Published in Vadose Zone Journal 3:858-866 (2004)
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
SPECIAL SECTION: RESEARCH ADVANCES IN VADOSE ZONE HYDROLOGY THROUGH SIMULATIONS WITH THE TOUGH CODES
Vadose Zone Remediation of Carbon Dioxide Leakage from Geologic Carbon Dioxide Sequestration Sites
Yingqi Zhang*,
Curtis M. Oldenburg and
Sally M. Benson
Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley CA 94720
* Corresponding author (yqzhang{at}lbl.gov)
Received 2 March 2004.
 |
ABSTRACT
|
|---|
In the unlikely event that CO2 leakage from deep geologic CO2 sequestration sites reaches the vadose zone, remediation measures for removing the CO2 gas plume may have to be undertaken. Carbon dioxide leakage plumes are similar in many ways to volatile organic compound (VOC) vapor plumes, and the same remediation approaches are applicable. We present here numerical simulation results of passive and active remediation strategies for CO2 leakage plumes in the vadose zone. The starting time for the remediation scenarios is assumed to be after a steady-state CO2 leakage plume is established in the vadose zone, and the source of this plume has been cut off. We consider first passive remediation, both with and without barometric pumping. Next, we consider active methods involving extraction wells in both vertical and horizontal configurations. To compare the effectiveness of the various remediation strategies, we define a half-life of the CO2 plume as a convenient measure of the CO2 removal rate. For CO2 removal by passive remediation approaches such as barometric pumping, thicker vadose zones generally require longer remediation times. However, for the case of a thin vadose zone where a significant fraction of the CO2 plume mass resides within the high liquid saturation region near the water table, the half-life of the CO2 plume without barometric pumping is longer than for somewhat thicker vadose zones. As for active strategies, results show that a combination of horizontal and vertical wells is the most effective among the strategies investigated, as the performance of commonly used multiple vertical wells was not investigated.
Abbreviations: NAPL, nonaqueous phase liquid • SVE, soil vapor extraction • VOC, volatile organic compound
|
INTRODUCTION
|
|---|
GEOLOGIC CARBON SEQUESTRATION is the direct injection of CO2 deep into geological formations for long-term storage for the purpose of reducing the rate of increase of atmospheric CO2 concentrations due to energy production from fossil fuels. Although there are many mechanisms to trap the injected CO2 (Bachu et al., 1994; Oldenburg and Unger, 2003), there is the risk that injected CO2 will migrate away from the primary target formation (Holloway, 1997), a process referred to as "leakage" by Oldenburg and Unger (2003). Possible leakage pathways include wells (abandoned or active), permeable faults and fractures, and unexpected fast-flow paths. Figure 1
is a conceptual diagram showing a variety of possible leakage pathways and processes. Leakage will likely lead to secondary trapping in shallower formations, or result in a flow path with a sufficiently long travel time so as to meet the sequestration objective. However, there is a risk that CO2 leakage will lead to rapid migration upward to the vadose zone. Seepage happens when leaked CO2 migrates through the vadose zone, reaches the ground surface, and escapes into the ambient air (Oldenburg and Unger, 2003). Seepage of CO2 can lead to locally high CO2 concentrations in the near-surface environment, which may cause health and environmental hazards. Although CO2 leakage to the vadose zone is highly unlikely, it is useful to demonstrate that effective remediation strategies exist for CO2 leakage plumes in the vadose zone, should such measures be necessary. In the context of Fig. 1, this study focuses on the uppermost part of the subsurface where a CO2 leakage plume exists within the vadose zone.
