A Field and Modeling Study of Fractured Rock Permeability Reduction

Oct 22, 2013 - We present results of the first field experiments applying MICP to reduce fractured rock permeability in the .... Journal of Environmen...
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A Field and Modeling Study of Fractured Rock Permeability Reduction Using Microbially Induced Calcite Precipitation Mark O. Cuthbert,*,†,∥ Lindsay A. McMillan,† Stephanie Handley-Sidhu,† Michael. S. Riley,† Dominique J. Tobler,‡,§ and Vernon. R. Phoenix‡ †

Water Sciences (Hydrogeology), School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, B15 2TT, U.K.. ‡ School of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 8QQ, U.K.. § Nano-Science Center, University of Copenhagen, Copenhagen, Denmark S Supporting Information *

ABSTRACT: Microbially induced calcite precipitation (MICP) offers an attractive alternative to traditional grouting technologies for creating barriers to groundwater flow and containing subsurface contamination, but has only thus far been successfully demonstrated at the laboratory scale and predominantly in porous media. We present results of the first field experiments applying MICP to reduce fractured rock permeability in the subsurface. Initially, the ureolytic bacterium, Sporosarcina pasteurii, was fixed in the fractured rock. Subsequent injection of cementing fluid comprising calcium chloride and urea resulted in precipitation of large quantities (approximately 750 g) of calcite; significant reduction in the transmissivity of a single fracture over an area of several m2 was achieved in around 17 h of treatment. A novel numerical model is also presented which simulates the field data well by coupling flow and bacterial and solute reactive transport processes including feedback due to aperture reduction via calcite precipitation. The results show that MICP can be successfully manipulated under field conditions to reduce the permeability of fractured rock and suggest that an MICP-based technique, informed by numerical models, may form the basis of viable solutions to aid pollution mitigation.

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INTRODUCTION Preventing unwanted fluid migration via fracture networks in otherwise low permeability formations is a priority in many environmental applications. For example, the eventual success of subsurface containment of nuclear waste and captured CO2 depends heavily upon adequately sealing host and cap rocks,1,2 and the construction of nuclear waste facilities also requires sealing of access shafts in the short to medium term. Microbially induced calcite precipitation (MICP) offers an attractive alternative to traditional grouting technologies3−6 due to its advantages as a low viscosity and low pressure technique. In addition to being a well understood natural phenomenon, MICP has been intensively studied over the past decade for its potential application in a range of environmental technologies including permeability reduction of porous and fractured media.3,7−10 A particularly promising biogeochemical pathway for MICP involves urea hydrolysis by the enzyme urease produced by ureolytic bacteria which, in the presence of sufficient calcium ions, drives calcite precipitation.8 Although many laboratory studies have advanced our understanding of MICP,5,6,9,11−14 without detailed field experiments its viability as an engineering solution remains unproven. Owing to the presence of ambient groundwater flows and flows induced by the density of the injected fluids, real-world applications will not provide the long static growth and © XXXX American Chemical Society

precipitation phases used in many laboratory experiments. Indeed, the long time scales of most laboratory studies are not compatible with practical engineering applications. To date, the only other reported example of a field test involving MICP was aimed at the solid phase capture of radionuclides in unfractured basalt and, being based on a single well test, was not designed to determine the location and nature of any permeability reduction.15 Indigenous subsurface bacterial communities may be stimulated to achieve significant rates of MICP but only at a high risk of inducing bioclogging.9 This is intrinsically less desirable than creating a mineralized fracture fill due to the low chemical and mechanical stability of biomass. Furthermore, during MICP, encapsulation of bacteria in the resulting precipitate limits further ureolysis and consequently mineralization.6,8,10,11 To overcome the difficulties of using indigenous bacteria and induced bioclogging from the addition of nutrient media, we developed a method of injecting fluids in a sequence of stages each comprising inoculation and fixing of bacteria (S. pasteurii, grown in the laboratory and then separated from the Received: June 12, 2013 Revised: October 16, 2013 Accepted: October 22, 2013

