Groundwater Depression Cone Research Articles

Variations of Groundwater Quality in the Multi-Layered Aquifer System near the Luanhe River, China

Nitrate pollution is an environmental problem in the North China Plain. This paper investigates the variation of groundwater levels and nitrate concentrations in an alluvial fan of the Luanhe river, northeast of the North China Plain. Three transects perpendicular to the riverbank were selected to investigate the exchange between river water and groundwater, and nitrate concentration with its isotopic composition (δ15N-NO3 and δ18O-NO3). The results showed that the groundwater level decreased slightly during the dry season, and increased regularly during the period of river stage rise. The groundwater is recharged by the river over 10 months each year. The nitrate concentration in the groundwater and river water varied with seasons. The nitrate concentration of groundwater in wells near the river is affected by the river water, which varied in basically the same way as the river. The nitrate concentrations in the zone of groundwater level depression cone were lower than those in the wells near the river, due to the long-term pumping of groundwater. However, the nitrate concentrations of river water have little influence on those of groundwater in wells far from the river. The values of δ15N-NO3 and the relationship between the two isotopes (δ15N-NO3 and δ18O-NO3) suggested that NO3-N was mainly attributable to sewage, livestock manure and natural soil organic matter. Due to the existence of a groundwater depression cone near the river, nitrate contamination can be transported into the aquifer with the flow. The average time lag of nitrate migration from the river to the zone of groundwater level depression cone is different in different sections, which shows an increasing trend from the upstream to downstream along the river, with an average of two to six months. It is mainly related to the stratigraphic structure, the migration distance, the hydraulic conductivities of the aquifer and the riverbed sediment. Compared with the case of considering the silt layer, the time lag of nitrate migration is greater than that of the case of ignoring the silt layer. The results will provide useful information for detecting nitrate concentrations in the alluvial fan area of the Luanhe river, northeast of the NCP (North China Plain).

Controlling factors and driving mechanisms of nitrate contamination in groundwater system of Bandung Basin, Indonesia, deduced by combined use of stable isotope ratios, CFC age dating, and socioeconomic parameters

Number of populations, industry, and economic activities in Indonesia are growing rapidly and these impacts on natural environments raise awareness about water quality issue over the country. Bandung Basin, one of the most growing rapidity urban areas in Indonesia, was assessed for NO3− contamination in groundwater systems, and its controlling factors and driving mechanisms were investigated with the aim to demonstrate novelty on the use of combination of parameters of stable isotope ratios in nitrate (δ15N and δ18O in NO3−), groundwater age (using CFC-12 age tracer), and socioeconomic parameters (land-use, population, and economic database). Groundwater NO3− concentrations at present time did not exceed HWO limit for all the analyzed samples (3.00 mg/L in average with maximum value of 20.69 mg/L, n = 102). Dual stable isotopic analysis together with CFC-12 groundwater age determination suggest that anthropogenic activities are the major causes for increasing NO3− concentrations in groundwater. Those activities under respective land-use are industrial and domestic wastes for urban areas and chemical fertilizers for paddy and plantations areas. In general shallow unconfined aquifer is more vulnerable to NO3− contamination compared with deep confined aquifer because denitrification partly occurs in deep anoxic aquifer and this led attenuation of NO3− pollution in groundwater flowing. However, it seems likely at groundwater depression cones in urban areas that more concentrated waters are transported from shallow aquifer into deep aquifer system through downward vertical fluxes due to excessive pumping. Principal component analysis (PCA) on NO3− concentrations with socioeconomic parameters indicated that industrial and population growths are the main factors related to groundwater NO3− contamination. This result corresponds to CFC-age dating which shows younger (more recently recharged) groundwaters as being more contaminated than older ones do. Our study implies that NO3− contamination in this area may become more severe in future with a lack of necessary controls and treatment for human-induced nitrogen sources. Proposed approach is useful to understand how the NO3− contaminant behaves in large basin aquifer system under urban environments and might be applicable in other developing regions too because increasing populations may be associated with increasing nitrogen loadings.