The objective of this work is to explore the effectiveness of various remediation strategies for CO2 leakage plumes in the vadose zone. The approach we take is numerical simulation of both passive and active remediation strategies that involve CO2 removal from the vadose zone. Passive strategies rely on natural attenuation processes to achieve site-specific remedial objectives, in our case, the mass reduction of leaked CO2 in the vadose zone, without human intervention and within a time frame that is reasonable compared with that offered by active methods. Both active and passive strategies have been established and implemented during the last 25 yr for the removal of contaminant vapors such as those from VOCs that have leaked into the vadose zone from surface leaks and spills or from underground storage tanks. In this study, we compare removal rates of CO2 plumes from the vadose zone under passive and active treatment processes. In so doing, we demonstrate that remediation of CO2 leakage plumes in the vadose zone can be accomplished effectively by standard approaches.
|
Background
|
|---|
The migration of CO2 through the vadose zone has some similarity to the transport of VOCs in the vadose zone. In particular, CO2 is a dense gas relative to air. Similarly, the high molecular weights and high vapor pressures of common contaminant VOCs give rise to dense VOC gas plumes emanating from nonaqueous phase liquid (NAPL) VOC leaks and spills (Falta et al., 1989). At a temperature of 25°C and a pressure of 1 atm, the density of air is 1.17 kg m–3, while the density of air saturated with VOCs ranges from 1.21 kg m–3 (Xylene) to 2.50 kg m–3 (Methylene chloride) (Falta et al., 1989), and pure CO2 has a density of 1.81 kg m–3. Falta et al. (1989) studied under what conditions the density-driven gas flow may dominate the transport of contaminants in the gas phase. The conclusion was that the magnitude of density-driven flows is mainly a function of organic liquid saturated vapor pressure and molecular weight, the gas phase permeability, and the gas phase retardation coefficient.
Although CO2 and VOC vapors share some similarities in density, there are very important differences between them relevant to remediation and transport. With regard to the need for remediation, the main difference between VOCs and CO2 is that VOCs are potentially harmful to humans and other animals even at low concentrations. In contrast, CO2 is naturally occurring, ubiquitous, and essential to life. The background atmospheric CO2 concentration is approximately 0.67 g m–3 (370 ppmv), and CO2 is relatively harmless even at concentrations several times the background concentration. However, long-term exposure to CO2 at concentrations of a few percent or higher can be harmful to humans, other animals, and the roots of plants. Another difference between CO2 leakage plumes and VOC contaminant plumes is the typical location of the source and transport direction. In general, vadose zone VOC gas plumes tend to diffuse away from NAPL or aqueous sources as they descend through the vadose zone from above due to leaking underground tanks and surface spills. In contrast, potential CO2 leakage plumes will typically arrive from below. Finally, CO2 dissolution in water forms carbonic acid, which leads to the lowering of pH and the potential for corrosion of metals in extraction systems, a feature that may require special attention in practical applications.
Soil vapor extraction (SVE) is a technology developed to remove VOCs and some semivolatile organic compounds from the vadose zone, a comprehensive review of which was presented by Wilson (1995). Soil vapor extraction system design options usually include vertical or horizontal wells screened in the contaminated zone, as well as trenches. Soil vapor extraction has been widely used for the remediation of spills, leaks, and hazardous waste sites during the past 25 yr because of its efficiency and relatively low cost. A significant number of modeling efforts including analytical solutions (Falta, 1995; Rossabi and Falta, 2002; Shan et al., 1992) and numerical simulations (Falta et al., 1992a, 1992b; Sleep and Sykes, 1989) have been made to understand gas flow and make better use of the technology. Recently, horizontal wells have been recognized to be a more effective alternative to vertical wells in some scenarios (Cleveland, 1994; Falta, 1995; Hunt and Massmann, 2000; Sawyer and Lieuallen-Dulam, 1998; Zhan and Park, 2002). The main factors that determine the effectiveness of SVE are:
Porous Medium Intrinsic Permeability.
The soil must be sufficiently permeable to permit the vapor extraction wells to draw soil gas through the contaminated domains at a reasonable rate.
Soil Water Content.
Water saturation must be low enough to allow sufficient gas flow.
Henry's Law Coefficient of Target Compound.
High solubility and low vapor pressure of contaminant requires higher gas flow rates or longer gas extraction times to extract contaminants from the aqueous phase.
