Showing papers on "Groundwater flow published in 2003"

A mechanism for sustained groundwater pressure changes induced by distant earthquakes

Abstract: [1] Large, sustained well water level changes (>10 cm) in response to distant (more than hundreds of kilometers) earthquakes have proven enigmatic for over 30 years. Here we use high sampling rates at a well near Grants Pass, Oregon, to perform the first simultaneous analysis of both the dynamic response of water level and sustained changes, or steps. We observe a factor of 40 increase in the ratio of water level amplitude to seismic wave ground velocity during a sudden coseismic step. On the basis of this observation we propose a new model for coseismic pore pressure steps in which a temporary barrier deposited by groundwater flow is entrained and removed by the more rapid flow induced by the seismic waves. In hydrothermal areas, this mechanism could lead to 4 � 10 � 2 MPa pressure changes and triggered seismicity. INDEX TERMS: 1829 Hydrology: Groundwater hydrology; 7209 Seismology: Earthquake dynamics and mechanics; 7212 Seismology: Earthquake ground motions and engineering; 7260 Seismology: Theory and modeling; 7294 Seismology: Instruments and techniques; KEYWORDS: earthquakes, triggering, time-dependent hydrology, fractures Citation: Brodsky, E. E., E. Roeloffs, D. Woodcock, I. Gall, and M. Manga, A mechanism for sustained groundwater pressure changes induced by distant earthquakes, J. Geophys. Res., 108(B8), 2390, doi:10.1029/2002JB002321, 2003.

Geomorphic controls on hyporheic exchange flow in mountain streams

Abstract: [1] Hyporheic exchange flows were simulated using MODFLOW and MODPATH to estimate relative effects of channel morphologic features on the extent of the hyporheic zone, on hyporheic exchange flow, and on the residence time of stream water in the hyporheic zone. Four stream reaches were compared in order to examine the influence of stream size and channel constraint. Within stream reaches, the influence of pool-step or pool-riffle sequences, channel sinuosity, secondary channels, and channel splits was examined. Results showed that the way in which channel morphology controlled exchange flows differed with stream size and, in some cases, with channel constraint. Pool-step sequences drove hyporheic exchange in the second-order sites, creating exchange flows with relatively short residence times. Multiple features interacted to drive hyporheic exchange flow in the unconstrained fifth-order site, where pool-riffle sequences and a channel split created exchange flows with short residence times, whereas a secondary channel created exchange flows with long residence times. There was relatively little exchange flow in the bedrock-constrained fifth-order site. Groundwater flow models were effective in examining the morphologic features that controlled hyporheic exchange flow, and surface-visible channel morphologic features controlled the development of the hyporheic zone in these mountain streams.

Factors controlling riffle‐scale hyporheic exchange flows and their seasonal changes in a gaining stream: A three‐dimensional groundwater flow model

Abstract: [1] Subsurface flow within a single riffle of a low-gradient gravel bed stream was modeled in three dimensions using MODFLOW, a finite difference groundwater flow model Model simulations showed that exchange flows can only occur in this low-gradient, gaining stream because of a zone of alluvial sediment around the stream that has much higher permeability than the surrounding catchment (K = 10 - 4 m s- 1 , compared with K = 10 - 6 to 10 - 8 m s - 1 ) The key factors controlling exchange flow within the alluvial zone were identified as the hydraulic conductivity of the alluvium, the hydraulic gradient between upstream and downstream ends of the riffle, and the flux of groundwater entering the alluvium from the sides and beneath In the study riffle each of these factors changes with season, causing a reversal of flow paths in the alluvium and a reduction in exchange flows from about 02-05 m 3 d-' per meter stream length in summer to about 0008-004 m 3 d - 1 per meter stream length during fall to spring The model also revealed that exchange flows are up to twice as strong, but more variable, at the sides of the stream than near the center, and that vertical flow paths beneath the channel are more persistent under the range of conditions modeled than lateral flow paths into the banks

Simulation of the development of karst aquifers using a coupled continuum pipe flow model

