Shallow Groundwater Flow Systems Research Articles

Groundwater Facies in the South Parts of Tegal-Brebes Groundwater Basin

The southern part of the Tegal Brebes Groundwater Basin has various rock units originating from the volcanic activity of the Slamet Volcano. The groundwater interacts with these rocks and influences the hydrogeochemical process that may alter its chemical components. The objective of this research is to assess the relationship between groundwater chemistry facies and the rock aquifer. The finding of this research will help us better understand how hydrogeochemical processes work, perform thorough research, and determine the water quality. The groundwater characteristics are determined based on the chemical composition of the water taken from the citizen’s dug wells and springs. These samples were collected to represent a difference in rock’s formation, namely Linggopodo Formation, Tapak, and Slamet Undifferentiated Volcanic Rock. The analysis of the Piper diagram shows that all the samples fall into one facies: the calcium bicarbonate type (Ca-HCO3). The groundwater’s physical and chemical properties in the study area indicate that groundwater quality has not interacted much with lithology and is still greatly influenced by meteoric water. The groundwater facies are dominated by the Ca-HCO3, a characteristic of shallow groundwater and local water flow systems.

Development of a conceptual groundwater model for a complex basement aquifer system: The case OF OSUN drainage basin in southwestern Nigeria

Sound knowledge of hydrogeological setting is required for effective groundwater resource management and protection. In the basement environment where groundwater occurrence is discontinuous and highly heterogeneous, hydrogeological conceptual model is required to understand the occurrence and flow pattern of groundwater. This study presents an integrated 2D and 3D GIS method for conceptual modeling of the hydrogeological setting of a basement complex aquifer system in Osun Drainage Basin, Southwestern Nigeria. The efficiency of conjunctive use of RockWorks, Suffer and ArcGIS for surface and subsurface 3D hydrogeological modeling of a basement aquifer system was evaluated.Three distinct aquifers (Weathered Layer Aquifer (WLA), Unconfined Fractured Aquifer (UFA) and Confined Fractured Aquifer (CFA)) were delineated. WLA is characterized by low hydraulic conductivity (K = 0.34–27.54md−1, mean = 4md−1), hence low yield potential. Transmissivity (T) range and mean (0.6–1368 m2d−1, 85 m2d−1) indicate restricted flow within WLA. UFA is hydraulically connected to the underlying multi-layered CFA in the contact zones. The occurrence of UFA is localized within meta-sedimentary environment and it acts as the recharge redistributing medium, connecting the deep seated aquifers with the highly porous surface flaggy rubbles. CFA is characterized by relatively high mean K (0.2–91md−1, mean = 11md−1) and relatively high T (1.6–7185 m2d-1, mean = 483 m2d−1). Comparative analyses of aquifers’ hydraulic properties and spatial pattern of occurrence affirm that regional groundwater flow only takes place within CFA. The variation of hydraulic properties along fault lines is attributed to alternating occurrence of bedrocks of different structural and physico-chemical characteristics. Three distinct groundwater flow systems were identified in the study basin: i) the radially concentric localized shallow groundwater flow systems that occur within the overburden, ii) the W-E oriented regional flow system that feeds the source of Ikogosi Warm Spring in the adjourning basin, and iii) the NNE-SSW trans-current flow system that traverses the study basin at the center. While the first flow system is controlled by surface topography and the hydraulic properties of the vadose zone, the second and third flow systems are structurally controlled. The efficiency and efficacy of the methodology presented in this study is sufficient to develop hydrogeological conceptual model for highly heterogeneous, discontinuous basement aquifer system. Therefore, the developed conceptual model could be transformed to a numerical model of the study basin.

Geophysical response to simulated methane migration in groundwater based on a controlled injection experiment in a sandy unconfined aquifer

