Unconfined Groundwater System Research Articles

Seasonal Groundwater Level Dynamics in Unconfined Aquifers across the United States

AbstractGroundwater hydrographs contain a rich set of information on the dynamics of aquifer systems and the processes and properties that influence them. While the importance of seasonal cycles in hydrologic and environmental state variables is widely recognized there has yet to be a comprehensive analysis of the seasonal dynamics of groundwater across the United States. Here we use time series of groundwater level measurements from 997 wells from the National Groundwater Monitoring Network to identify and describe groundwater seasonal cycles in unconfined aquifers across the United States. We use functional data analysis to obtain a functional form fit for each site and apply an unsupervised clustering algorithm to identify a set of five distinct seasonal cycles regimes. Each seasonal cycle regime has a distinctive shape and distinct timing of its annual minimum and maximum water level. There are clear spatial patterns in the occurrence of each seasonal cycle regime, with the relative occurrence of each regime strongly influenced by the geologic setting (aquifer system), climate, and topography. Our findings provide a comprehensive characterization of groundwater seasonal cycles across much of the United States and present both a methodology and results useful for assessing and understanding unconfined groundwater systems.

Coral Reveals a Transition in Sources of Seawater Vanadium From Submarine Groundwater Discharge to Upwelling in the Northern South China Sea

AbstractIn recent decades, vanadium (V) levels in the environment have dramatically increased and will continue to rise alarmingly in the future. Conventional methods for assessing oceanic V typically only involved the collection of seawater and sediments, which significantly limited our understanding of the dynamics of oceanic V and its relationship to environment and climate. Here, the interannual temporal variations in surface seawater V levels in the northern South China Sea, were reconstructed utilizing a 137‐year record of V/Ca ratios from Porites lutea coral from Xiaodonghai Bay off Sanya. We observed that anthropogenic activities had a limited impact on the V levels of surface seawater in the study area, whereas V variability exhibited significant disparities across different time periods. Precipitation‐related submarine groundwater discharge (SGD) recharge was the primary driver of V variation prior to 1990, with tropical cyclones (TCs) playing a detectable role in total precipitation during this period. Conversely, changes in surface seawater V levels after 1990 were predominantly linked to increasingly intense upwelling driven by summer winds. The recharge processes in unconfined groundwater systems within the study area might generate favorable oxic and alkaline conditions, which facilitated mineral dissolution and/or desorption reactions that elevated V concentrations along the groundwater flow path, which were recorded in coral skeletal V/Ca ratios. This study confirmed the seawater V was dominated by SGD and upwelling near Sanya, providing essential evidence for comprehending the geochemical behavior of V and the impact of ocean‐atmospheric climate interactions on long‐term variations in seawater V levels.

A comparative study of DNAPL migration and transformation in confined and unconfined groundwater systems

To explore the migration and transformation process of dense non-aqueous liquid (DNAPL) pollutants’ multiphase flow, specifically nitrobenzene (NB), in confined groundwater (CG) versus unconfined groundwater (UG), a two-dimensional sandbox experimental device was designed and constructed. This involved constructing a vadose zone-UG- aquitard-CG structure, which was then subjected to different scenarios. Real-time analysis and numerical simulation methods were established and employed, with a particular focus on the detailed investigation results of actual contaminated site. The study found that when the same amount of NB was injected, the special structure of the CG layer resulted in a more pronounced reverse diffusion of NB in both the dissolved and NAPL phases. This was especially true for the dissolved phase, which was more likely to diffuse reversely. Meanwhile, CG did not directly interact with the vadose zone, and there was no loss of gas phase NB after the leakage in CG. As a result, higher concentrations of dissolved phase NB were generated, leading to the emergence of a larger area of NB contaminant plumes with CG flow. Importantly, the simulation study of the actual site and the laboratory experimental results were found to be validated, further validating the conclusion that direct leakage of NB into CG results in a higher concentration and larger area of dissolved phase contaminant plume, causing more serious pollution to the groundwater environment.

