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Fluids Dynamics in Transient Air Sparging of a heterogenous unconfined aquifer

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A project for remediating a site with diesel contamination employing techniques such Air sparging, Soil Vapor Extraction, Pump and treat

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Fluids Dynamics in Transient Air Sparging of a heterogenous unconfined aquifer

  1. 1. ORIGINAL ARTICLE Fluids’ dynamics in transient air sparging of a heterogeneous unconfined aquifer Rashid S. Al-Maamari • Akihiko Hirayama • Tsuyoshi Shiga • Mark N. Sueyoshi • Mahfoodh Al-Shuely • Osman A. E. Abdalla • Anvar R. Kacimov Received: 15 July 2009 / Accepted: 7 October 2010 Ó Springer-Verlag 2010 Abstract Water table dynamics, dissolved oxygen (DO) content, electrical resistivity (ER) in monitoring wells and air pressure in the vadose zone are monitored in air sparging (AS) accompanied by soil vapor extraction (SVE) at a hydrocarbon-contaminated groundwater site in Oman, where a diesel spillover affected a heterogeneous uncon- fined aquifer. The formation of a groundwater mound at the early stage of air injection and potential lateral migration of contaminants from the mound apex called for an additional hydrodynamic barrier constructed as a pair of pump-and- treat (P&T) wells whose recirculation zone encompassed the AS and SVE wells. In all monitored piezometers the phreatic surface showed a rapid and distinct peak, which is attributed to the time of air breakthrough from the injection point to the vadose zone and a relatively mild recession limb interpreted as a decay of the mound. Tracer tests showed a layer of a relatively low hydraulic conductivity at an intermediate depth of the screened interval of the wells. Increased levels of DO and borehole air pressure that have been observed (as far as 50 m away) are likely mitigated by SVE and P&T. Radius of influence can be indirectly inferred from ER and DO changes in the AS operation zone. Salt tracer tests have shown that groundwater velocity within the AS zone decreases with the increase of air injection rate. Keywords Air sparging Á Heterogeneous formation Á Groundwater Á Hydrocarbon contamination Introduction Air sparging (AS) is a groundwater remediation technique widely used for treatment of VOCs (volatile organic compounds) in contaminated aquifers (Johnson 1998; Braida and Ong 2001). Air, injected under the water table, ascends and breaks through the saturated zone to the vadose zone, scavenging VOCs and releasing them to the unsaturated soil, from which air ventilation (soil vapor extraction, SVE) wells abstract the volatilized chemicals (Fig. 1 depicts schematically the corresponding wells in a vertical cross-section). Laboratory, field, and modeling studies of AS have been undertaken with a special focus on the effect of particle size of the aquifer porous medium, its heterogeneity, geo- logical and hydrogeological zonation, and engineering parameters, including the depth of the injector suction screens, distance between the wells, sizes of counter- shortcut impermeable caps on the ground surface, the injection-abstraction rates and regimes, maintained pres- sures, etc. (e.g., Angell 1992; Brown and Jasiulewicz 1992; Marley et al. 1992; Reddy et al. 1995, 2001; McCray and Falta 1996; Ng and Mei 1996; Bass et al. 2000; Adams and Reddy 2000; Berkey et al. 2003; Tomlinson et al. 2003; Geistlinger et al. 2006; Kim et al. 2006; Selker et al. 2007). Electronic supplementary material The online version of this article (doi:10.1007/s12665-010-0793-y) contains supplementary material, which is available to authorized users. R. S. Al-Maamari (&) Á M. Al-Shuely Á O. A. E. Abdalla Á A. R. Kacimov Sultan Qaboos University, P.O. Box 33, Al-Khoudh, PC 123, Sultanate of Oman e-mail: A. Hirayama Á M. N. Sueyoshi Shimizu Corporation, 1-2-3 Shibaura, Minato-ku, Tokyo 105-8007, Japan T. Shiga Taisei Kiso Sekkei Co., Ltd., 3-43-3 Sendagi, Bunkyo-ku, Tokyo 113-0022, Japan 123 Environ Earth Sci DOI 10.1007/s12665-010-0793-y
  2. 2. Of crucial importance is the flow type in the air sparged zone, which can be channel-type, pervasive (bubbly), or chamber-type (with amoeba- and tabular-form air invaded pockets) (Ji et al. 1993; Peterson 1999, 2001; Chao et al. 2008). The final output of AS–SVE is determined by the drop in concentrations of chemicals to be removed from groundwater–vadose zone. As regular sampling and chemical analysis of soil and water is costly, indirect indicators of remediation are used, in particular, the char- acteristics of the oval-shaped sparging plume, which are assessed by the air-pressure increment (air volumetric content), dissolved oxygen (DO) content in groundwater, VOC concentration in the vadose zone and groundwater mounding. These characteristics are further used in models for predictions of Darcian velocities of air, water, and free products as well as of physical and chemical transforma- tions (mostly volatilization) within the