Identification of groundwater recharge sources and processes in a heterogeneous alluvial aquifer: results from multi‐level monitoring of hydrochemistry and environmental isotopes in a riverside agricultural area in Korea

We evaluated sources and pathways of groundwater recharge for a heterogeneous alluvial aquifer beneath an agricultural field, based on multi‐level monitoring of hydrochemistry and environmental isotopes of a riverside groundwater system at Buyeo, Korea. Two distinct groundwater zones were identified with depth: (1) a shallow oxic groundwater zone, characterized by elevated concentrations of NO₃^? and (2) a deeper (>10–14 m from the ground surface) sub‐oxic groundwater zone with high concentrations of dissolved Fe, silica, and HCO₃^?, but little nitrate. The change of redox zones occurred at a depth where the aquifer sediments change from an upper sandy stratum to a silty stratum with mud caps. The δ¹⁸O and δ²H values of groundwater were also different between the two zones. Hydrochemical and δ¹⁸O? δ²H data of oxic groundwater are similar to those of soil water. This illustrates that recharge of oxic groundwater mainly occurs through direct infiltration of rain and irrigation water in the sandy soil area where vegetable cropping with abundant fertilizer use is predominant. Oxic groundwater is therefore severely contaminated by agrochemical pollutants such as nitrate. In contrast, deeper sub‐oxic groundwater contains only small amounts of dissolved oxygen (DO) and NO₃^?. The ³H contents and elevated silica concentrations in sub‐oxic groundwater indicate a somewhat longer mean residence time of groundwater within this part of the aquifer. Sub‐oxic groundwater was also characterized by higher δ¹⁸O and δ²H values and lower d‐excess values, indicating significant evaporation during recharge. We suggest that recharge of sub‐oxic groundwater occurs in the areas of paddy rice fields where standing irrigation and rain water are affected by strong evaporation, and that reducing conditions develop during subsequent sub‐surface infiltration. This study illustrates the existence of two groundwater bodies with different recharge processes within an alluvial aquifer. Copyright © 2009 John Wiley & Sons, Ltd.     

Influence of artificial groundwater lakes on the abundance and activity of bacteria in adjacent subsurface systems

Bacterial abundances and activity, estimated by 4′,6-diamidino-2-phenylindole staining (DAPI) and the reduction of 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium chloride (INT), were investigated in two oligotrophic artificial groundwater lakes and the surrounding aquifers. To evaluate the effect of lake water on groundwater downstream, samples were taken from wells at different distances from the lakes, and the total number of bacteria and the number of active bacteria in these samples were compared with samples collected upstream. In addition, sterilized sandy sediments were exposed in groundwater wells to measure the number and activity of bacteria attached to particles. At one of the study sites, where the lake sediments were disturbed by dredging, total bacterial abundance and the number of respiring bacteria in the groundwater aquifer was clearly influenced by the lake water. The average bacterial abundances decreased from 2.6 ± 1.9 × 10⁵ cells ml⁻¹ in the well closest to the lake (S2) to 2.9 ± 3.8 × 10⁴ cells ml⁻¹ in the most distant one (S4), which was equivalent to cell numbers in the upstream well. The number of respiring bacteria showed a similar tendency with 1.3 ± 2.7 × 10⁴ active cells ml⁻¹ in S2 and 1.9 ± 1.5 × 10³ active cells ml⁻¹ in S4. At the second study site, which was not influenced by dredging, bacteria in the downstream wells seemed not to be affected by the lake water. The number and activity of bacteria, which colonized exposed sediments, were not significantly different in the upstream and downstream wells, indicating a minor influence of lake water on this habitat. Our results suggest that gravel-pit lakes may influence the free living bacterial assemblages in nearshore groundwater systems, but do not visibly affect numbers and activity of bacteria attached to the surface of aquifer sediments.     