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 10years 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 3-4m 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 5-75mg/L before RW recharge to 50-130mg/L in 2years (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 in the zone increased from 1.8×103t 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. 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.

Application of Factor Analysis for Characterizing the Relationships between Groundwater Quality and Land Use in Taiwan’s Pingtung Plain

Although the average municipal water coverage in Taiwan is quite high, at 93.91%, only around half of the residents in the Pingtung Plain use tap water originating from the Taiwan Water Corporation to meet their needs. This means the exploitation of a substantial amount of groundwater as a source of water to meet drinking, agriculture, aquaculture, and industry requirements. Long-term groundwater quality surveys in Taiwan have revealed obvious contamination of the groundwater in several locations in the Pingtung Plain, with measured concentration levels of some groundwater quality parameters in excess of the permissible levels specified by the Taiwan Environmental Protection Administration. Clearly, establishing a sound plan for groundwater quality protection in this area is imperative for maximizing the protection of human health. The inappropriate use of hazardous chemicals and poor management of land use have allowed pollutants to permeate through unsaturated soil and ultimately reach the underlying shallow unconfined groundwater system. Thus, the quality of the water stored in shallow aquifers has been significantly affected by land use. This study is designed to characterize the relationship between groundwater quality and land use in the Pingtung Plain. This goal is achieved by the application of factor analysis to characterize the measured concentrations of 14 groundwater quality parameters sampled from 46 observation wells, the area percentages for nine land use categories in the neighborhood of these 46 observation wells, and the thicknesses of four unsaturated types of soil based on core samples obtained during the establishment of 46 observation wells. The results show that a four-factor model can explain 56% of the total variance. Factor 1 (seawater salinization), which includes the groundwater quality parameters of EC, SO42−, Cl−, Ca2+, Mg2+, Na+, and K+, shows a moderate correlation to land used for water conservation. Factor 2 (nitrate pollution), which includes the groundwater quality parameters of NO3−-N and HCO3−, shows a strong correlation to land used for fruit farming and a moderate correlation to the thickness of the gravel comprising unsaturated soil. Factor 3 (arsenic pollution), which is composed of groundwater quality parameters of total organic carbon (TOC) and As, is very weakly affected by land use. Factor 4 (reductive dissolution of Fe3+ and Mn2+), which involves Mn2+ and Fe3+, is weakly impacted by land use. Based on a geographic visualization of the scores for the four different factors and the patterns for land use, we can demarcate the areas where the groundwater in shallow unconfined aquifers is more vulnerable to being polluted by specific contaminants. We can then prioritize the areas where more intensive monitoring might be required, evaluate current land use practices, and adopt new measures to better prevent or control groundwater pollution.

Hydrochemical characterization and vulnerability assessment of unconfined groundwater systems in Ore area, Southwestern Nigeria

The inhabitants of Ore and its environs, Southwestern Nigeria, rely exclusively on groundwater for their domestic, industrial and irrigation purposes such that there is the need to identify and subject the recharge and discharge areas for pollution potential integrity assessment. A total of 144 dug wells were selected for the study. Topography, water table levels and hydrochemical trend methods were employed to identify twelve recharge and eighteen discharge areas. Wells in each of the areas were selected and subjected to hydrogeochemical tests to determine their hydrochemical facies types. Results showed that concentrations of cations and anions are in the order of Ca2+> Na+> Mg2+> K+ and HCO3− > Cl− > SO42− > NO3−, respectively. The dominant hydrochemical facies is Ca–HCO3 type. These results were integrated with DRASTIC parameters to determine their pollution potentials. The results showed that both the recharge and discharge areas have low pollution potential based on their DRASTIC index values. Water quality indices result of Ore groundwater fall within good to excellent/very good categories with 31.43% and 68.57%, respectively. This study indicated that groundwater sourced from both recharge and discharge areas is safe for domestic and agricultural purposes and that the groundwater is generally less vulnerable to pollution.