Geophysical methods have the capacity to detect and characterize gas-phase dynamics in groundwater. Suitable methods can be deployed at surface or within boreholes depending on the required depth of investigation, spatial/temporal resolution, and geologic conditions. While the application of geophysical methods to monitor immiscible phase contaminants in the subsurface has been extensively documented, the effects of hydraulic properties and flow system conditions on the nature of the geophysical responses used to elucidate multi-phase fluid flow remains underdeveloped. A series of numerical 2-dimensional multi-phase flow and geophysical model simulations based on a controlled methane release experiment in the Borden unconfined sand aquifer was carried out to assess the influence of porous media hydraulic properties and flow system conditions on geophysical signatures associated with transient gas-phase saturation and gas migration behaviour. Specifically, the utility of electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) to monitor gas-phase plume dynamics in shallow groundwater flow systems is examined. ERT and GPR responses to gas-phase distribution and migration during a 72-day methane gas injection and subsequent recovery period was calculated using a numerical multi-phase flow model (CFbio) simulating four distinct parameterizations of the sandy aquifer system. Geophysical models showed that ERT was effective at imaging the central position of the plume but was less effective at detecting thinner lateral migration pathways extending beyond the primary high gas saturation bulb. Conversely, GPR was able to detect thin gas pools emanating from the primary gas bulb and small-scale vertical preferential pathways arising from capillary boundaries with contrasting saturations; however, gradational boundaries proved to be more difficult to resolve using GPR. This study demonstrates that ERT and GPR can be very useful tools in combination for longer-term monitoring of stray gas leakage from decommissioned hydrocarbon wells in shallow granular media freshwater aquifers, especially given the likelihood of strong lateral migration.

Hydrologic interpretation of seasonally dynamic ambient temperature profiles in sealed bedrock boreholes

This study evaluates the utility of ambient temperature profiles collected in sealed bedrock boreholes to assess variability in groundwater flow in discretely fractured shallow bedrock environments. A conceptual model for groundwater flow and groundwater-surface water temperature conditions and their interaction in a temperate climate is developed through a statistical interpretation of time-lapse thermal deviation logs. Temperature profiles were collected in three angled and three vertical boreholes drilled to 24–32 mbgs (meters below ground surface) and temporarily sealed with an impermeable fabric liner in a fractured dolostone bedrock aquifer adjacent to and extending beneath a bedrock river to monitor seasonal hydrodynamics. Ambient borehole temperature profiles collected every 1–8 weeks over a 12 month period identified zones of hydraulic activity during periods of intra-seasonal stability without the interference of open borehole cross-connection. Signal cross-correlation and Fourier spectra analysis of thermal deviation logs provided a novel way to observe the shallow bedrock flow system’s temperature evolution due to advection along discrete fractures in response to seasonal transience, and to identify and isolate noise caused by free-convection cells within the sealed borehole. This approach represents a diagnostic tool that improves confidence in identifying depth discrete, hydraulically active fracture zones from thermal deviation data sets in a shallow, fractured sedimentary bedrock environment. Variably scaled free-convection cells were observed within the borehole water columns during the colder winter periods. Although these periods were accompanied by higher signal noise near the river/atmospheric interface, these cells led to a temporary thermal disequilibrium between the borehole water column and formation water deeper in the bedrock. These conditions increased the maximum depth of thermal detections associated with discrete groundwater flow features from 14 mbgs in the summer to 26 mbgs in the winter, thereby enhancing the understanding of shallow groundwater flow systems under the direct influence of surface water.

Monitoring the evolution and migration of a methane gas plume in an unconfined sandy aquifer using time-lapse GPR and ERT

Fugitive methane (CH4) leakage associated with conventional and unconventional petroleum development (e.g., shale gas) may pose significant risks to shallow groundwater. While the potential threat of stray (CH4) gas in aquifers has been acknowledged, few studies have examined the nature of its migration and fate in a shallow groundwater flow system. This study examines the geophysical responses observed from surface during a 72day field-scale simulated CH4 leak in an unconfined sandy aquifer at Canadian Forces Base Borden, Canada, to better understand the transient behaviour of fugitive CH4 gas in the subsurface. Time-lapse ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) were used to monitor the distribution and migration of the gas-phase and assess any impacts to groundwater hydrochemistry. Geophysical measurements captured the transient formation of a CH4 gas plume emanating from the injector, which was accompanied by an increase in total dissolved gas pressure (PTDG). Subsequent reductions in PTDG were accompanied by reduced bulk resistivity around the injector along with an increase in the GPR reflectivity along horizontal bedding reflectors farther downgradient. Repeat temporal GPR reflection profiling identified three events with major peaks in reflectivity, interpreted to represent episodic lateral CH4 gas release events into the aquifer. Here, a gradual increase in PTDG near the injector caused a sudden lateral breakthrough of gas in the direction of groundwater flow, causing free-phase CH4 to migrate much farther than anticipated based on groundwater advection. CH4 accumulated along subtle permeability boundaries demarcated by grain-scale bedding within the aquifer characteristic of numerous Borden-aquifer multi-phase flow experiments. Diminishing reflectivity over a period of days to weeks suggests buoyancy-driven migration to the vadose zone and/or CH4 dissolution into groundwater. Lateral and vertical CH4 migration was primarily governed by subtle, yet measurable heterogeneity and anisotropy in the aquifer.