Using stable water isotopes to identify spatio-temporal controls on groundwater recharge in two contrasting East African aquifer systems

Understanding the spatio-temporal variability in groundwater recharge is a prerequisite to sustainable management of aquifers. Spatial analysis of groundwater stable isotopes uncovered predominant controls on groundwater recharge in the Nairobi aquifer system (NAS) and the South Coast aquifer (SC), two exemplar East African aquifers relied upon by 7 million people. We analysed 368 samples for stable isotopes and basic physico-chemical parameters. The NAS groundwater isotopes are controlled by precipitation orographic effects and enriched recharge from impounded lakes/wetlands; the SC isotopes are correlated with water-table depth influencing evapotranspiration. Global Network of Isotopes in Precipitation (GNIP) data revealed groundwater recharge during months of heavy rains in the NAS, whilst the SC experiences spatio-temporally diffuse recharge. Inferred “isoscapes” show: in the NAS, (1) direct, rapid recharge favoured by faults, well-drained soils and ample rainfall in uplands, (2) delayed recharge from impounded lakes and wetlands in mid-lands, and (3) focused, event-based recharge in floodplains; and in the SC, diffuse recharge complicated by significant water-table evapotranspiration processes.     

Evidence of short-term groundwater recharge signal propagation from the Andes to the central Atacama Desert: a singular spectrum analysis approach

The effect of pressure-driven groundwater recharge signal propagation in the Andean-Atacamenian environment is investigated by assessing a record of 15 years of water table fluctuations of an unconfined–confined aquifer system. Based on a singular spectrum analysis of water table time series, it is shown that, in the given case, groundwater levels in the central Atacama Desert are hydraulically controlled by two distant recharge areas associated with the Andes. The maximum observed range of the pressure signal propagation is ~50 km over an elevation difference of more than 3000 m at a lag of ~25 months. Several findings indicate that an often-cited study misinterpreted a water level rise at the same site as an in-situ alluvial fan recharge. Thus, the effect’s impact on groundwater dynamics in complex aquifer systems can easily be overlooked. Singular spectrum analysis could be of use to investigate pressure effects at hydrologically comparable sites.     

Characteristics of preferential flow and groundwater discharge to Shingobee Lake,Minnesota, USA

Small‐scale heterogeneities and large changes in hydraulic gradient over short distances can create preferential groundwater flow paths that discharge to lakes. A 170 m² grid within an area of springs and seeps along the shore of Shingobee Lake, Minnesota, was intensively instrumented to characterize groundwater‐lake interaction within underlying organic‐rich soil and sandy glacial sediments. Seepage meters in the lake and piezometer nests, installed at depths of 0·5 and 1·0 m below the ground surface and lakebed, were used to estimate groundwater flow. Statistical analysis of hydraulic conductivity estimated from slug tests indicated a range from 21 to 4·8 × 10^?3 m day^?1 and small spatial correlation. Although hydraulic gradients are overall upward and toward the lake, surface water that flows onto an area about 2 m onshore results in downward flow and localized recharge. Most flow occurred within 3 m of the shore through more permeable pathways. Seepage meter and Darcy law estimates of groundwater discharge agreed well within error limits. In the small area examined, discharge decreases irregularly with distance into the lake, indicating that sediment heterogeneity plays an important role in the distribution of groundwater discharge. Temperature gradients showed some relationship to discharge, but neither temperature profiles nor specific electrical conductance could provide a more convenient method to map groundwater–lake interaction. These results suggest that site‐specific data may be needed to evaluate local water budget and to protect the water quality and quantity of discharge‐dominated lakes. Copyright © 2002 John Wiley & Sons, Ltd.     

Investigating Groundwater‐Lake Interactions by Hydraulic Heads and a Water Balance

Discharge of groundwater into lakes (lacustrine groundwater discharge, LGD) can play a major role in water balances of lakes. Unfortunately, studies often neglect this input path because of methodological difficulties in its determination. Direct measurements of LGD are labor‐consuming and prone to error. The present study uses both spatially variable hydraulic‐head data and meteorological data to estimate groundwater input by LGD and lake water output through infiltration. The study sites are two shallow, groundwater‐fed lakes without any surface inflows or outflows. Horizontally interpolated groundwater heads were combined with lake water levels to obtain vertical hydraulic gradients between the aquifer and the lake, which are separated by a thick layer of lake bed sediment which has a much lower hydraulic conductivity than the underlying aquifer. By fitting the hydraulic gradient to the results of a simple mass balance and considering the process of clogging, we were able to estimate the hydraulic conductivity of the lake bed sediments. We calculated groundwater inputs by LGD and lake water outputs by infiltration on an annual basis. Although our method requires several assumptions, the results are reasonable and provide useful information about the exchange between the aquifer and the lake, which can, for example, be used for the calculation of nutrient mass balances.     

