Direct Push Technology Research Articles

Indirect joint petrophysical inversion of shallow-seismic and multi-offset ground-penetrating radar field data

SUMMARY In near-surface surveys, shallow-seismic and ground-penetrating radar (GPR) full-waveform inversions (FWIs) have received increasing attention because of their ability to reconstruct high-resolution subsurface models. However, they have different sensitivities to the same targets and thus may yield conflicting geophysical parameter models. To solve this issue, we have developed an indirect joint petrophysical inversion (JPI) integrating shallow-seismic and multi-offset GPR data. These data are used to reconstruct porosity and saturation whereby we use only strong sensitivities between petrophysical and geophysical parameters. To promote its field application, we proposed an input strategy to avoid measuring rock matrix parameters and make indirect JPI more robust. We apply indirect JPI to the field data acquired in Rheinstetten, Germany and find that it reveals the mechanical, electrical and petrophysical properties more reliably than individual inversions. The reconstructed models are assessed by direct-push technology, borehole sample measurements and migrated GPR image. Indirect JPI can fit seismic and GPR observed data simultaneously and provide consistent multiparameter models, which are hard to achieve by FWIs and individual petrophysical inversions. We also find that the method is robust when there are uncertainties in petrophysical a priori information. Overall, the field example proves the great potential of using indirect JPI to solve real-world problems.

Pollution status and health risk assessment of a specific organophosphate pesticide-contaminated site

Organophosphate pesticide pollution is a severe global environmental issue, posing significant threats to human health. This study focuses on an organophosphate pesticide production site in North China. Controlled soil and groundwater were sampled using grid sampling and direct-push technology. Analytical methods such as headspace gas chromatography-mass spectrometry (GC-MS), gas chromatography (GC), and purge and trap GC-MS were employed to detect contaminants, including volatile organic compounds (VOCs), semi-volatile organic compounds (SVOCs), pesticides, and heavy metals. The results indicate that the maximum exceedances in the contaminated soil for benzene, xylene, tri-chloromethane, Parathion, phosphorodithioic acid, Phorate, and Terbufos were 490.63, 411.22, 3459.09, 507.89, 277.29, 1946.15, and 281.5 times the standard limits, respectively. In groundwater, benzene, dichloromethane, trichloromethane, and Phorate exceeded the standard limits by 220.83, 1374, 1853.33, and 806.67 times, respectively, severely contaminating the local soil and groundwater and damaging the eco-system. Additionally, the carcinogenic risk values in the contaminated soil for benzene, toluene, phenylethane, xylene, 1,3,5-trimethylbenzene, 1,2,4-trimethylbenzene, Parathion, 1,2,4-trichlorobenzene, tri-chloromethane, naphthalene, and aniline were 2.75E–3, 1.36E–2, 1.034E–2, 9.72E–1, 1.5E–3, 8.9E–3, 1.85E–3, 4E–3, 3.48E–2, 2.7E–2, 1.03E–3, respectively. In groundwater, toluene, xylene, dichloro-methane, and trichloromethane showed carcinogenic risk values of 8E–3, 1.86E–2, 1.23E–2, 3.07E–2, respectively, significantly exceeding the acceptable limits and posing severe threats to the health of nearby residents. Furthermore, the hazard quotient for non-carcinogenic effects of Cypermethrin in soil was 1.15, and 2,6-dichlorophenol in groundwater was 1.22, both slightly above the standard, indicating a mild impact on the health of nearby residents. Overall, this study provides a theoretical basis for subsequent remediation work at the contaminated site through pollutant analysis.