Development and application of long-term root zone salt balance model for predicting soil salinity in arid shallow water table area

A simple root zone salt balance model was developed for long-term soil salt prediction. A groundwater balance module was integrated to obtain the water flux at the bottom of the root zone, since groundwater level is easily and reliably measured data. An assumption that the net percolation at the bottom of root zone equals to the net recharge from root zone to unconfined groundwater system was used, which makes capillary rise and infiltration water leaching of root zone obtainable. The model accuracy and limitation were evaluated by comparing the simulation results with those from HYDRUS-1D under various soil texture and bottom boundary conditions. The mean relative error (MRE) of soil salt content ranged from −6.25% to 9.95%, and the root mean square error (RMSE) from 0.02 kg/100 kg to 0.05 kg/100 kg, which suggest the model accuracy. The determination coefficient (R2) of the cumulative percolation at the bottom of root zone and recharge to groundwater at the end of each calculation period was 0.99, which validates the model assumption. The model was then applied to predict soil salinity in the Longsheng irrigation district of Hetao Irrigation District, China. The soil salinity and water table depth data from 1999 to 2016 were used to calibrate and validate the model parameters. The MRE values of water table depth and average soil salt content were 10.08% and 10.84%, and RMSE values 0.37 m and 0.04 kg/100 kg in the validation period. Subsequently, the validated model was used to simulate the soil salinity in future 100 years under various water saving conditions. Water table depth and autumn irrigation for salt leaching were the two major factors to control soil salinity. The required autumn irrigation to keep root zone in mild salinity level (0.3 kg/100 kg) can decrease by 25.1 mm with 0.1 m water table depth increase, while it should increase by 64 mm with 1.0 kg/m3 groundwater salinity increase in the crop growing season. Moderate groundwater exploitation to increase water table depth is recommended to save autumn irrigation water and control soil salinity.

Investigating effects of climate change, urbanization, and sea level changes on groundwater resources in a coastal aquifer: an integrated assessment

Urbanization and climate change are causing numerous side effects on groundwater resources. In this study, an integrated modeling approach by linking soil and water application tool (SWAT), modular finite difference groundwater flow (MODFLOW), and three-dimensional variable-density groundwater flow coupled with multi-species solute and heat transport (SEAWAT) models were used to exhibit responses of groundwater systems, in terms of flow and salt concentrations to current and future climatic and anthropogenic changes. Future climate scenarios for periods of 2010-2040 were generated from the Canadian Global Coupled Model (CGCM) for scenarios A1B, B1, and A2 which was downscaled by the Long Ashton Research Station weather generator (LARS-WG) providing precipitation and temperature patterns for the period 2018-2040. The GCM's outputs were applied to SWAT model to estimate recharge rate for the ten scenarios designed to assess the sensitivity of the aquifer to urbanization and climate change. The estimated recharge rate from SWAT was utilized as an input in numerical groundwater model to evaluate saltwater intrusion (SWI), changes in freshwater storage within the aquifer system, and changes in groundwater level. Based on the results of each scenario's simulation, increase of pumping rate yield by future population growth will have more adverse effects on the unconfined aquifer. The derived information from this study can be used to improve future works by developing a better understanding of the managed and unmanaged response of freshwater storage and unconfined groundwater systems to climate change and anthropogenic activities.

SUBMARINE GROUNDWATER DISCHARGE MEASUREMENT ON THE SANDY UNCONFINED AQUIFER AT THE CARNAVAL BEACH, ANCOL (JAKARTA BAY)

Submarine groundwater discharge (SGD) is defined as all direct discharge of subsurface fluids into coastal zone. Components of SGD consist of fresh submarine groundwater discharge and the recirculated saline seawater discharge. SGD could act as a pathway for the transport of anthropogenic contaminants and nutrients to coastal waters. Measurement SGD at Carnaval Beach, Ancol, Jakarta was focussed on unconfined groundwater system. The method of quantified used automatic seepage meter for measured of SGD and installed conductivity temperature depth. The average SGD rate was 0.21 mm/min on March 20-31, 2009 consist of 19.05% fresh water, 80.95% recirculated seawater. April 1-23, 2009, the average SGD rate was 0.81 ml/min which consisted of 16.04% fresh water, 83.96% recirculated seawater. SGD fluctuation was opposite with the tide. As a result, submarine groundwater discharge at Jakarta coastal area was defined and can be measured to quantify.

