Field Injection Experiment Research Articles

Intra aquifer variations in pesticide sorption during a field injection experiment

A field injection experiment was performed in an anoxic sandy aquifer over 6 days to assess sorption characteristics of 7 commonly applied pesticides in agriculture and 2 frequently detected metabolites. Pesticide use changed considerably in the last decades, and there is insufficient knowledge of the fate of currently used pesticides in aquifers. Injected water arrival was monitored at 6 depth intervals of 1 m ranging from 11.4 to 32.2 m-below surface level with varying organic carbon contents (0.057–0.91%d.w.) to examine intra-aquifer variations in sorption. Observed pesticide concentrations were fit using a non-linear least squares routine to an advection-dispersion equation, from which retardation factors (R) were obtained. Pesticide degradation did not significantly influence the simulated R during the experiment. We observed that bentazon and cycloxydim were most mobile with R < 1.1 at all depths. Desphenyl chloridazon, methyl desphenyl chloridazon, and imidacloprid were, on average, less mobile, with maximum R of 1.5. Boscalid, chloridazon, fluopyram, and flutolanil showed a larger range of R, and R > 2.0 were observed in the shallowest part of the aquifer. Largest R were observed at the top of the aquifer and decreased with depth. Koc values varied similarly, which indicates that sorption is not only influenced by sedimentary organic matter (SOM) content but also by its sorption reactivity. Obtained sorption parameters were substantially lower than reported in a widely used pesticide sorption database, which suggests that sorption parameters are influenced by methodological differences and variations in the sorption reactivity of SOM. The large intra-aquifer variations in pesticide sorption highlights that aquifer heterogeneity should be considered in groundwater risk assessments.

Stable Hydrogen Isotope Fractionation of Hydrogen in a Field Injection Experiment: Simulation of a Gaseous H2 Leakage

Stable Hydrogen Isotope Fractionation of Hydrogen in a Field Injection Experiment: Simulation of a Gaseous H₂ Leakage

Extracting microphysical fault friction parameters from laboratory and field injection experiments

Abstract. Human subsurface activities induce significant hazard by (re-)activating slip on faults, which are ubiquitous in geological reservoirs. Laboratory and field (decametric-scale) fluid injection experiments provide insights into the response of faults subjected to fluid pressure perturbations, but assessing the long-term stability of fault slip remains challenging. Numerical models offer means to investigate a range of fluid injection scenarios and fault zone complexities and require frictional parameters (and their uncertainties) constrained by experiments as an input. In this contribution, we propose a robust approach to extract relevant microphysical parameters that govern the deformation behaviour of laboratory samples. We apply this Bayesian approach to the fluid injection experiment of Cappa et al. (2019) and examine the uncertainties and trade-offs between parameters. We then continue to analyse the field injection experiment reported by Cappa et al. (2019), from which we conclude that the fault-normal displacement is much larger than expected from the adopted microphysical model (the Chen–Niemeijer–Spiers model), indicating that fault structure and poro-elastic effects dominate the observed signal. This demonstrates the importance of using a microphysical model with physically meaningful constitutive parameters, as it clearly delineates scenarios where additional mechanisms need to be considered.

Fine particle retention within stream storage areas at base flow and in response to a storm event

AbstractFine particles (1–100 µm), including particulate organic carbon (POC) and fine sediment, influence stream ecological functioning because they may contain or have a high affinity to sorb nitrogen and phosphorus. These particles are immobilized within stream storage areas, especially hyporheic sediments and benthic biofilms. However, fine particles are also known to remobilize under all flow conditions. This combination of downstream transport and transient retention, influenced by stream geomorphology, controls the distribution of residence times over which fine particles influence stream ecosystems. The main objective of this study was to quantify immobilization and remobilization rates of fine particles in a third‐order sand‐and‐gravel bed stream (Difficult Run, Virginia, USA) within different geomorphic units of the stream (i.e., pool, lateral cavity, and thalweg). During our field injection experiment, a thunderstorm‐driven spate allowed us to observe fine particle dynamics during both base flow and in response to increased flow. Solute and fine particles were measured within stream surface waters, pore waters, sediment cores, and biofilms on cobbles. Measurements were taken at four different subsurface locations with varying geomorphology and at multiple depths. Approximately 68% of injected fine particles were retained during base flow until the onset of the spate. Retention was evident even after the spate, with 15.4% of the fine particles deposited during base flow still retained within benthic biofilms on cobbles and 14.9% within hyporheic sediment after the spate. Thus, through the combination of short‐term remobilization and long‐term retention, fine particles can serve as sources of carbon and nutrients to downstream ecosystems over a range of time scales.