The in situ treatment of PFAS within porewater at the air–water interface of a PFAS source zone

AbstractThe treatment of per‐ and polyfluoroalkyl substances (PFAS) within groundwater is an emerging topic, with various technologies being researched and tested. Currently, PFAS‐impacted groundwater is typically treated ex situ using sorptive media such as activated carbon and ion exchange resin. Proven in situ remedial approaches for groundwater have been limited to colloidal activated carbon (CAC) injected into aquifers downgradient of the source zones. However, treatment of groundwater within the source zones has not been shown to be feasible to date. This study evaluated the use of CAC to treat dissolved PFAS at the air–water interface within the PFAS source zone. Studies have shown that PFAS tends to preferentially accumulate at the air–water interface due to the chemical properties of the various PFAS. This accumulation can act as a long‐term source for PFAS, thus making downgradient treatment of groundwater a long‐term requirement. A solution of CAC was injected at the air–water interface within the source zone at a site with PFAS contamination using direct push technology. A dense injection grid that targeted the interface between the air and groundwater was used to deliver the CAC. Concentrations of PFAS within the porewater and groundwater were collected using a series of nine lysimeters installed within the vadose and saturated water columns. A total of six PFAS were detected in the porewater and groundwater including perfluorobutanoic acid (PFBA), perfluoropentanoic acid (PFPeA), perfluorohexanoic acid (PFHxA), perfluoroheptanoic acid (PFHpA), perfluorooctanoic acid (PFOA), and perfluorononanoic acid (PFNA). Detectable concentrations of PFAS within the pore and groundwater before treatment ranged from values greater than 300 µg/L for PFPeA to less than 3 µg/L for PFNA. Following the injection of the CAC, monitoring of the porewater and groundwater for PFAS was conducted approximately 3, 6, 9, 12, and 18 months postinjection. The results indicated that the PFAS within the porewater and groundwater at and near the air–water interface was effectively attenuated over the 1.5‐year monitoring program, with PFAS concentrations being below the method detection limits of approximately 10 ng/L, with the exception of PFPeA, which was detected within the porewater during the 18‐month sampling event at concentrations of up to 55 ng/L. PFPeA is a five carbon‐chained PFAS that has been shown to have a lower affinity for sorption onto activated carbon compared to the longer carbon‐chained PFAS such as PFOA. Examination of aquifer cores in the zone of injection indicated that the total organic carbon concentration of the aquifer increased by five orders of magnitude postinjection, with 97% of the samples collected within the target injection area containing activated carbon, indicating that the CAC was successfully delivered into the source zone.

Salinity distribution in the subterranean estuary of a meso-tidal high-energy beach characterized by Electrical Resistivity Tomography and direct push technology

Understanding the interaction of terrestrial freshwater and seawater in the subterranean estuary (STE) is an important factor when considering nutrient fluxes from land to sea. State-of-the-art research describes the STE by a tide-induced upper saline recirculation cell, a freshwater discharge tube and a deep saltwater wedge. However, recent numerical modelling and shallow hydrogeochemical investigations for high-energy beaches indicate that multiple saline recirculation cells may exist and affect the land-sea interaction. Electrical Resistivity Tomography (ERT) and Direct Push (DP) technologies are common tools to explore the subsurface. Due to their sensitivity to the electrical conductivity of pore water, they permit investigating the STE. This study combines ERT and DP to image the salinity distribution within the STE of a meso-tidal, high-energy beach. We actively incorporate the DP data into the ERT inversion and use geostatistical regularization for closing the resolution gap. For the first time, our experimental results confirm the existence of several 10–20 m deep reaching upper saline recirculation cells and corresponding brackish discharge locations generated by a pronounced runnel-ridge beach system in 2019, whereas in 2021 only a single cell was displayed for a flat topography at the time.