plume. One or several characteristics are combined in the so-called radius of influence (ROI) of the plume (McCray and Falta 1996; Mei et al. 2002). In modeling and conceptualization of AS–SVE two major approaches are utilized. One-phase sharp-interface model requires an intrinsic permeability and porosity as the only parameters and assumes a distinct free boundary between the air plume and groundwater (Nikolskii 1961). Two–three phase flow models consider combined air and groundwater (and sometimes NAPL) motion (McCray and Falta 1996; Mei et al. 2002; Jang and Aral 2009) that requires phase permeabilities and capillary pressure functions, which are difficult to measure in the field. Any intermittency of AS–SVE—as in our case—involves imbibition-drainage cycles. The hysteresis of pressure– saturation-permeability functions in these cycles exacer- bates the complexity of multiphase modeling and under- standing of the flow-transport dynamics. Consequently, AS–SVE ‘‘multiphase’’ plumes are often assumed to be steady-state and therefore only air motion is studied (groundwater is hydrostatically stagnant) that reduces the multiphase-flow models to apparently one-phase, albeit, with air pressure, content and density varying from one point to another (Philip 1998; Mei et al. 2002). Although periodic agitation (cyclic flooding) of a multiphase sub- surface system is widely practiced in secondary oil recovery (Tsynkova et al. 1993) and has been recently proved to be superior to steady air injection-suction (see for e.g., Balcke et al. 2009), for unconfined aquifers AS–SVE under these cyclostationary regimes has peculiarities, which are not encountered by petroleum engineers. Namely, groundwater is bounded from above by a fluctu- ating phreatic surface, which periodically mounds and slumps under the action of another free surface (the injected air ‘‘macrobubble’’). Clearly, in groundwater unconfined aquifers, the effect of the vadose zone where gas abstraction wells are located makes another major difference with deep and always confined porous pays. The gas phase depressions caused by these wells have their own effect on the phreatic surface similar to upcresting (upc- oning) in reservoir engineering. Saturated zone Monitoring well 1 Monitoring well 2 Air invaded zone (plume) Vadose zone Transient water table Saturated zone Monitoring well 1 Monitoring well 2 Vadose zone Saturated zone Monitoring well 1 Monitoring well 3 Vadose zone (a) (b) (c) A B C SVE well (with vacuum regulator/vacuum blower) SVE- induced upwelling AS-induced upwelling Air compressor Injection well Original water table Plume Plume A C CA Fig. 1 Dynamics of the sparged zone and water table mound at different stages of air injection: a incipient stage with a slightly bulging phreatic surface and almost circular cross-section of the bubble, b pre-breakthrough stage with an oval-shaped bubble almost reaching the maximally-upwelled phreatic surface, c post-breakthrough stage with a slumping phreatic surface and stabilized air- flooded zone Environ Earth Sci 123
  3. 3. Several AS–SVE systems in Oman have been operated under essentially intermittent conditions when groundwater and free/dissolved hydrocarbons are not only non-static but undergo cyclostationary agitations interrupted by calm periods. For example, the time regime of AS–SVE at Al-Kamil site (fuel spillover from a filling station) is practiced with a frequency of sparging varying from 1 to 2 days (with several hours of air injection per day). Our experience with intermittent AS–SVE gave evi- dence that free/dissolved hydrocarbons’ concentrations at contaminated sites rebound during dormant stages. Corre- spondingly, AS–SVE has been supplemented by a pump- and-treat (P&T) system at a Rustaq site (Oman), which served, during this study, as a benchmark for assessing the effects of AS–SVE–P&T on the characteristics of the plume. The remediation efforts at Rustaq site have shown significant effect of AS–SVE–P&T in the removal of the hydrocarbon contaminants from both groundwater and the vadose zone as indicated by the analytical results of the continuous monitoring (Al-Maamari et al. 2009). Apart from their efficacy as remediation tools, AS–SVE–P&T induces changes to the hydrodynamics of the remediated aquifer. These changes vary with injection/abstraction rates and may potentially cause an undesired migration of the contaminant plume outside the radius of influence. During the remediation process in Rustaq area variable air injec- tion rates were used accompanied by multiple scenarios of operation (e.g., AS alone, AS–SVE without P&T and AS– SVE–P&T). The response of groundwater to these variable injection rates will be of interest to remediation of other sites adopting different combinations of AS, SVE, and P&T. As indirect indicators of remediation, tracer tests (measured by ER), DO, and air pressure were measured during the tests which will also provide assessment on groundwater velocity, degree of heterogeneity/anisotropy, and radius of influence. Therefore, the