Spatio‐temporal analysis of groundwater recharge and mound dynamics in an unconfined aquifer: a GIS‐based approach

Groundwater recharge and mounding of water‐table is a complex phenomenon involving time‐ and space‐dependent hydrologic processes. The effect of long‐term groundwater mounding in the aquifer depends on soil, aquifer geometry and the area contributing to recharge. In this paper, a GIS‐based spatio‐temporal algorithm has been developed for the groundwater mound dynamics to estimate the potential rise in the water‐table and groundwater volume balance residual in an unconfined aquifer. The recharge and mound dynamics as predicted using the methodology recommended here were compared with those using the Hantush equation, and the differences were quite significant. The significance of the study is to assess the effectiveness of the basin in terms of its hydrologic and hydraulic properties for sustainable management of groundwater recharge. Copyright © 2007 John Wiley & Sons, Ltd.     

Assessing coupling between lakes and layered aquifers in a complex Pleistocene landscape based on water level dynamics

The biosphere reserve Schorfheide-Chorin is a scenic region with many lakes. Hydraulic coupling between lakes and groundwater is difficult to assess due to the very heterogeneous Pleistocene deposits with a complex layering of different aquifers, part of them being confined. Thus, a principal component analysis of time series of groundwater and lake water levels was performed. The first two principal components provided a quantitative measure of damping of the input signal, i.e., the extent to which time series of groundwater pressure heads or lake water levels are smoothed and delayed with respect to the input signal, i.e., groundwater recharge or precipitation minus evapotranspiration, respectively. The lakes differed substantially with respect to damping behaviour, indicating different impacts of deep groundwater contribution. For most of the groundwater wells, damping increased linearly with mean depth to water table. In contrast, some wells exhibited nearly identical behaviour independent of depth. High-pass filtered data of water table level from these wells were strongly and inversely correlated with those of barometric pressure fluctuations, pointing to a confined aquifer which was evidently not connected to the adjacent lake.     

Contribution of magnetic resonance sounding to aquifer characterization and recharge estimate in semiarid Niger

To improve the knowledge of the regionally important Continental Terminal 3 (CT3) aquifer in south-western Niger, fifteen magnetic resonance soundings (MRS) were carried out in December 2005 in the vicinity of wells and boreholes. The output MRS geophysical parameters, i.e. water content and decay constants versus depth, were compared to hydrogeological characteristics, i.e. water table depth, total porosity, specific yield and transmissivity estimated from direct measurements, pumping tests and transient groundwater modelling. The MRS-determined parameters were then used to estimate the rates of groundwater recharge.Contained in poorly consolidated Tertiary sandstones, the CT3 aquifer's water table has continuously risen by 4 m in total over the past four decades. Additionally, a significant portion of this increase has occurred in the past decade alone, with an annual rise now ranging between 0.1 and 0.3 m depending on the monitored well. Increase in groundwater recharge due to land clearance and deforestation explains this situation. According to previous estimations, the pre-clearing recharge ranged from 1 to 5 mm per year in 1950–60 s, while more recent recharge rates (1990s–2000s) range from 20 to 50 mm per year. These recharge values are directly affected by estimated aquifer specific yield value, while the spatial variation of rates of water table rise can be attributed to large scale hydrodynamic heterogeneities in the aquifer. However, few field measurements were available to confirm these assumptions.The main results of this study are: (1) The water table depth and aquifer transmissivity are estimated from MRS output parameters with an average accuracy of ± 10% and ± 9% respectively. (2) The MRS-determined water content is linked to both the total porosity and the specific yield of the aquifer, but no quantitative formulation can be proposed as yet. (3) Using the average MRS-determined water content over the investigated area, i.e. 13%, the groundwater recharge rates can be estimated to be ~ 2 mm per year in the 1950–1960s (pre-clearing period), and ~ 23 mm per year for the last decade. (4) The variations in specific yield and transmissivity cannot explain by themselves the spatial variability of the rise of the water table. (5) The ranges in transmissivity and water content obtained from MRS are more realistic than the groundwater modelling outputs. Therefore, MRS could be used to better constrain the aquifer parameters in groundwater modelling with a dense site network.Finally, this work illustrates how MRS can successfully improve characterisation and transient multi-year groundwater balance of commonly found sedimentary aquifers, particularly when integrated with well observations and pumping tests.     

