Estimation of regional-scale groundwater flow properties in the Bengal Basin of India and Bangladesh

Quantitative evaluation of management strategies for long-term supply of safe groundwater for drinking from the Bengal Basin aquifer (India and Bangladesh) requires estimation of the large-scale hydrogeologic properties that control flow. The Basin consists of a stratified, heterogeneous sequence of sediments with aquitards that may separate aquifers locally, but evidence does not support existence of regional confining units. Considered at a large scale, the Basin may be aptly described as a single aquifer with higher horizontal than vertical hydraulic conductivity. Though data are sparse, estimation of regional-scale aquifer properties is possible from three existing data types: hydraulic heads, ¹⁴C concentrations, and driller logs. Estimation is carried out with inverse groundwater modeling using measured heads, by model calibration using estimated water ages based on ¹⁴C, and by statistical analysis of driller logs. Similar estimates of hydraulic conductivities result from all three data types; a resulting typical value of vertical anisotropy (ratio of horizontal to vertical conductivity) is 10⁴. The vertical anisotropy estimate is supported by simulation of flow through geostatistical fields consistent with driller log data. The high estimated value of vertical anisotropy in hydraulic conductivity indicates that even disconnected aquitards, if numerous, can strongly control the equivalent hydraulic parameters of an aquifer system.     

The geochemistry and mixing of leakage in a semi-confined aquifer at a municipal well field,Memphis, Tennessee,USA

The Memphis aquifer in southwestern Tennessee is confined to a semi-confined unconsolidated sand aquifer and is the primary municipal water source in the Memphis metropolitan area. Past studies have identified regions in the metropolitan area in which the overlying upper Claiborne confining unit lacks significant clay and provides a hydraulic connection between the shallow aquifer and the Memphis aquifer. In this study, major solute chemistry, ³H, and ³H/³He groundwater dating are used to investigate the extent and chemical effects of leakage through the confining unit to the Memphis aquifer in the vicinity of a municipal well field. The ³H/³He dates and geochemical modeling of the chemical data are used to constrain mixing fractions and the timing of modern recharge. Tritium activities of as much as 2.8 TU are observed in shallow production wells, but deeper production wells have ³H activities that approach the detection limit. Trends in water chemistry indicate vertical mixing in the aquifer of shallow Na–SO₄–Cl-rich water and deeper Ca–Mg–HCO₃-rich water. Water chemistry does not vary consistently with seasonal pumping, but ³H activity generally decreases during low use periods. Stable O and H isotopes show little variation and are not useful groundwater tracers for this study. The ³H-bearing, Na–SO₄–Cl-rich water is interpreted to reflect recharge of modern water through the upper Claiborne confining unit. The ³H/³He dates from 5 production wells indicate modern recharge, that infiltrated 15–20 a ago, is present in the shallow production wells. Geologic data and hydrologic boundary conditions suggest that the most likely source for continued leakage is a nearby stream, Nonconnah Creek. Geochemical reaction modeling using the NETPATH computer code suggests that proportions of shallow aquifer water leaking into the Memphis aquifer range from 6 to 32%. The ³H/³He dating and NETPATH modeling results correlate well, suggesting that these complementary analytical tools provide an effective means to evaluate proportions of modern water leaking into semi-confined aquifers. These results also indicate a need to carefully consider connections between surface water and semi-confined groundwater resources in wellhead protection programs.     

Analysis of Steady-State Flow To Multiscreened Wells Under Natural Vertical Flow Conditions

Water levels measured at multiscreened wells in unconfined aquifers may not coincide, in general, with the elevation of the water table. The presence of vertical gradients (as often is the case in recharge areas) or the existence of confining layers may cause the water levels to differ from local hydraulic heads in the aquifer. In these cases, a misinterpretation of water levels may lead to the erroneous conclusion that observed drawdowns are provoked by overpumping. In this paper, we analyze the effect that a natural vertical gradient has on water levels in wells screened over their entire saturated thickness. As one would expect, it is observed that, even without pumping, the water level in the wells lies below the water table. Type curves relating the steady-state drawdown to the vertical gradient and to the hydraulic conductivity anisotropy are presented. These curves were obtained using a groundwater flow numerical model (FREESURF: Neuman and Witherspoon, 1970). The theoretical results are checked with field data from deep wells in the detrital Madrid aquifer. In this particular aquifer, it is observed that the effect of vertical gradients is important both in terms of drawdowns and flow rates.