A

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container, stored overnight at 10 °C, and transported to the field. Well Hydraulics. Boreholes, 100 mm in diameter, were drilled to approximately 27 m depth into a fractured dacite formation at Whitwick Quarry, Leicestershire, U.K. The upper disturbed zone was sealed off to 10 m with plastic casing and a cement-bentonite grout. Packer testing indicated that flow between a cluster of four boreholes (B2−B5, Figure 1) was controlled by a single fracture approximately 25 m below ground level (bgl), which was targeted for the grouting experiments. Fluids were injected into borehole B2 by peristaltic pump via two parallel lines at 0.25 L/min per line using a double packer arrangement to isolate the fracture (Geopro BIMBAR). Abstraction from B5 (approximately 2.8 m away from B2) was via a rising main and submersible pump (Waterra WASP P2) at 2.7 L/min. Boreholes B3 and B4, which were used for hydraulic testing were isolated using packers during grouting experiments. Pressure transducers (Keller 36W), checked regularly against manual dip measurements, monitored pressure in B5 and in the packered injection interval in B2 during grouting, and in all boreholes for hydraulic testing. A series of preliminary tracer tests was conducted to understand the flow field and to choose pumping rates to ensure high recovery of injected solutes. Constant rate pumping tests were carried out to steady state in B2−B5 before and immediately after grouting as well as 12 weeks later, with pressure monitoring in all holes. The pumping tests were used to derive cross-hole conductances,16 which characterize the hydraulic connections between boreholes without requiring any prior assumptions to be made about the internal geometry or permeability distribution within the fractured rock. The steady state discharge, Q [L3T−1], between two boreholes is given by

nutrient media) in a fracture followed by a period of mineralization. Our objective was to test whether MICP is a viable solution for practical engineering applications for fractured rock permeability reduction. We present results from a novel experimental design based upon injection and abstraction boreholes and boreholes devoted to hydraulic characterization.16 This allowed us to control grouting effectively by engineering the groundwater flow field, while facilitating measurement of the changes in effluent chemistry and hydraulic connectivity of the system as grouting proceeded. The design also enabled inference, through subsequent modeling, of rates of ureolysis, calcite precipitation, bacterial transport, and temporal and spatial changes in transmissivity of a single fracture over an area of approximately 4 m2. These are the first published results of field experiments using MICP to seal fractured rock in the subsurface, and have significant implications for the future development of MICP technology.

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MATERIALS AND METHODS Overview. The experimental design followed the sequence: (1) hydraulic testing of boreholes to infer permeability distributions of the fractured rock mass; (2) an MICP grouting trial using an injection-withdrawal pumping arrangement (Figure 1); (3) retesting for permeability immediately after grouting and again 12 weeks later; and (4) the interpretation of the field results through numerical modeling.

Q = CABΔh

(1)

where CAB is the cross-hole conductance between boreholes A and B [L2T−1], and Δh is the head difference between the boreholes [L]. Field Injection and Monitoring. In the field, the bacterial concentrate (as obtained from centrifugation in the laboratory) was diluted with quarry sump water to obtain an optical density (OD) of 1.0 as measured spectrophotometrically at 600 nm with a UV-Vis spectrophotometer HACH DR 5000. Twenty minutes prior to bacterial injection, bacterial flocculation was induced by adding CaCl2 salts to the 1.0 OD bacterial suspension to yield a final concentration of 0.2 mM CaCl2. This mix was then shaken continuously to ensure the salts dissolved fully and to avoid the formation of large bacterial aggregates. Shortly before injection, a 1 mL sample of the flocculated bacterial suspension was removed to determine the ureolytic activity of the bacterial cells. For this, 10 min reactions in 1 M urea solutions at 15 °C were set up and the ureolytic activity was then calculated from the amount of ammonium determined by Nessler Assay. Ureolytic activities ranged between 0.6 and 7.3 mM urea/min/OD. The bacterial suspension (with no added nutrients) and urea at 0.4 M (urea salts were mixed with quarry sump water to achieve the desired concentration) were injected simultaneously through separate injection lines at 0.25 L/min into the packered interval. At regular time intervals throughout the injection procedure, samples of abstracted effluent were collected and analyzed for the amount of transported bacteria (OD measurements), the ammonium produced by ureolysis within the fracture (Nessler Assay) and the electrical conductivity