Abstract: [1] This paper is intended to provide insight into the controlling mechanisms of karst genesis based on an advanced modeling approach covering the characteristic hydraulics in karst systems, the dissolution kinetics, and the associated temporal decrease in flow resistance Karst water hydraulics is strongly governed by the interaction between a highly conductive low storage conduit network and a low-conductive high-storage rock matrix under variable boundary conditions Only if this coupling of flow mechanisms is considered can an appropriate representation of other relevant processes be achieved, eg, carbonate dissolution, transport of dissolved solids, and limited groundwater recharge Here a parameter study performed with the numerical model Carbonate Aquifer Void Evolution (CAVE) is presented, which allows the simulation of the genesis of karst aquifers during geologic time periods CAVE integrates several important features relevant for different scenarios of karst evolution: (1) the complex hydraulic interplay between flow in the karst conduits and in the small fissures of the rock matrix, (2) laminar as well as turbulent flow conditions, (3) time-dependent and nonuniform recharge to both flow systems, (4) the widening of the conduits accounting for appropriate physicochemical relationships governing calcite dissolution kinetics This is achieved by predefining an initial network of karst conduits (“protoconduits”) which are allowed to grow according to the amount of aggressive water available due to hydraulic boundary conditions The increase in conduit transmissivity is associated with an increase in conduit diameters while the conductivity of the fissured system is assumed to be constant in time The importance of various parameters controlling karst genesis is demonstrated in a parameter study covering the recharge distribution, the upgradient boundary conditions for the conduit system, and the hydraulic coupling between the conduit network and the rock matrix In particular, it is shown that conduit diameters increase in downgradient or upgradient direction depending on the spatial distribution (local versus uniform) of the recharge component which directly enters the conduit system

Groundwater Hydrology: Conceptual and Computational Models

Abstract: Preface1 IntroductionPART I: BASIC PRINCIPLES2 Background to Groundwater Flow3 Recharge due to Precipitation or Irrigation4 Interaction between Surface Water and GroundwaterPART II: RADIAL FLOW5 Radial Flow to Pumped Boreholes - Fundamental Issues6 Large Diameter Wells7 Radial Flow where Vertical Components of Flow are Significant8 Practical Issues of Interpretation and Assessing ResourcesPART III: REGIONAL GROUNDWATER FLOW9 Regional Groundwater Studies in which Transmissivity is Effectively Constant10 Regional Groundwater Flow in Multi-Aquifer Systems11 Regional Groundwater Flow with Hydraulic Conductivity Varying with Saturated Thickness12 Numerical Modelling InsightsAppendix: Computer Program for Two-zone ModelList of SymbolsReferencesIndex

Karstification and groundwater flow

Abstract: One of the principal aims of hydrogeology is to propose a reasonably adequate reconstruction of the groundwater flow field, in space and in time, for a given aquifer. For example, interpretation of the chemical and isotopic composition of groundwater, understanding of the geothermal conditions (anomalies) or forecasting the possible effects of industrial waste disposals and of intensive exploitation nearly always would require the knowledge of the regional and/or local groundwater flow systems such as defined by (Toth 1963). The problem of estimating the groundwater flow field in fractured and karstified aquifers is approached within the framework of a conceptual diagram showing the relationship between groundwater flow, hydraulic parameters (aquifer properties and boundary conditions), distribution of voids and geological factors. Autoregulation between groundwater flow and karst aquifer properties, duality of karst, nested model of geological discontinuities, scale effect on hydraulic parameters and use of numerical finite element models to check the interpretation of the global response of karst springs are some of the subjects addressed by the author. Inferences on groundwater flow regime with respect to the stage of karst evolution can be made only if the hydraulic parameter fields and the boundary conditions are known by direct observations, or estimated by indirect methods for the different types of karst. Practical considerations on the monitoring strategies applied for karst aquifers, and on the interpretation of the global response obtained at karst springs will complete the paper, which throughout reflects the point of view of a hydrogeologist. 1 Introductory remarks 1.1 Relation between groundwater flow field, hydraulic parameters and geological factors In hydrogeology we do not study karstification or karst evolution for themselves: we are interested in them only in so far as they exert an influence on the groundwater flow field. The reconstruction of a regional groundwater flow field, which is consistent with a given hydraulic conductivity field and with given boundary conditions, nearly always requires the use of numerical models. Presently we have a wide variety of equations describing the groundwater movement in various domains (see, for example, for saturated/unsaturated flow and groundwater/surface-water interaction Tregaro 2000; Ababou et al. 1998) and we can solve them quite accurately by numerical models if we know, in the modelled region, the field of the relevant hydraulic parameters (hydraulic conductivity, storage coefficient, efficient porosity, etc., in each point of the aquifer), as well as the initial and the boundary conditions (mainly infiltration and fixed head values, such as altitude of springs, lakes or rivers). With the use of numerical models explicitly appears a very important fact: as groundwater flow depends only on hydraulic parameters and on boundary conditions, the geological, geomorphological and climatic factors will exert their influence on the groundwater movement solely through the hydraulic parameter fields and the boundary conditions. If we 1 Publié dans Speleogenesis and Evolution of Karst Aquifers 1, issue 3, pp. 155-192, 2003 source qui doit être utilisée pour toute référence à ce travail

