Showing papers on "Groundwater flow published in 2000"

Environmental Tracers in Subsurface Hydrology

Abstract: List of Contributors. Preface. Acknowledgements. 1. Determining Timescales for Groundwater Flow and Solute Transport P.G. Cook, J.-K. Bohlke. 2. Inorganic Ions as Tracers A.L. Herczeg, W.M. Edmunds. 3. Isotope Engineering - Using stable isotopes of the water to solve practical problems T.B. Coplen, et al. 4. Radiocarbon Dating of Groundwater Systems R.M. Kalin. 5. Uranium-Series Nuclides as Tracers in Groundwater Hydrology J.K. Osmond, J.B. Cowart. 6. Radon-222 L. DeWayne Cecil, J.R. Green. 7. Sulphur and Oxygen Isotopes in Sulphate R. Krouse, B. Mayer. 8. Strontium Isotopes R.H. McNutt. 9. Nitrate Isotopes in Groundwater Systems C. Kendall, R. Aravena. 10. Chlorine-36 F.M. Phillips. 11. Atmospheric Noble Gases M. Stute, P. Schlosser. 12. Noble Gas Radioisotopes: 37Ar, 85Kr, 39Ar, 81Kr H.H. Loosli, et al. 13. 3H and 3He D.K. Solomon, P.G. Cook. 14. 4He in Groundwater D.K. Solomon. 15. Chlorofluorocarbons L.N. Plummer, E. Busenberg. 16. delta11B, Rare Earth Elements, delta37Cl, 32Si, 35S, 129I A. Vengosh, et al. Appendices. Index.

Arsenic toxicity of groundwater in parts of the Bengal basin in India and Bangladesh: the role of Quaternary stratigraphy and Holocene sea-level fluctuation

Abstract: Arsenic toxicity in groundwater in the Ganges delta and some low-lying areas in the Bengal basin is confined to middle Holocene sediments. Dissected terraces and highlands of Pleistocene and early Holocene deposits are free of such problems. Arsenic-rich pyrite or other arsenic minerals are rare or absent in the affected sediments. Arsenic appears to occur adsorbed on iron hydroxide-coated sand grains and clay minerals and is transported in soluble form and co-precipitated with, or is scavenged by, Fe(III) and Mn(IV) in the sediments. It became preferentially entrapped in fine-grained and organic-rich sediments during mid-Holocene sea-level rises in deltaic and some low-lying areas of the Bengal basin. It was liberated subsequently under reducing conditions and mediated further by microbial action. Intensive extraction of groundwater for irrigation and application of phosphate fertilizer possibly triggered the recent release of arsenic to groundwater. This practice has induced groundwater flow, mobilizing phosphate derived from fertilizer, as well as from decayed organic matter, which has promoted the growth of sediment biota and aided the further release of arsenic. However, the environment is not sufficiently reducing to mobilize iron and arsenic in groundwater in the Ganges floodplains upstream of Rajmahal. Thus, arsenic toxicity in the groundwater of the Bengal basin is caused by its natural setting, but also appears to be triggered by recent anthropogenic activities.

Subsurface denitrification in a forest riparian zone: interactions between hydrology and supplies of nitrate and organic carbon.

Abstract: The influence of hydrology andpatterns of supply of electron donors and acceptors onsubsurface denitrification was studied in a forestriparian zone along the Boyne River in southernOntario that received high nitrogen inputs from a sandaquifer. Two hypotheses were tested: (1) subsurfacedenitrification is restricted to localized zones ofhigh activity; (2) denitrification zones occur atsites where groundwater flow paths transportNO3 − to supplies of available organiccarbon. A plume of nitrate-rich groundwater withconcentrations of 10–30 mg N L−1 flowed laterallyat depths of 1.5–5 m in sands beneath peat for ahorizontal distance of 100–140 m across the riparianzone to within 30–50 m of the river. In situ acetyleneinjections to piezometers revealed that significantdenitrification was restricted to a narrow zone ofsteep NO3 − and N2O decline at theplume margins. The location of these denitrificationsites in areas with steep gradients of groundwater DOCincrease supported hypothesis 2. Many of thesedenitrification “hotspots” occurred near interfacesbetween sands and either peats or buried river channeldeposits. Field experiments involving in situadditions of either glucose or NO3 − topiezometers indicated that denitrification wasC-limited in a large subsurface area of the riparianzone, and became N-limited beyond the narrow zone ofNO3 − consumption. These data suggest thatdenitrification may not effectively removeNO3 − from groundwater transported at depththrough permeable riparian sediments unlessinteraction occurs with localized supplies of organicmatter.