Groundwater recharge through an alluvial fan in the Atacama Desert,northern Chile: mechanisms,magnitudes and causes

The Chacarilla fan in the Atacama Desert is one of several formed in the Late Miocene at the foot of the Pre‐Andean Cordillera overlying the large, complex, Pampa Tamarugal aquifer contained in the continental clastic sediments of the fore‐arc basin. The Pampa Tamarugal aquifer is a strategic source of water for northern Chile but there is continuing doubt over the resource magnitude and recharge. During January 2000 a 1 in 4 year storm in the Andes delivered a 34 million m³ flash flood to the fan apex where c. 70% percolated to the underlying aquifers. Groundwater recharge through the fan is calculated to be a minimum of 200 l/s or 6% of the long‐term catchment rainfall. These figures are supported by hydrochemical data that suggest that recharge may be 9% of long‐term rainfall. Isotopic data suggest groundwater less than 50 years old is transmitted westward through the permeable sheetflood sediments of the fan overlying the main aquifer. Analysis of this and other events shows that the hydrological system is non‐linear with positive feedback. The magnitude of groundwater recharge is dependent on climatic variations, antecedent soil moisture storage and changes in channel characteristics. Long‐term declines in groundwater level may partly result from climatic fluctuations and the causes of such fluctuations are discussed. Copyright © 2002 John Wiley & Sons, Ltd.     

Characterising groundwater–surface water interactions in idealised ephemeral stream systems

Transmission losses from the beds of ephemeral streams are thought to be a widespread mechanism of groundwater recharge in arid and semi-arid regions and support a range of dryland hydro-ecology. Dryland areas cover ~40% of the Earth's land surface and groundwater resources are often the main source of freshwater. It is commonly assumed that where an unsaturated zone exists beneath a stream, the interaction between surface water and groundwater is unidirectional and that groundwater does not exert a significant feedback on transmission losses. To test this assumption, we conducted a series of numerical model experiments using idealised two-dimensional channel-transects to assess the sensitivity and degree of interaction between surface and groundwater for typical dryland ephemeral stream geometries, hydraulic properties and flow regimes. We broaden the use of the term ‘stream–aquifer interactions’ to refer not just to fluxes and water exchange but also to include the ways in which the stream and aquifer have a hydraulic effect on one another. Our results indicate that deep water tables, less frequent streamflow events and/or highly permeable sediments tend to result in limited bi-directional hydraulic interaction between the stream and the underlying groundwater which, in turn, results in high amounts of infiltration. With shallower initial depth to the water table, higher streamflow frequency and/or lower bed permeability, greater ‘negative’ hydraulic feedback from the groundwater occurs which in turn results in lower amounts of infiltration. Streambed losses eventually reach a constant rate as initial water table depths increase, but only at depths of 10s of metres in some of the cases studied. Our results highlight that bi-directional stream–aquifer hydraulic interactions in ephemeral streams may be more widespread than is commonly assumed. We conclude that groundwater and surface water should be considered as connected systems for water resource management unless there is clear evidence to the contrary.     

