Groundwater Velocity Research Articles

Tritium in groundwater in the Black Hills of South Dakota

Tritium from 1950s and 1960s atmospheric hydrogen-bomb tests was dispersed into the stratosphere and around the globe. The fallout of tritium in precipitation was especially high in the northern hemisphere, peaking during a flurry of atmospheric nuclear testing in 1963. Rainout from hydrogen-bomb tests recharged aquifers, and this maximum of tritiated water inadvertently has provided hydrogeologists with a way to establish the time of recharge because prior to hydrogen-bomb explosions, practically no tritium existed in groundwater. In 1967, the US Geological Survey took numerous Inyan Kara Group groundwater samples in the Dewey/Burdock area of the southwestern Black Hills. They found tritium concentrations exceeding 200 tritium units and determined a groundwater velocity of 4.6 m per day in the Inyan Kara aquifer, assuming the tritium originated from rainout on nearby outcrops in 1963. In 2011, the US Geological Survey took groundwater samples in the same general area as the 1967 study. These samples from the Inyan Kara Group had tritium concentrations ranging from ≤ 0 to 15.3 tritium units, much lower than the 1967 study and almost back to values expected from natural production of tritium. Because the half-life of tritium is ~ 12.3 years, only 7% of the tritium generated from tests in 1963 would still be present 48 years later in 2011. The maximum tritium values found in the 2011 study are approximately the values expected from radioactive decay only; this could be interpreted that in some locations very slow groundwater movement has occurred between 1967 and 2011, while in other locations lower tritium values show there has been much faster movement. However, the great range of tritium concentrations indicates considerable variability in groundwater dispersion has occurred. This variability is caused by the complex stratigraphic units of irregular geometry and permeability in the Inyan Kara Group.

Laboratory validation of a point velocity probe for measuring horizontal flow from any direction

The point-velocity probe (PVP) quantifies groundwater speed and flow direction, i.e., velocity, at the centimeter scale. The first probe designs required that the flow direction be known a priori, within about 100° in order to position the probe during installation. This study introduces and assesses a ‘360° PVP’ that measures flow from any direction without foreknowledge of the groundwater velocity. In tests conducted in a Nested Storage Tank (NeST) aquifer simulator packed with sand, PVP-measured velocities matched expected velocities within ±9° in direction and ±15% in magnitude, on average, consistent with previously reported PVP performances in laboratory studies. In tests involving 17 repacked NeSTs, the measured and expected velocities were within ±30° and ±30% on average, illustrating the sensitivity of flow to porous medium packing, and the probes' ability to sense these changes; the porosity was found to vary considerably between packings i.e., n=0.34±0.2. For flow directions between 0° and 80° of an injection port, the experimental error on velocity magnitude was within the ranges reported above. At higher flow angles, experimental sources of error contributed to greater uncertainties. Fortunately, in these cases there were always alternative injection ports (with lower angles to flow) that could be used to circumvent any biases. At low experimental flow angles (<10°) the calculated values tended to overestimate the actual flow angles. Fortunately, these cases were identifiable by the detection of tracer at detectors on either side of the active injection port. In several tests designed with an expected flow direction of 0°, averaging the calculated directions from each side of the injection port resulted in improved matches to the expected flow direction.

Contaminant mass discharge to streams: Comparing direct groundwater velocity measurements and multi-level groundwater sampling with an in-stream approach

