Environ. Sci. Technol. 2008, 42, 2575–2581
Volatilization and Biodegradation of Naphthalene in the Vadose Zone Impacted by Phytoremediation R I K K E G . A N D E R S E N , †,‡ E L I Z A B E T H C . B O O T H , †,§ LINSEY C. MARR,† MARK A. WIDDOWSON,† AND J O H N T . N O V A K * ,† The Charles E. Via Jr. Department of Civil and Environmental Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, CH2M Hill, 2525 Airpark Drive, Redding, California 96001, and Golder Associates Inc., 6241 NW 23rd St., Suite 500 Gainesville, Florida 32653
Received June 14, 2007. Revised manuscript received December 31, 2007. Accepted January 9, 2008.
The combined remediation mechanisms of volatilization and biodegradation in the vadose zone were investigated for naphthalene remediation at a creosote-contaminated site where a poplar tree-based phytoremediation system has been installed. Concurrent field and laboratory experiments were conducted to study the transport and biodegradation of naphthalene in the vadose zone. Soil gas sampling showed that more than 90% of the naphthalene vapors were biodegraded aerobically within 5–10 cm above the water table during the summer months. Peak naphthalene soil gas concentrations were observed in the late summer, corresponding with peak naphthalene aqueous concentrations and the minimum saturated zone thickness. An analytical solution was developed for vapor transport where the diffusion coefficient and firstorder biodegradation rate vary vertically in two discrete zones. First-order aerobic biodegradation rates in laboratory columns using unsaturated site soil ranged from 5 to 28 days-1 with a mean rate of 11 days-1. The observed naphthalene mass flux at the source (3.3–22 µg cm-2 d-1) was enhanced by aerobic biodegradation and was greater than the mean observed flux in the abiotic control column and the maximum theoretical mass flux by factors of 7 and 28, respectively.
Introduction Phytoremediation of polycyclic aromatic hydrocarbon (PAHs) compounds using poplar trees has been studied for a period of 7 years at a creosote-contaminated site in north-central Tennessee (1). Naphthalene is the primary contaminant volatilized into pore air and dissolved into groundwater from the creosote, which is present as a dense nonaqueous phase liquid (DNAPL) in the surficial aquifer. Previous studies at the site demonstrated that naphthalene undergoes preferential remediation over the PAHs with three or more rings and enhanced biodegradation is occurring in areas impacted by poplar trees (1, 2). Direct volatilization of naphthalene to * Corresponding author e-mail:
[email protected]. † Virginia Polytechnic Institute and State University. ‡ CH2M Hill. § Golder Associates Inc. 10.1021/es0714336 CCC: $40.75
Published on Web 02/26/2008
2008 American Chemical Society
the atmosphere was measured at the site and found to be enhanced by the presence of the phytoremediation system because the trees reduce the saturated thickness separating the submerged DNAPL and the water table and thus enhance vertical fluxes of contaminants (3). The focus of this paper is to determine the significance of direct volatilization of naphthalene from groundwater to the vadose zone, to investigate naphthalene transport coupled with biodegradation in the vadose zone, and to quantify the naphthalene mass loss rate. Volatilization and rapid biodegradation in the vadose zone immediately above the water table can act as a significant mass removal mechanism for volatile organic compounds (VOCs) that are susceptible to aerobic biodegradation (4–7). Active biodegradation creates a steeper contaminant concentration gradient and this increases diffusion driven volatilization out of the saturated zone (6). A microbially enriched and aerated vadose zone is likely to have faster biodegradation rates than the oxygen-limited saturated zone. Lahvis et al. (6) quantified aerobic biodegradation and volatilization of hydrocarbons by analyzing vapor transport in the vadose zone at a gasoline spill site. In their study 68% of the volatilized mass of total hydrocarbons was biodegraded and mass losses were greatest within the capillary zone. Park et al. (8) found that 20–30% of the measured naphthalene loss was due to volatilization in an induced venting laboratory experiment. Phytoremediation may enhance biodegradation by promoting mass transfer of VOCs from the saturated zone to the vadose zone. The vertical movement of water caused by evapotranspiration and interception of downward infiltration can affect the vertical distribution of naphthalene in the vadose zone (9). These seasonal trends are enhanced by transpiring plants (10). In the case of naphthalene, solubility and diffusivity in water are limiting factors for loss by volatilization if the vertical distance from a contaminant source to the water table is significant (11). The upward migration of VOCs is proportional to the