Figure 1. Set-up of the experimental borehole array. The targeted fracture (in blue) is at approximately 25 m below ground level and has a dip of around 25 degrees.

Laboratory Cultures. A pure culture of the urease positive strain S. pasteurii (strain ATCC 11859) was grown at 30 °C in 1 L glass bottles containing tryptic soy broth and 2%wt urea. 400 mL of liquid containing cells in exponential growth phase, determined by measuring optical density at 600 nm using UVVis Spectrophotometer (WPA Lightwave S2000), was transferred to each of four vessels containing 8 L of sterilized growth media. The vessels were then sealed and incubated at 30 °C on an orbital shaker at 100 rpm. Cells at the late exponential growth stage (approx 24 h incubation) were harvested by centrifugation at 10 000 rpm for 10 min (Beckman J2-21). The pellet of biomass was then transferred to a sterile polypropylene B

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C. Bacterial Accumulation and Encapsulation. Filtration is modeled by

(EC). The EC was used in combination with the measured ammonium concentrations and known background groundwater geochemistry to calculate the CaCl2 concentration in the effluent water, using laboratory derived calibrations of EC against serial dilutions. A mass balance for Ca was carried out using the injected and effluent concentrations and volumes over time. This enabled an estimate of total calcite precipitation to be made, assuming any loss of Ca within the experiment was due to calcite precipitation. After successful bacterial fixing within the fracture, we continued to pump cementing solution comprising urea and calcium chloride to drive calcite precipitation; a summary of injection tests is given in SI Table S1. During these periods, we also regularly tested ureolysis and calcite precipitation rates using the ammonium production and electrical conductivity monitored in the effluent as proxies. Effluent was collected to avoid contamination of the quarry sump and sent for off-site disposal. Modeling. Modeling was carried out using COMSOL Multiphysics (v.4.2a) by coupling the following equations: A. Flow. The density-dependent groundwater flow equation in a planar fracture with dip θ and unit porosity can be written as shown:

∂(bs) ∂t

where φ is a filter factor [L ]. In addition to the accumulation process, we assume that the effective number of bacteria on the fracture surface, namely those available for ureolysis, reduces due to their encapsulation in the calcite precipitate. Thus, the accumulation of bacteria on the fracture surface is modeled by ∂(bs) ∂σ =− ∂t ∂t

+ filtration

∂σ ∂t

(6)

encapsulation

where σ is the number of bacteria per unit area attached to the fracture surface with units OD m [L]. The effective decay of attached bacteria due to encapsulation processes is assumed to take the form

∂σ ∂t

= −λσ (7)

encapsulation −1

where λ is a decay constant [T ]. D. Reactive Transport. The transport equation for urea is ∂(bc) ∂(bc) = ∇·(bD∇c − bcu) + ∂t ∂t

+R ureolysis

(8)

−3

where c is the concentration of urea [ML ], and R is a mass flux source term [ML−2T−1]. Ureolysis is modeled as a pseudo first-order reaction:

(2)

where x and η are the spatial coordinates along strike and down dip respectively; h0 is the equivalent freshwater head [L]; b is the fracture aperture [L]; T0 is the freshwater transmissivity of the fracture, calculated from the fracture aperture using the cubic law [L2T−1]; SSb is the freshwater storativity of the fracture [1]; ρ is the fluid density [ML−3]; C is the concentration of dissolved or suspended solids in the groundwater [ML−3]; and Q is a groundwater volume flux source term. The relationship between fluid density and concentration is given by