Groundwater geochemistry in bench experiments simulating CO2 leakage from geological storage in the Newark Basin

To learn the effect of CO2 leakage on shallow drinking water aquifers, laboratory experiments were conducted to measure the chemical changes of aquifer water, in which the fine fraction of sedimentary rock from the Triassic Newark Basin Series were immersed. Elevated CO2 concentrations caused a decrease in pH, an increase in the concentrations of major ions including Ca, Mg, Si, K, and alkalinity, an enhanced dissolution of carbonate minerals, and an increase in the concentrations of trace elements including Mn, Fe, Be, Cr, Co, Cu, Zn, Rb, Zr, Cd, Sb, Ba, Pb, and U in water under both continuous CO2 flow and one-time injection batch experiments. The magnitude of concentration increase was significantly lower in batch experiments with one-time CO2 injection than in experiments with continuous CO2 flow, although less rigorous agitation was maintained in the batch experiment. The logarithmic elemental release rates of Ca, Mg, Si, Mn, Fe and Zn normalized by reactive surface area, when plotted together with those from field injection experiments, decreased with the increase of pH (or the decrease of log (pCO2)) following linear trends. The laboratory elemental release rates were 1-3 orders of magnitude higher than field injection experiment rates extrapolated to incubation pH conditions. Hydrogeochemical parameters including pH, major and trace elements, particularly Fe, Mn, Cd, Tl, and sulfate, can be used as early indicators for leakage detection and need to be monitored in compliance with the U.S. Environmental Protection Agency (EPA) drinking water regulations.

Natural Attenuation Processes in Landfill Leachate Plumes at Three Danish Sites

This article provides an overview of comprehensive core and fringe field studies at three Danish landfill sites. The goal of the research activities is to provide a holistic description of core and fringe attenuation processes for xenobiotic organic compounds in landfill leachate plumes. The approach used is cross-disciplinary, encompassing integration of field-scale observations at different scales, field injection experiments, laboratory experiments, and reactive solute transport modeling. This is illustrated in examples from the most recently investigated site-the Sjoelund Landfill. The research performed serves as good case studies to conceptualize natural attenuation processes in landfill leachate plumes and also supports the notion that monitored natural attenuation (MNA) may be a possible remediation strategy at landfills. However, landfill leachate plumes challenge traditional approaches and tools used in the application of MNA. In particular, the use of in situ indicators to document mass removal in landfill leachate plumes is emphasized. In this article, we advocate the application of conceptual and numerical models as tools for the integration of data and testing of hypotheses.

Influence of Clastic Dikes on Vertical Migration of Contaminants at the Hanford Site

Clastic dikes are subvertical sedimentary features that cut through horizontally layered sediments, and they are common at the Hanford Site. Because of their cross‐cutting relationship with the surrounding matrix, they have been proposed as potential fast paths from former contaminant discharge sites at the surface to the water table. However, little was known of the detailed hydrogeologic properties of the dikes, and detailed modeling of flow and transport through the dikes had not been performed. We excavated a 2‐m‐wide clastic dike at the Hanford Site and characterized it using an air minipermeameter, infrared imagery, and grain size analyses. Field injection experiments were also used to characterize the system. The resulting data were used to prepare a detailed numerical model of the clastic dike and surrounding matrix for a portion of the excavation. Unsaturated flow through the system was modeled for several recharge rates. The highly heterogeneous nature of the system led to complex behavior, with the relative flux rates in the matrix and clastic dike being highly dependent on the recharge rates that were imposed on the system. The occurrence of saturation‐dependent complementary flow networks suggests that the contaminant release history may be important to the choice of remedial actions. Contaminants released under high flux conditions could be inaccessible under low fluxes, and vice versa. This phenomenon may also help explain the occurrence of complex breakthrough patterns of contaminants at compliance planes.