objectives of this study were to investi- gate and analyze the effect of AS at different injection rates on the fluids’ dynamics of a heterogeneous unconfined aquifer and to implement tracer tests/ER, DO, and air pressure as indirect indicators of remediation and tools for assessment of aquifer’s heterogeneity, anisotropy, and hydrodynamics. We present case-study data on imple- mentation of AS–SVE–P&T at different air–water injec- tion–abstraction rates. Indirect evidence of remediation by DO and air pressure measurements and contained plume (no lateral motion of groundwater from the agitated zone) by ER measurements is collected. We also measure and conceptualize pulse-type AS–SVE–P&T induced agitation of the fluids. We start from almost stagnant conditions (dormant stage) and monitor groundwater dynamics in response to an induced high hydraulic gradient (though short-duration) created by varying air injection rate. Description of study site, equipment, and protocol The study area is located in the Batinah Region of the Sultanate of Oman, about 150 km west of the capital, Muscat. It occupies approximately 7850 m2 of a low lying elevated plateau surrounded by wadis, sloped gently from south to north. Attributed to occasional rainfall, the wadis flow to the north following the topographic pattern. There is no irrigation or any other artificial input to the aquifer in the study area. The average annual rainfall in the study area is around 60 mm, direct recharge to the water table is assessed to be 5–15 mm/year (Al-Mushikhi 1999) and the evaporation rate for the existing groundwater depth ([10 m below the ground surface) is negligible. Therefore, natural descending moisture fluxes from the vadose zone can be neglected and the ascending air flow from AS wells is a major factor determining the groundwater dynamics in the remediation zone (Fig. 1). Groundwater is hosted in an alluvium layer formed during the Quaternary time and occupies the top of the geologic succession. The thickness of the alluvium increases toward the coast and is dominated by boulder/ gravels which are unconsolidated at the top and become compacted downwards, and are unconformably underlain by Tertiary limestone (Abdalla et al. 2010). The gravels are driven from the weathering of the ophiolites and the Ter- tiary limestones that are cropping out at the elevated pla- teaus flanking the study area. The limestones deposited under marine conditions whereas the ophiolites were formed when the Tethyan oceanic crust abducted against Arabian Plate in the Late Cretaceous between 90 and 105 Ma (Hanna 1995). The alluvium aquifer in the area lies under unconfining conditions and the regional groundwater flow is from the South to the North with varying hydraulic gradient. The aquifer exhibits clear heterogeneity and anisotropy owing to the spatial and vertical variations in grain size and shape. In November 1998, approximately 10,000 l of diesel were accidentally released from an above-ground storage tank, which was removed subsequent to the accident. As a corrective measure, approximately 200 m3 of soil was excavated to a depth of about 6 m below ground surface and transported off-site. The site was backfilled with a clean imported soil. In addition six polyvinyl chloride (PVC) pipes (4 in. diameter, 6 m length) were installed for venting the soil gas into the atmosphere. One AS well and four surrounding SVE wells were eventually commis- sioned. Additional ten boreholes (BHs) were drilled for monitoring purposes in addition to the one existing BH. Table 1 shows the distances of BHs with respect to the AS well, the diameters of casing, BH depths, and well screen parameters. Figure 2d indicates the positions of treatment BHs at the site (plan view). P&T injection–abstraction Environ Earth Sci 123
  4. 4. wells (BH-19 and -20) are 5.2 m apart symmetrically located down gradient of AS BH-16 that comprises the center of the contamination source. Pumped groundwater was passed through activated carbon tanks and reinjected into the aquifer up gradient at BH-41. Drilling logs and visual inspection of collected core samples showed the conglomerate–gravel–conglomerate– gravel–conglomerate sequence from the depth of 11 to 20 m beneath the ground surface, i.e., heterogeneous rock in the saturated zone. The hydraulic conductivity K deter- mined from slug tests in piezometers was in the range of 0.01–1 m/day. Water table measurements conducted before a sparging event using different triads of piezometers have indicated water depth of 13–15 m beneath ground surface. The undisturbed natural groundwater heads (counted from mean sea level) before AS operation are plotted in the map (Fig. 3). Groundwater heads indicate a general flow direction from the SSE to the NNW. Widely spaced head contours in the north compared to relatively closely spaced ones in the south indicate variation in hydraulic gradient which was estimated to be 0.015 in the south and 0.003 in the north. Variation in hydraulic heads results in variation in