Effect of air migration on volatile contaminant concentration in subsurface environment

 

N. EGUSA, T. HIRATA

 

Dept. of Environmental Systems, Wakayama University

930 Sakaedani, Wakayama 640-8510, Japan

TEL:+81-734-57-8331, FAX:+81-734-57-8335

e-mail:egusa@sys.wakayama-u.ac.jp

 

K. FUKUURA and T. MATSUSHITA

 

Maezawa Industries Inc.

5-11 Nakamachi, Kawaguchi 332-8556, Japan

TEL:+81-48-253-0910, FAX:+81-48-254-2328

e-mail:dojyou@beige.ocn.ne.jp

 

 

ABSTRACT

The in-situ air sparging technology devised to facilitate contaminant volatilization and to shorten a remediation period with air injected into groundwater, was applied to the site contaminated with volatile substances like tetrachloroethylene, toluene etc. In this site, the short-term pilot test for air sparging was carried out to examine the migration of injected air in groundwater, the radius-of-influence of injected air and the change of contaminant concentration. The result showed that injected air flowed smoothly and parabolically around the injection well in groundwater and the radius-of-influence was about 5 m at groundwater table. In addition, contaminant concentration declined in some monitoring wells which are placed within the radius-of-influence.

 

Keywords: groundwater contamination, volatile substance, remediation technology, in-situ air sparging, short-term pilot test

 

INTRODUCTION

As a remediation technology for subsurface contaminated by volatile substances, recently, instead of groundwater extraction, air sparging coupled with soil vapor extraction is developed and applied to remove a contaminant from subsurface environment (Lundegard and LaBrecque, 1995). The groundwater extraction technology needs water treatment with aeration on the ground. Contrary to this, the air sparging technology facilitates contaminant volatilization with air injected into groundwater, and removes contaminants in injected air by soil vapor extraction. Consequently, the advantages offered by this technology are that water disposal is eliminated and the same facilities of soil vapor extraction are used. However, groundwater flow is stirred by air injected into groundwater, so there is a potential risk that contamination area is enlarged due to artificial diffusion in subsurface environment (Johnson et. al., 1993). In this context, this study is designed to understand the mechanism of gas-water two phase flow concerning volatilization in subsurface environment. This paper describes the results of the short-term pilot test to establish the air sparging technology.

 

FIELD MEASUREMENT

The short-term pilot test for air sparging was applied to the site contaminated with volatile substances like tetrachloroethylene, toluene etc. These have been utilized to produce industrial chemicals for many years. Fig.1 illustrates the vertical geological feature and the locations of soil vapor extraction well, air injection wells, and monitoring wells both in the unsaturated and groundwater zones. The permeability test confirmed that unconfined shallow groundwater is nearly isotropic, so that each monitoring well was placed in the same depth to observe the migration of injected air and the change of contaminant concentration. The contaminants penetrated perched groundwater to shallow groundwater, and consequently groundwater concentration became high even at GL.-12.0m. Therefore, air sparging coupled with soil vapor extraction was employed to clean up shallow groundwater in this site.

 

 

Table 1 Experimental conditions for short term pilot test.

 

 

Fig.1 Vertical geological feature and locations of extraction well, injection wells and monitoring wells in a same section

 

At the beginning, the mounding test was implemented to reveal the detailed air migration in groundwater and the radius-of-influence of injected air. The air injection rate from one injection well (Sd 2) was 120 L min-1, and the operation has continued for an hour. In this test, the pressure head of shallow groundwater was measured in each monitoring well. On the other hand, the short-term pilot test for air sparging was conducted to examine the change of groundwater concentration due to volatilization into injected air. The short-term operations were carried out four times (see Table 1), and each operation time was 20 hours. The first operation (RUN 1) has injected air from the time of 11 00 on 25th March to 07 00 on 26th March (RUN 1-1), and from 14 00 on 26th March to 10 00 on 27th March (RUN 1-2) in Sd 1 well. During the operation of RUN 2, air has been injected from 11 00 on 1st April to 07 00 on 2nd April (RUN 2-1), and from 14 00 on 2nd April to 10 00 on 3rd April (RUN 2-2) in Sd 2 well. The air injection rate was 120 L min-1 and the extraction rate of soil gas was up to 180 L min-1. Soil vapor extraction started in February, 1997 prior to the mounding and the pilot tests of air sparging, and has been continuously undertaken. The contaminants in extracted soil vapor was treated with activated carbon adsorption.