Spatial and temporal distributions of submarine groundwater discharge rates obtained from various types of seepage meters at a site in the Northeastern Gulf of Mexico

Abstract: Direct measurements of submarine groundwaterdischarge (SGD) were taken by three different(continuous heat, heat pulse, and ultrasonic)types of automated seepage meters as well asstandard Lee-type manually operated meters. SGD flux comparisons and the spatial andtemporal variations in groundwater flow wereanalyzed. Seepage rates measured by thedifferent meters agree relatively well witheach other (more than 80% agreement). Comparisons of flux rates as a function ofdistance offshore using exponentialapproximations show that more than fivemeasurement locations (200 m offshore) areneeded for a precise integrated estimation ofSGD offshore within an accuracy of ±10%. Thedominant period of seepage variations isestimated to be about 12 hours, which closelymatches the semidiurnal tides in this area. Our analysis also shows that short durationmeasurement periods may cause significantunderestimates or overestimates of the dailyaveraged groundwater flow rates (±25%–±60% difference when the measurement durationis less than 12 hours). Thus, continuousmeasurements of SGD using automated seepagemeters with high time resolution should enableus to evaluate temporal and spatial variationsof dissolved material transports viagroundwater pathways. Such inputs may affectbiogeochemical phenomena in the coastal zone.

Effects of season and man-made changes on baseflow and flow recession: case studies

Abstract: Discharge hydrographs of rivers carry the cumulated information on the various hydrological processes in catchments and the influences imposed on them. From the analysis of observed baseflow recessions, direct and groundwater flow can be separated and time series of the main components of the underlying groundwater balance, namely discharge, evapotranspiration losses, abstractions, storage and recharge, can be derived. This inverse, or downward, approach was applied to daily streamflow data of rivers in different climate zones (Germany, Western Australia, Turkey) under different influences. The mostly shallow, unconfined aquifers were found to react as nonlinear reservoirs and an appropriate recession function was used instead of the traditional exponential function of the linear reservoir. However, though the actual storage–discharge relationships of aquifers will hardly change, flow recession properties are subject to seasonal variation and changes due to evapotranspiration and other fluxes and abstractions from the groundwater. As demonstrated in the case studies, recession analysis and baseflow separation permits the detection and quantification of these changes. In a separate application, the method is used to identify groundwater flow intruded into leaky sewer lines. Copyright © 2003 John Wiley & Sons, Ltd.

Deformation mechanisms and hydraulic properties of fault zones in unconsolidated sediments; the Roer Valley Rift System, The Netherlands

Abstract: In general, faults cutting through the unconsolidated sediments of the Roer Valley Rift System (RVRS), The Netherlands, form strong barriers to horizontal groundwater flow. The relationships between deformation mechanisms along fault zones and their impact on the hydrogeological structure of the fault zone are analyzed in a shallow (0–5 m below land surface) trench over one of the faults in the study area. Recently developed digital-image-analysis techniques are used to estimate the spatial distribution of hydraulic conductivity at the millimeter-scale and to describe the micromorphologic characteristics of the fault zone. In addition, laboratory measurements of hydraulic conductivity on core-plug samples show the larger-scale distribution of hydraulic conductivity in the damage zone flanking the main fault plane. Particulate flow is the deformation mechanism at shallow depths, which causes the damage zone around the fault, in the sand-rich parts, to have a relatively enhanced hydraulic conductivity. The fault core is characterized by reduced hydraulic conductivity due to clay smearing, grain-scale mixing, and iron-oxide precipitation.

