Modelling the seasonal variation in bioavailability of residual NAPL in the vadose zone Arturo A. Keller & Patricia. A. Holden Bren School of Environmental Science and Management University of California, Santa Barbara, CA 93106 e-mail:
[email protected]
Alicia M. Wilson Department of Geological Sciences, University of South Carolina 701 Sumter St., Columbia SC 29208
Abstract. A multiphase model (UTCHEM v.6) has been modified to account for boundary conditions present in the vadose zone, as well as adding an analytic solution for the soil temperature profile, the temperature dependence of physicochemical properties, and revised interphase mass transfer correlations. The model also includes biodegradation, which is limited by moisture content, oxygen and pollutant availability and temperature. The model is then used to study the effect of seasonal variations in soil temperature and moisture, driving by actual climatic conditions. A similar Non-Aqueous Phase Liquid (NAPL) spill is simulated under two climates, one appropriate for Mediterranean conditions (mild rainy winters and dry summers) and another one typical of temperate climates with higher rainfall spread throughout the year and longer, colder winters. Based on these modifications, the water table is allowed to rise and fall following seasonal variations in rainfall infiltration and atmospheric temperature. We also allow the exchange of chemicals across the soil-atmosphere interface, which has a significant effect on the predicted rate of disappearance. The results indicate that although biodegradation is an important process, the water table fluctuations and diffusive processes also produce significant transfer to the atmosphere. There is considerable difference in the rate at which pollutant disappears between these two climates, requiring more than double the time for natural attenuation in the temperate climate relative to the Mediterranean climate. Key words hydrocarbon, natural attenuation, biodegradation, volatilization, multiphase flow
INTRODUCTION A vadose zone management strategy that has recently gained more acceptance by regulators is natural attenuation (Rice et al., 1995; Chappelle, 1999). Although many pollutants can naturally biodegrade in the vadose zone, the physical, chemical and biological factors that control natural attenuation are not fully understood. In most circumstances, the practical approach to natural attenuation as a remediation strategy has been to assume constant site conditions throughout the life of the project, without taking into consideration that the biogeochemical processes involved are strong functions of soil temperature and moisture. Thus, there is considerable uncertainty about the time needed to bring the site to closure. Such uncertainty is costly to owners of contaminated property, to regulators, and to the public, while representing ongoing risks to human and ecosystem health. The mechanisms involved in natural biodegradation must be understood so that the predictability and reliability necessary to protect human and ecosystem health can be achieved.
Although it is generally understood that seasonal changes in temperature and moisture in the vadose zone can significantly affect the rate of natural processes, biotic and abiotic, incorporating these driving forces is generally considered too complex to take into consideration, or the effects are dismissed as being second order. However, as our understanding of these biogeochemical processes increases, we must incorporate the effect of these cyclical variations on our predictive ability. Diffusion, dissolution, volatilization and multiphase flow processes are all dependent on temperature, and moisture content can significantly modify the pathways for contaminant transport (Schwarzenbach et al., 1993; Watts, 1998; Falta et al., 1989; Mendoza and Frind, 1990). In addition, biodegradation is a strong function of both temperature and moisture. For this work, we modified UTCHEM version 6.0 (Pope et al., 1999), a three-phase, multicomponent, finite difference numerical model for predicting fluid flow, solute transport, and biodegradation to predict seasonal changes in natural attenuation of NonAqueous Phase Liquid (NAPL) spills in the vadose zone and groundwater. We added boundary conditions appropriate for vadose zone conditions, temperature dependent physicochemical relationships and interphase mass transfer to the gas phase, accounting for the mass of organic compounds lost through the gas phase to the atmosphere. We present some sample simulations, which serve to illustrate the importance of considering the seasonal variations in temperature