Thermal intensification of microbial Fe(II)/Fe(III) redox cycling in a pristine shallow sand aquifer on the Canadian Shield

This investigation evaluates spatial relationships between summer (July) groundwater temperatures and Fe(II)/Fe(III) biogeochemical cycling over a five year period in a shallow pristine sand aquifer at Meilleurs Bay near Deep River, Ontario, Canada. A warm subsurface thermal island of 12.5–16.1 °C, compared to background conditions of 10–11 °C, was manifest in contour maps of average groundwater temperature over the study period. The warm zone coincided with an area of convergent groundwater flow, implicating horizontal heat transfer by advective convection as the reason for elevated temperatures. Additionally, high concentrations of dissolved Fe(II) and Fe(III) overlapped the warm thermal island, indicative of increased rates of bacterial Fe(II)-oxidation and Fe(III)-reduction. A depletion in the modal abundance of Fe(II)-bearing minerals, notably amphibole and biotite, inside the area of the warm thermal island was also observed, suggesting enhanced mineral dissolution owing to chemoautotrophic Fe(II)-oxidation coupled to the reduction and fixation of dissolved inorganic carbon as biomass. Throughout the aquifer, redox conditions were poised in terms of Eh and pH close to equilibrium with respect to the Fe(II)/Fe(OH)3 couple, feasibly enabling simultaneous bacterial Fe(II)-oxidation and Fe(III)-reduction with an adequate supply of electron acceptors and donors, respectively. The significance of higher groundwater temperature as a determinant of elevated dissolved Fe(II) and Fe(III) concentrations induced by thermal intensification of microbial biogeochemical activities yielded Pearson product-moment correlations in which temperature alone, as a single independent variable, explains almost 30 to nearly 60 percent of the variation in the measured dissolved Fe(II) and Fe(III) concentrations in the groundwater. These results emphasize the important influence of thermal conditions on biogeochemical processes in aquifers coupled to the development of steep gradients in groundwater quality over short distances in shallow unconfined groundwater systems.

Seasonal variation, chemical behavior and kinetics of uranium in an unconfined groundwater system

The seasonal changes in the concentration of uranium in an unconfined groundwater system in Cyprus have been investigated and compared to corresponding changes of boron and salinity, to better understand the chemical behavior of uranium in the respective system. Uranium concentration measurements were performed by alpha spectroscopy after selective pre-concentration, whereas boron concentration analysis and electrical conductivity measurements were carried out by photometry using azomethine-H and an electrical conductivity electrode, respectively. The experimental data show that seasonal variations are mainly related to rainwater infiltration and the specific chemical behavior of a species. Increased levels of uranium and boron in natural water systems are attributed to the increased stability of the uranium(VI)-carbonato complexes and the boric acid, which are in natural waters the predominant chemical species for uranium and boron, respectively. Dilution/dissolution processes govern the seasonal concentration changes of uranium and boron in a groundwater system, however redox-reactions resulting in the reduction of U(VI) to U(IV) affect significantly the concentration of uranium in the respective system, particularly under suboxic conditions.