Land subsidence caused by groundwater exploitation in Suzhou City, China

Abstract: Suzhou City, located at the lower reaches of the Yangtze River in southeastern Jiangsu Province, is one of the few cities in China which suffer from severe ground settlement. A research project was carried out to investigate this problem. Geological and hydrogeological studies show that there is a multi-layered aquifer system with three distinct, soft mud layers of marine and lagoonal origins. An examination of historical records of groundwater extraction, water levels, and ground settlement shows that the ground subsidence is associated with the continuously increasing groundwater extraction in the deep, confined aquifer. It is believed that the consolidation of the soft mud layers, especially the third layer which is thick and close to the main pumped aquifer, contributes to the ground settlement. A three-dimensional finite difference numerical model representing the multi-layered aquifer system was developed to study the ground settlement in response to groundwater extraction. By calibrating the model with both the measured groundwater level and ground settlement, the aquifer parameters were estimated. The model outputs fit reasonably well with the observed results, which indicates that the numerical model can reproduce the dynamic processes of both groundwater flow and soil consolidation. The hydraulic conductivity of the third mud layer near the center of the ground settlement has been reduced by over 30% in the last 14 years. The gradual deterioration in the hydraulic conductivity of the mud may have significant adverse effect on the sustainable groundwater resource of the deep confined aquifer, since the recharge from the shallow aquifers through the mud layer is the only source of water to the deep aquifer. An analysis of the spatial distributions of groundwater drawdown and ground settlement shows that the area with maximum drawdown is not necessarily the area with maximum ground settlement due to the occurrence of the soft mud layer. A simple reallocation in pumping rates on the basis of the spatial distribution of the thick mud layer could significantly reduce the ground settlement.

Groundwater flow, heat transport, and water table position within volcanic edifices: Implications for volcanic processes in the Cascade Range

Abstract: [1] The position of the water table within a volcanic edifice has significant implications for volcano hazards, geothermal energy, and epithermal mineralization. We have modified the HYDROTHERM numerical simulator to allow for a free-surface (water table) upper boundary condition and a wide range of recharge rates, heat input rates, and thermodynamic conditions representative of continental volcano-hydrothermal systems. An extensive set of simulations was performed on a hypothetical stratovolcano system with unconfined groundwater flow. Simulation results suggest that the permeability structure of the volcanic edifice and underlying material is the dominant control on water table elevation and the distribution of pressures, temperatures, and fluid phases at depth. When permeabilities are isotropic, water table elevation decreases with increasing heat flux and increases with increasing recharge, but when permeabilities are anisotropic, these effects can be much less pronounced. Several conditions facilitate the ascent of a hydrothermal plume into a volcanic edifice: a sufficient source of heat and magmatic volatiles at depth, strong buoyancy forces, and a relatively weak topography-driven flow system. Further, the plume must be connected to a deep heat source through a pathway with a time-averaged effective permeability ≥1 × 10−16 m2, which may be maintained by frequent seismicity. Topography-driven flow may be retarded by low permeability in the edifice and/or the lack of precipitation recharge; in the latter case, the water table may be relatively deep. Simulation results were compared with observations from the Quaternary stratovolcanoes along the Cascade Range of the western United States to infer hydrothermal processes within the edifices. Extensive ice caps on many Cascade Range stratovolcanoes may restrict recharge on the summits and uppermost flanks. Both the simulation results and limited observational data allow for the possibility that the water table beneath the stratovolcanoes is relatively deep.