Six pilot‐scale studies evaluating the in situ treatment of PFAS in groundwater

AbstractPer‐ and polyfluoroalkyl substances (PFAS) have been identified by many regulatory agencies as emerging contaminants of concern in a variety of media including groundwater. Currently, there are limited technologies available to treat PFAS in groundwater with the most frequently applied approach being extraction (i.e., pump and treat). While this approach can be effective in containing PFAS plumes, previous studies of pump and treat programs have met with limited remedial success. In situ treatment studies of PFAS have been limited to laboratory and a few field studies. Six pilot‐scale field studies were conducted in an unconfined sand aquifer coimpacted by petroleum hydrocarbon along with PFAS to determine if a variety of reagents could be used to attenuate dissolved phase PFAS in the presence of petroleum hydrocarbons. The six reagents consisted of two chemical oxidants, hydrogen peroxide (H2O2) and sodium persulfate (Na2S2O8), and four adsorbents, powdered activated carbon (PAC), colloidal activated carbon (CAC), ion‐exchange resin (IER), and biochar. The reagents were injected using direct push technology in six permeable reactive zone (PRZ) configurations. Groundwater concentrations of various PFAS entering the PRZs ranged up to 24,000 µg/L perfluoropentanoic acid, up to 6,200 µg/L pentafluorobenzoic acid, up to 16,100 µg/L perfluorohexanoic acid, up to 6,080 µg/L perfluoroheptanoic acid, up to 450 µg/L perfluorooctanoic acid, and up to 140 µg/L perfluorononanoic acid. Performance groundwater sampling within and downgradient of the PRZs occurred for up to 18 months using single and multilevel monitoring wells. Results of groundwater sampling indicated that the PFAS were not treated by either the persulfate nor the peroxide and, in some cases, the PFAS increased in concentration immediately following the injection of peroxide and persulfate. Concentrations of PFAS in groundwater sampled within the PAC, CAC, IER, and biochar PRZs immediately after the injection were determined to be less than the method detection limits. Analyses of groundwater samples over the 18‐month monitoring period, indicated that all the PRZs exhibited partial or complete breakthrough of the PFAS over the 18‐month monitoring period, except for the CAC PRZ which showed no PFAS breakthrough. Analysis of cores for the CAC, PAC, and biochar PRZs suggested that the CAC was uniformly distributed within the target injection zone, whereas the PAC and biochar showed preferential injection into a thin coarse‐sand seam. Similarly, analysis of the sand packs of monitoring wells installed before the injection of the CAC, PAC, and biochar indicated that the sand packs of the PAC and biochar preferentially accumulated the reagents compared with the reagent concentrations within the surrounding aquifer by up to 18 times.

Distribution of Colloidal and Powdered Activated Carbon for the <i>in Situ</i> Treatment of Groundwater

The use of in situ technologies for the treatment of groundwater containing various compounds of concern are widely accepted. These technologies include chemical reduction, chemical oxidation, anaerobic and aerobic bioremediation, and adsorption, among others. One requirement for the successful application of these technologies is the delivery of the remedial reagent(s) to the compounds of concern. A rapidly evolving in situ technology is the injection of adsorptive media such as activated carbon and ion-exchange resin including powdered or colloidal activated carbon. Activated carbon has a long-demonstrated history of effectiveness for the removal of various organic and inorganic compounds in above ground water treatment systems. However, due to constraints related to the particle size and physical properties of the activated carbon, the in situ application of activated carbon has been limited. Recent developments in the manufacturing of activated carbon have created a smaller particle size allowing activated carbon to be applied in situ. To evaluate if powdered and colloidal activated carbon can be effectively distributed in aquifers, the two types of carbon were injected using direct push technology adjacent to each other at four sites with varying geology. Evaluation of distribution was completed by sampling the aquifer prior to and post-injection for total organic carbon. The results of the studies indicated that both forms of activated carbon were effectively delivered to the targeted injection zones with both carbon types being detected at least seven meters away from the point of injection. The colloidal form of the activated carbon showed good distribution throughout the four targeted zones of injection with 93 percent of the samples collected having colloidal activated carbon present within them whereas the powdered activated carbon cells were more susceptible to aquifer heterogeneity with only 67 percent of the samples collected having activated carbon present. Preferential accumulation of activated carbon was observed in high horizontal hydraulic conductivity seams, especially within the powdered activated carbon cells. These results suggested that the powdered form of activated carbon was more suspectable at the four sites to heterogeneity within the aquifer than the colloidal form of activated carbon. Sampling of monitoring well screens installed prior to the injection of the two forms of activated carbon showed preferential accumulation of powdered activated carbon within the sand pack, which could result in sampling bias.

