ANDRE J. A. UNGER, EDWARD A. SUDICKY &. PETER A. FORSYTH. Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo,. Ontario ...
Groundwater Quality: Remediation and Protection (Proceedings of the Prague Conference, May 1995). IAHSPubl.no. 225, 1995.
449
Efficiency of air sparging for remediation of heterogeneous formations contaminated by dense nonaqueous phase liquids
ANDRE J. A. UNGER, EDWARD A. SUDICKY & PETER A. FORSYTH Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Abstract A numerical model is used to study the efficiency of air sparging, coupled with vacuum extraction, as a means of remediating heterogenous formations contaminated by dense non-aqueous phase liquids. Two dominant mechanisms were previously demonstrated to control this remediation technology. First, at early times, the gas phase directly contacts the DNAPL, particularly in the unsaturated zone, causing rapid vaporization of contaminant and subsequent removal by the vacuum extractor. Second, at later times, pumping of liquid and vaporized water by the vacuum extractor effectively acts as a vertical pump-andtreat technology. This late-time removal mechanism is shown to be controlled by contaminant dissolution which is a slower transfer process than direct DNAPL vaporization that occurs at early time. The efficiency of this technology is examined by comparing its contaminant mass removal rates with that by other remediation technologies. In particular, dewatering combined with vacuum extraction as well as water flooding combined with pump-and-treat are used as remediation alternatives. The former technology is chosen to test whether enlarging the unsaturated zone improves the contact between the gas and non-aqueous phases. This should then extend the operation time of the early-time mechanism controlling air sparging and possibly enhance the removal efficiency. The latter technology is chosen to determine whether the second late-time mechanism controlling air sparging can be more efficiently and economically implemented using a traditional pump and treat approach.
INTRODUCTION Many hazardous organic chemicals are produced or stored at facilities such as petrochemical plants, wood treatment facilities, underground gasoline tanks and decommissioned town gas plants. Often, these chemicals leak into the subsurface causing groundwater contamination and many are known to be toxic to human health, even at minute concentrations. Upon entering the subsurface, these contaminants preferentially form a non-aqueous phases (NAPL) although they volatilize readily into the ambient gas phase and dissolve slightly into the aqueous phase. Numerical simulation of NAPL contamination of the subsurface is aimed at providing a detailed understanding of both the evolution and remediation of site
450
André J. A. Unger et al.
contamination. Evolution of NAPL contamination requires knowledge of the advection and dispersion of contaminant mass lying above, at and below the water table and mass transfer processes between phases. In general, numerical simulation of the evolution and remediation of NAPL contamination involves coupling mass conservation equations; one for each of the water, contaminant and air components. For the problem at hand, the mass conservation equations take into account advective and dispersive mass transport in all phases. Mass transfer between phases involves the partitioning of air, water and contaminant into the gas phase, water and contaminant into the aqueous phase, and only contaminant into the non-aqueous phase. The intent of this work is to provide an analysis, through the use of a comprehensive numerical model, of the efficiency of air sparging coupled with vacuum extraction as a means of remediating dense non-aqueous phase liquids (DNAPL) in heterogeneous porous media. The mathematical theory and discretization are described in Forsyth (1993) and will not be repeated here. The dominant mechanisms controlling this technology were first demonstrated by using a computational domain containing two horizontal air sparging wells located near the bottom of the computational cell and one vertical vacuum extractor located at the centre of the cell within the unsaturated zone (Unger et al, 1995). Trichloroethylene (TCE) is spilled at the top centre of the cell and the resulting DNAPL is then allowed to migrate until and equilibrium position is achieved. Next, the air sparging and vacuum extraction wells are operated for a fixed period of time during which the progress of the remediation strategy is monitored. To determine the relative efficiency of this technology, various strategies are used concerning the operation of the wells and the control the boundary-conditions. Also, the efficiency of the technology is examined by comparing its contaminant mass removal rates with that by other technologies. In particular, dewatering combined with vacuum extraction as well as water flooding combined with pump-and-treat are used as remediation alternatives.