Multiobjective design of aquifer monitoring networks for optimal spatial prediction and geostatistical parameter estimation

[1] Effective sampling of hydrogeological systems is essential in guiding groundwater management practices. Optimal sampling of groundwater systems has previously been formulated based on the assumption that heterogeneous subsurface properties can be modeled using a geostatistical approach. Therefore, the monitoring schemes have been developed to concurrently minimize the uncertainty in the spatial distribution of systems' states and parameters, such as the hydraulic conductivity K and the hydraulic head H, and the uncertainty in the geostatistical model of system parameters using a single objective function that aggregates all objectives. However, it has been shown that the aggregation of possibly conflicting objective functions is sensitive to the adopted aggregation scheme and may lead to distorted results. In addition, the uncertainties in geostatistical parameters affect the uncertainty in the spatial prediction of K and H according to a complex nonlinear relationship, which has often been ineffectively evaluated using a first-order approximation. In this study, we propose a multiobjective optimization framework to assist the design of monitoring networks of K and H with the goal of optimizing their spatial predictions and estimating the geostatistical parameters of the K field. The framework stems from the combination of a data assimilation (DA) algorithm and a multiobjective evolutionary algorithm (MOEA). The DA algorithm is based on the ensemble Kalman filter, a Monte-Carlo-based Bayesian update scheme for nonlinear systems, which is employed to approximate the posterior uncertainty in K, H, and the geostatistical parameters of K obtained by collecting new measurements. Multiple MOEA experiments are used to investigate the trade-off among design objectives and identify the corresponding monitoring schemes. The methodology is applied to design a sampling network for a shallow unconfined groundwater system located in Rocky Ford, Colorado. Results indicate that the effect of uncertainties associated with the geostatistical parameters on the spatial prediction might be significantly alleviated (by up to 80% of the prior uncertainty in K and by 90% of the prior uncertainty in H) by sampling evenly distributed measurements with a spatial measurement density of more than 1 observation per 60 m × 60 m grid block. In addition, exploration of the interaction of objective functions indicates that the ability of head measurements to reduce the uncertainty associated with the correlation scale is comparable to the effect of hydraulic conductivity measurements.

Analytical model for environmental tracer transport in well catchments

In this paper, we present analytical solutions and discuss them for simplified groundwater systems with decaying environmental tracers such as 3H, including the formation of a decay product such as tritiogenic 3He. The developed solutions are applicable for shallow, unconfined groundwater systems, which can be conceptually described by a steady‐state, two‐dimensional, semiconfined groundwater flow model with constant thickness, recharge rate, and porosity. The prerequisite for the applicability of our solutions is that the pumping wells and observation wells at which tracer information is available are fully screened over the entire aquifer thickness. The sampling by pumping from such wells produces the complete mixing of water of different age and origin. The application of our solution to the Baltenswil (Zurich, Switzerland) groundwater field site shows that the simplified model is able to catch essential dynamics of the transient concentration development of 3H and 3He.

阿蘇カルデラ内における地下水の流動機構

The groundwater flow system in Aso caldera is divided into four areas ( ( I ) somma of Aso volcano, ( II ) central cones side of the Aso volcano, ( III ) caldera low land unconfined groundwater and ( IV ) caldera low land artesian system) based on hydrogeochemical characteristics such as stable isotope values ( δ D and δ18O) and water chemistry. Estimated recharge elevations for groundwaters in areas ( I ) and ( III ) were relatively low and of Ca-HCO3 water type. On the other hand, groundwaters in areas ( II ) and ( IV ) belong to relatively a large groundwater flow system with high SO42- content and groundwaters were recharged in the high elevation area of the Aso volcano. We estimated the residence times of groundwaters collected from the four representative areas by using groundwater age tracers, tritium and chlorofluorocarbons (CFCs). The groundwater ages were estimated to be within 3 years under piston flow model by tritium analysis. Besides, longterm observed record analysis of tritium concentration were applied for areas ( I ) and ( II ) to estimate more precise groundwater age. Based on this analysis, the groundwater age was limited to approximately 20 years with piston flow model in the area ( I ) , and this result is consistent with CFCs ages. However in the area ( II ), estimated tritium piston flow groundwater age is different from CFCs ages. When we consider binary mixing flow in this area, the estimated groundwater age by CFCs was approximately 3 years. This longer groundwater age is harmonized with hydrogeochemical characteristics and estimated groundwater flow system. In the area ( III ), the groundwater age was not estimated because of excess addition of urban CFCs. In the area ( IV ), the estimated groundwater age was approximately 2 years by CFCs with piston flow model, however, considering the high dissolved chemical components and the estimated recharge area, the residence time may be longer.