the groundwater natural velocity which is assessed based on an average effective porosity of 0.1 which was esti- mated based on the lithological logs with reference to Freeze and Cherry (1979) and Fetter (2000). Therefore, we estimated the maximum natural groundwater velocity at the macroscopic scale of the aquifer without consideration of natural conduits to be 0.15 m/day in the south and 0.03 m/ day in the north based on the maximum K of 1 m/day. Benzene of concentrations 5, 4, and 2.5 ppm was observed at four out of the six pre-existing PVC venting pipes located within the area between BHs 17–20. Soil samples analyzed for total petroleum hydrocarbons (TPH) gave 490 mg/kg (BH-09, at the depth of 310.5 m above sea level (maSL)), 610 mg/kg (BH-09, 308.5 maSL), 809 mg/kg (BH-16, 308.6 maSL), 770 mg/kg (BH-20, 308.6 maSL), 1456-470-959 mg/kg (BH-41, 313.1-310.6- 308.2 maSL), and 1582 mg/kg (BH-42, 307.5 maSL). Groundwater sampling and analysis started in February 2002, and indicated a contamination level of 1.3 mg/l at BH-01. A thin free product was observed in this borehole. AS–SVE plant was commissioned in November 2002, and a full (continuous) AS operation was first conducted from August 2004 to June 2005. Between November 2002 and August 2004, several intermittent AS–SVE runs (active periods lasting from several weeks to 2 months) were implemented to optimize the recovery. Figure 2a shows a contour map of groundwater con- tamination (in mg/l) plotted in July 2004, i.e., before the first continuous run (background contamination but—we recall—after several trial cycles). An off-spillover point shift of the contaminated zone in Fig. 2a is in agreement with the natural groundwater gradient prevailing during the dormant stages. Figure 2b illustrates the decontamination pattern in June 2005, after continuous AS with the rate of 100 lpm combined with SVE and P&T. Figure 2c depicts Table 1 Air sparging test borehole details BH no. Function Distance from AS (m) Casing diameter (mm) BH GL (maSL) BH depth (mbGL) Top of screen depth (mbGL) Screen length (m) Screen open area (%) 16 AS 0 20 319.57 20 18 2 4 41 MW only 1.1 100 319.61 20 8 12 20 09 TW RW 2.1 50 319.62 20 9 11 20 19 SVE/P&T 3.1 100 319.50 18 7 11 7 20 SVE/P&T 3.7 100 319.57 18 7 11 7 18 SVE 5.2 100 319.56 18 7 11 7 17 SVE 6.4 100 319.58 18 7 11 7 42 MW only 13.4 100 319.51 20 8 12 20 44 MW only 14.3 100 319.37 20 8 12 20 01 MW only 17.2 225 319.51 36 – – – 43 MW only 19.4 100 319.33 20 8 12 20 45 MW only 20.5 100 319.30 20 8 12 20 40 MW only 35.8 100 319.84 20 8 12 20 22 MW only 43.5 50 318.86 20 8 12 20 21 MW only 46.1 50 318.92 20 8 12 20 10 MW only 50.1 50 319.03 20 8 12 20 AS air sparging, MW monitoring well, SVE soil vapor extraction, TW RW treated water return well, P&T pump and treat, GL ground level, maSL meters above sea level, mb meters below Environ Earth Sci 123
  5. 5. contamination’s rebound in December 2005 (after 6 months of dormancy in AS remediation). Changes in groundwater levels (GWLs), DO levels, and borehole air pressures were all monitored at AS rates of 10, 25, 50, 100, and 200 l per minute (lpm). Groundwater levels were measured using In Situ Minitroll and Solinst Levelogger Model 3001 water level probes. For tracer tests at AS rates 10, 25, and 50 lpm, table salt was introduced into BH-09 and electrical resistivity (ER) was monitored in the same borehole, up gradient of the AS BH-16. For tracer tests at AS rate 100 lpm, salt was introduced into BH-19 and BH-43, and ER was monitored in the same boreholes, down gradient of BH-16. (see Fig. 2 for BH locations.) Salt was introduced by placing it in a small cloth sack attached BH01 BH09 BH10 BH16 BH17 BH18 BH19 BH20 BH21 BH22 BH40 BH41BH42 BH43 BH44 BH45 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 (a) (b) (c) (d) 0 10 20 30 40 m 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13 BH01 BH09 BH10 BH16 BH17 BH18 BH19 BH20 BH21 BH22 BH40 BH41BH42 BH43 BH44 BH45 0 10 20 30 40 m 0 5 10 15 20 25 30 35 40 45 50 55 60 65 BH01 BH09 BH10 BH16 BH17 BH18 BH19 BH20 BH21 BH22 BH40 BH41BH42 BH43 BH44 BH45 0 10 20 30 40 m BH09 Treated Water Return BH16 AS BH17 SVE BH18 SVE BH19 SVE/P&T BH20 SVE/P&T BH41 0 1 2 3 4 m Fig. 2 Contour maps of groundwater TPH contamination (mg/l) at different stages of treatment and a plan view of the air sparging area. a July 26, 2004-Before AS100/SVE/P&T. b June 27, 2005-After AS100/SVE/P&T. c Dec 24, 2005-Rebound after P&T. d Details of AS area Environ Earth Sci 123