vertical thickness of groundwater due to the much slower diffusion of naphthalene in water compared to diffusion in gas (12). Thus, lowering of the water table as plants transpire water should enhance the potential for volatilization from saturated-zone contaminants. Once in the vadose zone, transport of gasphase VOCs may be further enhanced by plants. Because soil moisture affects gas diffusion in the vadose zone, water consumption will promote an increase in the air-filled porosity during the growing season (12). In addition, macropores from decaying roots also facilitate faster transport of volatilized contaminants to the atmosphere and replacement with fresh air into the subsurface (11). Jury et al. (13) showed that water evaporation may significantly increase volatilization fluxes for shallow surficial aquifers that do not receive frequent recharge. The objective of this study was to evaluate and quantify the coupled mechanisms of volatilization and biodegradation of naphthalene in the vadose zone. Field measurements of soil gas and groundwater parameters were collected along with hydrologic data over one growing season at different locations across the site. The data were analyzed to evaluate the seasonal variations and the impact of transpiring phytoremediation systems on the combined volatilizationbiodegradation mechanism. Laboratory column experiments were conducted in parallel to the field measurements to determine rates for volatilization with and without biodegradation in the vadose zone. The column study also provided information on enrichment of the naphthalene-degrading VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Deep (0–0.8 m above bedrock) naphthalene groundwater concentrations July 2004 (µg L-1) and data sampling locations. microorganisms in the vadose zone by long-term exposure to naphthalene.
Materials and Methods Site Description. The study site located in Oneida, Tennessee, was contaminated with creosote from a railroad tie operation active from the early 1950s to 1973. In 1997 a system of 1146 hybrid poplars, Populus deltoides × nigra DN34, were planted to provide hydraulic control of the dissolved contaminant plume and to stimulate remediation of PAHs in groundwater. The aquifer underlying the site consists of sand and sandy clay to a depth of 3.0–3.5 m below ground with dense shale underneath. The creosote is present as a DNAPL, up to 30 cm in vertical thickness that resides on the confining layer of bedrock at the bottom of the aquifer. The groundwater concentration and plume size of PAHs decreased over time beginning in the third growing season of the phytoremediation system and remained relatively stable since 2001. Field Measurements. Five clusters of soil gas probes were installed to measure soil gas concentrations of naphthalene, oxygen (O2), and carbon dioxide (CO2) in the vadose zone. The locations of the soil gas probe clusters are shown in Figure 1. The soil gas probes (SV) were constructed of galvanized steel pipes sealed with rubber stoppers and outfitted with fittings and clamps for sampling. The depths of the sampling intake points depended on the location of highly permeable sandy layers and varied from 0.7 to 1.1 m below ground for the shallow sampling points, 1.1–1.5 m below ground for the medium depths, and 1.5–2 m below ground for the deepest points. As the elevation of the water table declined during the summer, additional soil gas probes were installed down to depths of 2.15 m below land surface. The gas contents of the soil gas probes were evacuated prior to sampling using a vacuum pump. Soil gas was sampled by pumping 1 L of soil gas at a rate of 1 L min-1 onto XAD-2 sorbent tubes (SKC Inc., Eighty Four, PA) every 30–60 min until at least 15 L had been collected. This ensured a limited radius of influence for the sample (7–25 cm) and allowed time for the naphthalene present in the soil gas to be replaced. Direct sampling of the soil gas with low-flow pumps was used when the permeability of the soil was high (dry sandy soils). At low permeability, a high-flow vacuum pump was used to sample soil gas via a vacuum box containing a Tedlar 2576
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bag for soil gas collection (wet or clayey soils). Subsequently the gas sample was transferred to sorbent tubes at 1 L min-1. The sorbent tubes were capped, wrapped with aluminum foil, and stored at 4 °C for no longer than 24 h before analysis. The concentrations of O2 and CO2 in the soil gas were measured directly from the soil gas probes on the site with a hand-held GasTech GT series gas monitor. The O2 and CO2 sensor detection ranges were 0-30% and 0-5%, respectively, with accuracies of (0.5% and (1.0%, respectively. Most of the soil gas samples had CO2 above 5%, so dilution with clean air in Tedlar bags was necessary (14). The dilution resulted in uncertainties in the CO2 reading in the