Hydrometric and natural tracer (oxygen‐18, silica, tritium and sulphur hexafluoride) evidence for a dominant groundwater contribution to Pukemanga Stream, New Zealand

AbstractPukemanga is a small (3 ha) steep headwater catchment at the Whatawhata Research Station near Hamilton, New Zealand. The water balance (1996–2002) shows average annual rainfall of 1640 mm producing annual runoff of 440 mm (baseflow 326 mm, stormflow 114 mm) and ‘deep seepage’ loss of 450 mm (i.e. 450 mm of water not appearing in the stream). Oxygen‐18 (18O) concentrations were measured at weekly intervals for 8–15 months at six sites, ranging from Pukemanga Stream baseflow through wetland seepage to ephemeral streams and surface runoff. The first two showed no significant 18O variations. Inferred mean residence times within the catchment ranged from at least 4 years (for the stream baseflow and seepage) to a few weeks (for the ephemeral flows and surface runoff). Silica concentrations could also be used to distinguish deep flowpath water from near‐surface flowpath water. Tritium concentrations gave an estimated mean residence time of 9 years for Pukemanga Stream baseflow. Sulphur hexafluoride tended to give younger ages, while the chlorofluorocarbon ages were older, but are not considered as reliable for dating streamflow in this time range. These results show that deep pathways predominate with over 74% of runoff deriving from deep hillslope flowpaths via the wetland, and 87% of total drainage (baseflow and deep seepage) travelling via deep hillslope flowpaths. Our conception of the deep drainage process is that there is a large volume of slowly moving water in the system (above and below the water table), which reaches the wetland and stream via an unconfined groundwater system. Subsurface water equivalents are estimated to be 2·9 m for drainage at the weir and 4·1 m for drainage bypassing the weir, giving a total of 7 m depth over the catchment. The unsaturated zone plays an important role in storing water for long periods (about 4 years), while linking the surface with the groundwater water table to contribute to the fast streamflow response to rainfall. A schematic model of the various pathways with indicative residence times is given. Copyright © 2006 John Wiley & Sons, Ltd.

Quantitative hydrogeological studies of the Treviso alluvial plain, NE Italy

The alluvial plain of Treviso (NE Italy) is one of the most important aquifers in the VenetoRegion. Numerous hydrogeological studies of this unconfined groundwater system of the Venetian Plain provided qualitative and semi-quantitative data for its structural and hydrodynamic assessment. The unconfined aquifer changes from high plain southwards to multi-layered confined or semi-confined aquifers. The transition zone corresponds to a band 2–5 km wide where characteristic plain springs emerge with maximum discharge along a line known as the fontanili line. Important groundwater resources are exploited by widespread industry and agriculture, and also for public water supply including some large towns outside of study area. In recent years some test sites at which the knowledge of hydrodynamic parameters of the aquifer were improved, have been established in the study area. These, together with already available data (geometry, boundary conditions, recharge-discharge process, flow nets) allowed a water balance and a 2-D finite difference groundwater flow model to be constructed. A first calibration gave reasonable results. This flow model could be developed further to provide a tool for managing groundwaterresources including chemical water quality whencombined with chemical transport models.

Ground water flow analysis of a mid-Atlantic outer coastal plain watershed, Virginia, U.S.A.