PROBLEM DEFINITION We will now describe an example problem involving the spill of the dense solvent trichloroethylene, TCE, into a heterogeneous sand followed by an attempt to remove the TCE by a process known as air sparging (i.e. injecting air below the water table in the NAPL source zone) coupled with vacuum extraction of the contaminated gas phase above the water table. The physical system consists of a shallow heterogeneous sand deposit containing both a saturated and an unsaturated zone (Fig. 1). The computational domain is 10.0 X 10.0 X 5.0 m in the x-, y- and z-dimensions, respectively, and the water table is initially located 1.5 m below the surface. The problem is discretized using finite volume cells of dimension 0.5 x 0 . 5 x 0 . 1 m which is sufficient to capture the structure of the heterogeneous and statistically anisotropic permeability field. The geometric mean permeability is Kg = 1.3 X 10"11 m2, the variance is aY2 = 1.0, where Y = In K, and the correlation lengths of Y in the x-, y- and z-directions are \x = Xy = 5.14 m and Xz = 0.209 m, respectively, with the spatial self-correlation of F being assumed to decay exponentially. The mean permeability and correlation lengths are chosen to be representative of the Borden aquifer (Sudicky, 1986) while the variance is much greater. The variance of the permeability field was chosen to maximize the
Efficiency of air sparging for remediation
451
Fig. 1 Schematic of problem geometry.
spreading and pooling of the DNAPL on the low permeability lenses while minimizing the possibility that the DNAPL will actually reach the sides or bottom of the cell. The field was generated using the algorithm described by Robin et al. (1993). The simulation chronology is divided into three separate stages. First, in the injection stage, 0.8 m3 of TCEis spilled at a rate of 0.2 m3 day"1 for 4 days at the centre of the surface of the computational domain. The TCE forms a dense non-aqueous phase liquid (DNAPL) relative to the aqueous phase and sinks through the unsaturated and saturated zones. Second, during the equilibrium stage, the DNAPL is allowed to migrate to an equilibrium position over a period of 1 year. Third, for the remediation stage, air sparging and vacuum extraction wells are operated for a period of 1 year. The well geometry can be seen in Fig. 1. Here, the two horizontal wells inject air at a constant pressure of 135.0 kPa, which is slightly above the hydrostatic aqueous phase pressure, while the vertical well withdraws air at a constant pressure of 50.0 kPa. The ambient atmospheric pressure is 100.0 kPa. Horizontal air sparging wells were chosen to maximize the possibility that the injected air will contact the DNAPL below the water table, thereby increasing the efficiency of the sparging process. The 5.0 m distance between the two horizontal air sparging wells was selected because it was assumed a priori that this spacing would yield good contact between the injected air and DNAPL. Also, these wells were placed 0.3 m above the impermeable base of the domain to avoid unwanted boundary effects from influencing the air injection process. The vertical vacuum extractor was situated directly below the TCE injection point because it was felt that this location would be optimal for the collection of the contaminated vapour. Using the numerical model, the removal rate of the TCE from the computational domain using air sparging coupled with vacuum extraction is examined for three different scenarios. For scenario 1, water is allowed to flow into the domain along the bottom portion of the lateral boundaries, below the elevation of the horizontal air injectors, under a hydraulic pressure equivalent to the original height of the water table
452
André J. A. Unger et al.