Evaluating shallow flow-system response to climate change through analysis of spring deposits in southwestern Wisconsin, USA

Tufa-depositing springs in the southern Driftless Area of Wisconsin, USA, are used to inform the response of shallow and local groundwater flow systems to changes in climate over the last 3,000 years. The springs emanate from a shallow, unconfined, sedimentary bedrock aquifer and at stratigraphic positions similar to a shallow, perched aquifer that was identified in the eastern Driftless Area. The perched aquifer was shown to be stable under current climate conditions and over decadal time scales. This study provides further evidence of the significance of the stratigraphic interval in controlling shallow groundwater flow patterns in the region and in the stability of shallow and local groundwater flow systems over thousands of years. The tufa carbonates in three cores collected from the mounds adjacent to the springs show variations in stable isotope (δ13C, δ18O) and elemental (Mg/Ca) values that agree with well-established paleoclimate records for the region, suggesting that the springs were active and depositing tufa in the past, during climate conditions that were similar to the present and during conditions that were drier than the present.

Interaction between shallow and deep aquifers in the Tivoli Plain (Central Italy) enhanced by groundwater extraction: A multi-isotope approach and geochemical modeling

Abstract In the Tivoli Plain (Rome, Central Italy) the interaction between shallow and deep groundwater flow systems enhanced by groundwater extraction has been investigated using isotopic and chemical tracers. A conceptual model of the groundwater flowpaths has been developed and verified by geochemical modeling. A combined hydrogeochemical and isotopic investigation using ion relationships such as DIC/Cl − , Ca/(Ca + Mg)/SO 4 /(SO 4 + HCO 3 ), and environmental isotopes ( δ 18 O, δ 2 H, 87 Sr/ 86 Sr, δ 34 S and δ 13 C) was carried out in order to determine the sources of recharge of the aquifer, the origin of solutes and the mixing processes in groundwater of Tivoli Plain. Multivariate statistical methods such as principal component analysis and Cluster analyses have confirmed the existence of different geochemical facies and the role of mixing in the chemical composition of the groundwater. Results indicate that the hydrochemistry of groundwater is characterized by mixing between end-members coming directly from carbonate recharge areas and to groundwater circulating in a deeply buried Meso-Cenozoic carbonate sequence. The travertine aquifer is fed by both flow systems, but a local contribution by direct input in the Plain has also been recognized. The stable isotope data ( 18 O, 2 H, 13 C and 34 S) supports the flow system conceptual model inferred from the geochemical data and represents key data to quantify the geochemical mixing in the different groundwaters of the Plain. The results of numerical modeling (PHREEQC) are consistent with the flowpaths derived from the hydrogeochemical conceptual model. The inverse models performed generated the main geochemical processes occurring in the groundwater flow system, which also included mixing. Geochemical and isotope modeling demonstrate an increasing influence of groundwater from the deeply buried aquifer in the travertine aquifer, enhanced by lowering of the travertine aquifer water table due to quarry pumping.

Interaction between deep and shallow groundwater systems in areas affected by Quaternary tectonics (Central Italy): a geochemical and isotope approach

Hydro- and isotope geochemistry are used to refine groundwater conceptual models in two areas of central Italy (Acque Albule Basin and Velino River Valley) affected by extensional Quaternary tectonics, where deep and shallow groundwater flow systems are interacting. The role of geology, of recent deposits filling the plains and of main tectonic features controlling groundwater flowpaths and deep-seated fluids emergences are investigated and discussed. Environmental isotopes (2H and 18O) confirm recharge in the surrounding carbonate aquifers, and meteoric origin of both shallow and deep groundwater. Major ion chemistry indicates a mixing between shallow Ca-HCO3 groundwater from carbonate aquifers and deep Ca-HCO3-SO4 groundwater, characterised by higher salinity and temperature and high concentration in sulphates. Isotopic composition of dissolved sulphates (δ 34S and δ 18O) and dissolved inorganic carbon (δ 13C), henceforth indicated as DIC, are used to verify the presence of different sources of groundwater, and to validate the mixing model suggested by the major ion analyses. Sulphate isotope composition suggests a marine origin for the groundwater characterised by elevated sulphate concentration, whose source is present in the deep buried sequences. Carbon isotope composition confirms the role of a DIC source associated to CO2 degassing of a deep reservoir. Groundwater conceptual models are improved underlining the importance of Plio-Pleistocene sequences filling the tectonic depression. In the Acque Albule area, the travertine plateau represents a mixing stratified aquifer, where deep groundwater contribution is spread into the shallow aquifer. The alluvial–clastic–lacustrine leaky aquifer of Velino Valley enables a complete mixing of shallow and deep groundwater allowing spot-located discharge of deep groundwater along tectonic patterns and facilitating sulphate reduction in the lacustrine sediments, explaining locally the presence of H2S.