Determining Timescales for Groundwater Flow and Solute Transport

Abstract: One of the principal uses of environmental tracers is for determining the ages of soil waters and groundwaters. (We may refer to this as ‘hydrochronology’by analogy with the dating of solid materials known as geochronology.) Information on soil water and groundwater age enables timescales for a range of subsurface processes to be determined. For example, ‘groundwater stratigraphy’is used increasingly to decipher past recharge rates and conditions in unconfined aquifers, in much the same way that sedimentary stratigraphy yields information about past depositional environments. The use of environmental tracers to determine water ages allows groundwater recharge rates and flow velocities to be determined independently, and commonly more accurately, than with traditional hydraulic methods where hydraulic properties of aquifers are poorly known or spatially variable. Studies of groundwater residence times in association with groundwater contamination studies can enable historic release rates of contaminants and contaminant transport rates to be determined. Where input rates are known, measurements of groundwater contaminant concentrations, together with groundwater dating, can sometimes be used for estimating chemical reaction rates. The combination of these dating methods with stable isotope measurements has sometimes allowed changes in contaminant sources over time to be determined.

Mechanism and rate of denitrification in an agricultural watershed: Electron and mass balance along groundwater flow paths

Abstract: The rate and mechanism of nitrate removal along and between groundwater flow paths were investigated using a series of well nests screened in an unconfined sand and gravel aquifer. Intensive agricultural activity in this area has resulted in nitrate concentrations in groundwater often exceeding drinking water standards. Both the extent and rate of denitrification varied depending on the groundwater flow path. While little or no denitrification occurred in much of the upland portions of the aquifer, a gradual redox gradient is observed as aerobic upland groundwater moves deeper in the aquifer. In contrast, a sharp shallow redox gradient is observed adjacent to a third-order stream as aerobic groundwater enters reduced sediments. An essentially complete loss of nitrate concurrent with increases in excess N2 provide evidence that denitrification occurs as groundwater enters this zone. Electron and mass balance calculations suggest that iron sulfide (e.g., pyrite) oxidation is the primary source of electrons for denitrification. Denitrification rate estimates were based on mass balance calculations using nitrate and excess N2 coupled with groundwater travel times. Travel times were determined using a groundwater flow model and were constrained by chlorofluorocarbon-based age dates. Denitrification rates were found to vary considerably between the two areas where denitrification occurs. Denitrification rates in the deep, upland portions of the aquifer were found to range from <0.01 to 0.14 mM of N per year; rates at the redoxcline along the shallow flow path range from 1.0 to 2.7 mM of N per year. Potential denitrification rates in groundwater adjacent to the stream may be much faster, with rates up to 140 mM per year based on an in situ experiment conducted in this zone.

Nitrate Dynamics in Relation to Lithology and Hydrologic Flow Path in a River Riparian Zone

Abstract: The efficiency with which riparian zones remove nitrate (NO 3 ) from contaminated ground water can vary with landscape setting. This study was conducted to determine the influence of flood plain geometry, lithology, hydrologic flow path, and nitrate transport on mechanisms of nitrate depletion of contaminated ground water. Patterns of NO - 3 -N, chloride, and dissolved organic carbon (DOC) concentrations and δ 15 N-NO - 3 and δ 18 O-NO 3 values in combination with detailed piezometric head measurements were investigated in a river floodplain connected to a large upland sand aquifer in an agricultural region near Alliston, Ontario, Canada. Ground water discharging to the forested floodplain from the sand aquifer exhibited large spatial variability in NO 3 -N concentrations (10-50 mg/L). The transport and depletion of NO 3 was strongly influenced by floodplain geometry and lithology. Little ground water flow occurred through the low-conductivity matrix of peat in the floodplain. Plumes of NO 3 -rich ground water passed beneath the riparian wetland peat and flowed laterally in a 2- to 4-m-thick zone of permeable sands across the floodplain to the river. Analyses of the distribution of the NO + 3 -N concentrations, isotopes, and DOC within the floodplain indicate that denitrification occurred within the sand aquifer near the river where nitrate-rich ground water interacted with buried channel sediments and surface water recharged from peat to the deeper sands. This study shows that the depth of permeable riparian sediments, ground water flow path, and the location of organic-rich subsurface deposits may be more important than the width of vegetated strips in influencing the ability of riparian zones to remove nitrate.