as it might exist outside of the cutoff walls. This scenario can be viewed as one in which the TCE source is partly isolated by cutoff walls that are impermeable but partially-penetrating. The inflow of water will attempt to maintain a water table elevation within the domain during remediation that is representative of the outside environment. For scenario 2, the vertical cutoff walls are fully-penetrating such that the spilled TCE and contaminated groundwater is completely isolated from the outside environment. For both scenarios, the top and bottom boundaries are assumed to be impermeable. For scenario 3, water is allowed to flow into the domain as with scenario 1, except in this case the top boundary is considered open to the atmosphere and the vacuum extractor is operated at a pressure of 95 kPa. This problem geometry is typical of commonly used implementation techniques for air sparging in field situations. For scenarios 4 and 5, dewatering combined with vacuum extraction are used to remove the TCE. Following the dewatering, three vertical wells extending from the top to the bottom of the domain, at two opposite sides, are used to passively allow air to enter the cell at one side and alternatively vacuum extract it at the other side. Each pair of three wells are positioned at the two corners and centre of their respective sides to ensure lateral flow of air across the entire width of the cell. The vacuum extraction wells are maintained at a constant pressure of 50 kPa and 95 kPa for scenarios 4 and 5 respectively while all boundaries are considered impermeable. For scenario 6, flooding of the domain followed by pump-and-treat is used to remove the TCE. The two horizontal wells are used to inject water at a pressure sufficient to raise the water table to the surface. The vacuum extractor well is operated at 95 kPa to pump water and air from both the saturated and unsaturated zones effectively implementing a vertical pumpand-treat. All boundaries are considered impermeable as well. NUMERICAL SIMULATION RESULTS The removal of TCE for scenario 1 with partially-penetrating cutoff walls begins with the DNAPL distribution as it exists at the end of the equilibrium stage. Figure 2(a) depicts an isosurface of the 1 % non-aqueous phase saturation front, shown in dark grey, at this stage. The light grey bubbles represent gas-phase saturations; the larger they are, the greater the gas phase saturation. The medium grey zones are the layers having the lowest permeability with the values for these layers ranging from 7.71 x 10"13 m2 to 4.89 X 10"11 m2. These layers are highlighted because they exert considerable control on the flow pattern of the three mobile phases. It can be seen from Fig. 2(a) that the non-aqueous phase exhibits significant pooling on the low permeability lenses. Very little of the DNAPL has reached the lateral boundaries and none has reached the bottom. The capillary fringe can be seen at the transition between the saturated and unsaturated zones where the light grey "bubbles" are smallest. Note that the capillary fringe only represents a small fraction of the thickness of the unsaturated zone and that residual DNAPL exists in the unsaturated zone, the capillary fringe and saturated zone of the remediation cell. Although not evident in Fig. 2(a), pools of DNAPL with saturations as high as Sn = 0.4 are perched on several of the low-permeability lenses. Figure 2(b) shows the phase distribution after one day of remediation for scenario 1. The majority of the residual DNAPL in the unsaturated zone has been removed by vaporization while significant DNAPL remains below the water table. This point marks
Efficiency of air sparging for remediation
453
F/ 8 ' 2 D , N ^ L / i S t 1 r i l ? U t i 0 1 } a n d S a s -P h a s e saturations at: (a) end of the equilibrium stage, and (b) after 1 day of remediation for scenario 1. e4uuiurmm
the transition between vaporization and dissolution as mechanisms controlling remediation and dissolution. The vacuum extractor is continually pumping both liquid and vaporized water from the domain. This water is then replaced by water flowing underneath the partially penetrating cutoff walls. The active flushing of water through the cell acts to induce mass transfer of contaminant from the non-aqueous phase to the aqueous phase where it is subsequently removed by the vacuum extractor In order to assist with the interpretation of the fate of the TCE Fig 3 shows the fraction of TCE remaining in the non-aqueous phase as a function of remediation time Although not shown on this figure, some portion of the TCE also resides in the aqueous ' gas and solid phases. This data is not shown because these quantities are relatively minor and the general objective of these remediation technologies is to remove as much TCE as possible in a minimal time interval. Figure 3(a) shows the fate of TCE for scenario 1. In this case, the non-aqueous phase has been removed in 70 days. Figure 3(a) shows the fate of TCE for scenario 2, which involves air sparging in a cell surrounded by fullypenetrating cutoff walls. For this case, substantial quantities of TCE remain in the cell after one year of remediation. This is because after all the mobile aqueous phase is pumped out of the cell by the vacuum extractor, significant stagnant pools remain
André J. A. Urtger et al.
454 C
: (a) g 10 < 10
z
PH
10"
OÙ
c
'5 '3 £ Pu
\
i
,
•
:
10° : (b)
io- ! 10"
Q c _o 10"" o
:
10"
\
\
scenario 4 scenario 5 scenario 6
]
\ 1 \
j « \ \ \1
i '• i
:
aj j~
1
i