Shan et al. (1992) found that the well screen locations and porous medium anisotropy have strong effects on gas flow patterns. In particular, they found that wells should be screened near the bottom of the vadose zone to avoid short-circuiting of the gas flow by ground-surface inflows of ambient air, although some provision should be made for counteracting the upwelling of the water surface to the gas extraction well. Furthermore, the effective horizontal radius of an SVE well in a medium having a large anisotropy ratio (kh/kv) is much larger than that of a well in a medium having an isotropic permeability.
For active methods of removing CO2, we analyzed similar strategies to those used for VOCs by SVE. We show simulation results for horizontal wells, a vertical well, and a combination of both vertical and horizontal wells.
|
METHODS
|
|---|
Numerical simulations were performed using T2CA, a special module of the TOUGH2 simulator (Pruess et al., 1999), that models flow and transport of CO2–air mixtures. T2CA models five components (H2O, brine, CO2, gas tracer, and air), along with heat. T2CA uses real gas mixture properties for density and viscosity, and a Henry's Law formulation for CO2 solubility. Although capable of nonisothermal simulations, all of the results presented here are isothermal at 15°C.
To compare the effectiveness of the various remediation strategies and scenarios, we define a half-life of the CO2 plume as the time required for one-half of the initial CO2 mass to be removed from the domain as a convenient measure of the CO2 removal rate. The initial CO2 distribution in the model vadose zone corresponds to a steady-state leakage scenario in which CO2 flowed upward through the saturated zone and vadose zone, and ultimately seeped out at the ground surface, a process discussed in detail by Oldenburg and Unger (2003). We present simulation results first for a radial two-dimensional system to examine passive approaches and vapor extraction with a vertical well. Next, we present results for an equivalent three-dimensional system to examine the effectiveness of horizontal wells.
|
RESULTS
|
|---|
Two-Dimensional Radial Simulations
The model domain consists of a cylindrical, vertical section of unsaturated and saturated porous media with a radius of 2100 m. The ground surface is at 35 m elevation, and the water table is at 5 m elevation. The bottom boundary is held at constant hydrostatic pressure, and the top of the system is held at atmospheric pressure. The outer radial boundary is held at constant pressure corresponding to the initial CO2–free gravity-capillary equilibrium. Properties of the system are shown in Table 1. The porous medium properties correspond to typical, poorly sorted, unconsolidated sediments. We impose a constant water infiltration rate of 10 cm yr–1 with a CO2 mass fraction of 6.86 x 10–7 corresponding to water in equilibrium with air with CO2 concentration equal to 0.67 g m–3 (370 ppmv). Because of the high solubility of CO2 in water, this downward water flux is capable of transporting CO2 as a dissolved component downward to the water table. A leakage rate of CO2 is set at 4.0 x 105 kg yr–1 over a circular area at the water table with radius 100 m, corresponding to a leakage flux of 4.0 x 10–7 kg m–2 s–1. Compared with this rate, the CO2 flow rate due to infiltration, which is about 950 kg yr–1, will not be an important factor. After this leaking system has come to steady state, we obtain the initial condition used for the remediation simulations for different scenarios. This initial condition for the remediation simulations assumes that a leakage event occurred that brought CO2 to the vadose zone, but that this event was then stopped, for example by an intervention such as lowering the reservoir pressure by producing CO2 or water from the reservoir. The initial CO2 plume of approximately 9 x 105 kg (900 t) of CO2 is shown in Fig. 2
, where the shaded contours represent CO2 mass fraction in the gas phase, white contours represent aqueous phase saturation, and vectors represent gas velocities. The initial pressure at the water table is about 5.0 kPa higher than the atmospheric pressure. This high pressure difference makes the density effect of CO2 relatively unimportant in this study.