Evolution of shallow groundwater flow systems in areas of degrading permafrost

The recent increase in fresh‐water discharge during low‐flow conditions as observed in many (sub‐)Arctic Rivers has been attributed to a reactivation of groundwater flow systems caused by permafrost degradation. Hydrogeological simulations show how groundwater flow conditions in an idealized aquifer system evolve on timescales of decades to centuries in response to climate warming scenarios as progressive lowering of the permafrost table establishes a growing shallow groundwater flow system. Ultimately, disappearance of residual permafrost at depth causes a sudden establishment of deep groundwater flow paths. The projected shifts in groundwater flow conditions drive characteristic non‐linear trends in the evolution of increasing groundwater discharge to streams. Although the subsurface distribution of ice will markedly influence the system response, current modeling results suggest that late‐stage accelerations in base flow increase of streams and rivers, are to be expected, even if surface air temperatures stabilize at the current levels in the near future.

The stability of groundwater flow systems in unconfined sandy aquifers in the Netherlands

Summary Groundwater flow is temporally variable and uncertain, due to climatologically or anthropogenically induced variation in boundary conditions that result in changes in the drainage network, and uncertainties in hydraulic model parameters used in the quantification of groundwater flow. The quantification and mapping of the variation and uncertainty in groundwater flow is especially essential in relatively flat areas where flow direction is sensitive to decimetre-scale head variations. In these areas, the variability and uncertainty of groundwater flow directions may therefore have important implications for the uncertainties in the spatial configuration of groundwater flow systems. In this study we aim to quantify and map the sensitivity of shallow groundwater flow systems to uncertainties in aquifer anisotropy and drainage resistance, and variations in drainage level and groundwater recharge for a sandy unconfined aquifer in the Salland region, the Netherlands. For this purpose, the most probable configuration of current groundwater flow systems was mapped using particle tracking and Monte Carlo analysis. Sensitivity was represented by the membership of each model cell to the defined groundwater flow systems given the uncertainties and variations in the hydraulic parameters and boundary conditions. In addition, the current configuration of groundwater flow systems was compared to the historical situation without artificial drainage. The average groundwater flow system size was found to be in the order of a few square kilometres, with a relatively stable configuration. In contrast to the intrinsic and temporally invariant hydraulic parameters, which were shown to have a minor influence on the spatial configuration of groundwater flow systems, natural variation in recharge and variations in drainage level management exert a large influence.

The mobility of anthropogenic129I in a shallow sand aquifer at Sturgeon Falls, Ontario, Canada

SummaryThe transport and partitioning of129I has been examined for a shallow groundwater flow system at Sturgeon Falls in northeastern Ontario. Recharge into a sandy surficial postglacial deltaic aquifer occurs in the vicinity of a seasonally-wet boreal forest. Concentrations of stable iodine,129I, tritium, radiocarbon, stable isotopes and major ions were measured on samples of groundwater and precipitation. The present-day tritium profile delineates the position of the early 1960's above-ground nuclear weapons pulse at a depth of approximately 12 m.The concentrations of stable iodine for groundwaters above, near and below the above-ground nuclear weapons pulse range from 0.07 to 1.7 ppb. Over that same interval129I concentrations decreased from 1.9 × 106atoms/L at 9 m, to approximately 1.9 × 105atoms/L in tritium-depleted waters at 35 m, below the present-day depth of the recharging above-ground nuclear weapons peak at approximately 12.7 m. No substantial increases in the levels of129I were evident in waters sampled near the location of the above-ground nuclear weapons peak.The129I values in the lower aquifer are 1 to 2 orders of magnitude greater than published estimates of pre-bomb129I (103to 104atoms/L), and indicate contribution of129I from the fissiogenic decay of238U in the basal granite bedrock or from leaching of129I from the aquifer grains.A precipitation sample collected at Sturgeon Falls had concentrations of stable iodine and129I of 0.2 ppb and 8.5 × 107atoms/L, respectively. Concentrations of129I in Sturgeon Falls groundwaters are lower than those reported in other studies in central Canada, suggesting that the levels of129I in these groundwaters may have been attenuated by complexation or ion exchange with organic materials in the near-surface soil horizons. Such processes have been documented during a previous study on the behavior of129I in a shallow aquifer near a low-level, radioactive waste management area.Along a 30 cm soil profile at piezometer nest 28 at the Sturgeon Falls study site, the concentrations of129I ranged from approximately 4.3 × 108atoms/g in the uppermost soil litter layer to 5.6 × 107atoms/g in the siltier bottom soil horizons. Over that same profile, stable iodine varied from 4.7 ppm in the upper layers to 3.9 ppm in the lower layers.