Investigation of soil and groundwater environment in urban area during post-industrial era: A case study of brownfield in Zhenjiang, Jiangsu Province, China

ABSTRACT With the adjustment of industrial structure, many high pollution enterprises will gradually shut down. This process will produce a large number of high-risk pollution plots and cause a series of environmental problems. In this study, geophysical detection and direct push technology were applied to investigate soil and groundwater pollution in a typical brownfield in Zhenjiang, Jiangsu province, China. The results showed that Chlorotoluene and its ramification were the main pollutants for the brownfield. Pollution decreased with depth and was quite uneven in the brownfield. Production and wastewater treatment areas were heavily polluted, where volatile organic compounds contaminated and semi-volatile organic compounds were found 12 m and 15 m below the land surface, respectively. About 18880 m3 groundwater was contaminated. This study is significant to develop a geological survey work mode for the postindustrial era.

Characterising alluvial aquifers in a remote ephemeral catchment (Flinders River, Queensland) using a direct push tracer approach

The availability of reliable water supplies is a key factor limiting development in northern Australia. However, characterising groundwater resources in this remote part of Australia is challenging due to a lack of existing infrastructure and data. Here, direct push technology (DPT) was used to characterise shallow alluvial aquifers at two locations in the semiarid Flinders River catchment. DPT was used to evaluate the saturated thickness of the aquifer and estimate recharge rates by sampling for environmental tracers in groundwater (major ions, 2H, 18O, 3H and 14C). The alluvium at Fifteen Mile Reserve and Glendalough Station consisted of a mixture of permeable coarse sandy and gravely sediments and less permeable clays and silts. The alluvium was relatively thin (i.e. < 20 m) and, at the time of the investigation, was only partially saturated. Tritium (3H) concentrations in groundwater was ∼1 Tritium Unit (TU), corresponding to a mean residence time for groundwater of about 12 years. The lack of an evaporation signal for the 2H and 18O of groundwater suggests rapid localised recharge from overbank flood events as the primary recharge mechanism. Using the chloride mass balance technique (CMB) and lumped parameter models to interpret patterns in 3H in the aquifer, the mean annual recharge rate varied between 21 and 240 mm/yr. Whilst this recharge rate is relatively high for a semiarid climate, the alluvium is thin and heterogeneous hosting numerous alluvial aquifers with varied connectivity and limited storage capacity. Combining DPT and environmental tracers is a cost-effective strategy to characterise shallow groundwater resources in unconsolidated sedimentary aquifers in remote data sparse areas.

Remedial investigations of karst aquifers: a case study at former Marietta Air Force Station, Lancaster County, Pennsylvania

A phased multi-disciplinary approach was used to investigate potential environmental impact on the overburden aquifer and karst (bedrock) aquifer from the historic use of a suspected fire training pit at the former Marietta Air Force Station in Lancaster County, Pennsylvania. Investigation techniques have included direct push technology, real-time sample analysis via use of a mobile laboratory, earth resistivity imaging/induced polarization, geophysical borehole logging, packer testing, installation of one multi-port well with five sampling ports set in the karst aquifer, installation of multiple conventional monitoring wells, and completion of multiple rounds of groundwater sampling and data analysis. Both the vertical and lateral extents of groundwater contamination by chlorinated volatile organic compounds were delineated in the overburden and karst aquifer. The pathway-focused human health risk assessment has indicated that groundwater remediation is required to meet the federal and state health standards for volatile organic compounds. Based on the hydrogeological conditions and the nature and extent of the delineated contamination, plumes in both the overburden and karst aquifers, chemical oxidation (in the karst aquifer), and enhanced bioremediation (in the overburden aquifer) have been selected as the preferred alternatives for groundwater remediation, with monitored natural attenuation included as a common component of both alternatives.

Improving resistivity survey resolution at sites with limited spatial extent using buried electrode arrays