Seepage area and rate of bedrock groundwater discharge at a granitic unchanneled hillslope

Abstract: [1] Recent studies have demonstrated the importance of water movement through the bedrock in the rainfall-runoff process on steep hillslopes However, quantitative information on this process is still limited The objective of this study was to address the following questions: (1) How large is the area where bedrock groundwater seeps into the soil layer, and (2) what is the rate of water flow out of the bedrock? To address these questions, detailed hydrological, hydrochemical, and thermal measurements were conducted at a forested steep unchanneled granitic concave slope in the Tanakami Mountains, central Japan The relationship between the amplitude of annual soil temperature variation and the measurement depth showed that in a normal low-flow period, the seepage area ranged between 14 and 21 m2 and the ratio of this area to that of the whole catchment was about 20% In a drought period the seepage area ranged between 35 and 55 m2, and the ratio to the whole catchment was around 05% The variation in the area of seepage was controlled both by the short-term precipitation pattern during the preceding several weeks and by the long-term pattern over several preceding months A two-component geochemical hydrograph separation indicated that the ratio of bedrock groundwater to streamflow was about 082 for the normal low-flow periods and 090 for the drought period The rate of flow out of the bedrock into the soil layer ranged from 05 to 33 m3 d−1 That is, although the seepage area was small (05–20% of the catchment), the contribution of bedrock groundwater was considerable (50–95% of streamflow)

Submarine groundwater discharge estimates at a Florida coastal site based on continuous radon measurements

Abstract: The direct discharge of groundwater into thecoastal zone has received increased attentionin the last few years as it is now recognizedthat this process represents an importantpathway for material transport. Assessingthese material fluxes is difficult, as there isno simple means to gauge the water flux. Weestimated the changing flux of groundwaterdischarge into a coastal area in the northeastGulf of Mexico (Florida) based on continuousmeasurements of radon concentrations over aseveral day period. Changing radon inventorieswere converted to fluxes after accounting forlosses due to atmospheric evasion and mixing. Radon fluxes are then converted to groundwaterinflow rates by estimating the radonconcentration of the fluids discharging intothe study domain. Groundwater flow was also assessed via seepagemeters, radium isotopes, and modeling duringthis period as part of an ``intercomparison''study. The radon results suggest that the flowis: (1) highly variable with flows ranging from∼5 to 50 cm/day; and (2) strongly influenced bythe tides, with spikes in the flow every 12hours. The discharge estimates and pattern offlow derived from the radon model matches theautomated seepage meter records very closely.

Pleistocene hydrogeology of the Atlantic continental shelf, New England

Abstract: Salinity data from the Atlantic continental shelf off New England indicate that the freshwater/saltwater interface is far out of equilibrium with modern sea-level conditions. More than 150 km offshore of Long Island, New York. aquifer salinity levels are less than 5 parts per thousand (5 ppt). Salinity levels within confining units beneath Nantucket Island, Massachusetts, are 30%. 70% of seawater levels and exhibit a par abolic profile consistent with ongoing vertical diffusion. Here, we evaluate two fluid-flow-inducing mechanisms that could explain the apparent flushing of these coastal-plain aquifers: (1) meteoric recharge during Pleistocene sea-level low stands, and (2) subglacial recharge from the Laurentide Ice Sheet. Analytical models of vertical solute diffusion for the Nantucket confining units suggest that flushing of aquifers beneath Nantucket began in the late Pleistocene between ca. 195 and 21 ka; the models assume a diffusion coefficient of 3.0 × 10 - 1 1 m 2 /s. Cross-sectional numerical models of variable-density groundwater flow, heat, and solute transport could not reproduce the relatively low-salinity groundwaters observed off Long Island by applying boundary conditions consistent with Pleistocene seal-level fluctuations. Observed salinity conditions were most closely matched in the models by also including the effects of sub-glacial recharge from the Laurentide Ice Sheet and allowing groundwater to discharge from Miocene aquifers along submarine canyons near the continental slope. Simulated recharge induced by Laurentide Ice Sheet meltwater was probably short lived hut, on average, about two to ten times greater than modern subaerial levels. A sensitivity analysis performed using our cross-sectional model suggests that a narrow range of hydrologic conditions can drive fresh water long distances offshore across the continental shelf.