diluted samples of up to 25%. The moisture content in the vadose zone was measured with probes (Watermark Moisture Sensors, Riverside, CA) installed in the same locations and depths as the initial soil gas probes. The probes report soil–water potential in units of pressure, and these results were converted to volumetric soil moisture percentages as in Marr et al. (3) using a measured soil porosity of 0.37. The moisture measurements were then converted to gas saturation percentages, defined as the volume of gas (versus liquid) per volume of total pore space. Groundwater elevations were measured in piezometers using a water level indicator, and groundwater samples for PAH analysis were collected from multilevel sampling wells (ML) near where the soil gas and surface flux measurements were made. Laboratory Column Experiments. Column experiments were designed to study biodegradation in and flux through the vadose zone under controlled conditions. Soil was collected from both contaminated and clean locations at the phytoremediation site, SV7 and SV25, respectively (Figure 1). The soil was obtained from sand layers directly above the water table at a depth of 0.9-1.5 m below ground surface at the contaminated location and a depth of 1.5-2.1 m at the clean location. A portion of each soil type was autoclaved more than 15 times over a period of 40 days to create abiotic controls. The water content of the autoclaved soil was adjusted back to 0.11 g g-1 to account for water lost during autoclaving, and the live soil (not autoclaved) was air-dried and brought back up to a water content of 0.11 g g-1 with distilled water. Samples from the two locations were each split in half. Soil samples
FIGURE 2. Concentration of naphthalene in the upper 0.6 m of the saturated zone and water table elevations in the most highly contaminated areas of the plume (ML7, left; ML11, right) during the study period. were packed into two separate columns, and autoclaved samples were packed into control columns. Each column was 71 cm in height and 5 cm in diameter and was constructed of PVC (polyvinyl chloride) pipe. A mesh screen was used to support the soil above a headspace at the bottom of the column. Five grams of naphthalene crystals were placed in aluminum cups sitting on the bottom of each column. There was no pressure gradient along the length of the column, so naphthalene vapors were transported by diffusion only. Seven sampling ports, vertically spaced 5 cm apart at the bottom and 10 cm apart at the top, consisted of 3 mm diameter stainless steel tubing extending 2.5 cm into the column. The columns were maintained at a constant temperature (23.1 °C) and humidity (38.4%) and monitored over a period of 231 days. The naphthalene flux at the top of the column was captured by pumping the top headspace air only, i.e. no pressure gradient or advection was introduced along the length of the column, through two XAD-2 sorbent tubes in series at a flow rate of 0.5 L min-1. Make-up air passed through a sorbent tube to remove background contaminants and a water bath to humidify the air. Used sorbent tubes were handled using the same procedures described above. Analytical Methods. Sorbent tubes from the field soil gas measurements and from the columns were extracted following the NIOSH 5515 extraction procedure with toluene. Microcosms and column soil were extracted with methylene chloride as described in Robinson et al. (15), and groundwater samples were extracted as described in Widdowson et al. (1). The extracts were analyzed on an automated Hewlett-Packard 5890 gas chromatograph with flame ionization detection (GCFID) and a DB-5 capillary column. The injector, detector, and oven temperatures were set to 250, 310, and 80 °C, respectively, with a temperature ramp of 10 °C min-1. Liquid naphthalene standards were used for calibration. Vertical gas concentration profiles in the columns were obtained biweekly by using a gastight syringe to pull 100–200 µL samples of air from each column sampling port. In order to acquire a representative sample, 20 µL of stagnant air were pulled out of the port and expelled before the actual sample was taken. The syringe was then reinserted into the port and slowly pumped to gently mix the soil gas and obtain a uniform sample. Samples were taken from each port until the relative standard deviation of at least two measurements fell below 20%. Samples were manually injected into the GC-FID. Carbon dioxide and O2 gas samples were analyzed biweekly alternating with weeks when naphthalene was sampled using the same sampling method as described above. Oxygen samples of 100–200 µL were analyzed on a GOW-MAC GC series 580 instrument (Bridgewater, N.J.) with
a thermal conductivity detector (TCD) and Propaq Q packed column. For CO2, samples of 100–200 µL were analyzed on a Shimadzu GC-14A (Kyoto, Japan) with a TCD detector and Propaq Q packed column.
