Control and Prevent Land Subsidence Caused by Foundation Pit Dewatering in a Coastal Lowland Mega City: Indicator Definition, Numerical Simulation and Regression Analysis

Coastal mega cities are often commercial centers because of convenient traffic. Safe elevation above sea level is vital for their sustainable development. Global climate change and sea level rising increase flood risk especially in the lowland subsidence area. Shanghai of China was selected as research background. Although groundwater exploitation had been strictly restrained to control land subsidence and reserve safe elevation, lowering groundwater level during underground excavation cannot be avoided. Foundation pit dewatering (FPD) was intensively performed in underground exploitation during urbanization and city renewal. The FPD settlement accelerated land subsidence. Controlling FPD subsidence was urgent. Normally, the maximum horizontal influence radius of foundation pit excavation was less than three times excavation depth (H), and the 3H settlement was only caused by the FPD. The 3H maximum settlement was defined as the evaluating indicator of FPD land subsidence, and the corresponding 3H drawdown was defined as the control indicator of land subsidence. The FPD conceptual models were established on the basis of estimation and investigation of foundation pit information, including pit area, pit shape, pit depth, and curtain depth. Numerical models were established and a total of 5650 FPD numerical simulations were performed to investigate the land subsidence and FPD drawdown. Multi-factor regression analysis was conducted to obtain relations between land subsidence and FPD drawdown. Regression models were established between the 3H drawdown and the shape, area, depth, and curtain depth of foundation pit on the basis of the numerical simulations. A typical example introduced to verify the regression models. The regression models were used to manage the FPD land subsidence by controlling the 3H FPD drawdown. The results can provide reference for the land subsidence control in a coastal lowland city.

Groundwater exploitation in Awka (Anambra – Nigeria) and environs: Prospects, and challenges while drilling and its mitigation measures

Groundwater exploitation (borehole drilling) was carried out around Awka and environs in Anambra State, Southeastern Nigeria, to understand the underlying rock units encountered while drilling, differentiate boreholes with confined aquifers from those with unconfined aquifers, delineate the probable aquiferous zones from the borehole data, evaluate the challenges encountered while drilling (both geologic and technical), and identify mitigation measures employed to address these challenges. Detailed geologic log information of the boreholes was produced to illustrate the rock units encountered while drilling. Four rock units were identified, namely: shale, sandstone, clay, and gravel. These rock units were exposed within the Imo Formation and the Nanka Formation that underlie the study area. Results from the geologic log information of the boreholes indicate that the water table within the study area ranges from 11.2 m to 56.5 m from the soil surface, and the probable aquiferous zones vary from 6.8 m to 23.3 m in thickness. A detailed look at the lithologic logs of the boreholes show that 50% of the drilled boreholes possess confined aquifers while the remaining 50% have unconfined aquifers. A careful appraisal of the challenges encountered, which are mainly geologic, is strictly attributed to the geologic formation of the study area. Other technical challenges have been derived from mechanical faults developed during drilling.

Geoelectrical Survey for Evaluation of Groundwater Potential Within the Basaltic Terrain of Chikotra River Basin, Maharashtra (India)

Geoelectrical data was acquired using Wenner array over 23 sites with constant electrode separation of 70 m over Chikotra Basin, Dist. Kolhapur, Maharashtra (India). The spatial variation maps of resistivity at depths from 2 to 70 m were plotted using Inverse Distance Weighted (IDW) technique for interpolation in ArcGIS 10.5 to obtain a comprehensive subsurface hydrogeological representation of the study area. High resistivity (>140 Ωm) up to 20m depth, indicative of massive basalts is deciphered towards the NE part of the study area, while the NW sector reveal low resistive (up to 40 Ωm) feature at shallow depths due to fractured basalts, thus conducive for groundwater exploration. Alluvium deposits and columnar jointed basalts in the central part depicts as EW trending conductive (< 30 Ωm) feature suggesting prospective groundwater zone. Low resistivity (6-50 Ωm) from shallow to deeper depths (up to 70m), in the southern region can be identified as potential aquifer system. Longitudinal geoelectric cross-sections are generated over four profiles to identify the lateral and vertical variation in geology and groundwater potential zones. The western and central part of the northern profile (A-A') is highly resistive with resistivity of the order of 80-140 Ωm constituting compact basalts and thus devoid of water. Low resistive zone (30 Ωm) in the eastern part suggests groundwater at shallow depths. Low resistivity zones ranging from 10-50 Ωm is observed at different depth levels over the central profile (B-B') which can be tapped for groundwater exploitation. Several sites over profiles C-C' and the southern-most D-D' suggest promising aquifer zones. Because defining prospective groundwater zones in hard rock terrain is difficult, it’s crucial to look into a river basin’s hydrogeological arrangement early on in the planning process.