Estimating groundwater recharge in lowland watersheds

Little attention has been given to the role of groundwater in the hydrological cycle of lowland watersheds. Our objective in this study was to estimate total recharge to groundwater by analysing water table response to storm events and the rate at which water was transferred into the shallow aquifer. This was conducted at three sites in a rural watershed in the lower Atlantic coastal plain near Charleston, South Carolina, USA. A novel version of the water table fluctuation method was used to estimate total recharge to the shallow aquifer by comparing hourly data of water table position following storm events and measuring water table recession behavior, rather than subjective graphical analysis methods. Also, shallow aquifer recharge rates (vertical fluxes) were estimated using Darcy's Law by comparing static water levels in a water table well and in a shallow piezometer during dry periods. The total annual recharge estimated ranged from 107 ± 39 mm·yr^–1 (5–10% of annual precipitation) at a poorly drained topographic low area to 1140 ± 230 mm·yr^–1 (62–94% of annual precipitation) for a moderately well‐drained upland site. The average aquifer recharge rate was 114 ± 60 mm·yr^–1, which is similar to previous estimations of base flow for the ephemeral third‐order streams in this watershed. The difference in the two methods may have been caused by processes not accounted for in the Darcy flux method, soil moisture deficits, and average evapotranspiration demand, which is about 1100 mm·yr^–1 for this region. Although other factors also can affect partitioning of recharge, an integrated approach to inspecting easily gathered groundwater data can provide information on an often neglected aspect of water budget estimation. We also discuss the effects of land use change on recharge reduction, given a typical development scenario for the region. Copyright © 2011 John Wiley & Sons, Ltd.     

地下水补给型湖泊表层沉积物矿物组成及其形成机制——以巴丹吉林沙漠湖泊群为例

盐湖中的矿物沉积记录着丰富的环境气候变化信息,是古环境研究的重要对象.在无地表径流补给的盐湖中,其矿物组成及沉积特征与有地表径流补给的湖泊相比是否有一定的特殊性,是值得探讨的问题.采集巴丹吉林沙漠33个不同矿化度地下水补给型湖泊的表层沉积物和10个地表风积砂样品,通过X衍射的方法,分析样品的矿物组成.结果显示:湖泊表层沉积物主要为石英、长石、辉石、云母等碎屑矿物,部分湖泊含有少量的碳酸盐和氯化物盐类矿物.湖泊沉积物的矿物组成与湖水矿化度的关系较为密切,淡水湖仅分布碎屑矿物,微咸水湖含有碎屑矿物和碳酸盐类矿物,盐湖含有碎屑矿物、碳酸盐类矿物和氯化物.风积砂样品中主要为碎屑矿物,占总矿物含量的90%,对湖泊沉积物的矿物组成影响较大,但对湖泊沉积物中的盐类矿物没有贡献,表明湖泊表层沉积物中盐类矿物主要是自生作用形成的.虽然本地区湖泊边缘的沉积物中盐类矿物种类相对较少并且含量较低,但其盐类矿物组成与分布能够响应湖水矿化度的变化,其环境指示意义与有径流补给的盐湖相同,可以指示其湖水的盐度.因此,可以从巴丹吉林沙漠地下水补给型湖泊沉积的盐类矿物中提取相应的古环境信息,用于恢复古气候和古环境的研究.     

Interannual variations in the areas of thermokarst lakes in Central Yakutia

At present, close attention is given to studies of thermokarst lakes from the standpoint of cryolite zone response to global climate warming. However, other factors, which have an effect on the variation in areas of lakes, in particular, the atmospheric precipitation, receive little attention, while the atmospheric precipitation is one of the main sources of water recharge for many water objects. In this connection, interannual variations in the areas of lakes as a function of precipitation were analyzed on the territory of Central Yakutia. The studies were based on the analysis of space images taken at different time. As the result, it was shown that precipitation has a considerable effect on variations in the areas of thermokarst lakes; there is a direct relationship between variations in lake areas and precipitation; only some single high-water or low-water years have no effect on these variations. Variations in areas of lakes during summer were also analyzed with a view to use the results in choosing the images for studies of interannual variations. It has been revealed that the areas of thermokarst lakes decrease from June to August; this means that the images obtained within a short period at the end of the summer period should be used.     