A preliminary assessment of the effects of groundwater flow on closed-loop ground source heat pump systems

Abstract: A preliminary study has been made of the effects of groundwater flow on the heat transfer characteristics of vertical closed-loop heat exchangers and the ability of current design and in-situ thermal conductivity measurement techniques to deal with these effects. It is shown that an initial assessment of the significance of groundwater flow can be made by examining the Peclet number of the flow. A finite-element numerical groundwater flow and heat transfer model has been used to simulate the effects of groundwater flow on a single closed-loop heat exchanger in various geologic materials. These simulations show that advection of heat by groundwater flow significantly enhances heat transfer in geologic materials with high hydraulic conductivity, such as sands, gravels, and rocks exhibiting fractures and solution channels. Simulation data were also used to derive effective thermal conductivities with an in-situ thermal conductivity estimation procedure. These data were used to design borehole fields of different depths for a small commercial building. The performance of these borehole field designs was investigated by simulating each borehole field using the pre-calculated building loads over a ten-year period. Results of these simulations, in terms of the minimum and peak loop temperatures, were used to examine the ability ofmore » current design methods to produce workable and efficient designs under a range of groundwater flow conditions.« less

Radiocarbon Dating of Groundwater Systems

Abstract: Groundwater is an increasingly important water resource in arid or semi-arid regions, as well as a conjunctive resource in humid environments. Because of the long residence time for groundwater in the hydrologic cycle, the last few decades have seen expanding study of groundwater systems. It is therefore important to continually refine our interpretation of hydrogeologic, geochemical and isotopic data to better understand the spatial and temporal movement of water in the subsurface. With our ever-increasing understanding of the magnitude of climate variations during the last 40 000 years and the impact of our industrialised society on groundwater quality and quantity, hydrogeologists will continue to require more information about the rate of groundwater movement on scales from the subannual to millenium. The 5730 year half-life of 14C and the ubiquity of carbon (as organic and inorganic forms) in groundwater, makes it a potentially ideal tracer on these timescales.

A mechanistic study of nonlinear solute transport in a groundwater–surface water system Under steady state and transient hydraulic conditions

Abstract: Two laboratory experiments were conducted to investigate the effects of tides and buoyancy on beach hydraulics in the presence of a seaward groundwater flow due to an elevated “regional” water table. In the first experiment, case 1, the difference in concentration between the salt water at sea and the water of the regional aquifer was small, 2.4 g L−1, such that it did not engender density gradients; the salt acts as a tracer in this case. In the second experiment, case 2, the difference was ∼32.0 g L−1, which creates a significant density gradient. This case corresponds to the presence of fresh groundwater in the subsurface of the coasts of the continental United States. The experiments were numerically simulated by the marine unsaturated (MARUN) model, a numerical model for density-and-viscosity-dependent flows in two-dimensional variably saturated media. The long-term experimental and numerical results showed that the seawater plume entered the beach from the sea and occupied most of the intertidal zone. The maximum depth of the seawater plume was near the midsection of the intertidal zone, and it decreased near the low and high tide lines. When viewed in the context of case 2, these results indicate an inverted salinity distribution in beaches subjected to tides with salt water from sea overtopping the freshwater lens. For both cases, water from the regional aquifer moved seaward beneath the seawater in the intertidal zone and pinched out near the low tide mark. We also noted that beach hydraulics are highly two dimensional with water entering the beach at a near-vertical angle and leaving it at a near-horizontal angle, which casts doubts on analyses of beach hydraulics based on the Dupuit assumption. Findings from this work have direct implications within the practice of bioremediation of oil spills on beaches. We found that applying dissolved nutrients on the beach surface at low tide is superior to applying them in a trench landward of the beach. This is because the residence time of the nutrient plume in the bioremediation zone of the beach in the prior situation is longer than that in the latter.