 

RESULTS AND DISCUSSIONS

 

INJECTED AIR MIGRATION IN GROUNDWATER

Fig.2 illustrates the time-varied changes of pressure head in monitoring wells during the mounding test. The pressure heads in many monitoring wells begin to rise just after air injection starts (at 10 00). After air injection stops (at 11 00), they decline smoothly and then gradually get back to the pre-sparging values. The pressure head pattern depicts clearly that its rise is becoming smaller, being away from the injection well (Sd 2). In Md 2-2 well, very close to the injection well, there can be seen the largest pressure head rise among the monitoring wells. And the pressure head has been under the pre-sparging value for a few hours after air injection stopped. The same patterns can be recognized in some wells, however, there is the most remarkable in Md 2-2 well. In Md 3-1 to -3 wells, the large pressure head rise is produced in deeper well. The values are nearly equal to in Md 1-1 and -2 wells which are placed symmetry to the injection well. This fact shows that, shallow groundwater is in nearly isotropic condition. In Md 4-1 to -3 wells, which are placed at the lateral distance of 14 m from the injection well, there are no pressure head rises. Consequently, there is no influence of injected air flow 14 m away from the injection well, which means the radius-of-influence of injected air to be less than 14 m.

 

 

Fig.2 Time varied change of pressure head during mounding test

 

 

Fig.3 Schematic illustration of pressure head rise and hydraulic gradient between monitoring wells

 

Fig.3 illustrates the pressure head rises and vertical hydraulic gradients of groundwater between monitoring wells when air injection stops (at 11 00 in Fig.2). Based on no difference of initial pressure head in monitoring wells, the pressure head rise during air injection makes the hydraulic gradient between wells. The hydraulic gradient during air injection exhibits that groundwater is stirred by air injection and injected air flows in the area. In Md 3-1 to -3 wells which are laterally 4 m away from the injection well, the section between Md3-2 and -3 wells is closer to the injection well than between Md 3-1 and -2 wells, however, the vertical hydraulic gradient in the former section is one fourth of the latter. This fact shows that, at the place of 4 m away from the injection well, injected air does not mainly flow at the deeper area of shallow groundwater, i.e. extends parabolically from the injection well due to the balance between injected air pressure and buoyancy force in groundwater. In other words, injected air flows parabolically upwards in narrow area around near Md 2-2, Md 1-2 and Md 3-2 wells, and reaches the unsaturated zone.

 

Fig.4 Time varied changes of contaminant concentration in groundwater

 

CHANGE OF GROUNDWATER CONCENTRATION

Fig.4(a)-(f) demonstrates the time-varied changes of contaminant concentration in groundwater. There is a little change of contaminant concentration in each monitoring well during RUN 1. Considering no changes of contaminant concentrations in Md 7-1 to -3 wells, injected air does not mainly flow at the place which are laterally 5 m away from the injection well (Sd 1). Therefore, the radius-of-influence of injected air at groundwater table is less than 5 m in this pilot test. In RUN 2, contaminant concentrations clearly decline in Md 2-1 to -3 and Md 3-1 wells, which are placed within the radius-of-influence of injected air. In the contrast of this, there can be recognized no concentration changes in Md 3-2 and -3 wells where injected air does not mainly flow due to the mounding test. Therefore, the area that contaminant concentrations decline with volatilization into injected air can be easily presumed from the radius-of-influence of injected air due to the mounding test.

The concentrations in Md 2-1 to 3 wells, which are laterally in the closest position to the injection well (Sd 2) in RUN 2, tend to recover after air injection stops. Ji et al.(1993) pointed out that narrow air channels are formed in the domain of the radius-of-influence of injected air by air injection in groundwater. Because the contaminants are only removed near air channel with volatilization into injected air, the removal area of contaminants dose not extend in subsurface in the case that injection time is short. And consequently, after air injection stops, it seems for the concentrations in these wells to recover the previous values under the influence of the high concentration in the surroundings.

 

CONCLUSION

The short-term pilot test for air sparging coupled with soil vapor extraction was applied to the site contaminated with volatile substances. It resulted in making understandings of the details of injected air migration, the radius-of-influence of injected air and the change of groundwater concentration. After this, the full scale field experiment is now beeing undertaken to interpret the detail effect for air sparging and to achieve the evaluation as a remediation technology, considering the injected air migration, and the radius-of-influence obtained by this pilot test.

 

REFERENCES

Ji, W., A., A. Dahmani, D.P. Ahlfeld, J.D. Lin and E, Hill III, 1993. Laboratory study of air sparging: air flow visualization, Groundwater Monitoring and Remediation, Fall 1993, pp.115-126.

Johnson, R.L., Johonson, P.C., McWhorter, D.B. and Goodman I., 1993. An overview of in situ air sparging, Groundwater Monitoring and Remediation, Fall 1993, pp.115-126.

Lundegard, P.D. and LaBrecque, 1995. Air sparging in a sandy aquifer (Florence, Oregon, U.S.A.): Actual and apparent radius of influence, Journal of Contaminant Hydrology, Vol.19, pp.1-27.