  6. 6. to a weighted rope and moved up and down within the saturated zone in the boreholes until all salt was dissolved. Electrical resistance was measured by using digital groundwater conductivity tester, Kyowa KCM-200C, at 50 cm depth intervals within the same boreholes where the salt was introduced. Salt tracer was assumed to be flowing out of the borehole as ER increased. Changes to ER under natural conditions were compared with changes to ER under different AS rates to indicate the effect of AS on groundwater flow. DO measurements during two AS experiments with rate 100 lpm, with and without SVE and P&T were taken. DO was measured by in Situ MPTroll 9000 water quality probe. Borehole air pressure measured directly by pressure gauge from borehole covers was recorded during six tests at AS rates 50, 100, and 200 lpm with and without SVE and P&T. Results and interpretation The GWLs were monitored in BHs 1 and 17–20, for AS rate 200 lpm. GWLs rose to peak levels within the first 5 min of AS operation, after which they began to decline after 15–30 min from the start, stabilizing after 1.5 h (Fig. 4). The effect was found greatest in descending order, for BH-18 (5 m from AS BH-16), for BHs 19–20, (3–3.5 m from AS BH-16), for BH-17 (6.5 m from AS BH-16), and for BH-01 (17 m from AS BH-16). Other than for BH-18, for which there must be a special connectivity to AS BH- 16, the effect of AS was proportional to the distance from BH-16. Stabilized GWLs after 1.5 h of operation ranged from 0.2 to 0.6 m higher than the undisturbed GWL. Similar rise and decay of the mound was monitored in BHs 1, 17, and 20, for AS rates 50 and 100 lpm (Fig. 5), and for smaller injection rates (not shown). The maximum rise of GWL during AS with a rate of 200 lpm was 1.1 m observed in BH-18 (Fig. 4) which is located in the northern part that is characterized by a lower hydraulic gradient (Fig. 3). Consequent to GWL rise, the hydraulic gradient in the north changed to 0.013 (taking BHs 18 and 10 as reference) resulting in a maximum velocity of 0.13 m/day. Such velocity indicates a larger travel time of groundwater significantly beyond the normal duration of AS. Therefore, in case of a homogeneous iso- tropic aquifer the uncontrolled migration of the plume due to AS is unlikely. However, increased velocities through conduits related to aquifer’s heterogeneity and anisotropy shall be assessed (here we used tracer tests, ER, DO, and air pressure as tools for such an assessment). Realizing three possible types of motion of injected air (indicated above) and multiphase dynamics of three involved phases we upscale the monitored dynamics of the water table in the manner related to observable (by avail- able instruments) groundwater characteristics (Fig. 1). At the very early stage (Fig. 1a), air plume ABC expands around the screen of BH-16. The plume boundary can be considered as a classical free boundary (Muscat 1949; Nikolskii 1961; Polubarinova-Kochina 1977) similar to oil–water and gas–water contacts in petroleum engineering models of secondary recovery (Charny 1963). The free boundary expands from a ‘‘point’’ source (the screen of BH-16). Although water displacement in the plume is not complete, the Muscat (1949) model postulates that flow inside and outside the plume are one-phase with an abrupt jump of viscosity and density of the moving phases (air and groundwater) across the plume boundary. At this earliest stage, outside ABC groundwater moves as a result of almost radial displacement close to the expanding interface between a high air-pressure zone and nearby groundwater (at almost hydrostatic conditions). Far away from the AS source, groundwater dynamics are affected by gravity similarly to classical problems of sink-induced phreatic surface flows (Polubarinova-Kochina 1977). What makes our problem mathematically much more complicated than the Muscat problem in reservoir engineering (Muscat 1949; Polubarinova-Kochina 1977) is the existence of the second free boundary, viz. the water table, which forms a transient Fig. 3 Undisturbed natural groundwater heads in the study area before AS operation (not to scale to show the individual boreholes in the air sparging area) Environ Earth Sci 123
  7. 7. (rising-decaying) mound. At a certain time (Fig. 1b) the plume apex A and C reaches the water table and the injected air breaks through into the vadose zone, i.e., points A and C separate. We interpret this moment as the peak time tp which is found to be 24.25, 11, 12.5, 24, and 11.9 min in BHs 01 and 17–20 (Fig. 4), respectively. After tp, the BH hydrographs show a recession limb, which corresponds to the slumping stage of the mound (Fig. 1c). During this stage the air flow is almost steady-state from the source to the vadose zone but groundwater is still adjusting to the created excess air-pressure by transferring the volume of water from the displaced plume (the interior of ABC in Fig. 1c) i.e., groundwater spreads laterally from the crater-shaped mound. The contaminants move radially from BH-16 together with groundwater during the period of effacing of the AS-up welled periphery of the ‘‘crater’’ (Fig. 1c). It explains the purpose of P&T as a hydrody- namic barrier to this spread. As Fig. 4 peaks show the created hydraulic gradients are very high (*0.1) but short- lived due to the decay of the mound. The pulse-type convective displacement of groundwater particles based on the assessed hydraulic conductivity and corresponding linear average velocity will not exceed several tens of centimeters during one injection pulse. We note that the most