View larger version (39K):
[in this window]
[in a new window]
|
Fig. 2. Initial conditions shown by CO2 gas mass fraction shaded contours, white liquid saturation contours, and gas velocity. Maximum vector represents 8.5 x 10–6 m s–1 (7.3 x 10–1 m d–1).
|
|
We analyze four remediation scenarios in the two-dimensional radial system as follows:- Natural attenuation (passive remediation) without barometric pumping.
- Natural attenuation (passive remediation) with barometric pumping.
- A 30-m-long vertical well whose screen is from elevation 5 to 20 m.
- Identical well configuration to Scenario 3, with an impermeable surface cover of 50-m radius added.
The daily barometric pressure record used in the barometric pumping scenario corresponds to an actual pressure variation measured in the Central Valley of California for the year 1997 (Zawislanski et al., 1999). The amplitude of this yearly data fluctuates between –1.2 and 1.8 kPa around the average pressure of 99.708 kPa, which is the same as we used for the atmospheric pressure in the scenarios without barometric pumping. The pressure data are applied repeatedly every year during the whole remediation period. Simulation results after 10 yr of remediation for the four cases are shown in Fig. 3 through 6
by the shaded contours of CO2 gas mass fraction and gas velocity vectors. Although in Fig. 4 (barometric pumping scenario) the gas vectors point upwards, oppositely directed vectors occur on other days during the year when the pressure at the surface is high. The half-life time for each scenario is calculated and listed in Table 2. A pumping rate of 5.0 x 10–4 kg s–1 for Scenarios 3 and 4 is used.
View larger version (34K):
[in this window]
[in a new window]
|
Fig. 3. Scenario 1: CO2 plume after 10 yr of natural attenuation without barometric pumping. Maximum vector represents 4.1 x 10–8 m s–1 (3.5 x 10–3 m d–1).
|
|
View larger version (44K):
[in this window]
[in a new window]
|
Fig. 6. Scenario 4: CO2 plume after 10 yr of pumping with a cover. Maximum vector represents 4.5 x 10–4 m s–1 (39 m d–1).
|
|
View larger version (42K):
[in this window]
[in a new window]
|
Fig. 4. Scenario 2: CO2 plume (CO2 gas mass fraction) after 10 yr of natural attenuation with barometric pumping. Maximum vector represents 8.0 x 10–7 m s–1 (6.9 x 10–2 m d–1).
|
|
View larger version (38K):
[in this window]
[in a new window]
|
Fig. 5. Scenario 3: CO2 plume (CO2 gas mass fraction) after 10 yr of pumping. Maximum vector represents 4.5 x 10–4 m s–1 (39 m d–1).
|
|
As shown in Table 2, for passive remediation strategies, barometric pumping (Scenario 2) increases the removal rate of CO2. This is because when pressure at the ground surface is lower than the average pressure during barometric pumping, more CO2 seeps out of the ground surface than the case without barometric pumping. This portion of CO2 will be diluted immediately by the air in the atmospheric gridblocks. When the pressure at the ground surface becomes larger than the average atmospheric pressure, pure air (i.e., no CO2) will flow back into the subsurface from the atmospheric gridblocks. We can also see that pumping from a vertical well (Scenarios 3 and 4) increases the CO2 removal rate slightly. There are two reasons for the limited improvement. First, the gas production from the well (pumping rate) is low and limited by the high aqueous phase saturations around the well. Second, the radius of influence of a single well is too small to efficiently recover a large gas plume.
In Scenario 4, an impermeable cover is used at the ground surface. The half-life for this case is 6.18 yr, which is longer than for Scenario 3. This result occurs because the cover at the top decreases the gas pressure gradients and thereby decreases the vertical gas flows needed to remove the CO2 immediately beneath the cover. In short, while the sweep is more horizontal and therefore potentially more effective in removing CO2 for the case of an impermeable cover, the vertical flow rates are smaller and the half-life of the plume correspondingly longer. This can, of course, be remedied by a well with multiple screen zones.