Electrical resistivity tomography (ERT) surveys are widely used in geological, environmental and engineering studies. However, the effectiveness of surface ERT surveys is limited by decreasing resolution with depth and near the ends of the survey line. Increasing the array length will increase depth of investigation, but may not be possible at urban sites where access is limited. One novel method of addressing these limitations while maintaining lateral coverage is to install an array of deep electrodes. Referred to here as the Multi-Electrode Resistivity Implant Technique (MERIT), self-driving pointed electrodes are implanted at depth below each surface electrode in an array, using direct-push technology. Optimal sequences of readings have been identified with the “Compare R” method of Wilkinson. Numerical, laboratory, and field case studies are applied to examine the effectiveness of the MERIT method, particularly for use in covered karst terrain. In the field case studies, resistivity images are compared against subsurface structure defined from borings, GPR surveys, and knowledge of prior land use. In karst terrain where limestone has a clay overburden, traditional surface resistivity methods suffer from lack of current penetration through the shallow clay layer. In these settings, the MERIT method is found to improve resolution of features between the surface and buried array, as well as increasing depth of penetration and enhancing imaging capabilities at the array ends. The method functions similar to a cross-borehole array between horizontal boreholes, and suffers from limitations common to borehole arrays. Inversion artifacts are common at depths close to the buried array, and because some readings involve high geometric factors, inversions are more susceptible to noise than traditional surface arrays. Results are improved by using errors from reciprocal measurements to weight the data during the inversion.

Field Investigation of a New Recharge Approach for ASR Projects in Near-Surface Aquifers.

Aquifer storage and recovery (ASR) is the artificial recharge and temporary storage of water in an aquifer when water is abundant, and recovery of all or a portion of that water when it is needed. One key limiting factor that still hinders the effectiveness of ASR is the high costs of constructing, maintaining, and operating the artificial recharge systems. Here we investigate a new recharge method for ASR in near-surface unconsolidated aquifers that uses small-diameter, low-cost wells installed with direct-push (DP) technology. The effectiveness of a DP well for ASR recharge is compared with that of a surface infiltration basin at a field site in north-central Kansas. The performance of the surface basin was poor at the site due to the presence of a shallow continuous clay layer, identified with DP profiling methods, that constrained the downward movement of infiltrated water and significantly reduced the basin recharge capacity. The DP well penetrated through this clay layer and was able to recharge water by gravity alone at a much higher rate. Most importantly, the costs of the DP well, including both the construction and land costs, were only a small fraction of those for the infiltration basin. This low-cost approach could significantly expand the applicability of ASR as a water resources management tool to entities with limited fiscal resources, such as many small municipalities and rural communities. The results of this investigation demonstrate the great potential of DP wells as a new recharge option for ASR projects in near-surface unconsolidated aquifers.

Laboratory Study of Low-Flow Rates on Clogging Processes for Application to Small-Diameter Injection Wells

A new concept in aquifer storage and recovery (ASR) aims to lower costs by using gravity-induced infiltration and small diameter wells installed with direct-push technology. Such systems will have significantly lower injection rates than traditional ASR. Clogging is a major concern for ASR systems; however, previous investigations of injection well clogging assumed a high injection rate. To address this bias, this study investigates the potential for clogging in low-flow (<0.38 m3/min) ASR injection wells. The effects of low-flow rate injection on hydraulic conductivity and geochemistry were examined using laboratory columns packed with sand and gravel taken from an experimental low-flow ASR site. Changes in hydraulic conductivity and geochemistry were monitored over 17 days using two types of source water (treated surface water and native groundwater) at three different flow rates (Darcy velocities 1 m/d, 3 m/d, 5 m/d). The hydraulic conductivity of the lowest flow rate columns decreased below initial levels by at least one order of magnitude during the experiment. Observations of biofilm in effluent tubing suggest bacterial clogging reduced the hydraulic conductivity in medium and low flow treated surface water columns, but bacterial abundance was not quantified in this study. Clay dispersion was estimated to be an important process, partially reversing the bacterial clogging. Further understanding of clogging factors at low flow rates will aid in the selection of the most beneficial clogging prediction tests and pretreatment and redevelopment methods for low-flow ASR systems.

Evaluation of direct push probes: Sensor interface analysis of DC resistivity probes

In near surface sedimentary exploration direct push technology has become popular for geophysical logging. The method is thought to have great potential to offer accurate information about the variability of physical parameters since the region of disturbed sedimentary formation due to probe injection is considered to be smaller compared to disturbances by classical borehole measurements. Technical and experimental design of direct push probes follow often those of established borehole probes. A systematic appraisal of the suitability of such tools for direct push logging procedure exposing the probes to a very high mechanical stress and rapid aging process has been missing in the past. Following a recently developed general framework for direct push system decomposition we analyze two different DC resistivity direct push probes with regard to their sensor interface. Simple laboratory experiments validate the setup of a numerical simulation of both probes revealing significant differences on the suitability of the chosen electrode arrangement. Differences in robustness with regard to surface abrasion result in changing probe responses which could, depending on the experimental design of the probe, cause resistivity value changes of almost 25% within approximately 15 operational hours, which leaves severe doubts about the suitability of established direct push logging probes for quantitative geophysical probing.