Simulation of field injection experiments in heterogeneous unsaturated media using cokriging and artificial neural network

Simulations of moisture flow in heterogeneous soils are often hampered by lack of measurements of soil hydraulic parameters, making it necessary to rely on other sources of information. In this paper, we develop a methodology to integrate data that can be easily obtained (for example, initial moisture content, θi, bulk density, and soil texture) with data on soil hydraulic properties via cokriging and Artificial Neural Network (ANN)‐based pedotransfer functions. The method is applied to generate heterogeneous soil hydraulic parameters at a field injection site in southeastern Washington State. Stratigraphy at the site consists of imperfectly stratified layers with irregular layer boundaries. Cokriging is first used to generate three‐dimensional heterogeneous fields of bulk density and soil texture using an extensive data set of field‐measured θi, which carry signature about site heterogeneity and stratigraphy. Soil texture and bulk density are subsequently input into an ANN‐based site‐specific pedotransfer function to generate three‐dimensional heterogeneous soil hydraulic parameter fields. The stratigraphy at the site is well represented by the estimated pedotransfer variables and soil hydraulic parameters. The parameter estimates are then used to simulate a field injection experiment at the site. A relatively good agreement is obtained between the simulated and observed moisture contents. The spatial distribution pattern of observed moisture content as well as the southeastward moisture movement is captured well in the simulations. In contrast to earlier work using an effective parameter approach (Yeh et al., 2005), we are able to reproduce the observed splitting of the moisture plume in a coarse sand unit that is sandwiched between two fine‐textured units. The simple method of combining cokriging and ANN for site characterization provides unbiased prediction of the observed moisture plume and is flexible so that additional measurements of various types can be included as they become available.

Stochastic analysis of moisture plume dynamics of a field injection experiment

A vadose zone field injection experiment was conducted in the summer of 2000 at the Hanford Site, Washington. The unique moisture content (θ) database is used to identify the lithology at the field site and to interpret, visualize, and quantify the spatiotemporal evolution of the three‐dimensional (3‐D) moisture plume created by the injection experiment. We conducted a hierarchical geostatistical analysis to examine the large‐scale geologic structure for the entire field site and then investigated small‐scale features within different layers. Afterward, variogram analysis was applied to the θ field measured for seven different days during the injection experiment. Temporal variations of sills and ranges are related to the observed moisture plume dynamics. A visualization of the 3‐D moisture plume evolution illustrates effects of media heterogeneity. Statistics of changes in moisture content as a function of distance reveal large variance near the wetting front; the coefficient of variation increases with decreasing mean. These findings support the gradient‐ and mean‐dependent variability in the moisture content distribution as reported by existing stochastic theories. Spatial moment analysis is also conducted to quantify the rate and direction of movement of the plume mass center and its spatial spreading. The ratio of horizontal to vertical spreading at varying moisture contents suggests moisture‐dependent anisotropy in effective unsaturated hydraulic conductivity, confirming existing stochastic theories. However, the principal directions of the spatial moments are found to vary as the moisture plume evolves through local heterogeneity, a feature that has not been recognized in the theories.

A combined parameter scaling and inverse technique to upscale the unsaturated hydraulic parameters for heterogeneous soils

Determining a large number of soil hydraulic parameters for heterogeneous soils remains a challenge because inverting for too many parameters can lead to parameter values that are nonunique. Furthermore, such inversions may need very long simulation times, for example, months or more when inverting field‐scale problems. In this research, a combined parameter scaling and inverse technique (CPSIT) is proposed to upscale hydraulic parameters from the local scale to the field scale. The CPSIT approach includes two steps: (1) parameter scaling and (2) inverse modeling. In step 1 the number of parameters to be estimated at field scale (FS) is reduced by applying parameter scaling whereby a heterogeneous soil is treated as a composition of multiple equivalent homogeneous media (EHMs). In step 2 the FS parameters for the reference EHM are determined using the inverse technique and observations from well‐designed field experiments. The advantages of the CPSIT approach are that the number of parameters to be inverted is reduced by a factor of the number (M) of EHMs, and the simulation time is reduced by a factor of about M2. The CPSIT approach was tested by upscaling the hydraulic parameters using a field injection experiment at the Hanford Site. Results show that when the CPSIT upscaled parameters were used to simulated flow, the mean squared residual was reduced by 83.2% relative to that when the local‐scale parameters were used.

Degradation of the ( R)- and ( S)-enantiomers of the herbicides MCPP and dichlorprop in a continuous field-injection experiment

An aerobic field-injection experiment was performed to study the degradation and migration of different herbicides at trace levels in an aerobic aquifer at Vejen, Denmark. Mecoprop (MCPP) and dichlorprop monitored in a dense network of multilevel samplers were both degraded within a distance of 1 m after a period of 120 days. The study showed that no preferential degradation of the ( R)- and ( S)-enantiomers of MCPP and of dichlorprop took place as the enantiomeric forms of the phenoxy acids were degraded simultaneously in the aquifer.