A sensitivity analysis of how vadose zone thickness affects remediation rates was done for the two passive strategies. The results are shown in Fig. 7
. As expected, when the vadose zone is thicker, it takes longer for the CO2 to be removed because more of the CO2 is located farther from the ground surface for thicker vadose zones. However, the results show an exception in the trend for the case where the vadose zone thickness is 5 m without barometric pumping. In this scenario, the half-life for the CO2 plume was longer than it was for the 10- and 15-m-thick vadose zones. This reversal in the trend occurs because of the difficulty of removing CO2 from areas with high aqueous phase saturation near the capillary fringe. When the water table is close to the ground surface (as in this 5-m-thick vadose zone case), most of the vadose zone has a very large liquid saturation. This implies a large percentage of the CO2 plume resides in high liquid saturation regions. Because of the high water saturation, diffusive and advective transport is limited by low gas-phase saturation, and a significant amount of CO2 is dissolved in the aqueous phase. In the other cases, a large percentage of the CO2 is located in regions with relatively lower liquid saturations, where transport is faster, and a small proportion of the plume is dissolved in groundwater.
In an attempt to increase the CO2 removal rate for active remediation strategies, we tried two extra scenarios. In Scenario 5, we first increased the pumping rate. However, permeability and high water saturation limited the pumping rate. Therefore, we increased the base-case permeability to 1.0 x 10–11 m2 and raised the position of the well screen to be between 20 and 30 m. By doing this, a higher pumping rate of 1.0 x 10–2 kg s–1 could be applied, and significantly increased CO2 removal was found. To prevent short-circuiting, as discussed by Shan et al. (1992), we also used a second method (referred to as Scenario 6) in which an impermeable surface cover with a radius of 50 m exists around the well with the same screen configuration. The half-life time for the case without cover is 2.16 yr and the one with cover is 1.55 yr. Simulation results for these two scenarios are shown in Fig. 8 and 9
, respectively. When the screen is higher, a cover around the well prevents ambient air at the ground surface from flowing directly to the well, short-circuiting past the contaminated zone. The half-life in this case is shortened by using a cover. And the water saturation around the well screen is lower in this case because of the enhanced horizontal flow. In summary, a high well screen and impermeable surface cover together improve vadose zone CO2 leakage remediation effectiveness. Nevertheless, for the case of a single vertical extraction well, significant CO2 mass remains in the system away from the pumping well even after 10 yr of pumping.
View larger version (26K):
[in this window]
[in a new window]
|
Fig. 8. Scenario 5: CO2 plume after 10 yr of pumping. Maximum vector represents 3.0 x 10–5 m s–1 (2.6 m d–1).
|
|
View larger version (28K):
[in this window]
[in a new window]
|
Fig. 9. Scenario 6: CO2 plume after 10 yr of pumping. Maximum vector represents 3.0 x 10–5 m s–1 (2.6 m d–1).
|
|
Three-Dimensional Cartesian Simulations
The radial simulations presented above point out limitations of a single vertical well. For example, the influence of radius is small and gas velocities are inversely proportional to distance from the well. Using horizontal wells is another potential option. We used a three-dimensional Cartesian system to simulate remediation scenarios with horizontal wells. Because of the symmetry of the system, only one-quarter of the system is modeled. To reduce the computational effort, the horizontal domain was reduced to 300 m in both x and y directions since CO2 concentration (on the order of 10–4 by mass fraction) in the gas phase is negligible at r = 300 m. To make the three-dimensional simulations comparable with the radial cases, the initial plume for the three-dimensional system is obtained by mapping the initial conditions of the radial system into a quarter of a three-dimensional cylindrical domain.
Figure 10
shows the conceptual model for the three-dimensional Cartesian system consisting of a thick vadose zone and vertical and horizontal wells used for CO2 extraction. The water table is 30 m below the ground surface. Five scenarios are considered:
- Vertical well only, with the well screen from 20 to 30 m.
- Horizontal wells only, of length 90 m aligned with the x and y axes at an elevation of 20 m above the bottom of the domain.