Atmospheric Noble Gases

Abstract: A large fraction of gases dissolved in surface and groundwater, mainly N2, O2 and the noble gases He, Ne, Ar, Kr and Xe, originate from the atmosphere. Whenever water comes into contact with the atmosphere, or other phases such as natural gas, oil and solid organic matter, gases are exchanged and gas concentrations of the individual phases record some characteristics of these processes. Even in the absence of a separate gas phase, dissolved gas concentrations in a water parcel may change as a result of molecular diffusion, and mixing on a variety of space and time scales. However, in many cases the impacts of most of these processes on the dissolved gas concentrations are small and the dominating processes may be reconstructed from the measured concentrations of the dissolved gases. This chapter deals with atmospheric noble gases dissolved in groundwater, which reliably record information on certain physical processes due to the lack of chemical reactions which affect them. As has been shown in studies performed over the past 40 years, atmospheric noble gases dissolved in groundwater yield valuable information on palaeoclimate, in particular temperature at the time of recharge, dynamics of groundwater flow, and denitrification and oxygen consumption rates.

Optimal management of flow in groundwater systems

Abstract: Preface. List of Variables. Groundwater Flow Management. Groundwater Simulation. Building the Management Formulation. Solving the Management Formulation. Using the Management Model. Advanced Linear Formulations. Formulations with Binary Variables. Formulations with Nonlinear Functions. Appendix. References. Index.

Some analytical solutions of the linearized Boussinesq equation with recharge for a sloping aquifer

Abstract: Subsurface flow from a hillslope can be described by the hydraulic groundwater theory as formulated by the Boussinesq equation. Several attempts have been made to solve this partial differential equation, and exact solutions have been found for specific situations. In the case of a sloping aquifer, Brutsaert [1994] suggested linearizing the equation to calculate the unit response of the hillslope. In this paper we first apply the work of Brutsaert by assuming a constant recharge to the groundwater table. The solution describes the groundwater table levels and the outflow in function of time. Then, an analytical expression is derived for the steady state solution by allowing time to approach infinity. This steady state water table is used as an initial condition to derive another analytical solution of the Boussinesq equation. This can then be used in a quasi steady state approach to compute outflow under changing recharge conditions. | Subsurface flow from a hillslope can be described by the hydraulic groundwater theory as formulated by the Boussinesq equation. Several attempts have been made to solve this partial differential equation, and exact solutions have been found for specific situations. In the case of a sloping aquifer, Brutsaert [1994] suggested linearizing the equation to calculate the unit response of the hillslope. In this paper we first apply the work of Brutsaert by assuming a constant recharge to the groundwater table. The solution describes the groundwater table levels and the outflow in function of time. Then, an analytical expression is derived for the steady state solution by allowing time to approach infinity. This steady state water table is used as an initial condition to derive another analytical solution of the Boussinesq equation. This can then be used in a quasi steady state approach to compute outflow under changing recharge conditions.

Deciphering groundwater flow systems in Oasis Valley, Nevada, using trace element chemistry, multivariate statistics, and Geographical Information System

Abstract: The origin of groundwater discharging via evapotranspiration and from springs within Oasis Valley, Nevada, is of concern owing to the close proximity of the Nevada Test Site (NTS) and the possible contamination of groundwater as a result of underground nuclear testing. Principal components analysis, cluster analysis, and population partitioning, along with a Geographical Information System, were used to decipher groundwater flow patterns in Oasis Valley, Nevada. These multivariate statistical techniques were applied to the trace element chemistry of groundwater samples collected from 26 springs and wells within Oasis Valley, the NTS, and the Nellis Air Force Range. The results of all statistical analyses showed similar geographical trends in the trace element chemistry of the groundwaters included in this study. Differences are observed between the groundwaters from the NTS and those of Oasis Valley based on the concentrations of the elements Li, Ge, Mo, Rb, Ba, U, and Ru. A concentration gradient is observed from lower concentrations in the NTS to increasing concentrations toward Oasis Valley suggesting groundwater flow in an overall southwestward direction from the NTS. Also, a different trace element signature is observed for the waters collected in the northern and western region of Oasis Valley, suggesting another source of groundwater to this area.