Hydrogeochemical Evolution of an Ordovician Limestone Aquifer Influenced by Coal Mining: A Case Study in the Hancheng Mining Area, China

Statistical analysis was used to study the hydrogeochemical evolution of an Ordovician limestone aquifer group in the Hancheng mining area. Before mining, the groundwater flowed from northwest to southeast, the water type was primarily SO4–HCO3, and was mainly controlled by the tectonic structure and the specific hydrogeological conditions. After 40 years of mining, two large groundwater depression cones had formed, centered on the Sangshuping and Xiangshan coal mines in the north and south zones, respectively. The groundwater dropped by ≈ 20 m in the center of the depression cones due to over-exploration and mine water inrush, which changed the groundwater flow field significantly. Both the total dissolved solids and the concentrations of major ions increased 2.3- to 4.7-fold, and the water type changed to SO4–Cl. The saturation indices (SI) of the minerals along the two simulated paths both increased, indicating that the groundwater would dissolve minerals as it flowed, which verified the groundwater flow field. Groundwater quality deteriorated due to a mixture of old acidic pit water and hypersaline water intruding from the deep district. When studied vertically, the concentrations of major ions and SI of calcite and limestone increased, due to the limited cycling of water from shallow to deep. The coincidental Ca2+ and Mg2+ increases were caused by calcite-replacing dolomitization reactions. To summarize, long-term coal mining adversely affected the area’s groundwater flow field and hydrogeochemical evolution, and effective action should be taken to prevent the Ordovician groundwater from continuing to deteriorate.

Vertical Displacements Driven by Groundwater Storage Changes in the North China Plain Detected by GPS Observations

The North China Plain (NCP) has been experiencing the most severe groundwater depletion in China, leading to a broad region of vertical motions of the Earth’s surface. This paper explores the seasonal and linear trend variations of surface vertical displacements caused by the groundwater changes in NCP from 2009 to 2013 using Global Positioning System (GPS) and Gravity Recovery and Climate Experiment (GRACE) techniques. Results show that the peak-to-peak amplitude of GPS-derived annual variation is about 3.7~6.0 mm and is highly correlated (R > 0.6 for most selected GPS stations) with results from GRACE, which would confirm that the vertical displacements of continuous GPS (CGPS) stations are mainly caused by groundwater storage (GWS) changes in NCP, since GWS is the dominant component of total water storage (TWS) anomalies in this area. The linear trends of selected bedrock-located IGS CGPS stations reveal the distinct GWS changes in period of 2009–2010 (decrease) and 2011–2013 (rebound), which are consistent with results from GRACE-derived GWS anomalies and in situ GWS observations. This result implies that the rate of groundwater depletion in NCP has slowed in recent years. The impacts of geological condition (bedrock or sediment) of CGPS stations to their results are also investigated in this study. Contrasted with the slight linear rates (−0.69~1.5 mm/a) of bedrock-located CGPS stations, the linear rates of sediment-located CGPS stations are between −44 mm/a and −17 mm/a. It is due to the opposite vertical displacements induced by the Earth surface’s porous and elastic response to groundwater depletion. Besides, the distinct renewal characteristics of shallow and deep groundwater in NCP are discussed. The GPS-based vertical displacement time series, to some extent, can reflect the quicker recovery of shallow unconfined groundwater than the deep confined groundwater in NCP; through one month earlier to attain the maximum height for CGPS stations nearby shallow groundwater depression cones than those nearby deep groundwater depression cones.