ρ − ρ0 = γ(C − C0)

∂(bc) ∂t

(9)

where k is the reaction rate [T ] given by k = ωσ where ω is a constant [L−1T−1]. E. Aperture Change. The rate of change of aperture due to calcite precipitation is assumed to be given by β ∂b = −k bc ∂t ρB

(3)

filtration

= −kbc ureolysis −1

(10)

where β is the mass of calcite produced per unit mass of urea [1], and ρB is the bulk density of the calcite precipitate [ML−3]. Aperture reduction due to biomass accumulation was assumed negligible since the volume of calcite precipitate was orders of magnitude greater than the volume of bacteria added. Furthermore, no nutrients were added with the bacteria, and thus no significant increase in biomass could have occurred on the time scale of the experiments. F. Ammonium Transport. Groundwater at the site is almost anoxic (1 mg/L DO). Limited hydrogeological studies (e.g., refs 17 and 18) suggest that, while significant in groundwater over longer time spans, ammonium removal via anaerobic ammonium oxidation will not be significant over the short residence times (less than 30 min) during the MICP tests. Thus, ammonium transport is modeled simply by

where C0 is the background value of C, ρ0 is a reference fluid density and γ is a constant [1]. We have assumed a constant initial aperture which is reasonable since the variation in crosshole conductances measured between all pairs of boreholes used in the experiment is less than approximately 10%. B. Bacterial Transport. Fixing of bacteria within the fracture is assumed to be dominated by physical filtration of large aggregates and, in the absence of added nutrients, the short time scales (i.e., tens of minutes) mean that bacterial growth and decay in the aqueous phase can be assumed to be unimportant. So, the bacterial transport equation is given simply by ∂(bs) ∂(bs) = ∇·(bD∇s − bsu) + ∂t ∂t

(5) −1

∂h ∂ ⎡ ∂h0 ⎤ ρSSb 0 = ⎢ρT0 ⎥ ∂t ∂x ⎣ ∂x ⎦ ⎡ ⎞⎤ ρ − ρ0 ∂ ⎢ ⎛ ∂h0 ρT0⎜⎜ + + sin θ ⎟⎟⎥ ρ0 ∂η ⎢⎣ ⎝ ∂η ⎠⎥⎦ ∂C ∂b − bγ −ρ + ρQ ∂t ∂t

= −φbs|u| filtration

(4)

∂(bm) ∂(bm) = ∇·(bD∇m − bmu) + ∂t ∂t

where s is the aqueous (number) concentration of bacteria expressed in terms of optical density (OD) [1], u is the groundwater velocity derived from solving the flow equation [LT−1], and D is the usual dispersion tensor [L2T−1].

production

(11)

where m is the aqueous concentration of ammonium [ML−3]. C

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The ammonium production equation is derived from the s t o i c h i o m e t r y o f t h e o v e r a l l ur e o l y s i s r e a c t i o n urease NH2CONH2+2H2O+CaCl2⎯⎯⎯⎯⎯→2NH4++CaCO3(s)+2Cl− as follows: ∂(bm) ∂t

= αkbc production

(12)

where α is the mass of ammonium produced per mass of urea hydrolyzed [1]. A flow diagram of the equation coupling is given in SI Figure S1. Due to very low ambient groundwater flow rates, a constant head initial condition was assumed. Initial conditions for solute and bacterial concentrations were also set to zero since the background concentrations were orders of magnitude lower than the injected concentrations. A distant constant head boundary condition was implemented. The injection well used prescribed flux boundaries for both flow and transport. The abstraction well used prescribed flux boundary for flow, and the calculated sink term for transport terms set equal to the advective flux across the boundary. The complete set of tests carried out between 11th and 14th June 2012 (SI Table S1), including the intervening periods, were simulated. The initial fracture aperture, b0, was established by calibration from simulating the cross hole pumping tests conducted prior to the MICP tests. This resulted in b0 = 670 μm (equivalent to T0 = 19 m2/d using the cubic law). Longitudinal and transverse dispersivity were assumed to be 0.1 and 0.01 times the flow length between injection and abstraction boreholes, respectively. The transport model was calibrated manually to reproduce the observed ammonium concentration, the inferred cumulative mass of ammonium abstracted during the last two days of testing, and the postgrouting hydraulic tests, by varying φ (filter factor), ω (constant relating bacterial mass to ureolysis reaction rate), λ (the attached bacteria decay constant) and ρB (bulk density of the calcite precipitate). Calibration using the hydraulic test data ensures the simulated distribution of calcite is plausible.