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Geochemical and isotopic investigation of the aquifer system in the Djerid-Nefzaoua basin, southern Tunisia

In the Djerid-Nefzaoua region, southern Tunisia, about 80% of agricultural and domestic water supply is provided by the complex terminal (CT) aquifer. However, 20% of this demand is provided by other hydraulically connected aquifers, namely the continental intercalaire (CI) and the Plio-Quaternary (PQ). Overexploitation of the CT aquifer for agricultural practices has contributed to the loss of the artesian condition and the decline of groundwater level which largely increased the downward leakage from the shallow PQ aquifer. Excess irrigation water concentrates at different rates in the irrigation channels and in the PQ aquifer itself. Then, it returns to the CT aquifer and mixes with water from the regional flow system, which contributes to the salinization of the CT groundwater. A geochemical and isotopic study had been undertaken over a 2-years period in order to investigate the origin of waters pumped from the CT aquifer with an emphasis on its hydraulic relationships with the underlying and the overlying CI and PQ aquifers. Geochemistry indicates that groundwater samples collected from different wells show an evolution of the water types from Na-Cl to Ca-SO₄-Cl. Dissolution of halite, gypsum and anhydrite-bearing rocks is the main mechanism that leads to the salinization of the groundwater. Isotopic data indicate the old origin of all groundwater in the aquifer system. Mixing and evaporation effects characterizing the CT and the PQ aquifers were identified using δ²H and δ¹⁸O relationship and confirmed by the conjunction of δ²H with chloride concentration.     

Installation of a vertical slurry wall around an Italian quarry lake: complications arising and simulation of the effects on groundwater flow

Slurry walls are non-structural barriers that are constructed underground to impede groundwater flow or manage groundwater control problems. The study area is in the Piemonte plain (Italy), close to the River Po. Quarrying works carried out below the piezometric surface created two big quarry lakes. The local groundwater system is characterized by a lower semi-confined aquifer, which is overlain by a semi-permeable bed of clayey peat (aquitard) and an upper unconfined aquifer. Locally, the peat fades away and the granulometry of this horizon becomes silty sandy. A planned enlargement of the quarry will increase the size and depth of the quarry lakes. So the aquitard bed between the two aquifers will be damaged, creating a mixing rate of groundwater. Such a procedure would not be compatible with the presence of two municipal wells upstream from the quarries. Consequently, the installation of a vertical diaphragm (slurry wall) is recommended to separate the aquifers and to act as a filter for the groundwater flowing from the unconfined to the semi-confined aquifer. To predict the consequences caused by the installation of the vertical diaphragm separating the unconfined aquifer and the semi-confined one, a specifically adjusted finite-difference model was used. The model showed a maximum rising of the water table equal to 12 cm, just upstream of the diaphragm and for a distance of about 100 m, and a maximum lowering of 2 cm just downstream of the diaphragm. However, the slurry wall would not cause any change in the piezometric head in the area where there are municipal wells and, hence, will not have any negative effect on the functionality of the municipal wells. Moreover, the migration of water from the unconfined aquifer through the vertical diaphragm will stimulate a series of attenuation and auto-depuration processes of eventual contaminants. These processes are due to the higher crossing time that the groundwater flow takes to go through the vertical barrier (t _a = 96.5 days, whereas for the horizontal semi-permeable layer t _a = 9.6 days). So, the vertical diaphragm can be a resolutive element, representing a mediation and separation factor between the unconfined and the semi-confined aquifers along the border of the quarrying areas, and a protective barrier for the water quality of the quarry lake and the semi-confined aquifer.     

Effects of human-induced alteration of groundwater flow on concentrations of naturally-occurring trace elements at water-supply wells