Predicting the impact of heavy groundwater pumping on groundwater and ecological environment in the Subei Lake basin, Ordos energy base, Northwestern China

Abstract Groundwater is increasingly exploited for energy production in arid areas globally, which will inevitably disrupt the natural equilibrium of groundwater and the ecological environment. A groundwater flow model for Subei Lake basin, Ordos energy base, was developed and calibrated to predict groundwater levels' variation and the impact of heavy groundwater pumping on the ecological environment for the period 2010–2039 under two different pumping scenarios. Results showed that rainfall infiltration and groundwater evapotranspiration were the major source/sink terms for the groundwater system. The obvious groundwater depression cone will be formed in the production field at the end of 30 years and the maximum drawdown will be 11.70 m if the waterworks maintains the present situation. However, recovery of groundwater level will be obvious and the groundwater depression cone will disappear as a result of the implementation of the water diversion project. The increased volume of groundwater pumping between the two scenarios was derived from storage depletion, the activated lateral inflow, the captured groundwater evapotranspiration, lateral outflow and discharge into Subei Lake. Groundwater pumping from Haolebaoji waterworks has caused the decline of the Subei Lake and the noticeable degradation of phreatophyte.

Changes in flow and chemistry of groundwater heavily affected by human impacts in the Baiyangdian catchment of the North China Plain

To identify impacts of air pollution, sewage drainage, agricultural production, over-pumping and reservoir storage on groundwater, a field survey was conducted in the Baiyangdian catchment of the North China Plain. Major ions and water isotopes were measured. Results show that hydrological processes and hydrogeochemical evolution of shallow groundwater were greatly disturbed by human activities. Excessive pumping resulted in significant declines of groundwater levels over the study area. This also induced infiltration of surface water into groundwater. A groundwater depression cone was the conflux center of groundwater surrounded by recharge zones including alluvial fans and surface water in alluvial plain. Pumping almost was the only way to discharge groundwater. Emission of SO x and NO x contributed at least 11% of rock weathering by dissolving into infiltrating precipitation. Surface waters containing sewage replenished ambient groundwater with an average mixing ratio of 74 ± 17% due to groundwater level drawdown. As a result, groundwater had elevated concentrations of Na+ and SO4 2− with Na+ exchanged into aquifer sediments. About 29 ± 16% of Na+ was exchanged from groundwater into soil matrix. Agriculture nitrate was high only in the recharge zones. The most important result is that the transformation of the study area from a place rich in water resource into an area lack of water just took several decades with the joint action of the heavily human activities. Our study also indicates that shallow groundwater could sensitively respond to and record environmental changes.

Identifying the impact of Energy Base Water Project on groundwater using high-frequency monitoring data in the Subei Lake basin, Ordos, Northwestern China

Groundwater is increasingly exploited for energy production in arid regions, which necessitates a deeper insight into the impact of the enhanced human pressure on the groundwater. This study applied an integrated method (statistical analysis, water table fluctuation method, hydrograph analysis and remote sensing) to identify the impact of Energy Base Water Project on the groundwater in the Subei Lake basin. Groundwater levels in eight observation wells at 30 min intervals during the 2013–2014 period were monitored using automatic groundwater monitoring data loggers. Results showed that precipitation infiltration, irrigation return flow, groundwater pumping and evapotranspiration controlled the hydrodynamics of unconfined groundwater. The average evapotranspiration rates in the Quaternary phreatic aquifer and the Cretaceous phreatic aquifer were 6.15 and 12.48 mm/d. The unusual hourly hydrographs fall into three patterns (mutational, irregular and gradual hydrographs). Different recovery times after being influenced by pumping may be related to the presence of the mudstone lenses. The extent of the groundwater depression cone was qualitatively identified by gradual hydrographs, which may spread from the center area to the western boundary. Only some individual wells from Haolebaoji waterworks had conducted the intermittent pumping activities at random times and caused the decline of the lakes.

Determining the area of influence of depression cone in the vicinity of lignite mine by means of triangle method and LANDSAT TM/ETM+ satellite images