Quantitative knowledge of contaminant mass discharge is useful when assessing the risk posed to streams by point sources. However, due to a multi-directional flow field at the groundwater-surface water interface, reliable estimates close to streams are particularly challenging to obtain. Moreover, since the “true” value of the total contaminant mass discharge across a defined control plane is typically unknown at field sites, it is difficult to assess the accuracy of estimates at the field scale. We estimated the mass discharge of a chlorinated ethene plume entering a low-land Danish stream across a control plane on the stream bank. This was done using multi-level groundwater sampling combined with 1) specific discharge obtained from Darcy’s law, and 2) two different specific discharge fields (a constant equal to the mean, and a varying) obtained from direct groundwater velocity measurements using point velocity probes (PVPs). The methods yielded contaminant mass discharges ranging from 204 to 372 kg/y (PCE equivalents). To help account for the entire contaminant mass, we also quantified the contaminant mass discharge of chlorinated ethenes from measured contaminant concentrations in the stream water and the stream discharge to obtain an in-stream contaminant mass discharge. By subtracting contaminant input upstream from the plume discharge zone, as well as input from two culverts discharging contaminated water to the stream, this estimate was used to calculate a groundwater-borne in-stream contaminant mass discharge of 558 kg/y (PCE equivalents). This method has not previously been explored to access the accuracy of stream bank contaminant mass discharge estimates. Differences between the groundwater-borne in-stream value and those on the stream bank may be due to high concentration zones located in fast flow conduits or missed high concentration zones in the control plane. It is also possible that the groundwater-borne in-stream mass discharge was overestimated due to uncertainty in the input from two discharging culverts. The agreement of these results is sufficient to be of practical usefulness for risk assessment. It is concluded that the combination of direct groundwater velocity measurements and multi-level groundwater sampling can provide a useful extension of Darcy-based methods when quantifying the contaminant mass discharge to streams. Moreover, this study demonstrates the efficacy and usefulness of the in-stream contaminant mass discharge in risk assessment if fully mixed stream water concentration and corresponding stream discharge are available.

One-dimensional analytical solution for hydraulic head and numerical solution for solute transport through a horizontal fracture for submarine groundwater discharge

Submarine groundwater discharge (SGD) has been recognized as a major pathway of groundwater flow to coastal oceanic environments. It could affect water quality and marine ecosystems due to pollutants and trace elements transported through groundwater. Relations between different characteristics of aquifers and SGD have been investigated extensively before, but the role of fractures in SGD still remains unknown. In order to better understand the mechanism of groundwater flow and solute transport through fractures in SGD, one-dimensional analytical solutions of groundwater hydraulic head and velocity through a synthetic horizontal fracture with periodic boundary conditions were derived using a Laplace transform technique. Then, numerical solutions of solute transport associated with the given groundwater velocity were developed using a finite-difference method. The results indicated that SGD associated with groundwater flow and solute transport was mainly controlled by sea level periodic fluctuations, which altered the hydraulic head and the hydraulic head gradient in the fracture. As a result, the velocity of groundwater flow associated with SGD also fluctuated periodically. We found that the pollutant concentration associated with SGD oscillated around a constant value, and could not reach a steady state. This was particularly true at locations close to the seashore. This finding of the role of fracture in SGD will assist pollution remediation and marine conservation in coastal regions.

Thermal Impact Assessment of Groundwater Heat Pumps (GWHPs): Rigorous vs. Simplified Models

Groundwater Heat Pumps (GWHPs) are increasingly adopted for air conditioning in urban areas, thus reducing CO2 emissions, and this growth needs to be managed to ensure the sustainability of the thermal alteration of aquifers. However, few studies have addressed the propagation of thermal plumes from open-loop geothermal systems from a long-term perspective. We provide a comprehensive sensitivity analysis, performed with numerical finite-element simulations, to assess how the size of the thermally affected zone is driven by hydrodynamic and thermal subsurface properties, the vadose zone and aquifer thickness, and plant setup. In particular, we focus the analysis on the length and width of thermal plumes, and on their time evolution. Numerical simulations are compared with two simplified methods, namely (i) replacing the time-varying thermal load with its yearly average and (ii) analytical formulae for advective heat transport in the aquifer. The former proves acceptable for the assessment of plume length, while the latter can be used to estimate the width of the thermally affected zone. The results highlight the strong influence of groundwater velocity on the plume size and, especially for its long-term evolution, of ground thermal properties and of subsurface geometrical parameters.

