Characterization and Quantification of Pneumatic Fracturing Effects at

A field study of pneumatic fracturing in basal clay till indicates that injected tracers are found primarily in natural fractures within a distance of...
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Environ. Sci. Technol. 2008, 42, 570–576

Characterization and Quantification of Pneumatic Fracturing Effects at a Clay Till Site C A M I L L A M A Y M A N N C H R I S T I A N S E N , * ,† CHARLOTTE RIIS,‡ S T I N E B . C H R I S T E N S E N , †,4 METTE M. BROHOLM,† ANDERS G. CHRISTENSEN,‡ KNUD ERIK S. KLINT,§ J U D I T H S . A . W O O D , †,⊥ PETER BAUER-GOTTWEIN,† AND POUL L. BJERG† Institute of Environment & Resources, Technical University of Denmark, Bygningstorvet, DTU-Building 115, DK-2800 Kgs. Lyngby, Denmark, NIRAS Consulting Engineers A/S, and Geological Survey of Denmark and Greenland

Received June 1, 2007. Revised manuscript received October 22, 2007. Accepted October 24, 2007.

Environmental fracturing offers assistance to remediation efforts at contaminated, low-permeability sites via creation of active fracture networks, and hence, reduction of mass transport limitations set by diffusion in low-permeability matrices. A pilot study of pneumatic fracturing, focusing on direct documentation of fracture propagation patterns and spacing, was performed at a typical basal clay till site. The study applied a novel package of documentation methods, including injection of five tracers with different characteristics (bromide, uvitex, fluorescein, rhodamine WT, and brilliant blue), subsequent tracer-filled fracture documentation via direct and indirect methods, and geological characterization of the fractured site. The direct documentation methods consisted of Geoprobe coring, augering, and excavation. A mass balance and conceptual model have been established for the distribution of the injected tracers in the subsurface. They reveal that tracer was distributed within 2 m of the fracturing well, mainly in existing fractures above the redox boundary (2 to 4 m.b.s.; 5 to 10 cm spacing). Spacing of observed tracer-filled fractures was large (>1 m) at greater depths. The number of fractures induced/activated could possibly be increased via adjustments to the fracturing equipment design.

Introduction Much of the soil contamination that represents a source of leaching to groundwater in North America and Europe is found in naturally fractured, low-permeability deposits, such as clay till. Short-term, severe contaminant leaching occurs via the fractures that act as preferential flow paths (e.g., ref 1). Long-term, less severe, but still significant leaching occurs * Corresponding author phone: +45 45 25 14 58; fax: +45 45 93 28 50; e-mail: [email protected]. † Technical University of Denmark. ‡ NIRAS Consulting Engineers A/S. § Geological Survey of Denmark and Greenland. 4 Current address: Orbicon. ⊥ Current address: Environmental Protection Agency City of Copenhagen. 570

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because a diffusive exchange of contaminants driven by concentration gradients takes place between the contaminated water traveling in the fractures and the almost immobile water in the surrounding low-permeability matrix of such deposits. When contaminant transport in the fractures ceases (either due to source depletion and/or effective remediation in the fractures) the contamination accumulated in the matrix will diffuse back into the fractures and leach to underlying groundwater for a long period of time (e.g., ref 2). The phenomenon is referred to as back diffusion and necessitates effective remediation of the contaminated low-permeability deposits. The primary challenge in remediating low-permeability deposits, either via mass transfer methods (e.g., soil vapor and dual phase extraction) or in situ mass destruction/ transformation methods (e.g., chemical oxidation and anaerobic reductive dechlorination), is to overcome the mass transport limitations set by the slow matrix-diffusion process (3, 4). Fracturing, traditionally used in the oil and water well industries, is the process of injecting a pressurized fluid at specific depths in boreholes to create fractures and has been introduced as a method to meet this challenge (5, 6). Injection of a water-based fluid is referred to as hydraulic fracturing, whereas injection of a pressurized gas is referred to as pneumatic fracturing. In the early 1990s, environmental fracturing, in which fractures are created for remedial purposes in contaminated, low-permeability deposits, emerged. Its aim is to facilitate remediation enhancements via creation of new fractures and opening of existing, but hydraulically inactive fractures not otherwise accessible to remediation efforts. The resulting network of hydraulically active fractures shortens the distances that contaminants must diffuse in the low-permeability matrix before meeting a fracture, from which extraction or degradation processes may occur, thus shortening overall remediation time (5, 6). Pneumatic fracturing seems promising, but focus has primarily been on indirectly measured permeability and short-term remediation enhancements (7–9) rather than direct documentation of fracture propagation patterns, spacing (i.e., lengths of diffusion pathways) and aperture (i.e., openings between fracture walls). General rules-ofthumb for fracture propagation patterns have been formed based on central geotechnical principles (5, 6), and Alfaro and Wong (10), among others, have conducted laboratory experiments to characterize the geomechanics related to initiation pressure and growth of fractures. However, only few fracturing field studies have employed direct documentation methods, and with variable results on key parameters (9, 11). Uncertain and variable estimates of fracture spacing, aperture, and radius may affect calculations of diffusion times, hydraulic conductivity, and advection by orders of magnitude, making reasonably accurate predictive modeling of expected remediation timeframes, etc., impracticable. The first step toward obtaining more in-depth knowledge of typical pneumatic fracture propagation patterns and induced fracture spacing has been taken in a recent pilot study of pneumatic fracturing, carried out at a clay till site in Denmark. The pilot study brought together a package of agents and methods that have all been used separately for many similar purposes before but never coupled to document pneumatic fracturing effects to the extent attempted here. The package included injection of five different chemical tracers during fracturing, subsequent Geoprobe coring, and various other direct and indirect methods for tracer and, hence, fracture detection, as well as thorough geological characterization of the fractured site via partial excavation. 10.1021/es071294s CCC: $40.75