The effects of human-induced alteration of groundwater flow patterns on concentrations of naturally-occurring trace elements were examined in five hydrologically distinct aquifer systems in the USA. Although naturally occurring, these trace elements can exceed concentrations that are considered harmful to human health. The results show that pumping-induced hydraulic gradient changes and artificial connection of aquifers by well screens can mix chemically distinct groundwater. Chemical reactions between these mixed groundwaters and solid aquifer materials can result in the mobilization of trace elements such as U, As and Ra, with subsequent transport to water-supply wells. For example, in the High Plains aquifer near York, Nebraska, mixing of shallow, oxygenated, lower-pH water from an unconfined aquifer with deeper, confined, anoxic, higher-pH water is facilitated by wells screened across both aquifers. The resulting higher-O₂, lower-pH mixed groundwater facilitated the mobilization of U from solid aquifer materials, and dissolved U concentrations were observed to increase significantly in nearby supply wells. Similar instances of trace element mobilization due to human-induced mixing of groundwaters were documented in: (1) the Floridan aquifer system near Tampa, Florida (As and U), (2) Paleozoic sedimentary aquifers in eastern Wisconsin (As), (3) the basin-fill aquifer underlying the California Central Valley near Modesto (U), and (4) Coastal Plain aquifers of New Jersey (Ra). Adverse water-quality impacts attributed to human activities are commonly assumed to be related solely to the release of the various anthropogenic contaminants to the environment. The results show that human activities including various land uses, well drilling, and pumping rates and volumes can adversely impact the quality of water in supply wells, when associated with naturally-occurring trace elements in aquifer materials. This occurs by causing subtle but significant changes in geochemistry and associated trace element mobilization as well as enhancing advective transport processes.     

Groundwater recharge in suburban areas of Hanoi,Vietnam: effect of decreasing surface-water bodies and land-use change

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

     

Impact of groundwater withdrawals on the interaction of multi-layered aquifers in the Viterbo geothermal area (central Italy)

The impact of groundwater withdrawals on the interaction between multi-layered aquifers with different water qualities in the Viterbo geothermal area (central Italy) was studied. In this area, deep thermal waters are used to supply thermal spas and public pools. A shallow overlying aquifer carries cold and fresh water, used for irrigation and the local drinking-water supply. Starting with a conceptual hydrogeological model, two simplified numerical models were implemented: a steady-state flow model of the entire groundwater system, and a steady-state flow and heat transport model of a representative area, which included complex interactions between the aquifers. The impact of increased withdrawals associated with potential future development of the thermal aquifer must be considered in terms of the water temperature of the existing thermal sources. However, withdrawals from the shallow aquifer might also influence the discharge of thermal sources and quality of the water withdrawn from the shallow wells. The exploitation of the two aquifers is dependent on the hydraulic conductivity and thickness of the intervening aquitard, which maintains the delicate hydrogeological equilibrium. Effective methods to control this equilibrium include monitoring the vertical gradient between the two aquifers and the residual discharge of natural thermal springs.     

Hydrogeologic framework of Pico Island, Azores, Portugal

Pico, the youngest island of the Azores archipelago, is composed of basaltic volcanic deposits less than 300,000 years old. The principal aquifer system consists mainly of recent lava flows that are very permeable and whose head is influenced by tidal fluctuations. Groundwater abstraction is almost entirely by drilled wells. The hydraulic gradient is very low, about 10^–4, which agrees with observations made on similar volcanic islands. Groundwater also occurs in perched-water bodies, but the spring discharge from them is very low, about 10^–3 L/s. The transmissivity of the volcanic rocks ranges from 9.44×10^–3 to 3.05×10^–1 m²/s, indicating the heterogeneity of the aquifers. The hydraulic diffusivity, estimated from observations of the effects of tidal fluctuations, also confirms the high permeability of the aquifer system; the average value is higher than published values for other volcanic islands. A mixing process for fresh water and seawater, often coupled with ion-exchange mechanisms, explains the groundwater composition, which is mainly of the sodium-chloride type. The water salinity influences the groundwater quality, resulting in a chloride content that exceeds the recommended chloride limit in 91% of the wells . Water–rock interactions are dominant in the chemical evolution of the perched water, which is characterized by bicarbonate-anion type water. Electronic Publication     

鄂尔多斯盆地都思兔河流域白垩系含水层特征及供水前景分析

在鄂尔多斯盆地都思兔河流域供水水文地质详查的基础上,通过水文地质钻探、抽水试验、样品分析等手段,查明了流域含水层的结构、埋深以及含水层和隔水层在水平和垂直方向上的变化规律,探讨了各含水层之间的水力联系,并求取了含水层的水文地质参数;对比圈定了包乐浩晓、巴彦布拉格、好勒包勒吉3处富水区;采用地下水流数值模拟方法对上述3处富水区地下水开采量进行计算,提出了27种地下水开采方案,并经过对比得到地下水开采推荐方案。结果表明：包乐浩晓、巴彦布拉格、好勒包勒吉3处富水区主要补给为侧向径流及大气降水入渗补给,排泄方式以潜水面蒸发及向河流排泄为主;在地下水浅埋区,结合含水层单井涌水量的大小,确定采用管井开采方式、中段悬挂式非完整井结构,平均布井,井深300m,单一开采白垩系环河组的地下水,最大限度夺取潜水蒸发排泄量;根据推荐方案,上述3个富水区总开采量达到123500m3／d,可为该地区地下水资源开发与利用提供技术保障。     

