1Dept. of Chem. Eng., Kansas State University, Manhattan, KS 66506, Phone: (785)532-4324, FAX: (785)532-7273, 2Dept. of Biochemistry, Kansas State University, Manhattan, KS 66506, Phone: (785)532-6124, FAX: (785)532-7278
To study the effect of air sparging in soil with trichloroethylene present as a dense nonaqueous phase, air was supplied through pipes installed at the bottoms of two chambers planted with alfalfa. Air input rate was 2.14 L/m2/day. The fate of trichloroethylene (TCE) was investigated by monitoring TCE concentration in both outflow groundwater and soil gas. Comparison of these results with those of our previous study without air sparging indicates that air sparging appreciably increases the groundwater concentration of TCE. The soil gas at the surface shows even greater concentration difference. The flux of TCE to the atmosphere is increased significantly by air input. Accordingly, we can conclude that air sparging improved mass transfer of TCE from the nonaqueous phase to groundwater phase. Air sparging appeared to negatively impact the health of the alfalfa because of the elevated TCE present in the vadose zone of the chamber.
Gasoline, petroleum products, and industrial solvents have contaminated aquifers and ground water at many sites because of spills or leakage of underground storage tanks. These contaminants are sparingly soluble in water and exist at contaminated sites mostly as nonaqueous phase liquids (NAPLs). The NAPLs may move from the site and enter adjacent ground water. If they are denser than water, they will migrate downward and come to rest and remain as a pool at the bottom of the aquifer. This pool becomes a long-term source of water pollution because the contaminants will continue to enter the water phase to replace that which is transported away from the site, degraded, or removed by some remediation technology.
A number of studies have shown that vegetation can enhance the biodegradation of contaminated groundwater and soils (Davis et al., 1993a; Davis et al., 1993b; Banks and Schwab, 1993; Narayanan et al., 1996; Zhang et al., 1996). Our previous study (Zhang et al., 1996) has indicated that alfalfa plants can grow well in the presence of trichloroethylene (TCE), and that vegetation can, to certain extent, improve TCE transformation or immobilization within the planted chambers. In situ air sparging (IAS) is a remediation technology primarily applied to the removal of volatile organic contaminants (VOCs) or biodegradable organic compounds from groundwater aquifers. Extreme care must be exercised in designing and implementing the air sparging system so that the contaminants are removed efficiently and without adverse effects on the subsurface environment, particularly the spread of the contaminant to the clean area (Lord et al., 1995; Reddy et al., 1995).
In order to study the effect of air sparging on the fate and transport of TCE under vegetation circumstances, we injected air through pipes installed at the bottoms of two planted channels. The groundwater concentration of TCE and gas flux rates into atmosphere were monitored. Experimental results and comparison of these results with those without air sparging are presented here.
The schematic diagram of the chamber system used in this study is shown in Figure 1. Features of this system were described previously (Zhang et al., 1996). The only difference here is that air pipes were installed at five positions along the bottoms of channels 1 and 6. At the buried end of each pipe, an alumina gas dispersion stone was connected to distribute air into very fine bubbles.
Because TCE, perchloroethylene (PCE), and other chlorinated solvents frequently make up at least 1% to 2% of the NAPL (Boersma et al., 1995), trichloroethylene (TCE) was selected as a representative compound. Initially 20 ml (29.2 g) TCE pure liquid was injected to the inlets at the bottoms of five channels (Ch 1,2,4,5 and 6). Channel 3 was used as control to examine the effects of contaminants on plants, and channel 4 was unplanted so as to compare the beneficial impact of vegetation on the removal of TCE.
According to the stoichiometric relationship for aerobic biodegradation of TCE:
2.25 moles of oxygen are consumed for each mole of TCE degraded. On a mass basis, this equates to one mass unit of TCE degraded for every 0.55 mass units of oxygen consumed. Therefore, in any unit volume, only 14.6 mg/L of TCE may be biodegraded from a maximum initial dissolved oxygen concentration of 8.0 mg/L at saturation. As a consequence, 29.2 g TCE needs 29.2*0.55 g O2, which is equal to 11.2 L of oxygen gas under standard state. If we assume that oxygen can be transferred to water by air sparging, then 11.2 L/0.21 = 53.3 L air is necessary to completely biodegrade the 29.2 g TCE in each channel. However, due to the limited mass transfer of both O2 and TCE into water, this amount of air can only serve as the minimum estimation. In practice, air injection was controlled by a syringe pump at a flow rate of 0.115 ml/min through each of five pipes and 0.235 L/day per channel was supplied. This flow rate corresponds to a superficial velocity of 0.214 cm/day. Thus, in 240 days, sufficient air would be introduced to fully degrade the input TCE. Based on results in Zhang et al. (1996), a somewhat higher rate would be needed to obtain complete aerobic degradation in the saturated zone at the largest rates of dissolution of the NAPL.