Des indications sur le transport de contaminants provenant de l’exploitation de réservoirs de pétrole et de gaz non conventionnels à partir de l’analyze d’un système analogue des sources thermales renfermant du méthane dans le nord des Rocheuses canadiennes

Natural gas is currently being produced from shales of the Montney and Liard basins in western Canada. Production requires hydraulic fracturing due to the low permeability of the shales in the basins. Stratigraphically equivalent shales are present in the northern Canadian Rocky Mountains. Thermal springs with notable hydrocarbon concentrations occur where large-scale faults intersect the same shale units that are the focus of gas development, indicating that under certain circumstances, connection of deep fractured shales to the land surface is possible. To constrain these conditions, simulations were conducted for the spring with the highest hydrocarbon flux (Toad River Spring), results of which indicate that in order to supply sufficient water to a fault to support measurable advection, the effective permeability of the shales in these structurally deformed areas must be one to four orders of magnitude higher than in areas of active gas production to the east. The spatial scale of enhanced permeability is much greater than that which is achieved by hydraulic fracturing and the mechanism of maintaining high pressures at depth is more persistent in time. Examination of groundwater velocities suggests that upward migration of solutes from hydraulic fracturing may take decades to centuries. Results also indicate that any temperature anomaly will be associated with transport along a fault at such velocities. No such temperature anomaly has been documented in regions with unconventional oil and gas development to date. Such an anomaly would be diagnostic of a deep solute source.

Analytical Solution of Advection-Dispersion Equation with Spatially Dependent Dispersivity

AbstractIn the dispersion theory of solute transport in groundwater flow, the dispersion coefficient is regarded as proportional to the nth power of groundwater velocity, where n varies from 1 to 2...

A spatial Markov model for the evolution of the joint distribution of groundwater age, arrival time, and velocity in heterogeneous media

AbstractThe evolution of the joint distribution of groundwater age, velocity, and arrival times based on a Markov model for the velocities of fluid particles in heterogeneous porous media has been quantified. An explicit evolution equation for the joint distribution of age, arrival time, and particle velocity is derived, which is equivalent to a continuous time random walk for age, velocity, and arrival time. The approach is fully parameterized by the correlation model and the distribution of groundwater flow velocities. The transition probability for subsequent particle velocities along streamlines is implemented by a Copula, which is an efficient method to generate a correlated velocity series with prescribed marginal distribution. We discuss different solution methods based on finite‐differences and random walk particle tracking. The latter is based on continuous time random walks, whose transition times are obtained kinematically from the flow velocities. Specifically, we discuss a renormalization scheme to accelerate the particle‐tracking simulations based on the definition of aggregate particle transitions while at the same time renormalizing velocity correlation. The impact of velocity correlation and velocity distribution on the evolution of age at different distances from the inlet plane is also studied. At distances of the order of the correlation length, persistent particle velocities give the same behavior as stochastic streamtube models. For velocity distributions which give rise to transition times with finite variance, the age distributions evolve toward an inverse Gaussian. For heavy‐tailed weighting times, they evolve toward stable distribution as the distance from the inlet increases.

Design and and Construction of Groundwater Monitoring Network at Shallow-depth CO2 Injection and Leak Test Site, Korea

The effectiveness of CO2 leakage characterization is highly dependent on reasonable monitoring network designs for detecting the CO2 leakage under various hydrogeological conditions. Also, the migration of CO2 through heterogeneous subsurface is very difficult to predict because small differences in material properties may result in vastly different migration patterns. Therefore, like other environmental contaminants, it is necessary to design optimized groundwater monitoring network to effectively characterize the migration of the CO2. Main objectives of this study is to construct optimized groundwater monitoring network considering hydrogeological characteristics in field-scale transport of CO2 originated from artificially-injected CO2 at the environmental impact monitoring test site in Eumseong, Korea (K-COSEM). 24 monitoring wells were installed at the site aligned with the regional groundwater flow direction. Various hydraulic tests were performed to estimate the hydraulic characteristics such as hydraulic conductivity, effective porosity, and groundwater velocity. Groundwater monitoring network for CO2 injection and leak test constructed with regard of the heterogeneity understood through the various field tests. Three multi-depth monitoring well nests each of them was composed of four monitoring well units drilled with different depths (each 18, 22, 26, 30 m deep) have been installed along the groundwater flow direction and dedicated to depth-discrete water quality monitoring in saturated zone. Additional, four multi-depth monitoring well nests each of them was composed of four unsaturated-zone monitoring well units drilled with different depths (each 5, 8, 11, 14 m deep) were also installed with a distance of 1.25 m away between saturated zone monitoring wells and were dedicated to depth-discrete gas monitoring in unsaturated zone. By these efforts, effective groundwater monitoring network for temporal and spatial CO2 leakage detection and groundwater safety management have been designed and constructed at the test site.