 2008 American Chemical Society

Published on Web 12/12/2007

FIGURE 1. Map of the fracturing site with (a) fracturing borehole (PF1); (b) monitoring wells (T1 to 4) on a 5 m perimeter of PF1, expected to constitute the radius of fracturing influence; (c) excavation with main profiles 1 and 2; and (d) locations of cores (KF0 to 12) and auger drillings (M1 to 6). Several of the injected tracers were fluorescent dyes, as these are often used to evaluate subsurface flow regimes when characterization and mapping of flow paths and conduit networks are required, and they have been shown to be useful in low-permeability media such as clayey and glacial till soils (12). The aim of this article is to (i) evaluate pneumatic fracturing effects in a clay till by use of chemical tracers; (ii) present a mass balance established for the distribution of tracers injected during the fracturing in the pilot study; and (iii) present a conceptual model derived for the tracer-filled fracture network. Both the mass balance and conceptual model highlight challenges anticipated in the future use of environmental fracturing.

Materials and Methods The Field Site is a former chemical distribution location in Vasby in the eastern part of Denmark where the lowpermeability setting and underlying aquifer are contaminated by chlorinated solvents, water miscible solvents, and oil compounds (13, 14). The pilot study was carried out in an uncontaminated part of the site. Figure 1 gives an overview of the site and fracturing installations. Geological Characterization. The site geology consists of 14 to 16 m of clayey till, underlain by 2 to 3 m of fine sand, and subsequently, a primary limestone aquifer (13, 14). The till is, based on study of the glacial history of the area, a basal clay till. Thus, an extensive set of natural fractures was expected, and these were expected to have some degree of influence on the orientation and form of additional fractures induced herein (6, 15). To accurately characterize the deposit and its natural fractures geologically, an excavation was, therefore, conducted on the periphery of the fracturing area (Figure 1). The excavation was carried out after fracturing to