Systematic description of direct push sensor systems: A conceptual framework for system decomposition as a basis for the optimal sensor system design

Systematic decomposition and evaluation of existing sensor systems as well as the optimal design of future generations of direct push probes are of high importance for optimized geophysical experiments since the employed equipment is a constrain on the data space. Direct push technologies became established methods in the field of geophysical, geotechnical, hydrogeological, and environmental sciences for the investigation of the near subsurface. By using direct push sensor systems it is possible to measure in-situ parameters with high vertical resolution. Such information is frequently used for quantitative geophysical model calibration of interpretation of geotechnical and hydrological subsurface conditions. Most of the available direct push sensor systems are largely based on empirical testing and consecutively evaluated under field conditions. Approaches suitable to identify specific characteristics and problems of direct push sensor systems have not been established, yet. We develop a general systematic approach for the classification, analysis, and optimization of direct push sensor systems. First, a classification is presented for different existing sensor systems. The following systematic description, which is based on the conceptual decomposition of an existing sensor system into subsystems, is a suitable way to analyze and explore the transfer behavior of the system components and therefore of the complete system. Also, this approach may serve as guideline for the synthesis and the design of new and optimized direct push sensor systems.

Stress history profiling in soils using OCD–G0anisotropy relationship

The stress history profile in soils is evaluated on the basis of paired shear wave velocity measurements in different propagation directions (vertical and horizontal) with both waves horizontally polarised. Specifically, using a specially compiled database of sites with well-defined stress histories, the level of prestress, or overconsolidation difference (OCD = [Formula: see text]), relates directly to the ratio of shear moduli (G0,HH/G0,VH) as measured by field geophysical testing. The G0,VHprofile is obtained from standard downhole tests using a horizontal surface source and either a cased borehole or direct-push technology, such as seismic cone or seismic dilatometer, whereas the G0,HHprofile is determined from a crosshole array using a torsional or rotary source at the same elevation as the geophones. In soil formations that are mechanically prestressed, the OCD is constant and permits the calculation of the profile of preconsolidation stress ( [Formula: see text]) with depth and corresponding overconsolidation ratio.

NovCare 2013 (Novel methods for subsurface characterization and monitoring: from theory to practice)

As safe and effective use of the subsurface environment is a major challenge facing our society, there is a great need to improve our understanding of the subsurface and to observe and investigate natural and anthropogenically influenced systems. Characterization of the subsurface includes the investigation of both groundwater and subsurface soils and their interaction processes. The range of monitoring tools used can be extended to include remote sensing tools [e.g., Fourier transform infrared spectroscopy (FTIR)], applicable at the near-surface to estimate gas–soil gas exchange processes. One example that highlights the importance of subsurface characterization and monitoring is the detection of groundwater contamination, to provide data for developing plans to prevent any further contamination and remediate existing contamination. With an increasing demand for investigation methods that have both high accuracy and resolution across a variety of spatial scales, there are several challenges for monitoring and exploration technologies in environmental research. These challenges include, among others, the identification and parameterization of system relevant physical, chemical, biological processes, as well as the determination of interactions between different scales. These challenges require high-resolution methods or novel data processing and common interpretation of data measured by different methods. In particular, cost-effective methods that are minimally destructive and can be rapidly applied are taken into consideration. Methods for subsurface characterization include e.g., geophysical, hydrogeological and direct push (DP) technologies, borehole-based, and tomographic methods. The NovCare conference (Novel methods for subsurface characterization and monitoring: from theory to practice— established in 2009) provides a platform to present and discuss state of the art developments and concepts. Existing methods are often refined and new methods are continually being developed. Therefore, this symposium presents recent developments, examples of field applications and introduces method concepts such as direct push, geophysical methods, adaptive (sensor) networks, and novel monitoring sensors. It also highlights the application of these methods in different thematic areas. The NovCare 2013 International Conference (Novel methods for subsurface characterization and monitoring: from theory to practice) took place from May 13–16th 2013 at the Helmholtz Centre for Environmental Research in Leipzig. This conference, which was organized by the Helmholtz Centre for Environmental Research, the University of Tuebingen, the Kansas Geological Survey and the Michigan State University, was the third successive annual conference. This conference series, which began in Leipzig in 2009 and then moved to Cape Cod in the US in 2011, has proven to be an excellent forum for exchanging ideas and experiences related to the challenges of subsurface characterization and monitoring. NovCare 2013 again provided a rare opportunity and an outstanding platform for researchers and practitioners from all over the world to exchange their ideas and experiences in dealing with the challenges of subsurface characterization and monitoring using a variety of methods. The conference had a strong interdisciplinary and international flavor, with around 100 participants from 13 countries taking part. Twelve companies and institutes presented their expertise and novel equipment and U. Sauer (&) P. Dietrich Department of Monitoring and Exploration Technologies, UFZ-Helmholtz Centre for Environmental Research, Permoserstrasse 15, 04318 Leipzig, Germany e-mail: uta.sauer@ufz.de