Results and Discussion Field Measurements. The relationship between water table elevation and naphthalene groundwater concentrations (Figure 2) shows that naphthalene concentrations increased as the elevation of the water table declined. Concentrations at ML7 increased from 3700 µg L-1 in the winter to a peak of 8200 µg L-1 in August. In ML11, concentrations increased from 380 µg L-1 in the winter to 4700 µg L-1 in July and August. The water table decline of approximately 0.5 m was attributed to direct transpiration and decreased recharge due to canopy interception. Two lines of evidence suggested that the lowering of the water table was the result of poplar tree transpiration. First, the diurnal cycle of the water table associated with direct transpiration by phreatophytes was observed in piezometers beginning in April (Supporting Information, Figure 1). Total rainfall in 2004 (144 cm) was above the annual average (138 cm) and, in particular, the period from May to December 2004 was 23 cm above normal. Despite the above-average rainfall, water table elevations progressively decreased over time (Supporting Information, Figure 2) during active periods of evapotranspiration (April-May) relative to a period when transpiration would be expected to be inactive (March). The horizontal hydraulic gradient was minimized within the phytoremediation system in April and May. The most complete time series for soil gas concentrations of O2 and CO2, soil gas saturation, and naphthalene concentrations was obtained at or near the plume center from probes located 1.3–1.6 m below land surface (Figure 3). The highest naphthalene soil gas concentrations occurred in August (0.38 and 0.44 µg L-1 at SV7 and SV22, respectively) but were an order of magnitude less than the highest concentrations at the deeper sampling depths, also in August (13 and 30 µg L-1 at SV7 and SV22, respectively). The soil gas saturation at SV7 increased from 2 to 28% and remained high through September. The peak naphthalene concentrations in the soil gas corresponded with the highest levels of soil gas saturation and the peak naphthalene concentrations in the groundwater. At SV7 and SV22, soil gas concentrations were below detection from March to early May and from November through the end of the study. These periods corresponded with decreased naphthalene groundwater concentrations and an increased saturated zone thickness. Aerobic biodegradation of hydrocarbon compounds in the vadose zone is known to result in oxygen consumption and carbon dioxide generation (16). At SV7, O2 declined from VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Soil gas data at the plume center (SV7), collected 1.4 m below ground (left) and at an adjacent location (SV22), collected 1.3–1.5 m below ground (right), from April to November 2004.