Figure 2. Bacterial fixing. Bacterial and ammonium breakthrough in the effluent (test carried out on 12th June 2012, see SI Table S1) in comparison with a conservative tracer (CaCl2), representing the behavior of the remnant ammonium from the growth medium. OD = optical density of bacterial suspension. Error bars represent typical analytical error for each method.

Permeability Reduction by MICP. Figure 2 shows the ammonium concentrations measured in the effluent during the bacterial emplacement carried out on 12th June 2012. The initial ammonium peak is partly due to the breakthrough of some ammonium remnant from the growth medium injected with the bacterial suspension. However, breakthrough for a conservative tracer (CaCl2) using identical pumping rates is also shown in Figure 2, and it is clear from the relative timings that the continuing presence of significant ammonium in the effluent after the initial pulse must be due to bacterial ureolysis. The field data also indicate that the ureolysis rate decreases through time during continuous injection of cementing solution (as shown by the decrease in ammonium concentrations with time, Figure 2) and that reinoculation is eventually necessary to maintain the ureolysis rate and continue to reduce the fracture aperture. After eight inoculation-mineralization cycles carried out over a total of approximately 17 h spread over 4 days (SI Table S1), each boosting the in-fracture ureolysis rate (Figure 3), the total mass of calcite precipitated in the fracture was around 750 g (approximately 0.5 L of bulk precipitate). Mass balance calculations made on ammonium and calcium chloride showed a 2:1 balance consistent with the stoichiometry of a single reaction model which has previously been shown to work well at high ureolysis rates as the saturation dependent kinetics can be considered not to be limiting.6 Cross-borehole hydraulic conductances16 derived from steady state pumping tests decreased significantly as a result of MICP, indicative of a strong transmissivity decrease within the fracture (Figure 4). Repeat cross-borehole conductance tests indicate this observed decrease in transmissivity remained after 12 weeks exposure to ambient groundwater flows and relatively aggressive test pumping. Modeling results for the ammonium breakthrough curves (Figure 5) predict ammonium production for the first two inoculation cycles is much higher than that observed; this is explained by the much lower bacterial activity during the first two days, as determined prior to injection. In order to obtain model parameters for optimal bacteria behavior, ammonium data from the last six inoculation cycles only were used during

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RESULTS AND DISCUSSION Bacterial Fixing. Bacterial fixing in fractured rocks can occur by a variety of mechanisms including reversible or irreversible attachment and mechanical filtration,19−21 but techniques for fixing bacteria reliably in real fractures have not been developed until now. “Fixing agents”, such as CaCl2 solutions, have been shown to enhance bacterial immobilization in porous media such as sand.10,21 However, preliminary trials showed that for a fracture aperture of around 670 μm, and a high optical density (OD) suspension of bacteria (OD 0.5 at 600 nm) in groundwater, measurable retention of bacteria failed to occur at our field site, even with the addition of a salt solution (0.1 M CaCl2). Although such a fixative encouraged observable flocculation of the bacteria under static conditions, under flowing conditions the shear stresses appeared to overcome the weak aggregation leading to dispersal of the bacterial flocs and therefore limited fixing of bacteria within the fracture. We therefore developed a technique to induce bacterial immobilization by also adding urea (0.2 M) together with the bacteria to begin driving calcite precipitation. The presence of calcite precipitates encouraged stronger aggregation of the bacteria during injection and thus filtration within the fracture. This led to 70% retention of injected bacteria and significant in-fracture ureolysis of the injected urea (Figure 2). D