Numerical Modeling of Changes In Groundwater Storage And Nitrate Load in The Unconfined Aquifer Near a River Receiving Reclaimed Water

Reclaimed water (RW) has been widely used as an alternative water resource to recharge rivers in mega-city Beijing. At the same time, the RW also recharges the ambient aquifers through riverbank filtration, and modifies the subsurface hydrodynamic system and hydrochemical characteristics. To assess the impact of RW recharge on the unconfined groundwater system, we conducted a 3D groundwater flow and solute transport model based on 10 years of sequenced groundwater monitoring data to analyze the changes of the groundwater table, Cl- loads, and NO3-N loads in the shallow aquifer after RW recharge to the river channel. The results show that the groundwater table around the river channel elevated by about 3~4 m quickly after RW recharge from Dec. 2007 to Dec. 2009, and then remained stable due to the continuous RW infiltration. However, the unconfined groundwater storage still declined overall from 2007 to 2014 due to groundwater exploitation. The storage began to recover after groundwater extraction reduction, rising from 3.76×108 m3 at the end of 2014 to 3.85×108 m3 at the end of 2017. Cl- concentrations varied from 5~75 mg/L before RW recharge to 50~130 mg/L in two years (2007–2009), and then remained stable. The zones of the unconfined groundwater quality-affected by RW infiltration increased from 11.7 km2 in 2008 to 26.7 km2 in 2017. Cl- loads of the unconfined groundwater increased from 1.66×104 t in 2008 to 3.8×103 t in 2017, while NO3-N loads decreased from 29.8 t in 2008 to 11.9 t in 2017 annually in the zones. We determined the maximum area of the unconfined groundwater quality affected by RW, and groundwater outside this area not affected by RW recharge keeps its original state. The RW recharge to the river channel in the study area is beneficial to increase the groundwater table and unconfined groundwater storage with lesser environmental impacts.

Aquifer Susceptibility to Groundwater Pumping in Kediri City, East Java Province, Indonesia

The development of Kediri City in various sectors, such as industry, agriculture, and population growth, also increases water. The utilization of groundwater is still a major mainstay in this area. The utilization of groundwater includes the construction of production wells for irrigation and raw water. The aquifer susceptibility should be considered during groundwater exploitation to minimize a negative impact on the environment. This research aims to analyze the susceptibility of the aquifer to pumping in Kediri City, which is helpful for planning and making decisions in the management of groundwater resources. The determination of aquifer susceptibility is based on aquifer response characteristics, aquifer storage characteristics, allowable subsidence of groundwater level, and depth to the groundwater table. Based on those parameters, it can be concluded that the aquifer susceptibility on groundwater utilization in Kediri City is at moderate and high levels. Areas with moderate aquifer susceptibility are located on the west side of Kediri City, and high aquifer susceptibility is in the middle to the eastern side of Kediri City.

Seawater Intrusion into Coastal Aquifers

This editorial presents a representative collection of 11 papers presented in the Special Issue on Seawater Intrusion into coastal aquifers. Coastal aquifers are one of the most important water resources in the world. In addition, the natural discharge of freshwater to the sea as submarine groundwater discharge (SGD) has an important role in the ecology of marine environments. The dynamics of seawater and freshwater within coastal aquifers are highly sensitive to disturbances, and their inappropriate management may lead to the deterioration of water quality. In many coastal aquifers, seawater intrusion has become the major constraint imposed on groundwater utilization. Groundwater exploitation and climate variations create dynamic conditions, which can significantly increase seawater intrusion into aquifers and may result in the salinization of wells.

Cost minimization of groundwater supply to a central tank

Minimization of groundwater exploitation cost is examined, considering: (a) Pumping from a system of wells up to a central water tank, including friction losses along the connecting pipe network and (b) amortization of network construction. Assuming that the wells are located symmetrically around the tank and directly connected to it, we derived analytically the distance between tank and wells, which minimizes the total cost. Then we compared the minimum cost of this well layout, with that of placing one well at the location of the tank and the rest symmetrically around it. Finally, we dropped any assumption on well layout, we considered that wells are connected to the tank using a minimum spanning tree and we optimized well locations and flow rates using genetic algorithms. For up to 8 wells, the resulting minimum cost is comparable to that of the symmetrical cases, even when the optimal well layout is quite different. Moreover, the analytical solution, derived for the symmetrical case, can serve to evaluate solutions achieved by sophisticated optimization techniques.