enable potential observation of tracer-filled fractures. Characterization work was done on profiles 1 and 2 based on techniques described by Klint (16), including fabric analyses at three depths (1, 2.2, and 3.2 m.b.s.) and strike and dip measurements of observed fractures at 2 depths (1 and 2.8 m.b.s.). Additional information on excavation and till characterization procedures is given in the Supporting Information. Pneumatic Fracturing was carried out by ARS Technologies Inc. in five 1 m intervals from 3 to 8 m.b.s. in a single borehole (PF1) in December 2005 (Figure 1) using a bottomup procedure. The fracturing fluid was nitrogen gas carrying with it an atomized tracer mixture. The initiation and propagation of fractures took approximately 30 s at each fracturing level with initiation and propagation pressures in the ranges of 40 to 125 psi (276 to 862 kPa) and 20 to 85 psi (138 to 586 kPa), respectively. The larger pressures were applied at the lower fracturing levels, and may have been excessive, as Schuring (5) states that 2 to 3 psi per foot of overburden (6 to 9 psi per meter) are typically required to initiate a fracture at a given depth. Simultaneous or immediately following injection of 50 L of atomized tracer mixture took approximately 130 s at each level. The 50 L volume was chosen to ensure recovery/observation of the tracers without injecting unnecessarily large tracer amounts into the subsurface. It is estimated that it would, in other/ remedial contexts, be possible to atomize 300 to 400 L of liquid to flow with the nitrogen gas within the typical short duration of pneumatic fracture propagation per fracturing interval. It is possible that the fracturing radius of influence (ROI) was larger than documentable via observed tracer extent (if tracer injection ended slightly before gas injection, and hence, tracer was not carried all the way to the end of opened/ induced fractures). This may have bearing on any density estimations made far from the borehole (>2 m), but not in its close vicinity. The latter would indicate that the tracer flowed along only select pathways instead of all pathways taken by the gas, and this is unlikely given that the tracer mixture was atomized, i.e., mixed integrally, into the gas. The entire fracturing process was monitored. More detailed information on equipment, procedures, and monitoring is given in theSupporting Information. Injected Chemical Tracers and Fracture Documentation. The aqueous chemical tracer mixture injected (in atomized form) during fracturing consisted of five tracers with different properties: rhodamine WT, fluorescein, and uvitex (fluorescent tracers); bromide (colorless tracer); and brilliant blue (dye tracer, turquoise/blue). See the Supporting Information for details. The resulting concentration of each tracer in the mixture was approximately 10000 mg/L. The tracer mixture was composed to ensure the greatest possible breadth in documentation possibilities, and allow the employed documentation methods to complement each other. Focus in this paper is on the direct documentation of fracturing effects, accomplished primarily via the fluorescent tracers rhodamine WT and fluorescein. Rhodamine WT and Fluorescein (CAS nos. 37299–86–8 and 518–47–8) were supplied by Colorey SAS (France) and Sigma Aldrich A/S (Denmark) and are, respectively, a strongly sorbing fluorescent dye and a more mobile fluorescent dye (12, 17, 18) of magenta/orange and yellow/green color. Both have a visibility detection limit of 1 to 10 mg/L under 312 nm UV-light (13). Direct Documentation of Fracturing Effects consisted of auger drilling, coring and excavation (Figure 1). Auger drillings (M1 to 6) from 0 to 8 m.b.s. (M3 to 9 m.b.s.) were performed in the days immediately following the fracturing, the excavation 6 days after, while cores (KF1 to 12) were obtained in two coring campaigns: one in the days following VOL. 42, NO. 2, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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fracturing and one approximately 4 months later. One reference core (KF0) was taken prior to fracturing (to ensure that natural sediment coloration at various depths was not mistaken for tracer coloration and vice versa). The cores were of varying lengths from between 2 and 10 m.b.s., and they were obtained with a Geoprobe in 1.2 m sections in clear plastic tubes. Auger drillings and cores were located within an expected 5 m fracturing ROI, while the excavation was on the northwestern periphery of the expected ROI (Figure 1). The expected 5 m ROI was a conservative estimate given the ruleof-thumb that fracturing ROI will be approximately 3 times the depth of the fracture initiation point at depths until 5 m.b.s (5). The term ROI is, however, generally used to indicate the furthest point from fracturing at which any influence is detected, whereas we were interested in the radius within which main fracturing effects, and hence, significant remediation enhancements could be expected after fracturing. Core Sampling and Analysis. The Geoprobe cores (in total 85 core sections with a total length of 102 m) were opened in the laboratory, each section being split into two halves. One half was stored in a dark room to minimize fluorescent tracer transformation and sediment dehydration before inspection and sampling. The other half immediately underwent visual inspection; photography under simulated daylight conditions and 312 nm UV-light, respectively; and geological description. If fluorescent tracer was observed during photography of a particular core half-section, its corresponding stored half-section was subsequently sampled. The samples were analyzed for presence of fluorescein and rhodamine WT on a GGUN-FL fluorometer. See the Supporting Information for details. In the auger drillings and excavation, tracers were primarily observed and recorded directly in the field under daylight conditions, but some soil samples were taken from the auger drillings and analyzed in the same manner as the core samples. Observed tracer-filled fractures could potentially have three origins: (a) new fractures induced by the pneumatic fracturing; (b) natural fractures hydraulically active prior to fracturing; and (c) natural fractures opened to hydraulic activity via fracturing. Distinguishing between the three can be difficult (published dye tracer and hydraulic experiments (e.g. ref 19) reveal that apparently identical fractures may have very different hydraulic conductivity), and it is not critical here, as our primary focus is to evaluate pneumatic fracturing effects in terms of the spacing of fractures to which tracer was spread, regardless of fracture origin, to obtain indication of the achievable distribution of in situ remedial agents. More information on the sampling procedure and analysis is given in the Supporting Information. Mass balance and Conceptual Model. Tracer concentrations measured in core and auger samples were used to set up a quantitative mass balance (information on procedure in the Supporting Information) for the distribution of tracer in the subsurface, whereas a qualitative conceptual model for pneumatic fracture propagation was constructed via the tracer-filled fracture observations in cores, auger drillings, and excavation.