The Characteristics of Hydrogeological Parameters of Unconsolidated Sediments in the Nakdong River Delta of Busan City, Korea

This study dealt with the characteristics and the interrelations of hydrogeological parameters such as hydraulic conductivity, dispersivity and effective porosity of unconsolidated sediments for providing the basic data necessary for the planning of the management and preservation of groundwater quality in the Nakdong River Delta of Busan City, Korea. Groundwater quality in this area has been deteriorated due to seawater intrusion, agricultural fertilizer and pesticide, industrial wastewater, and contaminated river water. The physical properties (grain size distribution, sediment type, sorting) and aquifer parameters (hydraulic conductivity, effective porosity, longitudinal dispersivity) were determined from grain size analysis, laboratory permeability test and column tracer test. Among 36 samples, there were 18 Sand (S), 7 Gravelly Sand (gS), 5 Silty Sand (zS), 5 Muddy Sand (mS), and 1 Sandy Silt (sZ). Hydraulic conductivity was determined through a falling head test, and ranged from <TEX>$9.2{\times}10^{-5}$</TEX> to <TEX>$2.9{\times}10^{-2}cm/sec$</TEX> (0.08 to 25.6 m/day). From breakthrough curves, dispersivity was calculated to be 0.35~3.92 cm. Also, effective porosity and average linear velocity were obtained through the column tracer test, and their values were 0.04~0.46 and 1.06E-04~6.49E-02 cm/sec, respectively. Statistical methods were used to understand the interrelations among aquifer parameters of hydraulic conductivity, effective porosity and dispersivity. The relation between dispersivity and hydraulic conductivity or effective porosity considered the sample length, because dispersivity was affected by experimental scale. The relations between dispersivity and hydraulic conductivity or effective porosity were all in inverse proportion for all long and short samples. The reason was because dispersivity was in inverse proportion to the groundwater velocity in case of steady hydrodynamic dispersion coefficient, and groundwater velocity was in proportion to the hydraulic conductivity or effective porosity. This study also elucidated that longitudinal dispersivity was dependent on the scale of column tracer test, and all hydrogeological parameters were low to high values due to the sand quantity of sediments. It is expected that the hydrogeological parameter data of sediments will be very useful for the planning of groundwater management and preservation in the Nakdong River Delta of Busan City, Korea.

A Velocity-Oriented Approach for Modflow

When considering three-dimensional groundwater flow, the hydrogeological community widely uses the MODFLOW model based on the head-oriented approach (HOA). However, a great deal of hydrogeological situations requires an extremely accurate assessment of the Darcy velocity in three dimensions. Despite the good numerical approximation of the piezometric head offered by MODFLOW, it does not guarantee sufficient accuracy in estimating all three components of the flow velocity. As an alternative, this article presents the MODFLOW-based velocity-oriented approach (VOA), which can approximate the three components of the groundwater velocity with high accuracy. The article gives comprehensive theoretical background and a comparison of the HOA and VOA approaches. Importantly, the authors show how the VOA equations with the VOA boundary conditions can be implemented and solved with MODFLOW. Recently, the VOA has become a basis for new version of MODFLOW. The two approaches are effectively combined and compared using Visual MODFLOW for a simple case of river–aquifer interaction.