pronounced humping of the mound is detected in different pairs of monitoring wells (1 and 2 in Fig. 1). The maximum of GWL in BH-01 (Fig. 4) is delayed as it should be in mound dynamics (Kacimov 1997) because BH-01 is much farther from the source of disturbance than other wells in Fig. 4. Clearly, the lower the injection rate the smaller is the amplitude of the peak in all wells. When AS is stopped the well hydrographs show a blip with a minimum reached at tm (time minimum) = 7.3, 7, 9.5, 5, and 7.2 min in BHs 01 and 17–20, correspondingly (Fig. 4). This is in qualitative congruity with the dynamics of phreatic surface troughs, which is up well to the initial flat position (Kacimov et al. 2009). The converging hydraulic gradient—upon cessation of sparging—may be responsible for the rebound of concentration observed in the AS zone (Fig. 2c). The GWLs from seven BHs 17–20, 41–42, and 44 were observed when both AS and P&T were operated for AS rates of 10, 25, 50, 100, and 200 and P&T pumping rate of 36 lpm. There was a high water level rise at the beginning of AS/P&T operation similar in pattern to the water table rise observed during AS operation but of lesser magnitude attributed to the effect of P&T. Salt (conservative tracer) was introduced and ER mea- surements were taken without AS and at AS rates of 10, 25, and 50 lpm, to investigate the effect of AS on groundwater flow in BH-09 ER measurements started when GWL sta- bilized, i.e., 60–90 min after commencing AS operation. Figure 6 shows ER as a function of depth without AS (solutes subject to the natural groundwater flow only). ER Elapsed Time (minutes) ChangeinGroundwaterLevel(m) Fig. 4 Groundwater levels at AS rate 200 lpm Elapsed Time (minutes) ChangeinGroundwaterLevel(m) Fig. 5 Groundwater levels at AS rates 50 and 100 lpm Environ Earth Sci 123
  8. 8. increases more rapidly at around 14–15 and 17 mbGL, confirming the existence of a relatively rapid groundwater flow at these depths and, consequently, layer-type hetero- geneity found from core-samples and drilling logs in this well. The dynamics of ER in BH-09 at the depth of 15 mbGL is shown in Fig. 7 for AS rates of 10, 25, and 50 lpm. For no-AS operation, ER rises immediately and after some 70 min, the water in the BH is almost completely replaced by natural groundwater. In contrast, this replacement does not progress as rapidly during AS operation, indicating reduced groundwater flow. The ER recovery becomes slower as the AS rate increases. Similar ER recovery curves were observed at every depth, including the layer of 17 mbGL that can be attributed to the effect of the air-flow zone in Fig. 1c i.e., BH-09 depending on the AS rate can be located either inside this zone (and then migration of salts from the tracered well is basically diffusive through the residual water) or close to the air–groundwater interface ABC. The ovalic (in a vertical section of Fig. 1) sparging zone acts as an apparent impermeable boundary for the natural groundwater flow and thereby impedes the advec- tive transport near the tracered well. Salt as a tracer was introduced in BH-19 (3 m away from AS BH-16) and BH-43 (19.5 m away from AS BH- 16), and ER measurements were taken at AS rate 100 lpm. The objective of this experiment was to assess the potential increase of groundwater velocity (dissolution of injected salts) in the wells far from the AS point. The increase of ER in BH-19 was decelerated by AS similarly to BH-09 (Fig. 7) while in BH-43 ER dynamics was unchanged as compared with the natural conditions, indicating that the ROI ends somewhere between 3 (BH-19) and 19.5 m (BH-43). The DO was measured at five different depths (0.05, 0.5, 1, 2, and 5 m below the water table (mbWT)) in fifteen BHs 1, 9, 10, 17–22, and 40–45, without AS and at AS rate 100 lpm. DO increase with AS in all monitored BHs from 4–6 to 6–9 mg/l. The increase in DO levels was greater in the BHs located closer to BH-16. ROI inferred on the basis of DO extended as far as 50 m, as evident from the increase of DO at BH-10. For the most general case of AS–SVE–P&T (100–150 lpm at four SVE BHs-26 lpm, correspondingly), at BHs in proximity to the SVE and P&T, DO levels were lower by 0.5–2.5 mg/l than those when AS was applied without SVE and P&T. However, for BHs more distant from the SVE and P&T BHs, SVE and P&T have little or no reducing effect on DO levels compared with the AS- only scenario. Borehole air pressure was measured at fourteen BHs 9, 10, 17–22, 40–45, at AS rates of 50 and 200 lpm, both without and with SVE–P&T (150 lpm at four SVE BHs- 26 lpm, correspondingly). For AS Rate 50 lpm without SVE and P&T, pressure increase was observed at BHs 9 and 17 only, but not at any other BHs including BH-41, the closest BH to AS BH-16 (1 m away). For AS rate of 200 lpm without SVE and P&T, pressure increase was observed at BHs 9, 17–20, 21 (46 m away from AS BH-16) and 45 (20.5 m away from AS BH-16). We surmise that in a single-source AS operation of Fig. 1c (and without liner covering the soil surface) air injected