Fate of herbicides in a shallow aerobic aquifer: A continuous field injection experiment (Vejen, Denmark)

A continuous, natural gradient, field injection experiment, involving six herbicides and a tracer, was performed in a shallow aerobic aquifer near Vejen, Denmark. Bentazone, (±)‐2‐(4‐chloro‐2‐methylphenoxy) propanoic acid (MCPP), dichlorprop, isoproturon, and the dichlobenil metabolite 2,6‐dichlor‐benzamide (BAM) were injected along with 2‐methyl‐4,6‐dinitrophenol (not discussed in this paper) and the tracer bromide. The injection lasted for 216 days and created a continuous plume in the aquifer. The plume was monitored in three dimensions in 96 multilevel samplers of 6–9 points each for 230 days, with selected individual points for a longer period. The bromide plume followed a complex path through the monitoring network downgradient of the injection wells. The plume movement was controlled by spatially varied hydraulic conductivities of the sand deposit and influenced by asynchronous seasonal variation in groundwater potentials. An average flow velocity of 0.5 m/d was observed, as depicted by bromide. Bentazone, BAM, MCPP, and dichlorprop retardation was negligible, and only slight retardation of isoproturon was observed in the continuous injection experiment and a preceding pulse experiment. No degradation of bentazone was observed in the aerobic aquifer during the monitoring period. BAM and isoproturon were not degraded within 5 m downgradient of the injection. The two phenoxy acids MCPP and dichlorprop were both degraded in the aerobic aquifer. Near the source a lag phase was observed followed by fast degradation of the phenoxy acids, indicating growth kinetics. The phenoxy acids were completely degraded within l m downgradient of the injection wells, resulting in the plumes being divided into small plumes at the injection wells and pulses farther downgradient. During the lag phase, phenoxy acids had spread beyond the 25 m long monitoring network. However, the mass of the phenoxy acids passing the 10–25 m fences never matched the corresponding bentazone or bromide masses, and the pulse was observed to shrink in size. This indicates that this pulse of phenoxy acids was being partially degraded at a low rate as it traveled through the aquifer.

Sorption and degradation of the herbicide 2-methyl-4,6-dinitrophenol under aerobic conditions in a sandy aquifer in Vejen, Denmark.

A pulse (7 days) and a continuous (216 days), natural gradient field injection experiment with herbicides, including 2-methyl-4,6-dinitrophenol (4,6-dinitro-o-cresol, abbreviated DNOC), and a bromide tracer were conducted in a shallow, aerobic aquifer near Vejen, Denmark. The pulse and continuous plume were monitored in a dense, three-dimensional monitoring network installed in the aquifer downgradient of the injection. The sorption and degradation of DNOC were evaluated based on moment analysis of breakthrough curves, cross sections, and snapshots of the DNOC plume and supported by results from laboratory experiments. Significant and spatially variable sorption of DNOC (Kd range, 0.10-0.98 L/kg) was observed due to a specific binding of DNOC to clay minerals. The spatial variation was mainly a result of variation in pH, with stronger sorption at lower pH, whereas other factors such as cation composition on the solid matrix appeared to be negligible. Significant degradation of DNOC in the aquifer was revealed by moment analysis of data from the continuous field injection experiment. Degradation of DNOC in the field was slow and/or subject to long lag phases, and the data suggested spatially varying degradation potentials. This was supported by the laboratory experiments. The potential for natural attenuation of DNOC in aerobic aquifers appears promising.

Degradation of herbicides in shallow Danish aquifers: an integrated laboratory and field study.

Degradation of pesticides in aquifers has been evaluated based on a number of co-ordinated field and laboratory studies carried out in Danish aquifers. These studies included investigations of vertical and horizontal variability in degradation rates from the vadose zone to an aquifer, the effects of aerobic versus anaerobic conditions, and the importance of concentration on degradation kinetics for a selected range of herbicides. The studies were based on different experimental approaches ranging from simple batch experiments to column studies to field injection experiments and, where appropriate, results were compared. Some herbicides were degraded under aerobic conditions (some phenoxy acids, DNOC and glyphosate) and others under aerobic conditions (other phenoxy acids, DNOC; there was some indication of atrazine transformation). Certain pesticides were not degraded in any investigations (dichlobenil, the dichlobenil metabolite 2,6-dichlorobenzamide (BAM), bentazone, isoproturon, metamitron and metsulfuron-methyl). The spatial variability was substantial, since hardly any of the investigated pesticides were degraded in all comparable samples. This means that it is very difficult to claim that a given pesticide is readily degradable in aquifers. However, the experimental approaches used (with incubations lasting more than a year) may not be sensitive enough to verify the low degradation rates that may be significant as a result of the long retention time of groundwaters.