Multi-scale aquifer characterization and groundwater flow model parameterization using direct push technologies

Direct push (DP) technologies are typically used for cost-effective geotechnical characterization of unconsolidated soils and sediments. In more recent developments, DP technologies have been used for efficient hydraulic conductivity (K) characterization along vertical profiles with sampling resolutions of up to a few centimetres. Until date, however, only a limited number of studies document high-resolution in situ DP data for three-dimensional conceptual hydrogeological model development and groundwater flow model parameterization. This study demonstrates how DP technologies improve building of a conceptual hydrogeological model. We further evaluate the degree to which the DP-derived hydrogeological parameter K, measured across different spatial scales, improves performance of a regional groundwater flow model. The study area covers an area of ~60 km2 with two overlying, mainly unconsolidated sand aquifers separated by a 5–7 m thick highly heterogeneous clay layer (in north-eastern Belgium). The hydrostratigraphy was obtained from an analysis of cored boreholes and about 265 cone penetration tests (CPTs). The hydrogeological parameter K was derived from a combined analysis of core and CPT data and also from hydraulic direct push tests. A total of 50 three-dimensional realizations of K were generated using a non-stationary multivariate geostatistical approach. To preserve the measured K values in the stochastic realizations, the groundwater model K realizations were conditioned on the borehole and direct push data. Optimization was performed to select the best performing model parameterization out of the 50 realizations. This model outperformed a previously developed reference model with homogeneous K fields for all hydrogeological layers. Comparison of particle tracking simulations, based either on the optimal heterogeneous or reference homogeneous groundwater model flow fields, demonstrate the impact DP-derived subsurface heterogeneity in K can have on groundwater flow and solute transport. We demonstrated that DP technologies, especially when calibrated with site-specific data, provide high-resolution 3D subsurface data for building more reliable conceptual models and increasing groundwater flow model performance.

Numerical assessment of ASR recharge using small-diameter wells and surface basins

Aquifer storage and recovery (ASR) methods are increasingly used to overcome the temporal imbalance between water demand and availability. Common ASR recharge methods utilize large-diameter injection wells or surface infiltration basins and trenches, and can be costly to implement. A new low-cost ASR recharge method is currently being developed. This approach is based on recharge via gravity in small-diameter wells installed with direct-push (DP) technology. Numerical modeling is used here to assess the potential of this new approach under conditions commonly faced in field settings. The primary objective is to investigate if a battery of small-diameter DP wells can serve as a viable alternative to a surface basin under typical field conditions, while the secondary objective is to assess which subsurface parameters have the greatest control on DP well performance. Simulation results indicate that gravity recharge via small-diameter wells appears to have a distinct advantage over recharge via surface infiltration basins. For example, two 0.05-m shallow vadose-zone wells with 9-m screens can recharge water at a greater rate than a 60 m2 basin. Also, results reveal that, contrary to an infiltration basin, the recharge rate in a DP well has a much stronger dependence on the horizontal component of hydraulic conductivity than on the vertical component. Moreover, near-surface layers of low hydraulic conductivity, which can significantly reduce the recharge capacity of a surface basin, have a relatively small impact on the recharge capacity of a well as long as a significant portion of the well screen is installed below those layers. Given that installation and operation costs can be low in comparison to common ASR recharge methods, this new approach appears to have great potential for recharging good quality water in shallow unconsolidated aquifers. A field investigation has recently been initiated to follow up the findings of this simulation assessment.