FIGURE 4. Vertical profiles of naphthalene, oxygen, and carbon dioxide in the soil gas and the level of gas saturation at SV7 and SV22 in September. 6 to 8% in April and May to below 2% in August (Figure 3). The O2 measured at SV22 similarly decreased from 20% in May to 2% in July and remained below 6% through October. Soil gas CO2 at both SV7 and SV22 increased starting in the spring and remained elevated through September, after which no more CO2 data were collected. The consumption of O2 and generation of CO2 combined with elevated naphthalene concentrations in the soil gas during the summer indicates that aerobic biodegradation was active in degrading naphthalene vapors in the vadose zone. Typical vertical profiles of naphthalene, O2 and CO2 in the soil gas and the level of gas saturation, are shown in Figure 4 at two highly contaminated locations in September. At both locations, the O2 content decreased with depth to below detection whereas the CO2 content and naphthalene concentrations increased with depth. Oxygen consumption and CO2 production with depth provided further evidence of aerobic biodegradation in the region immediately above the water table. This pattern is consistent with field data observed at other vaportransport sites defined by oxygen-limited biodegradation in the region adjacent to the water table (17). Inaccessibility of soil gas samples very close to the water table along with biodegradation in the vadose zone complicated estimation of the rate of volatilization from the saturated zone. Sampling of soil gas closer than 5–10 cm to the groundwater table was not practical because some water was also collected by pumping. Flux estimates based on the concentration profiles in the groundwater and in the soil 2578
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gas, ignoring loss mechanisms, indicated that the flux should be 1–2 orders of magnitude larger than what was measured exiting the ground surface (3). The difference between the measured flux and the estimated flux was thought to be due to loss mechanisms, including biodegradation and plant uptake. The removal of naphthalene by biodegradation would result in steeper naphthalene concentration profiles, which would increase the mass transfer rate. Column Studies. In an effort to supplement the field data, laboratory soil column experiments designed to closely replicate site conditions were conducted to quantify the rates of naphthalene volatilization and biodegradation in the vadose zone. The steady-state naphthalene profile in the control (autoclaved) column was nearly linear (Figure 5), suggesting that diffusion is the dominant process affecting naphthalene transport. The difference between the autoclaved and live (not autoclaved) profiles was attributed to biodegradation in the live columns (Figure 5). Oxygen profiles in the live columns decreased with depth from 21% at the upper sampling port to 15% at the lowest port), and CO2 levels were elevated (0.05–3.2%) relative to the control columns (0.05–0.14%), indicating hydrocarbon utilization by microorganisms (data not shown). In the live columns, naphthalene concentrations were consistently reduced by 80% within the first 5 cm and by at least 99% over the full column length of 71 cm. Naphthalene concentrations at the uppermost port were below detection in the live columns. Concentration differences in the live soil
τa ) θa2.33 ⁄ n2
(2b)
Pasteris et al. (18) expressed the apparent first-order biodegradation rate using C ) Co at z ) 0 kwθw k) H
(3)
where kw is the first-order biodegradation rate in the aqueous phase (day-1), θw is the volumetric soil moisture content (m3 water m-3), and H is Henry’s Law coefficient. The steady-state solution is derived using type I boundary conditions
FIGURE 5. Naphthalene vapor concentration profiles in the soil columns (autoclaved and live) after 89 and 151 days, respectively. Soil for SV7 and the sterilized control was collected inside the PAH plume, and soil for SV25 was collected in an uncontaminated area.
C ) Co at z ) 0
(4a)
C ) 0 at z ) L
(4b)
At the interface (x ) L1), the intermediate conditions are given as C1 ) C2 2