Geochemical Approaches to Understanding a Shallow Groundwater Flow in the Kanamaru Uranium Mineralization Area (Japan)

AbstractPredicting the behaviour of radioactive wastes can be facilitated by comparison with the evolution of natural groundwater systems. During a study of the Kanamaru U mineralization (Japan), geochemical approaches for understanding a shallow (0-50 m) fresh groundwater flow system are being assessed. Deep granitic waters are Ca-HCO3-dominated and slightly acidic to slightly alkaline. Shallow waters within sediments display lower pH and they are more dilute. Halide concentrations suggest the existence of a non marine Br-rich and Cl-depleted deep groundwater in the basement. 234U/238U and 230Th/234U activity ratios in the mineralized sedimentary rocks indicate that U mobilization took place within the last 350,000 years. U dissolution currently continues and it is controled by lateral groundwater flow whereas vertical diffusion appears negligible. Dissolved alkaline earths concentrations and the 87Sr/86Sr ratio indicate that solutes exchanges take place through uppermost low permeable granite followed by mixing with more dilute and Cl-type shallow groundwater.

Groundwater–surface‐water interactions in a braided river: a tracer‐based assessment

AbstractNatural tracers (alkalinity and silica) were used to infer groundwater–surface‐water exchanges in the main braided reach of the River Feshie, Cairngorms, Scotland. Stream‐water samples were collected upstream and downstream of the braided section at fortnightly intervals throughout the 2001–2002 hydrological year and subsequently at finer resolution over two rainfall events. The braided reach was found to exert a significant downstream buffering effect on the alkalinity of these waters, particularly at moderate flows (4–8 m3 s−1/≅Q30–70). Extensive hydrochemical surveys were undertaken to characterize the different source waters feeding the braids. Shallow groundwater flow systems at the edge of the braided floodplain, recharged by effluent streams and hillslope drainage, appeared to be of particular significance. Deeper groundwater was identified closer to the main channel, upwelling through the hyporheic zone. Both sources contributed to the significant groundwater–surface‐water interactions that promote the buffering effect observed through the braided reach. Their impact was less significant at higher flows (>15 m3 s−1/>Q10) when acidic storm runoff from the peat‐covered catchment headwaters dominated, as well as under baseflow conditions (<4 m3 s−1/<Q70), when upstream alkalinity was already buffered owing to headwater groundwater sources assuming dominance. The significant temporally and spatially dynamic influence of these groundwater–surface‐water interactions was therefore seen to have important implications for both catchment functioning and instream ecology. Copyright © 2004 John Wiley & Sons, Ltd.

Origin of methane in the Elk Valley coalfield, southeastern British Columbia, Canada