Problems with lowering of water table are common all over the world. Intensive pumping of water from aquifers for consumption, irrigation, industrial or mining purposes often causes groundwater depletion and results in the formation of cone of depression. This can severely decrease water pressure, even over vast areas, and can create severe problems such as degradation of agriculture or natural environment sometimes depriving people and animals of water supply. In this paper, the authors present a method for determining the area of influence of a groundwater depression cone resulting from prolonged drainage, by means of satellite images in optical, near infrared and thermal infrared bands from TM sensor (Thematic Mapper) and ETM+ sensor (Enhanced Thematic Mapper +) placed on Landsat 5 and Landsat 7 satellites. The research area was Szczercowska Valley (Pol. Kotlina Szczercowska), Central Poland, located within a range of influence of a groundwater drainage system of the lignite coal mine in Belchatow. It is the biggest lignite coal mine in Poland and one of the largest in Europe exerting an enormous impact on the environment. The main method of satellite data analysis for determining soil moisture, was the so-called triangle method. This method, based on TVDI (Temperature Vegetation Dryness Index) was supported by additional spatial analysis including ordinary kriging used in order to combine fragmentary information obtained from areas covered by meadows. The results obtained are encouraging and confirm the usefulness of the triangle method not only for soil moisture determination but also for assessment of the temporal and spatial changes in the area influenced by the groundwater depression cone. The range of impact of the groundwater depression cone determined by means of above-described remote sensing analysis shows good agreement with that determined by ground measurements. The developed satellite method is much faster and cheaper than in-situ measurements, and allows for systematic monitoring of the vast area in the vicinity of Belchatow lignite mine. Besides, this method could be useful as a helper in in-situ measurement allowing a significant reduction of the number of in-situ measurements by performing them only within problematic areas. Hence, the triangle method can be used as an effective supplement to field measurements. Although the research area is located in Poland, in the vicinity of lignite mine, the method of observation of depression cones provided in this study is universal and effective, and therefore could also be useful to an international audience.

Land subsidence due to groundwater withdrawal in the northern Beijing plain, China

Beijing is an international metropolis, where over-exploration of water resource makes land subsidence becoming more and more serious. The related problems cannot be avoided in the coming years because of the giant increase of population. The aims of this study are to quantify land subsidence over the period 2003 to 2010, grasp the evolution of the process, and investigate the relation with the triggering factors in the northern area of the Beijing plain. Various data, including deep compaction from vertical multiple borehole extensometers, land subsidence from Persistent Scatterer Interferometry and leveling surveys, groundwater levels, hydrogeological setting from wellbores, and Landsat TM image were collected and effectively used to detect the spatial and temporal features of land subsidence and its possible relation with groundwater level changes, compressible layer thickness, and urban development. Results show that land subsidence is unevenly distributed and continuously increased from 2003 to 2010. The average loss of elevation over the monitoring period amounted to 92.5mm, with rates up to 52mm/y. The distribution of the subsidence bowl is only partially consistent with that of the groundwater depression cone because of the variable thickness of the most compressible fine deposits. In fact, extensometers reveal that silty-clay layers account for the larger contribution to land subsidence, with the 15m thick silty-clay layer between 102 and 117m depth accounting for about 25% of the total subsidence. Finally, no clear correlation has been observed between the subsidence rates and the increase of the load on the land surface connected to the impressive urban development. This study represents a first step toward the development of a physically-based model of the subsidence occurrence to be used for planning remediation strategies in the northern Beijing plain.

Risk Assessment and Prediction of Heavy Metal Pollution in Groundwater and River Sediment: A Case Study of a Typical Agricultural Irrigation Area in Northeast China.

The areas with typical municipal sewage discharge river and irrigation water function were selected as study sites in northeast China. The samples from groundwater and river sediment in this area were collected for the concentrations and forms of heavy metals (Cr(VI), Cd, As, and Pb) analysis. The risk assessment of heavy metal pollution was conducted based on single-factor pollution index (I) and Nemerow pollution index (NI). The results showed that only one groundwater sampling site reached a polluted level of heavy metals. There was a high potential ecological risk of Cd on the N21-2 sampling site in river sediment. The morphological analysis results of heavy metals in sediment showed that the release of heavy metals can be inferred as one of the main pollution sources of groundwater. In addition, the changes in the concentration and migration scope of As were predicted by using the Groundwater Modeling System (GMS). The predicted results showed that As will migrate downstream in the next decade, and the changing trend of As polluted areas was changed with As content districts because of some pump wells downstream to form groundwater depression cone, which made the solute transfer upstream.