Active fault zones and groundwater flow

Abstract: Boundary-element studies of an active strike-slip fault zone subject to fault-parallel loading of 6 MPa show tensile stress concentration in large areas around the fault-zone tips. In these areas, tensile stress exceeds typical in situ tensile strengths of rocks, resulting in the formation or reactivation of tensile fractures. These fractures curve toward the tips of the fault zone, and if interconnected they increase the rock permeability. Fault slip also increases the temporary permeability of the fault zone, by as much as many orders of a magnitude. Its effects on the surrounding groundwater flow, however, is normally small if the fault trends at a high angle to the groundwater flow but gradually increases as the angle between the flow and the fault decreases. When the trend of the fault zone and the groundwater flow coincide, the upstream part collects groundwater whereas the downstream part expels it. It follows that the yield of springs decreases in the upstream part, but increases in the downstream part.

Conceptual model of the geometry and physics of water flow in a fractured basalt vadose zone

Abstract: A conceptual model of the geometry and physics of water flow in a fractured basalt vadose zone was developed based on the results of lithological studies and a series of ponded infiltration tests conducted at the Box Canyon site near the Idaho National Engineering and Environmental Laboratory. The infiltration tests included one 2-week test in 1996, three 2-day tests in 1997, and one 4-day test in 1997. For the various tests, initial infiltration rates ranged from 4.1 cm/d (4.75 ×10−7 m/s) to 17.7 cm/d (2.05×10−7 m/s) and then decreased with time, presumably because of mechanical or microbiological clogging of fractures and esicular basalt in the near-surface zone, as well as the effect of entrapped air. The subsurface moisture redistribution was monitored with tensiometers, neutron logging, time domain reflectrometry, and ground-penetrating radar. A conservative tracer, potassium bromide, was added to the pond water at a concentration of 3 g/L to monitor water flow with electrical resistivity probes and water sampling. Analysis of the data shows evidence of preferential flow rather than the propagation of a uniform wetting front. We propose a conceptual model describing the saturation-desaturation behavior of the basalt, in which rapid preferential flow occurs through the largest vertical fractures, followed by a gradual wetting of other fractures and the basalt matrix. Fractures that are saturated early in the tests may become desaturated thereafter, which we attribute to the redistribution of water between fractures and matrix. Lateral movement of water takes place within horizontal fracture and rubble zones, enabling development of perched water bodies.

Hydrogeological framework of the Deccan basalt groundwater systems, west-central India

Abstract: Deccan basalts of west-central India are hydrogeologically inhomogeneous rocks A proper understanding of the physical framework of the basalts within which groundwater resides and moves is a key to the hydrogeology of these rocks Two types of basalt, the vesicular amygdaloidal basalt and the compact basalt, occur as alternate layers in the volcanic pile Although the rocks are generally inhomogeneous, structures in the basalt, such as sheet joints and vertical joints, serve as zones of groundwater flow In the shallow subsurface, two groundwater systems are operative Groundwater system A consists of a vesicular amygdaloidal basalt underlain by a compact basalt, whereas groundwater system B consists of a vesicular amygdaloidal basalt overlain by a compact basalt Groundwater system A has a better developed network of openings and, as a consequence, this system has a higher transmissivity and storage coefficient than groundwater system B Wells tapping groundwater system A have higher yields on average and irrigate more hectares of cropland than do wells tapping groundwater system B This simple systems concept offers a practical methodology for understanding the geometry of the physical framework that contains groundwater in the Deccan basalts The efficacy of the concept is in its widespread utility for the region The concept may also be extrapolated to help understand the hydrogeology of deeper Deccan basalt groundwater systems

Groundwater flow and contaminant transport in carbonate aquifers

Abstract: Carbonate aquifers are an important source of water throughout the world They are complicated systems and not always easy to interpret Caves and channels form in the rock, leading to complex flow pathways and unpredictable contaminant behaviour This volume covers the range of techniques used to analyse groundwater flow and contaminant transport in carbonate aquifers The book opens with a review of thoughts and methods, and continues by discussing the use of tracers, hydrograph and hydrochemograph evaluation, estimation of aquifer properties from outcrop studies, numerical simulation, analogue simulation, and 3-D visualization of conduits Other papers address the critical evaluation of matrix, fracture and conduit components of flow and storage An understanding of these approaches is important to engineers or hydrogeologists working in carbonate aquifers