To measure the gas phase TCE flux rates from each channel soil surface into the atmosphere, six identical one-end-opened containers, 420 ml in volume and 38.4 cm2 in cross sectional area, were placed along the top of each channel at six different positions. Gas samples were taken from each container 40 minutes after placement. Sample compositions were then analyzed using a gas chromatograph equipped with a flame ionization detector.
By assuming and having verified that the TCE accumulation in the gas collection containers was linear within 40 minutes, we obtained the gas phase TCE flux rate data using the following equation:
where NTCE is the flux rate (mmoles/m2day), CTCE and CDCE are TCE and DCE concentrations (mM) in the gas containers measured after 40 minutes of accumulation, V is the container volume (L), A is the container cross sectional area (m2), and t is the accumulation time which was chosen as 40 minutes.
Groundwater samples were collected at the exit sampling ports (see Figure 1) and then analyzed for TCE concentration in the groundwater using head space analysis (Zhang et al., 1996). Mass balances of TCE were estimated using numerical integration of the aqueous TCE concentration versus effluent volume profiles and the estimated daily flux rates from the channel soil surfaces.
RESULTS AND DISCUSSIONS
The growth of alfalfa plants was negatively impacted in channels 1 and 6 during the early period of air sparging because of the elevated TCE present in the vadose zone of the chamber. After most of the injected TCE had been transported out, alfalfa plants began to recover. The monthly biomass production during exposure to TCE in groundwater is plotted in Figure 2, which shows the plant growth with and without air sparging and with and without contamination. The experiments of Ryu et al. (1996) show that exposure of whole plants to even 1 mM TCE leads to toxicity, though 1 mM TCE is tolerated by the root system for at least one day according to the work of Makepeace et al. (1996).
The comparisons of effluent TCE concentrations are shown in Figures 3,4,5 and 6, where Figures 3 and 4 are the concentration relationships with respect to time, and Figures 5 and 6 are concentration versus effluent water volume. Each figure includes the results with and without air sparging. Air sparging appreciably enhanced TCE mass transfer from the NAPL into the aqueous phase; during the first 55 days or in the first 22 L effluent water, the TCE aqueous concentration is much higher with than without air sparging.
Figures 7 and 8 represent the gas phase TCE fluxes into the atmosphere measured at six locations above the soils of channels 1,2,4,5, and 6. Dichloroethylene (DCE) was detected as well as TCE. The summation of TCE and DCE concentrations was counted as the total chlorinated solvent concentration because DCE was believed to be the product of TCE anaerobic degradation, which is an equimolar reaction.
When the aqueous phase TCE concentrations from the channels were larger, the gas flux rates of Ch1 and Ch6 were higher than the other channels (see Figure 7). But as TCE was washed out or evapotranspired, gas flux rates of Ch1 and Ch6 became lower than those from the other channels (Figure 8).
Integration of aqueous phase concentration over effluent groundwater volume gives the results in Table 1. With air sparging, the amount of TCE washed out by groundwater flow is significantly smaller than it was without air sparging.
The mass balances showed that more than 80% of the injected TCE was recovered in all 10 cases. The average amount recovered was 28.6 g compared to 29.2 g injected. More than 100% was recovered in four of the 10 cases.
In situ air sparging was used in conjunction with vegetation to remediate groundwater contaminated by NAPL pollutants. Air sparging significantly enhanced the mass transfer of TCE from the nonaqueous phase to the groundwater phase. The upward TCE flux into the atmosphere was increased by air sparging. Air sparging appeared to negatively impact the health of alfalfa when the contaminant flux rates were high. Once the contaminant concentrations in the groundwater and flux rates decreased, the health of the plants improved.
This research was partially supported by the U.S. EPA under assistance agreements R-815709 and R-819653 to the Great Plains-Rocky Mountain Hazardous Substance Research Center for regions 7 and 8 under project 94-27. It has not been submitted to the EPA for peer review and, therefore, may not necessarily reflect views of the agency and no official endorsement should be inferred. The Center for Hazardous Substance Research also provide partial funding.
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Table 1. Amount of TCE washed out by groundwater flow.
|With air sparging (g)||Without air sparging (g)|
Figure 1. Schematic diagram of the chamber system (channels 1,2,3,5 and 6 are planted
with alfalfa and channel 4 is unplanted; channels 1 and 6 were air sparged in this study).
Figure 2. Alfalfa biomass production during exposure to TCE contaminant in
groundwater, TCE was injected on Dec. 2, 1996.
Figure 3. Effect of air sparging on TCE concentrations in the effluent water from channel
1: air sparging enhanced TCE disolution into the groundwater.
Figure 4. Effect of air sparging on TCE concentrations in the effluent water from channel
6: air sparging enhanced TCE disolution into the groundwater.
Figure 5. Comparison of two experimental results of TCE concentrations in the effluent
water from channel 1.
Figure 6. Comparison of two experimental results of TCE concentrations in the effluent
water from channel 6.
Figure 7. TCE flux rates into the atmosphere at different locations, Measured 78-79 days
after TCE injection.
Figure 8. TCE flux rates into the atmosphere at different locations, Measured 130 days after TCE injection.