Fate of MCPP and atrazine in an anaerobic landfill leachate plume (Grindsted, Denmark)

The fate of the two pesticides MCPP (mecoprop) and atrazine was investigated in an anaerobic aquifer downgradient from the Grindsted Landfill, Denmark, by means of a field injection experiment, in situ microcosms and laboratory batch experiments. The redox conditions in the experiments were dominated by iron reduction, but also methanogenesis and sulfate reduction occurred closest to the landfill. No degradation nor sorption was observed for the two pesticides in any of the biologically active experiments. In microbially inhibited laboratory experiments, supplied with sodium azide as a sterilizing agent, a chemical degradation of atrazine occurred.

An anaerobic field injection experiment in a landfill leachate plume, Grindsted, Denmark: 2. Deduction of anaerobic (methanogenic, sulfate‐, and Fe (III)‐reducing) redox conditions

Redox conditions may be environmental factors which affect the fate of the xenobiotic organic compounds. Therefore the redox conditions were characterized in an anaerobic, leachate‐contaminated aquifer 15–60 m downgradient from the Grindsted Landfill, Denmark, where an field injection experiment was carried out. Furthermore, the stability of the redox conditions spatially and over time were investigated, and different approaches to deduce the redox conditions were evaluated. The redox conditions were evaluated in a set of 20 sediment and groundwater samples taken from locations adjacent to the sediment samples. Samples were investigated with respect to groundwater chemistry, including hydrogen and volatile fatty acids (VFAs) and sediment geochemistry, and bioassays were performed. The groundwater chemistry, including redox sensitive species for a large number of samples, varied over time during the experimental period of 924 days owing to variations in the leachate from the landfill. However, no indication of change in the redox environment resulting from the field injection experiment or natural variation was observed in the individual sampling points. The methane, Fe(II), hydrogen, and VFA groundwater chemistry parameters strongly indicated a Fe(III)‐reducing environment. This was further supported by the bioassays, although methane production and sulfate‐reduction were also observed in a few samples close to the landfill. On the basis of the calculated carbon conversion, Fe(III) was the dominant electron acceptor in the region of the aquifer, which was investigated. Because of the complexity of a landfill leachate plume, several redox processes may occur simultaneously, and an array of methods must be applied for redox characterization in such multicomponent systems.

An anaerobic field injection experiment in a landfill leachate plume, Grindsted, Denmark: 1. Experimental setup, tracer movement, and fate of aromatic and chlorinated compounds

A continuous, natural‐gradient field injection experiment, involving 18 xenobiotic compounds and bromide as tracers, was performed in the anaerobic region of a leachate plume downgradient from the Grindsted Landfill, Denmark. The injection lasted for 195 days, and within this period a continuous cloud was established. Over a period of 924 days the cloud movement was monitored in approximately 70 discrete sampling points in the central part of the cloud, and the spatial distribution was described by seven cloud snapshots involving 400–700 sampling points. The bromide cloud movement closely followed the varying flow direction predicted by the water table measurements. Moment analysis showed decreasing tracer flow velocities and reduced capture of bromide mass with time, which may be explained by varying flow conditions (direction, hydraulic gradient) and the heterogeneous geological conditions in the sandy aquifer. Naphthalene, having the highest log Kow value, was the most retarded compound, with a retardation of less than 10%. Therefore sorption was not considered to be a significant attenuation process for any of the compounds studied. Transformation under iron‐reducing conditions was observed for toluene, o‐xylene, TeCM, 1,1,1‐TCA, PCE, and TCE, while transformation of benzene and napthalene was not detected in the aquifer within the time frame of this study. First‐order transformation rates were in the range of 0.028–0.039 d−1 and 0.0014–0.0028 d−1 for the aromatic compounds toluene and o‐xylene, respectively. The rates for the chlorinated aliphatic compounds, tetrachloromethane, 1,1,1‐ trichloroethane, tetrachloroethylene, and trichloroethylene, were &gt;0.7 d−1, 0.0044–0.0054 d−1, 0.0012–0.0038 d−1, and 0.0003–0.001 d−1, respectively. Long lag periods and slow transformation rates were observed for some of the compounds, suggesting that lack of transformation reported in the literature may be attributable to short experimental periods in those studies.