and moisture on the overall natural attenuation processes. APPROACH UTCHEM uses a modified Implicit Pressure Explicit Saturation (IMPES) method to solve the coupled flow and transport equations. We added (1) default boundary configurations appropriate for fluid flow and contaminant transport in the vadose zone; (2) an analytic method for calculating the soil thermal profile driven by atmospheric conditions; (3) relationships for the temperature dependence of physicochemical properties; and (4) revised inter-phase mass transfer relationships, based on recent experimental and theoretical work by Zhou et al. (2000) and Keller et al. (2000). The most important modifications to the boundary conditions are (1) to allow the movement of gaseous and aqueous phases across the atmosphere-soil interface, tracking the inflow and outflow of components in each phase, and (2) to maintain a constant- head boundary on the left-hand side of the saturated zone, allowing groundwater fluctuations within the domain while allowing discharge on the right-hand side. In addition, we used an analytical solution for the soil temperature profile, which significantly accelerates simulation time while providing accurate temperatures for all depths. Allowing the flux of rainfall infiltration, driven by the actual precipitation record, creates a more realistic soil moisture profile throughout the simulation period. We also consider that gases can be dissolved in the rainfall, which are thus transported throughout the vadose zone. Soil gas can also flow out of the vadose zone to the atmosphere as a result of water table rise, and gas flows in during the drier periods. Gas-phase components can also diffuse in or out the vadose zone, depending on the concentration gradients. For simplicity, a specified rainfall infiltration rate is applied to exposed surface, without allowing it to exceed the ponded infiltration rate, determined using the maximum water max saturation, S wmax = 1 − S nres − S res is the maximum water saturation, S nres is g , where S w the residual NAPL saturation (note that Sn = 0 if the grid point has not being contaminated
by NAPL), and S gres is the residual gas phase saturation. Three-phase relative
permeabilities are used throughout the vadose zone. The assumption that there is some residual gas present can result in an underestimation of actual infiltration, since it is likely that after several rainfall events there is some significant dissolution of the residual gas phase (Keller et al., 1997). Using the three-phase relative permeability relationships also may underestimate infiltration, since they are smaller than the two-phase relative permeabilities, but the effect is less important for water and gas. Using detailed soil temperature data from the McClellan site in Sacramento, CA, USA (LBNL, 1999), the parameter values of an analytical equation for soil temperature as a function of depth were determined, based on Hillel (1980). This allows us to drive the surface temperature with the measured amplitude of the yearly temperature fluctua tion at land surface, while the deeper soil temperatures respond more slowly based on the thermal conductivity and heat capacity of the soil, which dampens the oscillations. Once an efficient solution to the heat equation was incorporated, via the analytic al expression, it became important to consider the temperature dependence of the physicochemical properties (e.g. density, diffusivity, viscosity, surface tension, equilibrium coefficients). A literature search (e.g. Reinhard and Drefahl, 1999, Montgomery, 2000) of experimental or calculated values of these properties at different temperatures was conducted, and the data was fitted using the methods proposed by Reinhard and Drefahl (1999), Scharwzenbach et al. (1993), and Lyman et al. (1990). To simplify the numerical simulation process, lookup tables were created, interpolating between temperatures every 5 o C. Although UTCHEM v.6 allows equilibrium or mass transfer limited dissolution of NAPL components, it considers a equilibrium volatilization. Our modification included the incorporation of mass transfer limited volatilization, as well as the temperature dependence of the partition coefficients. The model was also modified to consider temperature dependent evaporation of water and the dissolution of gas phase components such as carbon dioxide and oxygen under equilibrium conditions. This will allow us to more accurately simulate the biogeochemical processes that occur during natural attenuation, and be able to compare field measurements of pCO2 and other gas components, which are indicative of biological activity at