Using natural distributions of short‐lived radium isotopes to quantify groundwater discharge and recharge

Abstract: Radium activity in pore water of wetland sediments often differs from the amount expected from local production, decay, and exchange with solid phases. This disequilibrium results from vertical transport of radium with groundwater that flows between the underlying aquifer and surface water. In situations where groundwater recharge or discharge is significant, the rate of vertical water flow through wetland sediment can be determined from the radium disequilibrium by a combined model of transport, production, decay, and exchange with solid phases. We have developed and tested this technique at three sites in the freshwater portion of the Everglades by quantifying vertical advective velocities in areas with persistent groundwater recharge or discharge and estimating a coefficient of dispersion at a site that is subject to reversals between recharge and discharge. Groundwater velocities ( v) were determined to be between 0 and 20.5 cm d 21 for a recharge site and 1.5 6 0.4 cm d 21 for a discharge site near Levee 39 in the Everglades. Strong gradients in 223 Ra and 224 Ra usually occurred at the base of the peat layer, which avoided the problems of other tracers (e.g., chloride) for which greatest sensitivity occurs near the peat surface—a zone readily disturbed by processes unrelated to groundwater flow. This technique should be easily applicable to any wetland system with different production rates of these isotopes in distinct sedimentary layers or surface water. The approach is most straightforward in systems where constant pore-water ionic strength can be assumed, simplifying the modeling of radium exchange.

An Illustrated Handbook of LNAPL Transport and Fate in the Subsurface

Abstract: Executive summary Dense non-aqueous phase liquids (DNAPLs) such as creosote, coal tar, chlorinated solvents and polychlorinated biphenyl oils represent a particular class of soil and groundwater contaminant that exist as a separate liquid phase in the presence of water. DNAPLs come to rest in the subsurface as disconnected blobs and ganglia of liquid referred to as residual DNAPL, and in potentially mobile distributions referred to as pools. The region of the subsurface containing residual and pooled DNAPL is referred to as the source zone. Groundwater flowing through the source zone slowly dissolves the DNAPL, giving rise to aqueous phase plumes of contamination hydraulically down-gradient of the source zone. Some DNAPL compounds are resistant to biodegradation and sorb little; they can therefore give rise to substantial aqueous phase plumes. Other DNAPL compounds are relatively immobile in groundwater and, therefore, are highly retarded relative to the rate of groundwater flow. In unsaturated media, volatile DNAPLs give rise to vapour phase contamination. Because DNAPLs are only slightly soluble in water, DNAPL source zones can persist for many decades and, in some cases, even hundreds of years. Some DNAPLs are highly toxic and even very low concentrations in groundwater or the atmosphere can pose an unacceptable risk to human health or the environment. The fact that DNAPLs are denser than water allows them to migrate to substantial depths below the water table in both unconsolidated deposits and fractured bedrock. Delineating the spatial extent of the DNAPL source zone at a site can be a substantial undertaking, requiring at times several years of investigation and significant financial resources. Remediation strategies are site-specific, with separate approaches often warranted for the DNAPL source zone and its associated aqueous phase plume. There has been limited success in removing all DNAPL from below the water table at sites, particularly in a fractured rock environment. Remediation strategies are therefore often directed towards source zone containment or stabilisation, partial mass removal, plume management or plume interception, within the framework of appropriate risk-management objectives. The purpose of this handbook is to provide a user-friendly overview of the nature of DNAPL contamination in a UK context. It is intended to assist site investigators, site owners and regulators in conducting site investigations, conducting risk assessments and selecting remediation approaches. While this handbook reflects the state-of-the-art at the time of publication, it should be noted that the discipline of groundwater and soil contamination by hazardous organic liquids is evolving continuously and is relatively ‘young’ compared with many other areas of science and engineering. Readers are therefore advised to keep abreast of the new advances in understanding and approaches expected in the foreseeable future.