Evolution characteristics and influence factors of deep groundwater depression cone in North China Plain, China—A case study in Cangzhou region

The North China Plain (NCP) is one of the global hotspots of groundwater depletion, groundwater is almost the only source of water for agricultural, industrial and drinking water in this region. After long-term’s over-exploitation of deep groundwater, there appeared several deep groundwater depression cones, such as Cangzhou cone, Dezhou cone, Hengshui cone, Tianjin cone, etc., in which the Cangzhou cone is one of the typical cones for its special geography and hydrogeology conditions. In this study, the authors intended to analyze the evolution characteristics and influence factors of deep groundwater depression cone in Cangzhou region, especially the No. III aquifer depression cone, which is the main exploitation zone in this region. Analysis of the evolution of the groundwater depression cone of the No. III aquifer group in Cangzhou region showed that this process can be divided into four stages, namely, development, stable development, rapid expansion, and gradual recovery. The shape and evolution characteristics of the depression cone at different stages are described by analyzing the evolution of the −30, −40, and −50 contours of the groundwater table, for example the closed area of water table contour of −50 m has been enlarged from 95 km2 in 1985 to 6 528.5 km2 in 2005. The dominant factors that affect the evolution characteristics at different stages are proposed. The results showed that relatively long dry periods with less precipitation, special geological and hydrogeological conditions, and sharply increased water consumption for industrial and agricultural development are the main factors that cause the formation of deep groundwater depression cones. Meanwhile, an environmental response against groundwater exploitation is presented, and rational solutions are suggested to avert water crisis.

Potential impacts of climate change and regional anthropogenic activities in Central European mesoscale catchments

The Soil and Water Integrated Model (SWIM) was used to assess potential climate and land-use change impacts in the Central European catchments of Schwarze Elster, Spree and Lusatian Neisse which are heavily influenced by opencast lignite mining. To account for potential climate change, scenarios of two statistical regional climate models, STAR and WettReg, were used. Regional anthropogenic change was considered in terms of increasing cultivation of energy crops (oilseed rape, silage maize, sunflower and sorghum) and decreasing mining activities (decreasing groundwater depression cone). In the climate scenarios, decreased natural discharge, by up to 60% in the long-term average, was simulated. In simulations with climate scenarios and oilseed rape, this effect is halved; the other energy crops have a small additional impact on discharge. The decreasing groundwater depression cone slightly compensates for climate change impacts. Overall, potential impacts of regional anthropogenic activities are secondary to those of climate change.Editor Z.W. Kundzewicz; Guest editor M. White

Upward recharge through groundwater depression cone in piedmont plain of North China Plain

summary Whether a recharge was induced by groundwater depression cones is a crucial issue for water resource management. In the North China Plain, shallow groundwater had been over-pumped since 1970s and many groundwater depression cones formed. The groundwater depression cone, Daceying–Machang, occurred even in the piedmont plain. In the area, water levels of deep and shallow groundwater were observed since 2005 and field survey was conducted in the dry season 2010. The upward recharge induced by the depression cone is verified based on water level records, major ions, 2 H, 18 O and kinds of statistical analyses. Since August 2006, the water level of the deep groundwater ascended by 1.9 mm/d. High correlations (r = 0.86, s = 0.67) between the water level series of shallow and deep groundwater were found by two distinct correlation analyses only in the center of depression cone. Further, the reversion of hydraulic gradient of the depression cone occurred in dry seasons since September 2008. Hydro-geochemical features of the shallow groundwater are consistent with deep groundwater in the center of depression cone, which was demonstrated by the fuzzy C-means clustering based on principal components and paired t test, respectively. It is concluded that the deep groundwater recharged the shallow groundwater from the center of the depression cone. As a result, the groundwater mixture occurred that improves the quality of the shallow groundwater. Seasonally changed flow of shallow groundwater enhanced the mixture. The persistent overpumping of shallow groundwater and the large elevation difference (around 1200 m) between the recharge zone and the discharge point of deep groundwater facilitate the upward recharge.