Results Till Characterization. The basal till at the site was deposited by an ice sheet that transgressed the area on an initially northwestern and later more northbound trajectory. The natural fracture types identified in the our excavation showed root- and wormholes from 0 to 1 m.b.s. with a dense spacing of less than 1 cm. Contraction fractures were observed from 0 to 2 m.b.s., unsystematically oriented horizontally and vertically with a dense spacing of on the order of 1 cm. Desiccation fractures are expected to be present until the 572

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redox boundary at approximately 3.5 to 4 m.b.s., but at a reduced frequency. Horizontal subglacial-tectonic shear fractures were observed from 2 to 4 m.b.s., but are expected to traverse the entire till formation until 14 m.b.s. They are systematically horizontally oriented with an average spacing of 20 cm (ranging from 10 to 50 cm). Vertical subglacialtectonic shear and extension fractures were observed from 1 to 4 m.b.s., but are expected to continue until maximally 6 m.b.s., where the secondary water table is met. They are systematically vertically oriented with an average spacing of 12 to 14 cm from 1 to 3 m.b.s. and a spacing of approximately 1 m from 3 to 6 m.b.s. More details are given in the Supporting Information. Overall, fractures are more closely spaced above the redox boundary, i.e., above the depth at which redox conditions change from aerobic to anaerobic. The boundary is easily distinguishable in an excavation, as the sediment changes color from brown (oxidized) to gray (reduced/anaerobic), when fewer fractures are present to replenish oxygen levels over depth. Thus, the redox boundary serves as a good indicator of increased fracture spacing. The observed characteristics of the site are comparable to those of other clay till sites (3, 16, 20, 21), and the site constitutes a globally typical basal clay till. Tracer-Filled Fractures Observed in the Excavation. The northwestern edge of the expected fractured area (including monitoring well T1) and two areas, where surface venting had been observed during fracturing, were included in the excavation. No evidence of tracer was observed in either of the surface venting locations when excavated. Two tracerfilled fractures were, however, clearly visible on the SE-side of the excavation (Figure 1). The fractures were exposed at a distance of 4.8 and 6.4 m, respectively, from PF1, making their spacing 1.6 m. The visible length of the fracture closest to PF1 was over 4 m at an approximate depth of 2.2 m.b.s., whereas the other fracture was visible over a distance of almost 2 m at approximately 2.5 m.b.s. The two fractures appeared as tortuous blue-purple traces surrounded by graycolored, silty-clayey sediment in the otherwise brown overburden (Figure 2a). The gray areas are silty-clay-filled hydraulically active natural fractures. Hydraulic activity was confirmed by significant seepage of tracer during the 2 days the excavation was open. The tracer-filled regions of the fractures (purple regions) varied from tenths of a millimeter to several centimeters. The planes of the tracer-filled fractures were steeply dipping, i.e. the fractures were subvertical in orientation, rising toward ground surface away from the fracturing borehole. Tracer Observations in Auger Drillings. Tracer-affected areas were easily observable in cuttings from auger drillings M1 to 6. Sixteen areas were identified over a depth range of 2 to 8 m.b.s. (the majority within 2.5 to 6 m.b.s.), and were assumed to be concurrent with tracer-filled fractures. In auger drillings M1, M2, and M3, areas with extensive tracer coloration were observed (2.5 to 2.6 m.b.s., 2.4 to 5 m.b.s., and 6.5 to 7 m.b.s., respectively). This may indicate dense fracture network formation, but fracture orientation, apertures and origins were not distinguishable, due to the disturbance of the augered samples. Tracer-Filled Fractures Observed in Cores KF1 to 3 Retrieved 1 to 2 Days after Fracturing. Geoprobe cores provided less disturbed samples of the subsurface, and a total of thirteen clearly tracer-filled fractures were observed within 3 to 6 m.b.s. in cores KF1 to 3. The fractures appeared as magenta-colored stripes/zones with a dark blue edge, ranging in width from about 0.01 to 1 cm. A single magenta zone was 1 to 2 cm wide. A few of the tracer-filled fractures were cracked open, but it is uncertain whether this occurred due to core handling. A single green-colored fracture was observed. A thin yellow-green colored halo was also associ-