An improved analytical model for vertical borehole ground heat exchanger with multiple-layer substrates and groundwater flow

An improved analytical model for vertical borehole ground heat exchanger (BHE) which takes into account effect of groundwater flow in multiple-layer geologies is applied to simulate and analyze the long-term temperature response of soil around BHE with the unbalanced seasonal dynamic load. The present model was validated by experiment for a single borehole with good agreement, and it can find that the average of temperature integral along the borehole wall depth is taken as the representative temperature of the borehole wall will be more reasonable. Moreover, the influence of groundwater flow velocity on soil temperature around the borehole was investigated, the results show that the deformation of plane temperature field caused by groundwater advection, as equivalent velocity reach a certain order of magnitude, groundwater flow can effectively alleviate the heat (cold) accumulation. Finally, the temperature change of soil around borehole at different operation time was simulated, results indicate that the higher the groundwater velocity is, the faster time make temperature field to achieve stability. This analytical model provides better flexibility and versatility to research further the effect of groundwater flow in layered geologies.

The transport dynamics of chloride and sodium in a ladder fen during a continuous wastewater polishing experiment

Ladder fen peatlands have excellent potential for wastewater polishing as they naturally contain both open water (pools) and subsurface (peat) treatment landforms; however, there is a poor understanding of solute transport in ladder fens with and without the increased hydrological load imposed by wastewater discharge. To better understand solute transport in ladder fens under wastewater polishing conditions a continuous solute (NaCl) tracer experiment (38m3day−1 of water, chloride – 47.2mgL−1, and sodium – 25.3mgL−1) was conducted during the summer of 2014 (day of year 192–243) in a small ladder fen in the James Bay Lowland. The transmissivity distribution and effective porosity (average 0.5) of the peat ribs were determined through repeated bail tests and the drainable porosity of 18 peat cores at −100mb, respectively. Water samples were taken at least every 7days to capture the solute (sodium and chloride) plumes. Both solute plumes never reached the site outflow (∼250m downgradient) and displayed complex plume morphology, typically following the patterns of higher hydraulic conductivity within the upper 0.1m of the saturated peat, rather than the microtopography. Based on the 50% breakthrough isotherms, sodium and chloride were transported at an average solute velocity of 1.9 and 1.1mday−1, respectively (average linear groundwater velocity=2.1mday−1); thus, the solutes were retarded by a factor of 2.1 and 1.2 for sodium and chloride, respectively. Due to the inherent retardation of solutes into inactive pores and relatively high solute residence times, this study demonstrates the potential for wastewater polishing in ladder fens.

Characterization of Thin Alluvial Bed Aquifers in Manggar River Balikpapan East Kalimantan Indonesia

Abstract Manggar Village is suburban, located in District of East Balikpapan, East Borneo Indonesia. In dry season, clean water crisis always hit this area. Therefore in order to full fill the need of water for domestic use the inhabitant of this area use dug well water. Dug well water comes from the upper most ground water that is stored and transmitted by relatively thin layer of alluvium deposit. In this research surface elevation measurement was done to control the elevation of dug well so that the depth of dynamic water level ( DWL ) could be determined accurately. The equipotential map was made based on the DWL , then flow line can be drawn and gradient hydraulics ( i ) can be calculated. Coefficient of permeability ( k ) that were resulted from fall head test has shown value 1.284 x 10 -3 up to 11.45 x 10 -3 cm/sec which gradually increases toward beach. According to the results of serial test and calculation the ground water transmissibility and velocity increases toward the beach. Generally groundwater has high hardness. The groundwater close to the beach has been mixed with sea water.