Coalbead methane is part of the non-conventional gas reservoirs and makes a significant contribution to gas production in some parts of the world. Initially, it was assumed that coalbed methane was of thermogenic origin, but most recent studies based on isotope and chemical data and taking into account the hydrogeology of the basin have demonstrated that secondary biogenic gases are formed in many coal-bearing basins. This study, using a similar approach, evaluated the origin of the gas in the Elk Valley coalfield located in British Columbia, Canada. Isotope data in methane samples collected from testhole wells and piezometers show a range that varies from −51.8‰ to −65.4‰ and −303‰ to −415‰ for δ 13C and δ 2H, respectively. The δ 2H data, which are among the most depleted data reported for coalbed methane, have to be related to the very depleted δ 2H values of the groundwater (−148‰ to −163‰). Isotope and chemical data collected from DIC show a trend of increasing δ 13C values (−11.9‰ to +34.9‰) associated with an increase in DIC concentration (216 to 1650 mg/l). The most 13C depleted DIC and low DIC waters are found in the shallow groundwater flow system representing conditions close to recharge areas, while the most 13C enriched DIC and high concentration DIC waters are found in the discharge areas associated with a deep groundwater flow system. The DIC pattern, which is typical for methanogenic aquifers, and the isotope data obtained in methane samples clearly indicate that the gas found in the Elk Valley coalfield is mainly biogenic in origin. This study reaffirms that an approach that combines an evaluation of the groundwater flow system, the isotopic characterization and concentration pattern of the main carbon pools (CH 4 and DIC), and the isotopic characterization of the groundwater is needed to fully evaluate the origin of gases in coal basins.

Hydrogeological constraints on riparian buffers for reduction of diffuse pollution: examples from the Bear Creek watershed in Iowa, USA

Riparian Management Systems (RiMS) have been proposed to minimize the impacts of agricultural production and improve water quality in Iowa in the Midwestern USA. As part of RiMS, multispecies riparian buffers have been shown to decrease nutrient, pesticide, and sediment concentrations in runoff from adjacent crop fields. However, their effect on nutrients and pesticides moving in groundwater beneath buffers has been discussed only in limited and idealized hydrogeologic settings. Studies in the Bear Creek watershed of central Iowa show the variability inherent in hydrogeologic systems at the watershed scale, some of which may be favorable or unfavorable to future implementation of buffers. Buffers may be optimized by choosing hydrogeologic systems where a shallow groundwater flow system channels water directly through the riparian buffer at velocities that allow for processes such as denitrification to occur.

Ownership of Mine‐Tunnel Discharge

AbstractWater discharging from numerous tunnels constructed during mining in the Wasatch Mountains near Salt Lake City, Utah, flows into nearby creeks. Disputes over ownership of water feeding the creeks have resulted in extensive litigation. In the course of a legal dispute over ownership of outflow from the Kentucky‐Utah (K‐U) Tunnel we evaluated the patterns and rates of ground water flow using an integrated study of the geology, chemistry, isotopes, and chlorofluorohydrocarbon (CFC) composition of the water. A sequence of sedimentary rocks with a range of hydraulic conductivity values has been folded, faulted, intruded by igneous rocks, and then eroded to create the rough topography of the Wasatch Mountains. The similarity of composition among tunnel discharge, springs, and base flow in the creek indicates that the creek is fed by ground water circulating in local, shallow flow systems. Results of numerical simulations of ground water flow indicate that the K‐U Tunnel likely intercepts ground water that, in the absence of the tunnel, would ultimately flow in the subsurface to Big Cottonwood Creek. CFC and tritium contents of the water indicate flow weighted average ground water travel times range from four to 23 years and support our conclusion that water discharging from the tunnel is moving within a shallow ground water flow system. Despite sparse data, the scientific understanding was deemed sufficient for the judge to rule that owners of the surface water also own the tunnel discharge because, in the absence of the tunnel, this water would supply the stream.

Interactions between groundwater and surface water in a virginia coastal plain watershed. 1. Hydrological flowpaths

AbstractA field study of surface water and groundwater interactions during baseflow and stormflow conditions was performed at the Reedy Creek watershed in the Virginia Coastal Plain. Three estimates of the average saturated hydraulic conductivity (Ks) of the unconfined aquifer were in reasonable agreement (ranging from 0.0033 to 0.010 cm/s), indicating that baseflow in the creek is entirely from the drainage of shallow groundwater from the relatively thin (1–6 m thick) unconfined aquifer. This relatively permeable surficial aquifer was found to be underlain by dark, olive grey, clay‐silt and diatomaceous Miocene deposits of low permeability known as the Calvert Formation, which is believed to function as a confining bed in the area.A chemical hydrograph separation technique was used to resolve the contributions of [old] (pre‐event) and [new] (event) water to stormflow. Results from a major rainstorm indicated that old water dominated the stormflow response of the watershed, although the new water contribution approached 40% at the hydrograph peak. Stormflow at Reedy Creek appears to result from saturation overland flow from variable source areas which include the stream channels and a significant part of the riparian wetland area. This response appears to be attributable to the transient dynamics of the shallow groundwater flow system and to the formation of localized groundwater mounds which raise the water‐table to the wetland surface.