FIGURE 2. (a) Photo (and close-up) of part of one of the fractures observed in the excavation 6.4 m from the fracturing borehole (PF1) at a depth of approximately 2.5 m.b.s. (b) KF2, 4 to 5 m.b.s. cored on the day of fracturing and sampled 1 day later. From left to right the figure shows the core section under daylight conditions, UV-light conditions and finally tracer concentrations (obtained via sampling and analysis) for the section. Three rhodamine WT-filled fractures are apparent in both photos, whereas the surrounding fluorescein halo is only visible in the UV-photo. The concentrations confirm beginning diffusion, but tracer mass is still primarily found in the fractures. (c) Fracture observed in KF10, 5.8 m.b.s., cored and sampled approximately 4 months after fracturing. Distinct rhodamine WT-filled fracture surrounded by substantial fluorescein halo. The concentrations reveal a clear diffusion profile for fluorescein moving into the matrix from the fracture. Rhodamine WT remains to a higher extent in the fracture, since it is retarded more by sorption in the clay matrix (12, 17, 18). (d) Fluorescent sand lense observed in KF8, 5 to 5.5 m.b.s., cored and sampled approximately 4 months after fracturing. Even tracer distribution observed throughout both visually and in sampling results. Photos taken under daylight conditions corresponding to the shown UV-photos are not shown in (c) and (d), as the fractures here are not visible under daylight conditions. ated with some of the fractures. In Figure 2b, a set of closely spaced fractures in KF2 (4 to 5 m.b.s.) is shown along with tracer concentrations in this core section. Tracer-Filled Fractures Observed in Cores KF4 to 12 Retrieved 4 Months after Fracturing. Thirty-one clearly tracer-filled fractures and several fluorescent sandy areas were observed in cores KF4 to 12. The fractures were found from 2 to 8 m.b.s., the majority distributed from 2 to 4 m.b.s., coincident with the zone above the redox boundary, where the largest density of natural fractures is observed. The fractures’ appearance was almost the same as that observed in the cores taken immediately after fracturing, but now typically with a significant yellow-green-colored halo. Figure 2c shows a fracture observed in KF10 at 5.74 m.b.s. The fracture is situated in a compact part of the clay till, thus the yellow-green halo surrounding it is representative of typical fluorescein diffusion into the sediment. Figure 2d shows a