Evaluation of the importance of clay confining units on groundwater flow in alluvial basins using solute and isotope tracers: the case of Middle San Pedro Basin in southeastern Arizona (USA)

As groundwater becomes an increasingly important water resource worldwide, it is essential to understand how local geology affects groundwater quality, flowpaths and residence times. This study utilized multiple tracers to improve conceptual and numerical models of groundwater flow in the Middle San Pedro Basin in southeastern Arizona (USA) by determining recharge areas, compartmentalization of water sources, flowpaths and residence times. Ninety-five groundwater and surface-water samples were analyzed for major ion chemistry (water type and Ca/Sr ratios) and stable (¹⁸O, ²H, ¹³C) and radiogenic (³H, ¹⁴C) isotopes, and resulting data were used in conjunction with hydrogeologic information (e.g. hydraulic head and hydrostratigraphy). Results show that recent recharge (<60 years) has occurred within mountain systems along the basin margins and in shallow floodplain aquifers adjacent to the San Pedro River. Groundwater in the lower basin fill aquifer (semi confined) was recharged at high elevation in the fractured bedrock and has been extensively modified by water-rock reactions (increasing F and Sr, decreasing ¹⁴C) over long timescales (up to 35,000 years BP). Distinct solute and isotope geochemistries between the lower and upper basin fill aquifers show the importance of a clay confining unit on groundwater flow in the basin, which minimizes vertical groundwater movement.     

Tritium�Chelium groundwater age used to constrain a groundwater flow model of a valley-fill aquifer contaminated with trichloroethylene (Quebec, Canada)

Tritium?Chelium groundwater dating was carried out in a trichloroethylene (TCE)-contaminated valley-fill aquifer system in Quebec, Canada, where a numerical groundwater flow model was developed. Forty seven discrete groundwater and dissolved gas samples were obtained along two flow paths originating from known TCE source zones whose related plumes converge down gradient to form a single plume. Sampling points in monitoring wells were projected onto vertical sections showing particle tracks along the two flow paths. At these points, simulated advective ages obtained from particle tracking were matched to tritium?Chelium ages using different porosity values; the best match was 0.35. Ages were also obtained above and below a prodeltaic silty aquitard in a portion of the aquifer where some source zones are located, which provide groundwater and TCE transit times through the aquitard as well as a mean vertical hydraulic conductivity that agrees with previous estimates used in the model. In certain locations, anomalously old ages associated with high terrigenic ⁴He indicate areas where groundwater from the underlying proglacial unit flows upward into the deltaic sand aquifer through aquitard windows. Upflow locations correspond with increased TCE concentrations, suggesting significant TCE provenance through the proglacial unit originating from a previously unrecognized TCE source zone.     

Assessing groundwater availability and the response of the groundwater system to intensive exploitation in the North China Plain by analysis of long-term isotopic tracer data

The use of isotope tracers as a tool for assessing aquifer responses to intensive exploitation is demonstrated and used to attain a better understanding of the sustainability of intensively exploited aquifers in the North China Plain. Eleven well sites were selected that have long-term (years 1985–2014) analysis data of isotopic tracers. The stable isotopes δ¹⁸O and δ²H and hydrochemistry were used to understand the hydrodynamic responses of the aquifer system, including unconfined and confined aquifers, to groundwater abstraction. The time series data of ¹⁴C activity were also used to assess groundwater age, thereby contributing to an understanding of groundwater sustainability and aquifer depletion. Enrichment of the heavy oxygen isotope (¹⁸O) and elevated concentrations of chloride, sulfate, and nitrate were found in groundwater abstracted from the unconfined aquifer, which suggests that intensive exploitation might induce the potential for aquifer contamination. The time series data of ¹⁴C activity showed an increase of groundwater age with exploitation of the confined parts of the aquifer system, which indicates that a larger fraction of old water has been exploited over time, and that the groundwater from the deep aquifer has been mined. The current water demand exceeds the sustainable production capabilities of the aquifer system in the North China Plain. Some measures must be taken to ensure major cuts in groundwater withdrawals from the aquifers after a long period of depletion.     