- Both vertical and horizontal wells (i.e., combination of Scenarios 1 and 2).
- Same as Scenario 3, except that horizontal wells have a length of 120 m.
- Same as Scenario 3, except that an anisotropy ratio (kz/ky, ky = kx) of 0.1 is used, where we keep the horizontal permeability unchanged while reducing vertical permeability.
A permeability of 1.0 x 10–11 m2 and a total pumping rate of 1.0 x 10–2 kg s–1 are used for all scenarios, with other properties the same as those shown in Table 1. Simulation results (after 10 yr of remediation) for Scenarios 1 through 3 are shown in Fig. 11 through 13
. Again, shaded contours represent mass fraction of CO2 in the gas phase. Note that different contour levels are used in different figures. Figure 14 and 15
are cross sections from Fig. 13, shown to elucidate regions hidden from view in three dimensions.
Table 3 lists the half-life for all scenarios. Among all of the remediation strategy Scenarios 1 through 4, excluding Scenario 5, which has permeability anisotropy, the most effective remediation strategy is Scenario 3 (both horizontal and vertical wells), with a horizontal well length of 90 m. Scenario 4 uses a longer horizontal well length of 120 m, but the same total pumping rate. The longer half-life for this scenario indicates that a longer horizontal well does not necessarily help if the well reaches beyond where most of the plume resides.
Another scenario (Scenario 5) is a special case to examine how permeability anisotropy affects CO2 removal. The remediation strategy is identical to that used in Scenario 3. As shown in Fig. 14 through 16
, at early times the removal rate is much faster. This is because the pressure contours for the anisotropic case are "stretched" by a factor of 101/2, and the flow tends to be more horizontal with a greater sweep radius. Later the removal rate is slower because vertical flow is relatively restricted, resulting in a lower overall removal during the simulation period. This is consistent with the results found by Shan et al. (1992). In summary, the results of the three-dimensional Cartesian simulations demonstrate the increased effectiveness of using both vertical and horizontal wells for removing potential CO2 leakage plumes from the vadose zone.
View larger version (21K):
[in this window]
[in a new window]
|
Fig. 16. Remaining CO2 vs. time for different scenarios. The solid horizontal line indicates half-life time.
|
|
|
CONCLUSIONS
|
|---|
The overall conclusion of these modeling studies is that standard passive and active vadose zone remediation strategies will be effective for remediating potential CO2 leakage plumes in the vadose zone. In detail, the simulation results presented here suggest the following conclusions regarding vadose zone CO2 leakage plume remediation:- Barometric pumping enhances the removal rate of CO2.
- Passive CO2 removal from high water saturation regions near the water table is limited by low gas saturation and high solubility in groundwater.
- For vapor extraction using a vertical well, the well screen should not be too close to the water table.
- A combination of an impermeable cover and vertical well will improve the removal rate of CO2 if the well screen is relatively shallow.
- The combination of horizontal and vertical wells is more effective than having either a single horizontal or vertical well.
- Permeability anisotropy (kx > kz) results in a faster removal rate at an early stage and slower rate later on.
- The combined vertical and horizontal well configuration would also be effective for VOC contaminants.
These conclusions are only for the scenarios investigated. We did not do sensitivity analysis to find the optimal well screen location and length. As mentioned above, Shan et al. (1992) suggested a well screen position closer to the water table because the area cleaned would be larger. On the other hand, if it is too close to the water table, the high water saturation limits the gas flow, hence the pumping rate. Also, the simulator we used, TOUGH2 uses a total mass flow rate for production resulting in water production when the screen is located in high liquid saturation regions. Future work could include other potential remediation options (e.g., multiple vertical wells, both production and injection wells), effects of well screen positions and well length, and economical considerations.
