancia et al, 2019b) comprising sand and silty sand (saprolite) and fractured to laminated bedrock. The weathered blanket constitutes a shallow aquifer distributed along the slope with an average thickness of 30 m. Groundwater flows parallel to the slope towards streams but anthropogenic elements such as roads or building foundations can interrupt the flow (Figure 4 (a)).…”

“…The dataset was interpolated through an ordinary kriging interpolation method (output cell size 0.01 degree; 12 number of points) and contoured with an interval of 50 mm/ year (Main Map). Geological and hydrogeological data are from maps at various scale (GGS, 1979(GGS, , 1981Kirk et al, 2006;Yang et al, 2007), integrated with sketches and drawings from research papers (Huang et al, 2018;Lancia et al, 2019aLancia et al, , 2019bZhang et al, 2019;Zhong et al, 2009). Numerous geological formations were merged into nine hydrogeological units, after detailed analysis and interpretation of the available data, based on local hydrogeological features.…”

“…The blanket is generally thinner when compared to the intrusive rocks. Recharge depends on many factors including the slope acclivity and exposure, occurrence of flow landslide as well as vegetation density (Figure 4(d)) (Lancia et al, 2019b). Along the surface, cataclastic bands (Figure 4(e)) or fracture zones are less developed than in the granite intrusions and the fissured aquifers have a lower transmissivity.…”

“…In the urban areas, pressurized water-supply pipes leak from fractures and cracks recharging the shallow aquifers (Lerner, 2000). On the other hand, urban sewers in free flow condition drain groundwater when the groundwater table close to the ground level (Lancia et al, 2019b). As a result of this favorable condition, leakage represent a minor issue throughout the delta.…”

Hydrochemical Evolution and Nitrogen Behaviors in Coastal Groundwater Suffered From Seawater Intrusion and Anthropogenic Inputs

“…As a strategic water resource and environmental asset, it is urgent to evaluate the water quality and major pollutants in the local aquifer. Although recent studies on the evolution of resources and chemistry in the groundwater have been conducted in Shenzhen city, including the effects of sewers on the amount of groundwater (Lancia et al, 2020), and the groundwater chemistry and its main geochemical factors (Shi et al, 2018), it is still unclear which major biogeochemical processes are likely to contribute to the environmental fate of major pollutants under the impacts of seawater intrusion and anthropogenic activities. Therefore, in this study, hydrogeochemical and isotope techniques were used in the coastal aquifer to (1) characterize the hydrogeochemistry, (2) identify the sources of major and trace elements, and (3) identify the major biogeochemical processes of the hydrochemical evolution suffered from seawater intrusion and anthropogenic disturbance.…”