System reliability analysis for near-surface radioactive waste disposal facilities

ABSTRACTThe low- and intermediate-level radioactive wastes are generally disposed in near-surface disposal facility (NSDF). The NSDF is composed of engineered barriers. The probabilistic safety assessment model has been developed to analyse the performance of NSDF. The endpoints of the assessment are the concentration of the radionuclide in the groundwater and the corresponding dose rate of the radionuclide. The barrier system can have multiple failure modes but practically the possible failure modes are failure of top cover, failure of waste container, degradation of waste form and failure of bottom cover, which are usually considered as independent failure events. Through a sensitivity analysis, the most critical parameters affecting the design reliability for failure criteria are identified as the groundwater velocity and distribution coefficient. The study shows that for the NSDF considered, there is a high degree of dependence between the failure modes, and demonstrates the probability of the simultaneous occurrence of failures. Thus, the need to consider the system reliability in the NSDF is highlighted. The study also advocates the use of optimisation techniques to evaluate the probability of failure, which provides a better estimate of the probability of failure, as validated from the results obtained from Monte Carlo simulations.

Application of 3D Printing to the Manufacturing of Groundwater Velocity Probes

AbstractFor the purposes of risk mitigation and remediation design, Darcy's Law calculations, that are typically used to estimate flow direction and magnitude, can be usefully supplemented with more direct measurements. The point velocity probe (PVP), which measures groundwater velocity by conducting a mini‐tracer test across a cylindrical probe surface, was developed to address this need. Laboratory and field experiments have shown that these probes can provide accurate velocity magnitude and direction measurements at the centimeter scale. In an effort to streamline the production of PVPs, three‐dimensional (3D) printing was investigated. The 3D printer produces a plastic probe body with designated detector and injection port locations. The subsequent installation of injection lines and detector wires requires, for a single‐port probe, no longer than 1–2 h of assembly time, compared to approximately 10 h to assemble previous designs (mainly determined by injection port installation). Probes can be printed in batches with the number of units made depending on the printer and probe size. Laboratory tests of the original PVP models yielded velocity magnitudes and directions with average errors (from expected values) of ±9% to ±15% and ±8°, respectively. A printed PVP was tested in a nested storage tank system (NeST) packed with sand that mimics flow conditions through a sandy aquifer. The probe was tested at multiple pumping rates with the injector port at angles of (α =) 30°, 45°, and 75° from the expected linear flow direction. The printed PVPs provided an average magnitude percent error of ±13.5% and an average direction error of ±4°. This shows that in laboratory tests, printed PVPs performed as well as the original probes previously reported, making them viable units for field applications as well as being quicker to assemble than the earlier designs.

Adsorption capacity of chosen sandy ground with respect to contaminants relocating with groundwater

Abstract One of the most important problems concerning contaminant transport in the ground is the problem related to the definition of parameters characterizing the adsorption capacity of ground for the chosen contaminants relocating with groundwater. In this paper, for chloride and sulfate indicators relocating in sandy ground, the numerical values of retardation factors (Ra) (treated as average values) and pore groundwater velocities with adsorption (ux/Ra) (in micro-pore ground spaces) are taken into consideration. Based on 2D transport equation the maximal dimensionless concentration values (C*max c) in the chosen ground cross-sections were calculated. All the presented numerical calculations are related to the unpublished measurement series which was marked in this paper as: October 1982. For this measurement series the calculated concentration values are compared to the measured concentration ones (C*max m) given recently to the author of this paper. In final part of this paper the parameters characterizing adsorption capacity (Ra, ux/Ra) are also compared to the same parameters calculated for the two earlier measurement series. Such comparison also allowed for the estimation of a gradual in time depletion of adsorption capacity for the chosen sandy ground.