sand lense observed in KF8, 5 to 5.5 m.b.s., with tracer seemingly evenly distributed throughout. Fluorescence sampling every 5 cm in the lense confirmed this. An overview of all fracture observations made in the auger drillings and cores is given in the Supporting Information. Observed Orientation, Spacing and Origin of Fractures Observed in Cores. The tracer-filled fractures observed in the cores were predominantly horizontal. Many had propagated in existing fractures, some apparently opened to hydraulic activity via the pneumatic fracturing. Some vertical, subvertical and tortuous fractures were also observed. Only these were deemed to be newly induced fractures. Fracture spacing ranged from about 1 cm to >2 m. Most closely spaced fractures (spacing 1 m apart. It thus seems that when fewer fractures are present over a given fracturing interval, one preferential pathway is chosen in basal clay till. The observed fracture spacings below 4 m.b.s. are not satisfactory, as the results of preliminary modeling studies (9, 13, 14) indicate that a fracture spacing on the order of 10 cm will be necessary to clean up a clay matrix saturated with the chlorinated solvent tetrachloroethylene (PCE) within a time frame of 10 years. Alterations to the pneumatic fracturing equipment to reduce the width of fracturing intervals (presently 90 cm) could possibly rectify this, as the gas injected in a given interval would then have a smaller likelihood of encountering the more widely spaced natural fractures, thus being forced to create its own path. Conceptual Model of Pneumatic Fracture Propagation in Clay Till. A conceptual model is proposed for typical pneumatic fracture propagation and, hence, the distribution of a fluid injected via pneumatic fracturing in a basal clay till formation (Figure 4). Focus has been placed on the significance of the natural fracture distribution, which can be directly assessed via excavation and geological characterization of any given clay till site. The conceptual model illustrates that a large part of an injected fluid will be distributed in natural, horizontal fractures above the redox boundary, which is typically found between 2.5 and 5.5 m.b.s. in clay till deposits (9, 16). In Figure 4, the redox boundary is indicated between 3.5 and 4 m.b.s., as it is at the pilot site. Fluid-filled horizontal fractures with 10 to 20 cm spacing can be expected locally within this area (from 2.5 to 4 m.b.s.). Fluid leak-off in the hydraulically active fractures above the redox boundary will lead mobile compounds (such as fluorescein) to leach further into the formation than the initial radius of pneumatic fracturing influence. Mobile compounds will typically also diffuse further into the matrix. Below the redox boundary horizontal fluid-filled fractures are expected to be encountered with a spacing of 0.5 to 1 m. Some vertically formed fractures must also be expected in locations where pressure relief is most easily achieved in the vertical direction, as illustrated in the lower part of Figure 4. In sand lenses, concentration equalization occurs over the entire volume of the lense by diffusion and advection/ dispersion. Generally, fractures will rise toward the surface when the applied propagation pressure is no longer sufficient to support horizontal propagation, in some cases resulting in surface venting. In theory the rise will progress along a curve toward the surface (such that the fractures acquire a bowlshape) (5, 6). With the many natural, often hydraulically active, subvertical fractures typically found in basal clay tills, however, it is expected that the horizontal progression of “induced” fractures in many cases will be stopped when they transect such a vertical fracture that offers a rapid route toward the surface and, hence, pressure relief. The tracerfilled fractures observed in the excavation at the pilot site are a good example of this phenomenon, see the Supporting Information. Pressure relief can be obtained before a fracture reaches the surface, if the vertical fracture crosses a horizontal fracture with higher permeability and/or aperture, through

FIGURE 4. Conceptual model of natural fractures in clay till and expected pneumatic fracture propagation. which the fluid is then lead. Such leak-off is primarily expected above the redox boundary where the fracture density is largest. VOL. 42, NO. 2, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Perspectives for Remediation of Contaminated Clay Till. At contaminated, naturally fractured, low-permeability sites, a good correlation is generally observed between the location of natural fractures and the bulk mass of contamination, since the contamination has diffused into the deposit via these fractures. Consequently, the “reactivation”/use of these fractures for remedial purposes, via fracturing, could be sufficient to ensure adequate remediation. However, the time frame for remediation (reliant on contaminant back diffusion out of the sediment) must then be expected to significantly exceed that which the contamination has had to diffuse into the sediment, since a constant, high level of back diffusion will be difficult to maintain. On the other hand, activation of the many previously inactive fractures typically present in clay tills (16) can serve to significantly reduce the length of diffusion pathways, and hence, the diffusion limitations on mass transfer and in situ removal. This could potentially make remediation timeframes significantly shorter than the timeframes that the contaminants have had to diffuse into the sediment. In our field trial, pneumatic fracturing has proven able to spread tracer into both previously hydraulically active and inactive natural fractures of a basal clay till. However, at depths below the redox boundary, the distance between activated fractures is large, and the pneumatic fracturing technology is, therefore, in its present state not considered able to overcome the mass transfer limitations set by diffusion in the this type of low-permeability matrix at depth. It is possible that the number of fractures induced/activated at deeper levels can be increased via a number of adjustments to the pneumatic fracturing equipment design to reduce fracturing interval widths. Prior to field scale application of pneumatic fracturing, site characterization is a prerequisite. This study reveals a need for further field trials with detailed documentation of fracture propagation using state-of-the-art chemical tracer techniques as well as characterization of geology (till type, presence of sand lenses and natural fractures) and geotechnical properties (in situ stress levels).

Acknowledgments The presented research is part of a collaborative research project funded by Copenhagen County, Denmark, and the Technical University of Denmark (DTU) through a PhD scholarship. We thank Julie Laurberg Lund Kofoed, Torben Dolin, Maria Heisterberg Hansen, and Anne-Marie Rasmussen at Institute of Environment and Resources, DTU; the technical staff at NIRAS; Carsten Bagge Jensen and Henriette Kerrn-Jespersen at Copenhagen County; and the ARS consultants.

Supporting Information Available Some aspects of the study are described in greater detail. This material is available free of charge via the Internet at http://pubs.acs.org.

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