D1 columns (SV7 versus SV25) were attributed to previous acclimation of soil microorganisms to naphthalene in the field at SV7, the more contaminated site of the two. Peak soil gas naphthalene concentrations measured at SV7 and SV22 in August (plotted in Figure 5) were similar to the concentrations measured in the live column (SV7) at the same vertical distance above the water table. Model for Estimating Biodegradation Rate Constants. Wilson (18) developed a one-dimensional analytical solution that describes vapor transport in the vadose zone above a NAPL source when molecular diffusion and biodegradation are governing processes. The solution has been employed to estimate biodegradation rate constants by fitting an analytical solution to petroleum hydrocarbon compound vapor concentration data in a field lysimeter (19) and laboratory column experiments (20). Application of the solution (18) to the live column laboratory data showed that the ratio of the apparent first-order biodegradation rate to the effective diffusion coefficient (k/D) was not uniform along the column height. In most cases, a change in slope occurred at a height of 15 cm above the source. A solution was developed to compensate for differences in k/D along the column in which the finite domain of length (L) is divided into two sections of lengths L1 and L2, each with a unique value of k (day-1) and D (m2 day-1) in each zone. Devaull (21) recently developed a two-zone model for vapor transport based on the assumption that aerobic biodegradation will not occur in the lowermost zone. The governing equation of vapor transport is expressed in terms of C1 and C2, the volatile organic compound concentration (g m-3) in zones 1 and 2, respectively. D1 D2
d2C1 dz2 d2C2 dz2
- k1C1 ) 0
(1a)
- k1C1 ) 0
(1b)
Pasteris et al. (19) calculated the diffusion coefficient of a VOC in soil gas based on the molecular diffusion coefficient of a VOC in air (Da (m2 day-1)) using d ) θaτaDa
(2a)
where θa is the volumetric soil air content (m3 air m-3), and τa is tortuosity, calculated using an empirical expression for a porous medium of porosity n as,
d C1 dz
2
(5a) 2
) d2
d C2
(5b)
dz2
The solution to eq with eq and eq yields C1(z) ) R1exp(k1 ⁄ d1)1⁄2(z) + β1exp - (k1 ⁄ D1)1⁄2(z) (6a) C2(z) ) R2exp(k2 ⁄ d2)1⁄2(z) + β2exp - (k2 ⁄ D2)1⁄2(z) (6b) where β2 )
{
β1 ) -Co
-Co(k1D1)1⁄2exp[L(k2 ⁄ d2)1⁄2] [(k2D2)(p - q) - (k1D1)(p + q)] 1
(exp[-2L1(k1 ⁄ D1)1⁄2] - 1)
(7a)
+
(k1D1)1⁄2 sinh[(k2 ⁄ D2)1⁄2L2] [(k2D2)(p - q) - (k1D1)(p ) q)] sinh[-(k ⁄ D )1⁄2L ] 1 1 1 R 1 ) Co - β 1
}
(7b) (7c)
R2 ) -β2exp2L(k2 ⁄ D2)1⁄2
(7d)
and where p ) sinh[(k2⁄D2)1⁄2L2 - (k1⁄D1)1⁄2L1] and q ) sinh[(k2⁄D2)1⁄2L2 + (k1⁄D1)1⁄2L1]. As a means to estimate D in each zone, the van Genuchten equation was utilized to calculate average values of the moisture content and θa. Using parameters approximate to a loamy sand (22) combined with eqs and indicates that D ranged from 80 to 112 cm2 day-1 for the lower 10–15 cm of the column compared to 240–250 cm2 day-1 in the region immediately above. Figure 6 shows the fit of the two-zone solution for the live column (SV7) from a vertical concentration profile collected at day 154. The apparent first-order biodegradation rate coefficients were 14 day-1 and 0.4 day-1, for the lower and upper zones, respectively. The biodegradation rates in the lower zone of the column varied over time (5–28 days-1) with mean rates of 11 days-1 and 0.3 days-1 for the lower and upper zones, respectively. Using estimates of the moisture content in the lower and upper zone (0.25 and 0.19, respectively), the mean aqueous-phase first-order biodegradation rates were 1.1 days-1 and 0.04 days-1 for the lower and upper zones, respectively. The relatively low naphthalene biodegradation rate in the upper zone may reflect suboptimal moisture conditions for VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Naphthalene vapor concentration profile represented by eq and experimental data in the live acclimated soil column (SV7) at day 154. microbial utilization of naphthalene as a substrate. In column SV25 (nonacclimated soil), the biodegradation rate in the lower zone also increased over time from 3 to 12 days-1, possibly due to microbiological acclimation to naphthalene or in the increase in the number of microorganisms. Firstorder rates in the control column also increased with time, initially showing no decay along the height of the column, but by day 154, the best-fit biodegradation rate reached a maximum value (k ) 0.4 days-1). However, the biodegradation rates in the lower zones of the control and live columns were statistically different at each sampling event. A comparison of naphthalene biodegradation rates with previous studies was made difficult due to the lack of studies on naphthalene in the vadose zone. The first-order rates for naphthalene measured early in our experiments were consistent with apparent first-order biodegradation rates reported for toluene vapor in laboratory (1.3 days-1) and field (3.2 days-1) experiments after 23 and 7 days, respectively, using nonacclimated soil (19). The derivative of eq provides a means to calculate the vapor flux at the source (Jo) using Fick’s Law Jo ) -D1