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Figure 5. Observed and modeled ammonium breakthrough at the abstraction borehole. Results are from all eight inoculation cycles (see SI Table S1 for details). Poor fit during first two cycles due to reduced bacteria activity cmpared with subsequent days. Model calibrated by least-squares method for fit against data from last six inoculations giving RMSE values of 1.42 mM/L and 9.38 g for modeled ammonium breakthrough concentration and cumulative ammonium mass abstracted, respectively.

Figure 3. Temporal changes in effluent chemistry and hydraulic head differences during staged inoculation and MICP. Step wise increases in ammonium production due to bacterial inoculation and decreases in calcium due to calcite precipitation; head gradient increase between injection and abstraction boreholes due to decreases in fracture transmissivity. Results are from the final three inoculation-mineralization cycles carried out on 14th June 2012, see SI Table S1 for test details.

feedback between the precipitation reaction and the transport of solutes between the bacteria and the fluid within the fracture. Here we have treated the inactivation as a first order process, characterized by a decay constant (eq 7). However, it is assumed that the decay constant takes a value λ1 when the cementing fluid is abundant, and hence is not limiting ureolysis, and a value λ2 otherwise, namely in the times between active testing in which ureolysis is driven by residual cementing fluid held in relatively immobile groundwater zones in the fracture. The final calibrated values λ1 = 10/d during cement injection and λ2 = 1/d overnight gave a good fit against observed ammonium data (Figure 5). Other calibrated parameters were φ = 1/d, ω = 45 000/OD/m/d, and ρB = 1626 kg/m3 giving RMSE values of 1.42 mM/L, 9.38 g and 0.69 m2/d for fits to the observed ammonium concentration, the cumulative mass of ammonium, and the cross hole conductance values respectively. The spatial distribution of the inferred reduction in transmissivity (Figure 4) shows that although a greater degree of aperture reduction occurs around the injection borehole (approximately 99%), a significant transmissivity decrease (approximately 35%, equivalent to approximately 90 μm aperture reduction) also occurred at a distance of 2 m. Physical confirmation of precipitation at distance was given by calcite observed on the packer used for sealing B4 at the depth of the fracture approximately 1.5 m from the injection point (SI Figure S3). The inferred spatial pattern in transmissivity reduction, with maximum decreases seen immediately surrounding the injection borehole, was relatively insensitive to the ureolysis reaction rate, the rate of bacterial decay and the calcite cement density. The limiting factor is the spatial pattern of bacterial accumulation which modeling indicated had to be strongly centered around the injection borehole in order to match the change in cross-borehole conductances observed (SI Figure S2). Implications for MICP as a Viable Engineering Technology. In combination, these are extremely important results indicating that significant MICP is viable under real field conditions at temporal and spatial scales useful for engineering subsurface flow barriers. The predominant control on the spatial distribution of precipitate in our experiments was the distribution of trapped bacteria; concentrations of urea and

Figure 4. Changes in cross-borehole conductance and modeled transmissivity (normalized by the initial value, T0) due to MICP. Streamlines emanating from B2 shown in gray. Vertical axis parallel to fracture dip direction.

calibration. Modeling suggests increasing dispersion of the cementing fluid beyond the region containing attached bacteria, due to the changing geometry of the fracture around the injection borehole, plays a minor role in decreasing the rate of ureolysis as each cementing phase progresses (Figure 5). However, this mechanism alone is not enough to explain the decrease in ammonium production observed in the data during periods between bacterial injections. We suggest that the decrease in ammonium production is due to the effective inactivation of attached bacteria by encapsulation in the calcite precipitate, which is a complex process6 involving nonlinear E