Hydrology and chemistry of groundwater and seasonal ponds in the Atlantic Coastal Plain in Delaware, USA

The hydrochemistry of small seasonal ponds was investigated by studying relations between ground-water and surface water in a forested Coastal Plain drainage basin. Observation of changes in the water table in a series of wells equipped with automatic water-level recorders showed that the relation between water-table configuration and basin topography changes seasonally, and particularly in response to spring recharge. Furthermore, in this study area the water table is not a subdued expression of the land surface topography, as is commonly assumed. During the summer and fall months, a water-table trough underlies sandy ridges separating the seasonal ponds, and maximum water-table altitudes prevail in the sediments beneath the dry pond bottoms. As the ponds fill with water during the winter, maximum water-table altitudes shift to the upland-margin zone adjacent to the seasonal ponds. Increases in pond stage are associated with the development of transient water-table mounds at the upland-margin wells during the spring. The importance of small local-flow systems adjacent to the seasonal ponds also is shown by the similarities in the chemistry of the shallow groundwater in the upland margin and water in the seasonal ponds. The upland margin and surface water samples have low pH (generally less than 5.0), and contain large concentrations of dissolved aluminum (generally more than 100 μg 1 −1), and low bicarbonate concentrations (2 mg l 4 or less). In contrast, the parts of the surficial aquifer that do not experience transient mounding have higher pH and larger concentrations of bicarbonate. These results suggest that an understanding of the hydrochemistry of seasonally ponded wetlands requires intensive study of the adjacent shallow groundwater-flow system.

和迩(わに)川デルタからの琵琶湖への地下水の漏出

Water levels of 39 wells and water qualities of samples from the wells have been measured to determine the groundwater movement in the Wani river delta, on the western shore of Lake Biwa. In the littoral zone around the delta, direct seepage measurements and sampling of seepage water and porewater have been carried out to clarify the seepage process of groundwater flow into Lake Biwa.Shallow groundwater is strongly recharged by effluent water from the Wani river an d moves into the lake in the direction perpendicular to the shore line. Slopes of water table near the shore range from 0.005∼0.01 at Wani beach and about 0.005 at Imajyuku beach throughout the year.Both seepage flux and electrical- conductivity of seepage water change spatially along the shore line. Areas along the transect perpendicular to the shore line show high values of seepage flux and electrical conductivity while near the shore, these values decrease abruptly with increasing distance from the shore. These patterns along the transect perpendicular to the shore line, however, do not show any significant spatial and seasonal changes throughout the study period. There are similar correlation between the quality of seepage waterwithin about 20m from the shore and that of well water near the shore. The distribution of chloride concentrations of groundwater in the vicinity of the lake indicates that the shallow groundwater (3∼5m depth below water table) in the aquifer discharges almost vertically into the lake within 20∼30m from the shore; the deeper water discharges into the lake at a distance of over 30m offshore.These results indicate that it is important to measure groundwater seepage at many sites near and along the shore line in order to estimate seepage amount into a lake and also suggest that the direct method employed is available to investigate shallow groundwater flow system near shore.

An integrated approach to quantify groundwater transport of phosphorus to Narrow Lake, Alberta

An integrated approach was used to quantify groundwater phosphorus flux to Narrow Lake, a small glacial-terrain lake in central Alberta. Data from a drilling program, major ion concentrations, environmental isotopes, and computer simulations indicated that the lake gains water through the nearshore region from a small, shallow groundwater flow system; at deep offshore regions, water moves from the lake to the groundwater flow system. Seepage flux was quantified by water budget, Darcy’s equation with data from wells near the lake, Darcy’s equation with data from minipiezometers in the lake, and seepage meters. Whole-lake seepage flux determined from minipiezometer data (30 mm yr−1) was only 10–25% of the other estimates (mean, 221 mm yr −1; range, 133–332 mm yr−1 from seepage meter and water budget data, respectively). Groundwater contributed ∼30% of the annual water load to the lake. The P concentration, [P], in pore water from lake sediments (mean, 175 mg m−3) was 8 times higher than groundwater from wells near the lake (mean, 21 mg m−1). Thus, if well water was used to estimate the [P] of the seepage water, the rate of groundwater P loading to the lake would be underestimated. The rate of groundwater P loading to the lake computed from average seepage flux and average pore-water [P] was 39 mg m−2 yr−1, and groundwater may be the largest single source of P to epilimnetic water in the lake.