Hydrological cycle and water balance estimates for the megadune–lake region of the Badain Jaran Desert,China

The hydrology and water balance of megadunes and lakes have been investigated in the Badain Jaran Desert of China. Field observations and analyses of sand layer water content, field capacity, secondary salt content, and grain size reveal 3 types of important natural phenomenon: (a) vegetation bands on the leeward slope of the megadunes reflect the hydrological regime within the sandy vadose zone; (b) seepage, wet sand deposits, and secondary salt deposits indicate the pattern of water movement within the sandy vadose zone; (c) zones of groundwater seeps and descending springs around the lakes reflect the influence of the local topography on the hydrological regime of the megadunes. The seepage exposed on the sloping surface of the megadunes and gravity water contained within the sand layer confirm the occurrence of preferential flow within the vadose zone of the megadunes. Alternating layers of coarse and fine sand create the conditions for the formation of preferential flows. The preferential flows promote movement of water within the sand layer water that leads to deep penetration of water within the megadunes and ultimately to the recharging of groundwater and lake water. Our results indicate that a positive water balance promotes recharge of the megadunes, which depends on the high permeability of the megadune material, the shallow depth of the surface sand layer affected by evaporation, the occurrence of rainfall events exceeding 15 mm, and the sparse vegetation cover. Water balance estimates indicate that the annual water storage of the megadunes is about 7.5 mm, accounting for only 8% of annual precipitation; however, the shallow groundwater per unit area under the megadunes receives only 3.6% of annual precipitation, but it is still able to maintain a dynamic balance of the lake water. From a water budget perspective, the annual water storage in the megadunes is sufficient to serve as a recharge source for lake water, thereby enabling the long‐term persistence of the lakes. Overall, our findings demonstrate that precipitation is a significant component of the hydrological cycle in arid deserts.     

Predicting average annual groundwater levels from climatic variables: an empirical model

On the basis of one-dimensional theoretical water flow model, we demonstrate that the groundwater level variation follows a pattern similar to recharge fluctuation, with a time delay that depends on the characteristics of aquifer, recharge pattern as well as the distance between the recharge and observation locations. On the basis of a water budget model and the groundwater flow model, we propose an empirical model that links climatic variables to groundwater level. The empirical model is tested using a partial data set from historical records of water levels from more than 80 wells in a monitoring network for the carbonate rock aquifer, southern Manitoba, Canada. The testing results show that the predicted groundwater levels are very close to the observed ones in most cases. The overall average correlation coefficient between the predicted and observed water levels is 0.92. This proposed empirical statistical model could be used to predict variations in groundwater level in response to different climate scenarios in a climate change impact assessment.     

Stream water infiltration, bank storage, and storage zone changes due to stream-stage fluctuations

During a flood period, stream-stage increases induce infiltration of stream water into an aquifer; subsequent declines in stream stage cause a reverse motion of the infiltrated water. This paper presents the results of the water exchange rate between a stream and aquifer, the storage volume of the infiltrated stream water in the surrounding aquifer (bank storage), and the storage zone. The storage zone is the part of aquifer where groundwater is replaced by stream water during the flood. MODFLOW was used to simulate stream–aquifer interactions and to quantify rates of stream infiltration and return flow. MODPATH was used to trace the pathlines of the infiltrated stream water and to determine the size of the storage zone. Simulations were focused on the analyses of the effects of the stream-stage fluctuation, aquifer properties, the hydraulic conductivity of streambed sediments, regional hydraulic gradients, and recharge and evapotranspiration (ET) rates on stream–aquifer interactions. Generally, for a given stream–aquifer system, larger flow rates result from larger stream-stage fluctuations; larger storage volumes and storage zones are produced by larger and longer-lasting fluctuations. For a given stream-stage hydrograph, a lower-permeable streambed, an aquitard, or an anisotropic aquifer of low vertical hydraulic conductivity can significantly reduce the rate of infiltration and limit the size of the storage zone. The bank storage solely caused by the stage fluctuation differs slightly between gaining and losing streams. Short-term rainfall recharge and ET loss in the shallow groundwater slightly influence on the flow rate, but their effects on bank storage in a larger area for a longer period can be considerable.     