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directly transferable to other MICP subsurface applications such as chemical sequestering of carbon by enhanced solubility trapping,4 enhancing soil stability,10,32 and solid phase capture of radionuclides.33,34

CaCl2 were not limiting. The ability to control an evenly distributed transmissivity reduction over a large area is more useful from an engineering perspective than simply reducing the permeability of a small zone near a borehole. Thus the results demonstrate clearly the need for ways of controlling the emplacement of the bacteria to ensure precipitation away from the injection point. Flushing of the injection port has recently been suggested as a way to reduce this effect7 and this is planned for testing in further field trials. However, we also propose that the use of separate boreholes for inoculation and injection of cementing solutions may be a practical solution. A variety of geometries dependent on the desired flow barrier design can be imagined, in line with borehole arrangements used for traditional cement grouting.22 The coupled flow modeling approach we have developed, being more parsimonious than other published approaches,23−25 is a practical blueprint to aid the design of such systems by varying injection rates, volumes and borehole designs to inform further field trials. The low viscosity and micron-sized bacterial cell make the technique applicable to fine aperture formations at much lower operating pressures than cementitious grouts.26 As well as having practical advantages, this reduces the risk of “heave”, whereby hydraulic jacking of the ground surface can actually act to increase the transmissivity of fine fractures to the detriment of any bulk permeability reduction due to grouting.22 For longterm engineered barriers, the mechanical and chemical nature of the precipitate is also of paramount importance.27 Mechanically, calcite is stronger than other types of cementitious grouts.28 Moreover it is stable on geological time scales as long as it is not subject to dissolution from low pH groundwaters, which is unlikely in the deep subsurface environment.29 That repeat hydraulic testing 12 weeks after MICP indicated no change in transmissivity in the intervening period demonstrates the chemical stability of the precipitate in the presence of an ambient groundwater flow field (at least in the short term), and its mechanical stability when subjected to moderate shear forces induced by repeated pump testing at high flow velocities. In comparison with other grouting techniques MICP has also added advantages of operating at modest pH, being nontoxic, not prone to shrinkage and nonexothermic. The resulting ammonium production generated during MICP does however warrant consideration for applications in scenarios where groundwater may be vulnerable. Nonmicrobial cementation using urease enzymes extracted from jack bean meal have been suggested as an alternative to MICP but currently it cannot be seen how the preferential clogging of the inlet can be overcome using this technique30 as enzyme and reactants have to be injected in parallel, simultaneously. The technique of MICP by staged injection of bacteria and cementing solution demonstrated here is more promising for controlling cementation away from the injection point as discussed above. Moreover, the multiple staged injections of new S. pasteurii inocula would also prevent a significant drop in ureolysis that would be caused by the inability of this organism to produce urease under anoxic conditions.31 These results represent a major step forward toward realizing MICP as an engineering solution to subsurface fracture sealing. Further studies are needed to test the behavior of this technology in a variety of other geological contexts, and tests of alternative injection strategies can be informed by the type of coupled models we have presented here. Our findings are also

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ASSOCIATED CONTENT

S Supporting Information *

Flow diagram of coupled model equations, cross-borehole conductance calibration, photograph of calcite observed on packer, table detailing all MICP grouting tests and table of model parameter values. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(M.O.C.) E-mail: [email protected]. Present Address ∥

(M.O.C.) Connected Waters Initiative Research Centre, The University of New South Wales, 110 King St, Manly Vale, NSW, 2093, Australia. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the members of the BANDD consortium for support in various aspects of this work over the last 3 years and Richard Greswell, Ban To & Mahmoud Jaweesh for technical/ laboratory assistance. The work was jointly funded by an Engineering and Physical Sciences Research Council (EPSRC) and Natural Environment Research Council grant (EP/ G063699/1). David Gazzard at Midland Quarry Products Ltd. kindly gave access to the fieldsite.

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REFERENCES

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dx.doi.org/10.1021/es402601g | Environ. Sci. Technol. XXXX, XXX, XXX−XXX