Mangroves buffer marine protected area from impacts of Bovoni Landfill, St. Thomas, United States Virgin Islands

One of the many ecosystem services that mangrove systems provide is their ability to act as buffers between the land and sea, protecting human development from storm surges while also trapping terrestrial pollutants. In St. Thomas, United States Virgin Islands, an ecologically-important mangrove system sits between Bovoni Landfill and a marine protected area, the St. Thomas East End Reserves. To characterize the physical processes driving this mangrove system, groundwater hydraulic head, sediment cores, sediment surface temperatures, and water and sediment chemistry were analyzed. Hydraulic head data from January to November 2014 were used to determine vertical and horizontal groundwater flow directions. Water and sediment samples were tested for heavy metals potentially originating from Bovoni Landfill. Stratigraphic context was provided by the sediment cores and used to infer past environmental conditions. Subsamples were taken from these cores and analyzed for dry bulk density, organic matter content (through loss on ignition), and heavy metals using electron microscopy. Vertical groundwater velocity and sediment porosity were determined by calibrating a one-dimensional finite difference heat transport model to near surface temperature data from depths of 0, 7, 14, and 21 cm. Groundwater was found to flow from the terrestrial upland, through the mangroves, and toward the ocean for the majority of the study. Flow reversal was seen after long periods of little precipitation. In the surface and shallow groundwater samples, trace metal concentrations were measured from 23 to 105 μg/L for Cr, Ni, Sn, and Zn. Sediment samples collected near the landfill contained Bi, Cr, Sn, Ti, and Zn. Very slow flushing of sediment pore water was indicated by the vertical groundwater velocities produced from the heat transport model, which ranged from ±10–7 to ±10–9 m/s. This study revealed that the mangrove system is an important buffer system protecting the outer lagoon of the marine protected area from terrestrial contaminants via sediment trapping and slowing of water fluxes from the upland area into the lagoon. The results presented here can be used as a baseline for future studies and are relevant to local managers and to landfill closure plans.

Evaluation of multiple tracer methods to estimate low groundwater flow velocities

Four different tracer methods were used to estimate groundwater flow velocity at a multiple-well site in the saturated alluvium south of Yucca Mountain, Nevada: (1) two single-well tracer tests with different rest or “shut-in” periods, (2) a cross-hole tracer test with an extended flow interruption, (3) a comparison of two tracer decay curves in an injection borehole with and without pumping of a downgradient well, and (4) a natural-gradient tracer test. Such tracer methods are potentially very useful for estimating groundwater velocities when hydraulic gradients are flat (and hence uncertain) and also when water level and hydraulic conductivity data are sparse, both of which were the case at this test location. The purpose of the study was to evaluate the first three methods for their ability to provide reasonable estimates of relatively low groundwater flow velocities in such low-hydraulic-gradient environments. The natural-gradient method is generally considered to be the most robust and direct method, so it was used to provide a “ground truth” velocity estimate. However, this method usually requires several wells, so it is often not practical in systems with large depths to groundwater and correspondingly high well installation costs. The fact that a successful natural gradient test was conducted at the test location offered a unique opportunity to compare the flow velocity estimates obtained by the more easily deployed and lower risk methods with the ground-truth natural-gradient method. The groundwater flow velocity estimates from the four methods agreed very well with each other, suggesting that the first three methods all provided reasonably good estimates of groundwater flow velocity at the site. The advantages and disadvantages of the different methods, as well as some of the uncertainties associated with them are discussed.

Electric-field response based experimental investigation of unsaturated soil slope seepage

Rainfall is one of the important factors causing the failure of slope, such as the occurrence of transverse cracks and localized slumps. The process of rainfall seepage was studied with an indoor soil slope model based on the Network Parallel Electrical Method. The responses of geoelectric-field parameters were analyzed to infer the evolution process of rainfall seepage path. The variations of geoelectric-field parameters also contribute to our understanding of the behavior of groundwater seepage. The results show that the seepage velocity and seepage position of groundwater can be obtained according to the exciting current and primary field potential response characteristics of seepage field. The primary field potential, exciting current, spontaneous potential and apparent resistivity are sensitive to the water flow. When the position of the seepage surface reaches a certain electrode, the spontaneous potential, primary field potential and exciting current rapidly increase, while the apparent resistivity decreases gradually. The result of apparent resistivity can reflect the variation of the water content in the 3D structural soil slope and the position of infiltration surface. The results of study can provide the theoretical basis for studying the behavior of moisture flow in soil slope under rainfall condition.