Where can cone penetrometer technology be applied? Development of a map of Europe regarding the soil penetrability.

Over the past decades, significant efforts have been invested in the development of push-in technology for site characterization and monitoring for geotechnical and environmental purposes and have especially been undertaken in the Netherlands and Germany. These technologies provide the opportunity for faster, cheaper, and collection of more reliable subsurface data. However, to maximize the technology both from a development and implementation point of view, it is necessary to have an overview of the areas suitable for the application of this type of technology. Such an overview is missing and cannot simply be read from existing maps and material. This paper describes the development of a map showing the feasibility or applicability of Direct Push/Cone Penetrometer Technology (DPT/CPT) in Europe which depends on the subsurface and its extremely varying properties throughout Europe. Subsurface penetrability is dependent on a range of factors that have not been mapped directly or can easily be inferred from existing databases, especially the maximum depth reachable would be of interest. Among others, it mainly depends on the geology, the soil mechanical properties, the type of equipment used as well as soil-forming processes. This study starts by looking at different geological databases available at the European scale. Next, a scheme has been developed linking geological properties mapped to geotechnical properties to determine basic penetrability categories. From this, a map of soil penetrability is developed and presented. Validating the output by performing field tests was beyond the scope of this study, but for the country of the Netherlands, this map has been compared against a database containing actual cone penetrometer depth data to look for possible contradictory results that would negate the approach. The map for the largest part of Europe clearly shows that there is a much wider potential for the application of Direct Push Technology than is currently seen. The study also shows that there is a lack of large-scale databases that contain depth-resolved data as well as soil mechanical and physical properties that can be used for engineering purposes in relation to the subsurface.

Diagnostic monitoring to identify preferential near-surface structures for CO2 degassing into the atmosphere: Tools for investigations at different spatial scales validated at a natural analogue site

Diagnostic monitoring tools applied at different scales are required to reliably detect and assess CO2 leakages from storage formations in the shallow subsurface, as well as playing an important role in helping to establish a risk assessment strategy at carbon dioxide capture and storage facilities. These tools incorporate method developments and applications that will enable large spatial areas to be consistently covered in sufficient spatial and temporal resolutions. The use of remote sensing (open-path Fourier-transform infrared (OP-FTIR) spectroscopy) in combination with regional measurements (geophysics and chamber based soil CO2 flux measurement) and local in situ measurements (Direct Push technology) enables a more reliable validation of risk, using a joint data interpretation approach. A promising tool currently in development for large-scale leakage detection and monitoring is Fourier transform infrared (FTIR) spectroscopy, which is used to determine spatial atmospheric CO2 distribution in the near-surface atmosphere. Sufficient geophysical techniques for meso-scale monitoring include geoelectrical and self potential (SP) surveys. These methods are useful for characterizing fluid flow and transport processes in permeable near-surface sedimentary layers and can yield important information concerning CO2 affected subsurface structures. The paper presents promising results achieved from measurements taken at a natural analogue site in the Czech Republic and indicates that the hierarchical monitoring approach represents a successful multidisciplinary modular concept to monitor physical and chemical processes taking place during CO2 migration and seepage. The application of FTIR spectroscopy in combination with soil gas surveys and geoelectrical investigations results in a comprehensive site characterization, including the atmospheric and near-surface CO2 distribution as well as subsurface structural features. Our data illustrate a correlation of higher CO2 concentration in the atmosphere with increased CO2 soil flux rates and soil CO2 concentrations. These soil gas anomalies coincide with structural units characterized by a distinct negative SP anomaly and a zone of decreased resistivity in the ERT result.