escapes more or less uniformly through the vadose zone and such that only at high AS rates the air pressure rises significantly in the boreholes. With added SVE four-sink operation, pressure in the contamination area became negative with an exception of positive pressure at BHs 21 and 45, at AS rate of 200 lpm. Discussion and conclusion In this article, experimentation at several AS rates in combination with SVE and P&T at a diesel contaminated site in Oman is presented. Conventionally, it is considered that AS is applicable only in homogenous formations while Depth(m) Fig. 6 Tracer test: changes in ER in salt-infused borehole (BH-09) with time (no AS) Elapsed Time (Minutes) Fig. 7 Tracer tests: ER in BH-09 (15 mbGL) for AS rates of 10, 25, and 50 l/min Environ Earth Sci 123
  9. 9. the tested site exhibited strong heterogeneity of the satu- rated zone. As debated by Mei et al. (2002) and McCray and Falta (1996) even for steady-state AS or AS–SVE the definition of ROI of the remediated zone is ambiguous. In transient or intermittent AS–SVE regimes (switching on and off the injection well and playing with the rates and combinations of AS–SVE and P&T) this ambiguity calls for thorough monitoring of direct indicators of remediation (TPH in groundwater) at different stages of intermittency and in different BHs and of indirect indicators of flow and transport, viz. phreatic surface fluctuations, tracer move- ment, and air/oxygen pressure/concentration. Turning on an AS source under an almost flat phreatic surface causes a strong wave-type GWL response. Crater- shaped groundwater mound was detected with a rapid growth of the water table and relatively slow decline after an air plume had established from a continuously injected point source of AS. The lateral hydraulic gradient caused by the upwelling of groundwater on the periphery of the air plume may generally induce contaminated water flow from the original contamination zone faster than without AS but in the tested regimes this was not the case because of a short time of existence of a single-hump mound in Fig. 1a and a relatively rapid (tens of minutes) recession in Fig. 1c. Moreover, water table upwelling caused by SVE in Fig. 1c may serve as another regional impediment to the natural groundwater flow. Stauffer et al. (2009) upscaled injection from individual paths of air, which can be monitored in laboratory experiments only, to a macroscale (‘‘air plume’’). Our upscaling is even cruder: it extends to observation of free surface dynamics only. Tracer tests (measured by ER) illustrated the replacement speed of wellbore storage by groundwater in a BH-09 located 2 m up gradient of the AS well. The dissolution rate of injected salts decreases with an increase of the AS rate, i.e., AS reduces the natural groundwater velocity in the vicinity of the AS well (similar effect is reported by Wardwell (1999)). DO levels increased with AS and DO-based ROIs of at least 50 m were monitored. At AS rate of 50 lpm, increases in vadose zone air pressure due to AS were counteracted by SVE and P&T. However, at an AS rate of 200 lpm, even with SVE and P&T, increased pressure was still evident in outlying BHs as far as 46 m from the sparging source. Remediation of strongly heterogeneous and anisotropic aquifers can potentially cause uncontrolled release of contaminants outside the ROI. Tracer tests, ER, DO, and air pressure can be effective tools in defining the ROI extension and at the same time to assess aquifer’s hetero- geneity and anisotropy. Overall, AS operations at a single air sparging well drilled in a heterogeneous formation showed a significant reduction of contamination in an unconfined aquifer. Combined with SVE and P&T, any subcatchment-scale spread in contamination due to AS is highly unlikely. Acknowledgments The authors would like to thank Japan Coop- eration Center, Petroleum (JCCP) for their generous sponsorship of this project, and Ministry of Regional Municipalities and Water Resources (MRMWR), Oman for their contribution to the project. References Abdalla OAE, Ali M, Al-Higgi K, Al-Zidi H, El-Hussain I, Al-Hinai S (2010) Rate of seawater intrusion estimated by geophysical methods in arid area of Al Khabourah, Oman. Hydrogeol J 18:1437–1445. doi:10.1007/s10040-010-0606-0 Adams JA,Reddy KR (2000) Removal of dissolved–and free-phase benzene pools from groundwater using in situ air sparging. J Environ Eng 126(8):697–707 Al-Maamari RS, Hirayama A, Sueyoshi MN, Abdalla OAE, Al- Bemani AS, Islam MR (2009) The application of air-sparging, soil vapor extraction and pump and treat for remediation of a diesel-contaminated fractured formation. Energy Sources, Part A 31:911–922. doi:10.1080/15567030801904236 Al-Mushikhi AAM (1999) Formation of subsurface water of the East- South Batinah Region (Sultanate of Oman) and perspectives of development. PhD thesis, Moscow State Geoexploration Uni- versity, Moscow (in Russian) Angell KG (1992) In situ remedial methods: air sparging. Natl Environ J 2(1):20–23 Balcke GU, Paschke H, Vogt