natural attenuation sites. To illustrate the effect of seasonal variations in temperature and moisture on the fate and transport of residual NAPL, we compared the rate at which the same NAPL spill would completely disappear under two climate regimes: (1) a Mediterranean-type climate typical of California, in which cool, wet winters follow warm, dry summers; and (2) a temperate climate with considerable more precipitation throughout the year, typical of the Midwestern USA, with larger variations in temperature throughout the year. For clarity, we considered a simple homogeneous soil, to understand the importance of various fate and transport processes. The model can be run with any level of soil heterogeneity, but the results are more difficult to interpret due to the added complexity. For the first climate, we used meteorological conditions from the area near McClellan Air Force Base in Sacramento, California (NCDC, 2001). The average precipitation in Sacramento is 0.44 m/yr, but nearly all rain falls between October and May. The average soil temperature is 21.3ºC, but the profiles show fluctuations of 30ºC at the surface throughout the year, with the most significant temperature changes occurring in the top 5 m (LBNL, 1999). The combination of temperature and precipitation fluctuations creates strong seasonal variations in soil moisture. The second climate type is based on soil surface temperatures and rainfall rates from Madison, Wisconsin (NCDC, 2001). Average precipitation is ~1.50 m/yr, with a more even distribution throughout the year. Soil temperature at land surface varies over ~28 °C
from summer to winter, with an average temperature of 8 °C. Soil properties are based on samples from McClellan Air Force Base, near Sacramento (LBNL, 1999) for both cases. The simulation starts with a clean soil in autumn, with an initial water table depth of 9 m. Autumn rainfall is allowed to infiltrate to create a typical moisture profile before the winter months. The spill of NAPL (100% toluene) occurs in the first 10 days of January, releasing 3000 L from an underground storage tank at 2-4 m in the domain. We then simulate the distribution of the NAPL spill until it reaches residual saturation, while mass transfer to the gas and aqueous phases occurs. We track toluene concentrations in all phases. RESULTS Figure 1 presents the results of the simulation for the Sacramento site. Note that due to editorial constraints we are only able to present a few snapshots of the simulations; more information is available at http://www.bren.ucsb.edu/~keller#Projects.
Fig. 1 Simulation of NAPL spill in Sacramento, showing NAPL saturation (left), aqueous toluene concentration (center) and gaseous toluene concentration (right) over time.
Rate of toluene loss (kg/d)
NAPL saturation decreases to residual saturation relatively rapidly, while it transfers to the aqueous and gaseous phases in the vadose zone as well as to the underlying groundwater. There is some lateral spreading of the NAPL, due to the rate of spillage, but the general movement is downward towards the underlying water table. NAPL spreads displacing both vadose zone gas and water, until each one of those phases reaches its residual saturation. The NAPL residual disappears by 380 days. Toluene is rather volatile, so a significant mass fraction partitions directly to the gas phase and begins to disperse through the vadose zone. As can be seen in the snapshots at 100 and 150 days, the concentration of toluene right below the surface (depth = 0 m) is rather high. The model predicts a significant amount of mass lost to the atmosphere via diffusion across the soil-atmosphere interface. As the NAPL phase shrinks, the gas phase concentrations drop fairly rapidly. The corresponding toluene concentrations in the aqueous phase are quite high in the vadose zone during the first half of the simulation, essentially at equilibrium with the surrounding gas phase concentrations, since the rate of mass transfer is sufficiently rapid. However, there is sufficient contact between the NAPL residual and the water table to transfer toluene to groundwater, forming an emerging plume. Not captured in Figure 1 is the additional spreading and pumping out of toluene to the atmosphere as the water table rises and falls throughout the seasons. These effects are more readily observed in the dynamic simulation results presented in the web page indicated above. 0.014 Sacramento
0.012
Wisconsin
0.01 0.008 0.006 0.004 0.002 0 0
100
200
300
400
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600
Time, t (days)
Fig. 2 Comparison of toluene disappearance under two climatic conditions.