Degradation of Aromatic and Chlorinated Aliphatic Hydrocarbons in the Anaerobic Part of the Grindsted Landfill Leachate Plume: In Situ Microcosm and Laboratory Batch Experiments

AbstractThe fate of seven aromatic and four chlorinated aliphatic compounds was studied using in situ microcosm (ISM) and laboratory batch (LB) experiments performed at six distances along a flow line in the anaerobic leachate plume downgradient of the Grindsted Landfill, Denmark. The redox conditions were dominated by iron reduction in this part of the plume (15 to 60 m downgradient of the landfill); however, methanogenesis and sulfate reduction also took place closer to the landfill (15 to 45 m). Generally, good accordance was found between the degradation of the compounds determined by the two experimental approaches, considering the limitations of the experimental periods (up to 220 days for the ISM and up to 537 days for the LB). Of the aromatic compounds, toluene, o‐ and m/p‐xylene were degraded, while benzene, ethylbenzene, and naphthalene were not degraded, neither in the ISM nor in the LB at any distance from the landfill. All the chlorinated aliphatic compounds, (TeCM, 1,1,1‐TCA, PCE, and TCE) were degrad‐able; however, the degradation potential for PCE and TCE depended on the distance from the landfill. For most compounds, both approaches gave results comparable to those of the natural gradient field injection experiment performed in the same plume. This suggests that laboratory batch experiments, which are easier to run than ISM experiments, may be a useful tool in determining the degradability of mono aromatic hydrocarbons and chlorinated aliphatic hydrocarbons under strongly anaerobic conditions in the investigation of natural attenuation in landfill leachate plumes.

Preliminary observations on bacterial transport in a coastal plain aquifer

A multidisciplinary research team, funded by the U.S. Department of Energy (DOE) Subsurface Science Program, initiated a field-scale bacterial transport study in a sandy aquifer on the coastal plain of Virginia in 1994. The purpose of the study was to evaluate the relative importance of hydrogeological and geochemical heterogeneity in controlling bacterial transport. Extensive geophysical and geochemical characterization of the site was accomplished using intact cores obtained during the construction of the flow field and in a nearby sand pit exposure of the sedimentary facies found in the flow field. Geophysical techniques, including ground penetrating radar and cross borehole tomography, were used to relate the depositional environment of the sand pit to the flow field as well as to produce a 3-dimensional depiction of the flow field to be used in modeling the site and the results of the injection experiments. The 30 m long flow cell consists of ground water production and injection wells, a tracer injection well, and 10 multilevel samplers screened every half meter from 4.0 to 10.5 m below ground surface. The organization that owns the field site required that only native microorganisms be introduced at the site, therefore, the injected bacterial strain was isolated from the indigenous community in the aquifer. Candidate strains were selected by a protocol that enriched for phenotypes of low adhesion and non-clinical antibiotic resistance which could be used to detect the organism on selective media. The bacteria were selected for low adhesion to site sediments so that they might be readily transported through the aquifer. For the field injection experiment detection and quantitation of the strain chosen by this screening process, PL2W31, was accomplished by isotopically enriching the cells with [13C]glucose. Forced gradient conservative (Br−) tracer tests were performed immediately prior to the bacterial injection experiment to provide a measure of non-reactive transport through the aquifer. The non-reactive tracer test indicated the presence of hydrogeological heterogeneities at the site that caused differential breakthrough of the tracer. Results from the bacterial transport experiment indicate that bacteria traveled the length of the flow field (4 m), but that the majority of the biomass injected was retained in the sediments between the injection well and the first multilevel sampler at 0.5 m. Preliminary bacterial transport models indicate that the observed behavior could be accounted for by the presence of two subpopulations within a single bacterial strain with differing transport properties.

Case history—excavation of the Brooklyn tunnel

The behaviour of injected fluids and hydraulic fractures in non-cohesive soils was investigated using laboratory experiments and two field experiments. A viscous guar gel was injected into fine-grained sand in the laboratory; of eight tests, two produced vertical fractures and a third produced both horizontal and vertical fractures. At one field site, an injection well was installed at 4.1 m in silty clay, and water and coarse-grained sand injected. Piping to the surface occurred beside the well. The second field injection experiment took place in sandy gravel at a depth of 12.2 m. Water and guar gel were injected, and infiltrated uniformly into the soil (verified using ground penetrating radar). During excavations, four processes were noted when discrete hydraulic fractures were not produced: fluid infiltration, piping, sloughing, and sandout. These processes can be controlled somewhat by adjusting fluid viscosity, injection strategy, and well construction technique.