Groundwater responses to controlled water releases in the limitrophe region of the Colorado River: Implications for management and restoration

Flow regulation and water diversion for irrigation have considerably affected the exchange of surface water between the Colorado River and its floodplains. However, the way in which both have impacted groundwater–surface water interactions is not completely understood. The objective of this study was to conduct a hydrologic analysis of the 2009–2010 winter flows released into the limitrophe region of the Colorado River in order to characterize the surface flows along this dry reach and, for the first time, study the impact of winter flows on the groundwater conditions in the area. The study used existing data on groundwater levels that was collected from regional piezometers on both sides of the limitrophe every five years from 1980 to 2005. Regional flow direction from NE to SW was observed in all years. A groundwater depression cone in the southwest part of the limitrophe was identified from 1980 to 1995. A general rise of groundwater levels was observed from 2000 to 2005 on both sides of the limitrophe, but during the same time period, a depression cone formed along the border between Arizona and Sonora, in the Mesa Arenosa on the Mexican side of the border. In order to identify the water table evolution within the limitrophe riparian zone, nine sets of piezometers were constructed in an arrangement perpendicular to the main river channel. Water table levels were measured automatically every 30min and were also manually measured periodically. Nine geohydrological cross sections were constructed using the topographic relief from LIDAR elevation points and depth to groundwater measurements during water discharges in the Colorado River channel at Morelos dam. Groundwater seepage from irrigation canals, irrigation returns, and river discharge flows were identified and the depth to groundwater and its influence on riparian vegetation was analyzed. A strong correlation between flow discharge (up to 60.49hm3 from November 2009 to April 2010) and groundwater elevation (average elevation change of 1.62m on January 22, 2010) in time and space was found. The percentage of water retained in the main river channel decreased from 100% in the first discharge event (December 12–13, 2009) to 36% after the last discharge event (April 9–17, 2010), due to remaining moisture in the unsaturated soil. The total volume of water retained, infiltrated, and evaporated, was 60% (36.6×106m3) of the total water discharged. The delay time of the groundwater front during a discharge event was on average 6:30, 20:06, and 28:53h from section 1 to section 2, 1–3, and 1–4 respectively. This historical study provides insight into how floods affect the groundwater system, which is the foundation for aquatic and riparian biodiversity. This issue is of increasing relevance given growing international interest in rehabilitating the riparian and aquatic ecosystems of the Colorado River delta through intentional flood releases.

Characterization and mechanism of regional land subsidence in the Yellow River Delta, China

In this paper, we discuss historical and recent land subsidence in the Modern Yellow River Delta. Integrated analysis of leveling and relevant background data, including groundwater level, oil extraction, and geological structure, has revealed that land displacement is driven by natural and induced components acting at various depths. Since the 1950s, intense settlements occurred in the modern estuary delta lobes. Between 2002 and 2008, the subsidence center of Dongying and Guangrao exhibited a typical subsidence area with subsidence rates of 28.2 and 64.7 mm/years, respectively. Higher magnitudes are associated with groundwater withdrawals and oil-gas field exploitations, which induce the compaction of a deep clayey layer. There existed a significant linear positive correlation between groundwater level and elevation in the center of the deep groundwater depression cone. The major contributor of natural subsidence is tectonic movements, while moderate sinking due to the natural consolidation of the recent delta subsoil is still acting.

Hydrogeochemical and isotopic evidences of the groundwater regime in Datong Basin, Northern China