Models for ground water flow (MODFLOW) and particle tracking (MODPATH) were used to determine ground water flow patterns, principal ground water discharge and recharge zones, and estimates of ground water travel times in an unconfined ground water system of an outer coastal plain watershed on the Delmarva Peninsula, Virginia. By coupling recharge and discharge zones within the watershed, flowpath analysis can provide a method to locate and implement specific management strategies within a watershed to reduce ground water nitrogen loading to surface water. A monitoring well network was installed in Eyreville Creek watershed, a first-order creek, to determine hydraulic conductivities and spatial and temporal variations in hydraulic heads for use in model calibration. Ground water flow patterns indicated the convergence of flow along the four surface water features of the watershed; primary discharge areas were in the nontidal portions of the watershed. Ground water recharge zones corresponded to the surface water features with minimal development of a regional ground water system. Predicted ground water velocities varied between < 0.01 to 0.24 m/day, with elevated values associated with discharge areas and areas of convergence along surface water features. Some ground water residence times exceeded 100 years, although average residence times ranged between 16 and 21 years; approximately 95% of the ground water resource would reflect land use activities within the last 50 years.

Oxygen supply and the adaptations of animals in groundwater

1. The first part of this review focuses on the oxygen status of natural groundwater systems (mainly porous aquifers) and hyporheic zones of streams. The second part examines the sensitivity of groundwater organisms, especially crustaceans, to low oxygen concentrations (&lt; 3.0 mg L−1 O2).2. Dissolved oxygen (DO) in groundwater is spatially heterogeneous at macro‐ (km), meso‐ (m) and micro‐ (cm) scales. This heterogeneity, an essential feature of the groundwater environment, reflects changes in sediment composition and structure, groundwater flow velocity, organic matter content, and the abundance and activity of micro‐organisms. Dissolved oxygen also exhibits strong temporal changes in the hyporheic zone of streams as well as in the recharge area of aquifers, but these fluctuations should be strongly attenuated with increasing distance from the stream and the recharge zone.3. Dissolved oxygen gradients along flow paths in groundwater systems and hyporheic zones vary over several orders of magnitude (e.g. declines of 9 × 10−5 to 1.5 ×10−2 mg L−1 O2 m−1 in confined aquifers and 2 × 10−2 to 1 mg L−1 O2 m−1 in parafluvial water). Several factors explain this strong variation. Where the water table is close to the surface, oxygen is likely to be consumed rapidly in the first few metres below the water table because of incomplete degradation of soil‐generated labile dissolved organic carbon (DOC) in the vadose zone. Where the water table is far from the surface, strong oxygen depletion in the vicinity of the water table does not occur, DO being then gradually consumed as groundwater flows down the hydraulic gradient. In unconfined groundwater systems, oxygen consumption along flow paths may be compensated by down‐gradient replenishment of DO, resulting either from the ingress of atmospheric oxygen or water recharge through the vadose zone. In confined groundwater systems, where replenishment of oxygen is impossible, the removal time of DO varies from a few years to more than 10 000 years, depending mainly on the organic carbon content of the sediment. Comparison of the hyporheic zones between systems also revealed strong differences in the removal time and length of underground pathways for DO. This strong variability among systems seems related to differences in contact time of water with sediment.4. Although groundwater macro‐crustaceans are much more resistant to hypoxia than epigean species, they cannot survive severe hypoxia (DO &lt; 0.01 mg L−1 O2) for very long (lethal time for 50% of the population ranged from 46.7 to 61.7 h). In severe hypoxia, none of the hypogean crustaceans examined utilized a high‐ATP yielding metabolic pathway. High survival times are mainly a result of the combination of three mechanisms: a high storage of fermentable fuels (glycogen and phosphagen), a low metabolic rate in normoxia, and a further reduction in metabolic rate by reducing locomotion and ventilation. It is suggested here that the low metabolic rate of many hypogean species may be an adaptation to low oxygen and not necessarily result from an impoverished food supply.5. An interesting physiological feature of hypogean crustaceans is their ability to recover from anaerobic stress and, more specifically, rapidly to resynthesize glycogen stores during post‐hypoxic recovery. A high storage and rapid restoration of fermentable fuels (without feeding) allows groundwater crustaceans to exploit a moving mosaic of suboxic (&lt; 0.3 mg L−1 O2), dysoxic (0.3–3.0 mg L−1 O2) and oxic (&gt; 3 mg L−1 O2) patches.6. It is concluded that although hypogean animals are probably unsuited for life in extensively or permanently suboxic groundwater, they can be found in small or temporarily suboxic patches. Indeed, their adaptations to hypoxia are clearly suited for life in groundwater characterized by spatially heterogeneous or highly dynamic DO concentrations. Their capacity to survive severe hypoxia for a few days and to recover rapidly would explain partly why ecological field studies often reveal the occurrence of interstitial taxa in groundwater with a wide range of DO.