The evolution of groundwater in the Tyrrell catchment, south-central Murray Basin, Victoria, Australia

The Tyrell catchment lies on the western margin of the Riverine Province in the south-central Murray Basin, one of Australia’s most important groundwater resources. Groundwater from the shallow, unconfined Pliocene Sands aquifer and the underlying Renmark Group aquifer is saline (total dissolved solids up to 150,000 mg/L) and is Na-Cl-Mg type. There is no systematic change in salinity along hydraulic gradients implying that the aquifers are hydraulically connected and mixing during vertical flow is important. Stable isotopes (¹⁸O+²H) and Cl/Br ratios indicate that groundwater is entirely of meteoric origin and salts in this system have largely been derived by evapotranspiration of rainfall with only minor halite dissolution, rock weathering (mainly feldspar dissolution), and ion exchange between Na and Mg on clays. Similarity in chemistry of all groundwater in the catchment implies relative consistency in processes over time, independent of any climatic variation. Groundwater in both the Pliocene Sands and Renmark Group aquifers yield ages of up to 25 ka. The Tyrrell Catchment is arid to semi-arid and has low topography. This has resulted in relatively low recharge rates and hydraulic gradients that have resulted in long groundwater residence times.     

Identifying groundwater recharge connections in the Moscow (USA) sub-basin using isotopic tracers and a soil moisture routing model

Globally, aquifers are suffering from large abstractions resulting in groundwater level declines. These declines can be caused by excessive abstraction for drinking water, irrigation purposes or industrial use. Basaltic aquifers also face these conflicts. A large flood basalt area (1.1?×?10⁵ km²) can be found in the Northwest of the USA. This Columbia River Basalt Group (CRBG) consists of a thick series of basalt flows of Miocene age. The two major hydrogeological units (Wanapum and Grand Ronde formations) are widely used for water abstraction. The mean decline over recent decades has been 0.6 m year^?1. At present day, abstraction wells are drying up, and base flow of rivers is reduced. At the eastern part of CRBG, the Moscow sub-basin on the Idaho/Washington State border can be found. Although a thick poorly permeable clay layer exists on top of the basalt aquifer, groundwater level dynamics suggest that groundwater recharge occurs at certain locations. A set of wells and springs has been monitored bi-weekly for 9 months for δ¹⁸O and δ²H. Large isotopic fluctuations and d-excess values close to the meteoric water line in some wells are indicating that recharge occurs at the granite/basalt interface through lateral flow paths in and below the clay. A soil moisture routing (SMR) model showed that most recharge occurs on the granitic mountains. The basaltic aquifer receives recharge from these sedimentary zones around the granite/basalt interface. The identification of these types of areas is of major importance for future managed-aquifer recharge solutions to solve problems of groundwater depletion.     

穆兴平原北部地下水年龄及更新性

地下水年龄结构是了解一个地区地下水资源开采可持续性的重要基础。穆兴平原地下水开采量增加以及地下水环境恶化,对该地区可持续发展有一定制约,为此在2016年采集CFCs样品31组和3H样品60组,估算了研究区地下水年龄。结果表明,穆兴平原北部地下水年龄为21年到大于65年,由西北部和穆棱河向平原中部及乌苏里江逐渐变老,更新性变差,主要受到大气降水和地表河水补给,但是由于地表覆盖一层黏性土,地下水中缺失小于10年的水;不同井深样品中二者及NO_3~-浓度的变化,表明在60 m以上地下水的防污性能较差,而在100 m以下则较好,饮用水源井深需超过100 m。     

The groundwater age in the Middle-Upper Devonian aquifer system, Lithuania

³H, δ¹³C and hydrochemical data were used to estimate the corrected groundwater age derived from conventional ¹⁴C age of dissolved inorganic carbon (DIC). The Middle-Upper Devonian aquifer system from the Baltic upland recharge area in eastern Lithuania towards the discharge area on the Baltic Sea coast in the west was considered. The concentration of total dissolved solids (TDS) in groundwater changes from 300 to 24,000  mg/L and increases downgradient towards the coast. The other major constituents have the same trend as the TDS. The hydrochemical facies of groundwater vary from an alkali-earth carbonates facies at the eastern upland area to an alkali-earth carbonate-sulfate and chloride facies at transit and discharge areas. Meteoric water percolating through the Quaternary and Devonian aquifers regulate the initial ¹⁴C activities of groundwater involving two main members of DIC: soil CO₂ with modern ¹⁴C activity uptake and dissolution of ¹⁴C-free aquifer carbonates. Other sources of DIC are less common. ¹⁴C activity of DIC in the groundwater ranged from 60 to 108 pMC at the shallow depths. With an increase of the aquifers depth the dolomitization of aqueous solution and leakage of the “old” groundwater from lower aquifers take place, traced by lower activities (7–30 pMC).     

