Land subsidence is common in some regions of China. Various eco-environmental problems have arisen due to changes in water–rock interactions in these subsided areas, for which a comprehensive understanding of the hydrogeological setting is needed. This paper presents the general status of land subsidence in three typical subsided areas of China through the compilation of relevant data, and reviews some typical changes in the water–rock interactions in subsided areas along with related eco-environmental issues. It is found that the subsidence development and distribution are controlled by the groundwater-withdrawal intensity externally, and by the thickness and compressibility of unconsolidated sediments internally. The physical changes and related effects of water–rock interactions in subsided areas include: (1) the decreased ground elevation that caused floods, waterlogged farmland, etc.; (2) the differential subsidence that caused ground fissures; and (3) the change of seepage field that caused substantial reduction of the water resource. Chemically, the changes and related effects of water–rock interactions include: (1) the change to the chemical environment or processes due to the hydrogeologic structure alteration, which caused groundwater pollution; and (2) hydrologic mixing (seawater intrusion, artificial recharge; exchange with adjacent aquifers or aquitards), which degraded the groundwater quality. Further research on the subsided areas in China is suggested to reveal the mechanisms regarding biological and gaseous (meteorological) changes from the perspective of interacting systems among water, rocks, biological agents and gases.

Over-exploited groundwater is expected to remain the predominant source of domestic water in suburban areas of Hanoi, Vietnam. In order to evaluate the effect on groundwater recharge, of decreasing surface-water bodies and land-use change caused by urbanization, the relevant groundwater systems and recharge pathways must be characterized in detail. To this end, water levels and water quality were monitored for 3 years regarding groundwater and adjacent surface-water bodies, at two typical suburban sites in Hanoi. Stable isotope (δ¹⁸O, δD of water) analysis and hydrochemical analysis showed that the water from both aquifers and aquitards, including the groundwater obtained from both the monitoring wells and the neighboring household tubewells, was largely derived from evaporation-affected surface-water bodies (e.g., ponds, irrigated farmlands) rather than from rivers. The water-level monitoring results suggested distinct local-scale flow systems for both a Holocene unconfined aquifer (HUA) and Pleistocene confined aquifer (PCA). That is, in the case of the HUA, lateral recharge through the aquifer from neighboring ponds and/or irrigated farmlands appeared to be dominant, rather than recharge by vertical rainwater infiltration. In the case of the PCA, recharge by the above-lying HUA, through areas where the aquitard separating the two aquifers was relatively thin or nonexistent, was suggested. As the decrease in the local surface-water bodies will likely reduce the groundwater recharge, maintaining and enhancing this recharge (through preservation of the surface-water bodies) is considered as essential for the sustainable use of groundwater in the area.

The aim was to study density-driven groundwater flow and analyse groundwater mixing because of seasonal changes in groundwater temperature. Here, density-driven convection in groundwater was studied by numerical simulations in a subarctic climate, i.e. where the water temperature was <4 °C. The effects of soil permeability and groundwater temperature (i.e. viscosity and density) were determined. The influence of impermeable obstacles in otherwise homogeneous ground was also studied. An initial disturbance in the form of a horizontal groundwater flow was necessary to start the convection. Transient solutions describe the development of convective cells in the groundwater and it took 22 days before fully developed convection patterns were formed. The thermal convection reached a maximum depth of 1.0 m in soil of low permeability (2.71 · 10⁻⁹ m²). At groundwater temperature close to its density maximum (4 °C), the physical size (in m) of the convection cells was reduced. Small stones or frost lenses in the ground slightly affect the convective flow, while larger obstacles change the size and shape of the convection cells. Performed simulations show that “seasonal groundwater turnover” occurs. This knowledge may be useful in the prevention of nutrient leakage to underlying groundwater from soils, especially in agricultural areas where no natural vertical groundwater flow is evident. An application in northern Sweden is discussed.

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.

As in many places, groundwater in California (USA) is the major alternative water source for agriculture during drought, so groundwater’s availability will drive some inevitable changes in the state’s water management. Currently, agricultural, environmental, and urban uses compete for groundwater, resulting in substantial overdraft in dry years with lowering of water tables, which in turn increases pumping costs and reduces groundwater pumping capacity. In this study, SWAP (an economic model of agricultural production and water use in California) and C2VISim (the California Department of Water Resources groundwater model for California’s Central Valley) are connected. This paper examines the economic costs of pumping replacement groundwater during drought and the potential loss of pumping capacity as groundwater levels drop. A scenario of three additional drought years continuing from 2014 show lower water tables in California’s Central Valley and loss of pumping capacity. Places without access to groundwater and with uncertain surface-water deliveries during drought are the most economically vulnerable in terms of crop revenues, employment and household income. This is particularly true for Tulare Lake Basin, which relies heavily on water imported from the Sacramento-San Joaquin Delta. Remote-sensing estimates of idle agricultural land between 2012 and 2014 confirm this finding. Results also point to the potential of a portfolio approach for agriculture, in which crop mixing and conservation practices have substantial roles.

The aquifer beneath Chengdu Plain is a main water resource for drinking and industry, which has severe groundwater nitrate contamination due to the shallow water table and intensive agricultural activities. The aim of this study is to estimate nitrogen loading to the aquifer and to predict nitrate concentration in groundwater in response to changes in land-use and agricultural practices, i.e. nitrogen fertilizer application, in ‘Jingyang region’ of the Chengdu Plain, one of the most developed agriculture areas in southwestern China. The framework utilized a geographic information system to account for the spatial and temporal distribution of on-ground nitrogen sources and corresponding loadings, and employed an export coefficient model to estimate the corresponding nitrogen loss. The simulation of nitrate fate and transport in the aquifer was conducted by developing a groundwater mass transport model using MODFLOW and MT3DMS. A number of predictive simulations for year 2026 were carried out to evaluate the impact of land-use and proposed future management options on controlling groundwater nitrate contamination. The results show that a nitrate plume would extend over most of the study area with some local areas having concentrations exceeding 20 mg L⁻¹ under the current setting and the scenario of increasing agricultural land use. Reduction in fertilizer application rates did mitigate nitrate contamination; however, this effect diminished with the expansion of agricultural land use. Land-use planning should consider the significant effects of agriculture to improve the quality of groundwater in the Chengdu Plain.

Nitrate is found in groundwater due to natural and anthropic processes. Nitrate content in groundwater is associated with factors such as human activities, soil type, climate, geology and chemistry of groundwater. Some of these factors (climate and geology) coincide with those that determine the type of groundwater flow system (local, intermediate or regional) present in an area which, in turn, is influenced by climate, stratigraphy, and type of subsoil and surface rocks; therefore, it is expected that the concentration of nitrate is related to the type of groundwater flow. The relationship between the concentration of nitrate in groundwater samples and the type of flow was analyzed in an aquifer system located in the Trans-Mexican Volcanic Arc, within the Michoacán-Guanajuato volcanic complex. The system is composed of two hydrogeological units, one volcanic and the other sedimentary, with the presence of geological faults, in a context where there is agricultural activity and deficient domestic wastewater management. To improve understanding of the overall aquifer system, 34 groundwater samples (28 wells, 6 springs) were analyzed. The results indicate that each flow system presents characteristic patterns of nitrate concentration and groundwater chemical composition. A high nitrate concentration was found in local and local-intermediate flow systems. Nitrate concentration decreased from local to intermediate and regional flows. The nitrate concentration decreased depending on groundwater flow direction, so it is possible to use nitrate as a parameter to differentiate groundwater flow systems.