C, Schirmer M (2009) Pulsed gas injection: a minimum effort approach for enhanced natural attenuation of chlorobenzene in contaminated groundwater. Environ Pollut 157:2011–2018 Bass DH, Hastings NA, Brown RA (2000) Performance of air sparging systems: a review of case studies. J Hazard Mater 72:101–119 Berkey JS, Lachmar TE, Doucette WJ (2003) Tracer studies for evaluation of in situ air sparging and in-well aeration system performance at a gasoline-contaminated site. J Hazard Mater 98:127–144 Braida W, Ong SK (2001) Air sparging effectiveness: experimental characterization of mass transfer zone for VOC volatilization. J Hazard Mater 87(1–3):241–258 Brown RA, Jasiulewicz F (1992) Air-sparging used to cut remediation costs. Pollut Eng, July, pp 52–57 Chao K-P, Ong SK, Huang M-C (2008) Mass transfer of VOCs in laboratory-scale air sparging tank. J Hazard Mater 152:1098–1107 Charny IA (1963) Subterranean hydrogas-dynamics. Gostoptekhizdat, Moscow (in Russian) Fetter CW (2000) Applied hydrogeology fourth edition. Prentice Hall International, Englewood Cliffs, p 598 Freeze RA, Cherry JA (1979) Groundwater. Prentice Hall, Englewood Hills Geistlinger H, Krauss G, Lazik D, Luckner L (2006) Direct gas injection into saturated glass beads: transition from incoherent to coherent gas flow pattern. Water Resour Res 42:W07403. doi: 10.1029/2005WR004451 Hanna SS (1995) Field guide to the geology of Oman. Western Hajar Mountains and Musandam, vol 1. The Historical Association of Oman, Ruwi, p 180 Jang W, Aral MA (2009) Multiphase flow fields in in situ air sparging and its effect on remediation. Transp Porous Med 76:99–119. doi:10.1007/s11242-008-9238-4 Environ Earth Sci 123
  10. 10. Ji W, Dahmani A, Ahlfeld DP, Lin JD, Hill E (1993) Laboratory study of air sparging: air flow visualization. Ground Water Monit Rem 13(4):115–126 Johnson PC (1998) Assessment of the contributions of volatilization and biodegradation to in situ air sparging performance. Environ Sci Technol 32(1):276–281 Kacimov AR (1997) Dynamics of ground water mounds: analytical solutions and integral characteristics. Hydrol Sci J 42(N3):329–342 Kacimov AR, Sherif MM, Al-Jabri S, Al-Shidi S (2009) Slumping of groundwater mounds: revisiting the Polubarinova-Kochina the- ory and modeling by analytic element method. J Hydrol Sci 54(1):174–188 Kim H, Choi KM, Moon JW, Annable MD (2006) Changes in air saturation and air–water interfacial area during surfactant- enhanced air sparging in saturated sand. J Contam Hydrol 88:23–35. doi:10.1016/j.jconhyd.2006.05.009 Marley MC, Hazebrouk DJ, Walsh MT (1992) The application of in situ sparging as an innovative soil and groundwater remediation technology. Groundwater Monitor Remediat 12(2):137–144 McCray JE, Falta RW (1996) Defining the airsparging radius of influence for groundwater remediation. J Contam Hydrol Ground Water 24:25–52 Mei CC, Cheng Z, Ng CO (2002) A model for flow induced by steady air venting and air sparging. Appl Math Model 26:727–750 Muscat M (1949) Physical principles of oil production. McGraw Hill, New York Ng C-O, Mei CC (1996) Aggregate diffusion model applied to soil vapor extraction in unidirectional and radial flows. Water Resour Res 32:1289–1297 Nikolskii IS (1961) Seepage of air from a horizontal drain laid under a reservoir bottom. Fluids Dyn, pp 181–183 (in Russian) Peterson JW, Lepczyk PA, Lake KL (1999) Efect of sediment size on area of influence during groundwater remediation by air sparging: a laboratory approach. Env Geol 38(1):1–6 Peterson JW, Murray KS, Tulu YI, Peuler BD, Wilkens DA (2001) Air-flow geometry in air sparging of fine-grained sands. Hydrogeol J 9:176–188 Philip JR (1998) Full and boundary-layer solutions of the steady air sparging problem. J Contam Hydrol 33:337–345 Polubarinova-Kochina PYa (1977) Theory of ground-water move- ment. Nauka, Moscow (in Russian) Reddy KR, Adams JA (2001) Effects of soil heterogeneity on airflow patterns and hydrocarbon removal during in situ air sparging. J Geotech Geoenviron Eng 3:234–247 Reddy KR, Kosgi S, Zhou J (1995) A review of in situ air sparging for the remediation of voc-contaminated saturated soils and ground- water, Hazard. Waste Hazard Mater 12(2):97–118 Selker JS, Niemet M, McDufiie NG, Gorelick SM, Parlange J-Y (2007) The local geometry of gas injection into saturated homogeneous porous media. Transp Porous Med 68:107–127 Stauffer F, Kong X-Z, Kinzelbach W (2009) A stochastic model for air injection into saturated porous media. Adv Water Res 32:1180–1186 Tomlinson DW, Thomson NR, Johnson RL, Redman JD (2003) Air distribution in the Borden aquifer during in situ air sparging. J Contam Hydrol 67(1–4):113–132 Tsynkova OE, Myasnikova NA, Baishev BT (1993) Hydrodynamic methods of enhanced oil recovery. Nedra, Moscow (in Russian) Wardwell DA (1999) Groundwater flow sensor monitoring of air sparging. In: Proceedings from the fifth international in situ and on-site bioremediation symposium, in situ bioremediation of petroleum hydrocarbon and other organic compounds, pp 47–57 Environ Earth Sci 123