The water table in the Tokyo District

The aim of this study was to reveal the mechanisms of occurrence, movement, and distribution of unconfined water under the Tokyo District, Japan's largest city, and the outlying areas, where its people live and do business. As many as 15 simultaneous investigations were made to determine the exact level of the groundwater table and the long-term fluctuations of the water level. The work was aided by automatic recorders, aquifer testing, geological observations, and topographical and other related studies undertaken from 1966 to the present. Unconfined groundwater systems in the Tokyo area are controlled by the geology and topography of a diluvial upland and alluvial lowland. The former is represented by a deposit referred to as the Kanto loam, which is a volcanic ejecta originating from the volcanos of Fuji and Hakone. An aquifer, representing the unconfined water beneath the upland, is probably related to terrace gravels widely distributed throughout the upland area. It should be noted that in some sections of the area, the Kanto loam forms groundwater mounds, and shallow well zones have been presently confirmed which have been there since ancient times and are used as a partial source for household water supplies. This means that the unconfined water now available seems to come from several formations and is related to the topography and geology of that particular area. The massive movement of people to the suburbs of the western part of Tokyo began in the latter half of the 20th century. During this urbanization, there was a tremendous need for housing and subsequent need for additional and dependable water supplies. This need was met by the introduction of advanced equipment and technical knowledge. Changes in the hydrological environment caused by the urbanization of the area are, at present, so serious that immediate action needs to be taken for a more permanent and stable supply of groundwater. This can only be achieved by adequate maintenance of the surrounding environment as well as conservation of the underground water resources. This article discusses quantitatively the analytical reports of the changes found as a result of a hydrological survey of that environment.

Confined groundwater system in Tokyo

The Musashino Terrace and the Shitamachi Lowland, the main urban areas in the Tokyo metropolis, are in the eastern part of Tokyo and are underlain by thick Quaternary sediments. The geologic structrue of these sediments has a general strike of WNW—ESE, with a gentle dip to the east in the western part of the Musashino Terrace, and strikes generally to the E—W with a gentle dip to the north in the eastern part of the terrace and strikes WNW—ESE to N—S with a very gentle dip to the east in the Shitamachi Lowland. The basal sediments in this area consist chiefly of massive thick silty layers, whereas the sediments overlying them consist of alternating layers of silt, sand, and gravel, which act as confined aquifers throughout the Musashino Terrace and the Shitamachi Lowland. Confined groundwater systems are being developed in the Tama District, the western part of the terrace, and are mainly recharged with river water from the Tama River. The unconfined groundwater system is being developed in the surface layers such as terrace sand and gravel formations and the Kanto Loam Formation. These confined aquifers that are being developed throughout the terrace and the lowland have been exploited for household water supplies and industrial and air-conditioning use for nearly seven decades. The heavy utilization of the confined groundwater, however, has caused a major lowering of groundwater levels. As a result of the lowering of groundwater levels, land subsidence has occurred all over the Lowland and the terrace. At this time the national government and the Tokyo Metropolitan Government restricted groundwater withdrawals for industrial, air-conditioning use and the others except for household supply. Owing to these restrictions, not only have the water levels recovered, but land subsidence has rapidly declined since around 1973.