|
ACKNOWLEDGMENTS
|
|---|
We thank Joe Rossabi (Redox Tech), Chris Doughty (LBNL), Stefan Finsterle (LBNL), and two anonymous reviewers for helpful comments and suggestions. This work was supported in part by a Cooperative Research and Development Agreement (CRADA) between BP Corporation North America, as part of the CO2 Capture Project (CCP) of the Joint Industry Program (JIP), and the U.S. Department of Energy (DOE) through the National Energy Technologies Laboratory (NETL), and by the Ernest Orlando Lawrence Berkeley National Laboratory, managed for the U.S. Department of Energy under contract DE-AC03-76SF00098.
|
REFERENCES
|
|---|
- Bachu, S., W.D. Gunter, and E.H. Perkins. 1994. Aquifer disposal of CO2—Hydrodynamic and mineral trapping. Energ. Convers. Manage. 35:269–279.
- Cleveland, T.G. 1994. Recovery performance for vertical and horizontal wells using semianalytical simulation. Ground Water 32:103–107.[ISI]
- Falta, R.W. 1995. Analytical solutions for gas flow due to gas injection and extraction from horizontal wells. Ground Water 33:235–246.[ISI]
- Falta, R.W., I. Javandel, K. Pruess, and P.A. Witherspoon. 1989. Density-driven flow of gas in the unsaturated zone due to the evaporation of volatile organic compounds. Water Resour. Res. 25:2159–2169.
- Falta, R.W., K. Pruess, I. Javandel, and P.A. Witherspoon. 1992a. Numerical modeling of steam injection for the removal of nonaqueous phase liquids from the subsurface. 1. Numerical formulation. Water Resour. Res. 28:433–449.
- Falta, R.W., K. Pruess, I. Javandel, and P.A. Witherspoon. 1992b. Numerical modeling of steam injection for the removal of nonaqueous phase liquids from the subsurface. 2. Code validation and application. Water Resour. Res. 28:451–465.
- Holloway, S. 1997. Safety of the underground disposal of carbon dioxide. Energy Convers. Manage. 38:S241–S245.
- Hunt, B., and J. Massmann. 2000. Vapor flow to trench in leaky aquifer. J. Environ. Eng. 126:375–380.
- Oldenburg, C.M., and A.J.A. Unger. 2003. On leakage and seepage from geologic carbon sequestration sites: Unsaturated zone attenuation. Available at www.vadosezonejournal.org. Vadose Zone J. 2:287–296.[Abstract/Free Full Text]
- Pruess, K., C. Oldenburg, and G. Moridis. 1999. TOUGH2 user's guide. Version 2.0. LBNL-43134. Lawrence Berkeley Natl. Lab., Berkeley, CA.
- Rossabi, J., and R.W. Falta. 2002. Analytical solution for subsurface gas flow to a well induced by surface pressure fluctuations. Ground Water 40:67–75.[ISI][Medline]
- Sawyer, C.S., and K.K. Lieuallen-Dulam. 1998. Productivity comparison of horizontal and vertical ground water remediation well scenarios. Ground Water 36:98–103.[ISI]
- Shan, C., R.W. Falta, and I. Javandel. 1992. Analytical solutions for steady state gas flow to a soil vapor extraction well. Water Resour. Res. 28:1105–1120.
- Sleep, B.E., and J.F. Sykes. 1989. Modeling the transport of volatile organics in variably saturated media. Water Resour. Res. 25:81–92.
- van Genuchten, M.Th. 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44:892–898.[ISI]
- Wilson, D.J. 1995. Modeling of in situ techniques for treatment of contaminated soils. CRC Press, Boca Raton, FL.
- Zawislanski, P.T., C.M. Oldenburg, C.A. Doughty, and B.M. Freifeld. 1999. Application of the vadose zone monitoring system at a TCE-contaminated site: Field data and modeling summary. LBNL-44325. E.O. Lawrence Berkeley Natl. Lab., Berkeley, CA.
- Zhan, H., and E. Park. 2002. Vapor flow to horizontal wells in unsaturated zones. Soil Sci. Soc. Am. J. 66:710–721.[Abstract/Free Full Text]
TOP
ABSTRACT
INTRODUCTION