Hydrogeological interactions between fault zones and alluvial aquifers in regional flow systems

The hydrological influence of fault zones in tectonic areas is usually difficult to depict from field data. Numerical simulation allows representation of such flow systems and an estimation of flow lines and rates. This paper reports on simulations of the groundwater flow in a range‐and‐basin area affected by a regional fault zone, which may drain or recharge an overlaying alluvial aquifer. Different hydraulic conductivity values for the range rocks, the fault‐zone, and the sedimentary infill of the basin are considered, as well as different fault‐zone widths and boundary conditions. Results show that upward and downward fluxes develop in the upper part of the fault zone, controlled by the action of the alluvial aquifer, influencing the recharge of the sedimentary basin. This paper shows the hydrological efficiency of fault zones as preferential flow; it also analyses the constraints that determine groundwater recharge to the surrounding basins. These results contribute to the understanding of hydrogeological dynamics in tectonic areas. Copyright © 2008 John Wiley & Sons, Ltd.     

Analysis of variations in the regime of rivers and lakes in the Upper Volga and Upper Zapadnaya Dvina based on archaeological-geomorphological data

The geomorphological and altitudinal positions of occupational layers corresponding to 1224 colonization epochs at 870 archaeological sites in river valleys and lake depressions in southwestern Tver province. A series of alternating low-water (low levels of seasonal peaks, many-year periods without inundation of floodplains) and high-water (high spring floods, regular inundation of floodplains) intervals of various hierarchical rank was identified. In low-water epoch, an increase was recorded in the share of settlements on low elevations, including river and lake floodplains now subject to inundation. The archaeological epochs 2–3 Ky in length were found to form the following series from high-water to low-water: Mesolithic (11.8–8.0 Ky ago)-Iron age (2.8–0.3)-Neolithic (8.0–5.0)-Bronze epoch (5.0–2.8). The first half of the Iron age (2.8–1.8 Ky ago) was extremely water-abundant, while its second half (middle ages) was dry (relative to the present time). A correlation between the hydrological and temperature regimes was identified: low-water epochs closely correlate with warm epochs, while high-water ones correlate with cold epochs. This can be associated with the specific features of the present-day type of water regime with dominating spring flood; this regime is supposed to have existed during the most part of the Holocene: the runoff and the levels of floods decline during warming epochs and increase during cooling epochs.     

Monitoring of drawdown pattern during pumping in an unconfined physical aquifer model

In this study, we attempted to analyse a drawdown pattern around a pumping well in an unconfined sandy gravelly aquifer constructed in a laboratory tank by means of both experimental and numerical modelling of groundwater flow. The physical model consisted of recharge, aquifer and discharge zones. Permeability and specific yield of the aquifer material were determined by Dupuit approximation under steady‐state flow and stepwise gravitational drainage of groundwater, respectively. The drawdown of water table in pumping and neighbouring observation wells was monitored to investigate the effect of no‐flow boundary on the drawdown pattern during pumping for three different boundary conditions: (i) no recharge and no discharge with four no‐flow boundaries (Case 1); (ii) no recharge and reservoir with three no‐flow boundaries (Case 2); (iii) recharge and discharge with two no‐flow boundaries (Case 3). Based on the aquifer parameters, numerical modelling was also performed to compare the simulated drawdown with that observed. Results showed that a large difference existed between the simulated drawdown and that observed in wells for all cases. The reason for the difference could be explained by the formation of a curvilinear type water table between wells rather than a linear one due to a delayed response of water table in the capillary fringe. This phenomenon was also investigated from a mass balance study on the pumping volume. The curvilinear type of water table was further evidenced by measurement of water contents at several positions in the aquifer between wells using time domain reflectometry (TDR). This indicates that the existing groundwater flow model applicable to an unconfined aquifer lacks the capacity to describe a slow response of water table in the aquifer and care should be taken in the interpretation of water table formation in the aquifer during pumping. Copyright © 2001 John Wiley & Sons, Ltd.