Hydrologic factors controlling groundwater salinity in northwestern coastal zone,Egypt

The aim of this article is to assess the main factors influencing salinity of groundwater in the coastal area between El Dabaa and Sidi Barani, Egypt. The types and ages of the main aquifers in this area are the fractured limestone of Middle Miocene, the calcareous sandstone of Pliocene and the Oolitic Limestone of Pleistocene age. The aquifers in the area are recharged by seasonal rainfall of the order of 150 mm/year. The relationship of groundwater salinity against the absolute water level, the well drilling depth, and the ability of aquifer to recharge has been discussed in the present work. The ability of aquifer to locally recharge by direct rainfall is a measure of the vertical permeability due to lithological and structural factors that control groundwater salinity in the investigated aquifers. On the other hand, the fracturing system as well as the attitude of the surface water divide has a prime role in changing both the mode of occurrence and the salinity of groundwater in the area. Directly to the west of Matrouh, where the coastal plain is the narrowest, and east of Barrani, where the coastal plain is the widest, are good examples of this concept, where the water salinity attains its maximum and minimum limits respectively. Accordingly, well drilling in the Miocene aquifer, in the area between El Negila and Barrani to get groundwater of salinities less than 5000 mg/l is recommended in this area, at flow rate less than 10 m³/hr/well. In other words, one can expect that the brackish water is probably found where the surface water divide is far from the shore line, where the Wadi fill deposits dominate (Quaternary aquifer), acting as a possible water salinity by direct rainfall and runoff.     

Isotope geochemistry and modelling of the multi-aquifer system in the eastern part of Lithuania

A steady-state groundwater flow model of three Quaternary intertill aquifers in the eastern part of Lithuania has been compiled. The distinction of separate modelled layers is based on hydraulic and isotope-hydrochemistry data criteria. ³H data were used to estimate the corrected groundwater age and were coupled with a groundwater-flow-dynamics model of the Quaternary aquifer system along a cross-section flow pathway from the Baltic Upland recharge area in eastern Lithuania towards the discharge area in the lowlands near the city of Kaunas in central Lithuania. The bicarbonate content in groundwater (214–462 mg/l) increases downgradient towards the lowland area. The other major constituents and total dissolved solids (TDS) have a trend analogous to the bicarbonate. The ¹⁴C activity of dissolved inorganic carbon (DIC) in the groundwater ranges from 41.4 to 85.7 pMC. With aquifer-system depth, active precipitation of aqueous solution takes place by dissolving minerals of calcite and dolomite and leakage of “old” groundwater from lower aquifers; the process is also traced by lower ¹⁴C and ³H activities and by more positive δ¹⁸O values in lowland areas.     

Influence of Hydraulic Conductivity and Wellbore Design in the Fate and Transport of Nitrate in Multi-aquifer Systems

Nitrate concentrations in multi-aquifer systems are heavily affected by the presence of wellbores (active or abandoned) that are screened in several aquifers. The spatial variability of hydraulic conductivity in the confining layers has also an important impact on the concentrations. A synthetic three-dimensional flow and transport exercise was carried in a multi-aquifer system consisting of two aquifers separated by an aquitard in which 100 vertical wellbores had been drilled. To model the wellbores and the flow and transport connection between aquifers that they may induce, we assign a high vertical hydraulic conductivity and a low effective porosity to the cell blocks including the wells. With these parameters, a solute will travel quickly from one aquifer to the other without being stored in the well itself. The wellbores will act as preferential pathways, and the solute will move quickly between aquifers according to the hydrodynamic conditions. Not considering these preferential pathways could induce erroneous interpretations of the solute distribution in an aquifer. We also noted that when there are vertical wellbores that connect aquifers in a multi-aquifer system, low conductivity in the aquitard enhances the flow of solute through the wellbores. Time-varying pumping rates induce important fluctuations in nitrate concentrations; therefore, any estimate of the water quality of the aquifer will depend on the moment when the data has been recorded. Consequently, concentration maps obtained by interpolation of point samples are seldom a good indicator of the chemical status of groundwater bodies; alternatively, we recommend complementing the usual interpolated maps with numerical models to gain a true understanding of the spatial distribution of the solute concentration.