Datong Basin is one of the Cenozoic faulted basins in Northern China’s Shanxi province, where groundwater is the major source of water supply. The results of hydrochemical investigation show that along the groundwater flow path, from the margins to the lower-lying central parts of the basin, groundwater generally shows increases in concentrations of TDS, HCO3−, SO42−, Cl−, Na+ and Mg2+ (except for Ca2+ content). Along the basin margin, groundwater is dominantly of Ca–HCO3 type; however, in the central parts of the basin it becomes more saline with Na–HCO3-dominant or mixed-ion type. The medium-deep groundwater has chemical compositions similar to those of shallow groundwater, except for the local area affected by human activity. From the mountain front to the basin area, shallow groundwater concentrations of major ions increase and are commonly higher than those in medium-deep aquifers, due to intense evapotranspiration and anthropogenic contamination. Hydrolysis of aluminosilicate and silicate minerals, cation exchange and evaporation are prevailing geochemical processes occurring in the aquifers at Datong Basin. The isotopic compositions indicate that meteoric water is the main source of groundwater recharge. Evaporation is the major way of discharge of shallow groundwater. The groundwater in medium-deep aquifers may be related to regional recharges of rainwater by infiltrating along the mountain front faults, and of groundwater permeating laterally from bedrocks of the mountain range. However, in areas of groundwater depression cones, groundwater in the deep confined aquifers may be recharged by groundwater from the upper unconfined aquifer through aquitards.

Impact of artificial recharge on water quality in groundwater depression cone

More than 100 regional groundwater depression cones have developed because of groundwater over-exploitation in recent years in China. The total area of the cones is nearly 15 × 104 km2 and the Daqing groundwater depression cone considered in this paper is the second largest. In order to control groundwater level and its related environmental side effects, artificial aquifer recharge (AR) will be implemented in the Daqing groundwater depression cone. Recharge water imported from the River Nenjiang will interact with native groundwater and the aquifer matrix during the process of artificial aquifer recharge and this will influence groundwater quality and affect the groundwater environment in the region. The interaction between recharge water, native groundwater and aquifer matrix was forecast by groundwater geochemistry modelling. The modelled results show a tendency of water quality change in aquifer, which is helpful to identify any health risks and assess the feasibility of artificial recharge in this region of China.

Hydrogeochemical characteristics of groundwater depression cones in Yinchuan City, Northwest China

Groundwater in Yinchuan City has been heavily over-exploited, thus leading to the formation of depression cones in confined and phreatic groundwater environments. The depression cones have an important influence on the hydrodynamic and hydrochemical fields of groundwaters. The evolution of depression cones was analyzed on the basis of the monitoring data on groundwater level accumulated in the past 14 years. The ratio of rCl−/rCa2+ showed that phreatic water circulation was intensified, and confined groundwater was affected by external factors. Mass balance of Cl− showed confined water mixed with about 11% phreatic water. It is shown that the alternative function of confined water was affected by external factors. At last, the evolution of groundwater hydrochemical field on the basis of groundwater chemical composition showed that phreatic water quality has been improved whereas confined water quality has been deteriorated. Saturation indices of minerals with respect to phreatic and confined waters were calculated by using PHREEQC.

The effect of stratigraphic heterogeneity on areal distribution of land subsidence at Taiyuan, northern China

Taiyuan city has experienced serious land subsidence since the 1950s, with the maximum accumulated subsidence of 2,960 mm and a total affected area of 585 km2 by 2003. Tectonic settlement was found to contribute about 1% and the major cause is over-exploitation of groundwater. The spatial–temporal relationship between the areal distribution of land subsidence and that of groundwater drawdown from 1956 to 2000 indicates that although land subsidence centers overall match groundwater depression cones, there are local deviations, and that although the time series curves are similar, land subsidence always lags behind groundwater level decline, with different lag time at different sites. The major findings of this work on the control of stratigraphic heterogeneity on the spatial pattern of land subsidence at Taiyuan include: (1) land subsidence centers shift from the corresponding groundwater depression cones to the sides with thicker accumulated clay layers; (2) under the same pumping rate, land subsidence at places with more clay interlayers and thinner individual interlayers is greater and the lag time shorter; and (3) land subsidence is closely related to the physical properties of clay soils. The Interbed Storage Package-1, a modular subroutine of MODFLOW was employed to simulate the areal distribution of individual layer compression. The modeling results show that compression of different clay layers has different contribution to land subsidence. Pumping groundwater from water-bearing zones close to the most compressible clay layers should therefore be carefully controlled.