The results of the simulation of the same spill but under “Wisconsin climate” indicate a much slower rate of disappearance for toluene (Figure 2). Although mass is lost through the soil-atmosphere interface at roughly the same time, the rate of volatilization is slower for the colder climate and thus the mass transfer across that surface is much smaller. Note that the rate of disappearance considers losses via volatilization out to the atmosphere, outside the domain (although in these simulations we have extended the boundaries far enough not to have such losses), and through biodegradation, which is considered limited by oxygen availability, water limitations on bacterial growth and pollutant bioavailability. The bioavailability is a combination of mass transfer and transport processes that supply the pollutant from the residual NAPL to the microbes, slowed down by transport across a number of phase interfaces including the microbial exo-polymeric substance (Holden et al., 1997). The mass transfer processes are a function of temperature and of the interfacial area between NAPL-air, NAPL-water or water- microbes, which are functions of moisture
content in the soil. Similarly, phase mobility is a function of temperature through the viscosity and density of the phase, but more importantly the movement of the various phases is a function of their saturation, which is varying throughout the year due to rainfall infiltration and drying events. This is reflected in the fluctuations of the rate of disappearance of toluene in Figure 2. CONCLUSIONS Predicting the rate of natural attenuation requires a detailed understanding of the controlling processes, and their dependence on naturally fluctuation environmental conditions. We have presented the initial simulation results from a multiphase model capable of addressing a wide range of environmental conditions. Our comparison of two similar NAPL spills under different climates indicates that the seasonal fluctuations in temperature and moisture can play a major role in determining the rate of disappearance of NAPL, since these two parameters affect many biogeochemical processes, both in terms of fate and transport. Pollutant bioavailability is thus a strong function of seasonal variations in temperature and moisture. The results indicate that although biodegradation is an important process, the water table fluctuations and diffusive processes also produce significant transfer to the atmosphere. There is considerable difference in the rate at which pollutant disappears between these two climates, requiring more than double the time for natural attenuation in the temperate climate relative to the Mediterranean climate. The authors wish to acknowledge the funding by US EPA Exploratory Research Grant Number R826268 and US EPA Grant Number R827133. References Chappelle, F.H. (1999) Bioremediation of petroleum hydrocarbon-contaminated ground water: The perspectives of history and hydrology. Ground Water, 37(1): 122-132. Falta, R.W., Javandel, I., Pruess, K., Witherspoon, P.A. (1989) Density-driven flow of gas in the unsaturated zone due to the evaporation of volatile organic compounds. Water Resources Research, 25(10): 2159-2169. Hillel, D. (1980) Fundamentals of Soil Physics. Academic Press, Inc., Orlando, Florida, 413 pp. Holden, P.A., Hunt, J.R. and Firestone, M.K. (1997) Toluene diffusion and reaction in unsaturated Pseudomonas putida biofilms. Biotechnology and Bioengineering, 56(6): 656-670. Keller, AA, Blunt, M, Roberts, PV (1997). Micromodel Observation of the Role of NAPL Films on Multiphase Flow. Transport in Porous Media 23(14): 1-21. Keller, AA, Sirivithiyapakorn, S (2000) Pore Scale Determination of the Rate of NAPL Dissolution or Volatilization. Report R826268. US Environmental Protection Agency, Washington, D.C. LBNL (1999) Site S-7 Vadose zone monitoring system: Draft final report for McClellan AFB. LBNL Report 44325, Lawrence Berkeley National Laboratory, Berkeley, CA. Lyman, WJ, Reehl, WF, Rosenblatt, DH (1990) Handbook of chemical property estimation methods. American Chemical Society, Washington, DC. Mendoza, C.A.. Frind, E.O., (1990) Advective-dispersive transport of dense organic papers in the unsaturated zone, 1, Model development. Water Resources Research, 26(3): 379-387. Montgomery, JH (2000) Groundwater Chemicals, 3rd Edition. CRC Lewis, Boca Raton, FL, USA NCDC (2001) Climatic data set. National Climatic Data Center (www.ncdc.noaa.gov). Pope, G. et al. (1999) Three-dimensional NAPL fate and transport model. EPA Report 600/R-99/011, U.S. Environmental Protection Agency, Cincinnati, OH. Reinhard, M., Drehfahl, A (1999) Handbook for estimating physicochemical properties of organic compounds. Wiley Interscience, New York, NY. Rice, D. W. Dooher B. P. Cullen S. J. Everett L. G. Kastenberg W. E. Grose R. D. Marino M. A. (1995) Recommendations to Improve the Cleanup Process for California's Leaking Underground Fuel Tanks (LUFTs). Lawrence Livermore National Laboratory, Livermore, CA. Schwarzenbach, R, Gschwend, P, Imboden, D (1993) Environmental Organic Chemistry. John Wiley & Sons, Inc., New York, NY. Watts, R. J. (1998). Hazardous Wastes: Sources, Pathways, Receptors . John Wiley & Sons, Inc., New York, NY.