Use of water balance calculation and tritium to examine the dropdown of groundwater table in the piedmont of the North China Plain (NCP)

Abstract: The groundwater table in the piedmont plain was only about 1–2 m in depth in the 1950s and 1960s, but it lowered dramatically afterwards to about 25–27 m in depth (currently 21–23 m above sea level) due to overpumping of groundwater and drought in the region. This change has adversely affected the sustainable development and food supply of this important agricultural area. The groundwater table at Luancheng Experimental Station of the Chinese Academy of Sciences, located in the piedmont, dropped from 39.36 m in 1975 to 21.47 m above sea level in 1999, at an average rate of 0.72 m/year. Water balance components, such as daily rainfall, pan-evaporation, and evapotranspiration (by lysimeter after 1995) have been recorded since the 1970s, and they were used as variants to simulate monthly water table change based on a physically based statistical model. Groundwater samples were collected during the period 1998–2001, and tritium was measured in the laboratory to trace the groundwater flow from the Taihang Mountains to the piedmont. A reasonable exploitation rate of 150 mm/year was obtained from the model by assuming the annual water table is constant. The recharge and groundwater flow from the Taihang Mountains plays an important role in the water balance of the piedmont area, and it was estimated to be about 112.5 mm/year by using the variation of tritium with the depth, which followed a good exponential function. The simple water balance calculation indicated that the water table could recede at a rate of 0.8 m/year, which is close to the actual situation.

Calibration of regional groundwater flow models: Working toward a better understanding of site-specific systems

Abstract: [1] This work highlights the lack of unique solutions for regional groundwater flow models and quantifies the degree of freedom concerning hydraulic conductivities for models calibrated on measured hydraulic heads. The potential of 4He as an independent tracer at reducing the nonuniqueness problem is tested. Four different calibrated groundwater flow scenarios are presented for the Carrizo aquifer and surrounding formations in Texas. It is shown that variations of hydraulic conductivities up to 2 orders of magnitude in the Carrizo aquifer and overlying confining layer lead to similar calculated hydraulic heads. No clear-cut arguments are present to invalidate one groundwater flow scenario over a different one. In contrast, when tested with a 4He transport conceptual model, all groundwater flow scenarios except one failed to reproduce a coherent 4He transport behavior in the system. This study exemplifies possible future contributions of 4He at discerning which model most closely replicates natural conditions.

On the possibility for generic modeling of submarine groundwater discharge

Abstract: We simulate large-scale dynamics of submarine groundwater discharge (SGD) in three different coastal aquifers on the Mediterranean Sea. We subject these aquifers to a wide range of different groundwater management conditions, leading to widely different net groundwater drainage from land to sea. The resulting SGD at steady-state is quantifiable and predictable by simple linearity in the net land-determined groundwater drainage, defined as total fresh water drainage minus groundwater extraction in the coastal aquifer system. This linearity appears to be general and independent of site-specific, variable and complex details of hydrogeology, aquifer hydraulics, streamlines and salinity transition zones in different coastal systems. Also independently of site-specifics, low SGD implies high seawater content due to seawater intruding into the aquifer and mixing with fresh groundwater within a wide salinity transition zone in the aquifer. Increasing SGD implies decreasing seawater content, decreased mixing between seawater and fresh groundwater and narrowing of the salinity transition zone of brackish groundwater in the aquifer.

Groundwater-surface water interactions in a braided river: a tracer-based assessment.

Abstract: Natural 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/

Radon Tracing of Submarine Groundwater Discharge in Coastal Environments

Abstract: This chapter discusses the radon tracing of submarine groundwater discharge (SGD) in coastal environments. Direct discharge of groundwater into the coastal zone may be an important material flux pathway from land to sea in some areas. It has been largely ignored because of the difficulty in assessing its magnitude. While measurement problems persist, there is a growing recognition that groundwater flow into the sea is important. This chapter reviews an approach, using a simple one-dimensional model, to measure SGD via use of 222 Rn as a natural tracer. Radon has certain advantages over other potential geochemical tracers of groundwater discharge. Typically, it is greatly enriched in groundwater compared to seawater; it can be measured at very low concentrations, and is completely conservative. On the other hand, as a gas it is subject to losses at the air–sea interface which may limit its use in shallow water environments. The radon tracing is an excellent qualitative tool for identifying areas of spring or seepage inputs in most coastal environments. It is a good quantitative tool in shallow marine environments characterized by large amounts of SGD.