d2C1 dz2
|
x)0
) √k1D1(Co - 2β1)
(8)
Using eq 8, the naphthalene mass flux at the source (column SV7) varied from 3.3 to 22 µg cm-2 day-1 and was dependent on the source concentration (Figure 7). A comparison of naphthalene mass flux in the live column at day 154 (10.3 µg cm-2 day-1) with the maximum theoretical flux at the source with no biodegradation (0.37 µg cm-2 day-1) suggests that the microbial utilization of naphthalene significantly enhanced the rate of volatilization into the column. Consistent with this comparison, the naphthalene mass flux in the control column was less than in the live column by a mean factor of 6.5, varying from an initial rate of 0.24 µg cm-2 day-1 to a maximum rate of 3.4 µg cm-2 day-1 at day 222. Comparison of the laboratory-measured naphthalene mass flux rate at the source to the maximum field-measured rate at the land surface suggests that 98–99% of naphthalene vapors were biodegraded in the vadose zone. However, because oxygen was present in the laboratory columns near the source and was depleted near the water table at the field site, the naphthalene mass flux rates at the field site would be expected to be lower than the laboratory-derived values. Naphthalene Remediation. The mass removal rate of naphthalene at this site increased significantly after three growing seasons when the poplar tree roots reached the groundwater table (1). We believe that the loss of naphthalene 2580
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FIGURE 7. Naphthalene flux versus source concentration represented by eq 8 and experimental data in the live acclimated soil column (SV7) and the control column at day 154. was due to a combination of mechanisms associated with the phytoremediation system at this site which resulted in a substantial increase in the naphthalene mass flux from the saturated zone compared to the flux during the nongrowing season. Water transpiration by the poplar trees reduced rainwater infiltration, depleted moisture from the vadose zone, reduced the thickness of the saturated zone, and exposed more contaminated regions of the surficial aquifer. This, coupled with a rapid rate of biodegradation in the 5 cm above the groundwater surface where 90–95% of the volatilized naphthalene was degraded, resulted in an increased rate of naphthalene transfer from the groundwater to the vadose zone. Consistent trends in the concentration data collected in the laboratory columns and vadose zone vapor probes indicated that aerobic biodegradation, aided by the system of poplar trees, is the mechanism responsible for the significant removal of naphthalene vapors in the vadose zone. Loses by volatilization to the atmosphere and anaerobic degradation in the saturated zone were considered to be minor loss mechanisms compared to vadose zone degradation (23). It should be noted that the results of this study are unique to this site and contaminant. Little data exists for volatilization of naphthalene from aquifers, and the phyto-induced rates measured in this study are expected to be high compared to most other aquifers. The groundwater table and depth to bedrock were relatively shallow. Naphthalene is readily degradable under aerobic conditions, so degradation in the
vadose zone was rapid. For deeper groundwater tables, the impact of a decline in the groundwater table of 10–20% of the groundwater depth would likely have a much less dramatic impact on the volatilization rate. Similarly, if the DNAPL was a chlorinated compound where reductive dehalogenation was the major biotransformation mechanism, the enhanced volatilization rate associated with phytoremediation would be expected to be less because vadose zone biodegradation would be lower. For chlorinated solvents, phyto-induced direct volatilization could be an important loss mechanism, but would be much slower than the loss rate observed at this site for naphthalene.
Acknowledgments This research was supported by the Midwest Hazardous Substance Research Center at Purdue University and by a Via Fellowship from the Virginia Tech Department of Civil and Environmental Engineering. A summer Edna Bailey Sussman Fellowship supported the extensive fieldwork. We thank Julie Petruska and Jody Smiley for their technical expertise and the landowner for allowing access to the field site.
Supporting Information Available Figure 1 shows water table recession analysis, and Figure 2 shows the water table profile. This material is available free of charge via the Internet at http://pubs.acs.org.
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