Environ. Sci. Technol. 2007, 41, 2717-2722
Micrometeorological Measurements of Methane and Carbon Dioxide Fluxes at a Municipal Landfill A N N A L E A L O H I L A , * ,† T U O M A S L A U R I L A , † JUHA-PEKKA TUOVINEN,† MIKA AURELA,† JUHA HATAKKA,† TEA THUM,† MARI PIHLATIE,‡ JANNE RINNE,‡ AND TIMO VESALA‡ Climate and Global Change Research, Finnish Meteorological Institute, P.O. Box 503, FI-00101 Helsinki, Finland, and Department of Physical Sciences, University of Helsinki, P.O. Box 68, FI-00014 University of Helsinki, Finland
Continuous and area-integrating monitoring of methane (CH4) and carbon dioxide (CO2) emissions was performed for 6 and 9 months, respectively, at a municipal landfill in Finland with the micrometeorological eddy covariance (EC) method. The mean CH4 emission from June to December was 0.53 mg m-2 s-1, while the CO2 emission between February and December averaged 1.78 mg m-2 s-1. The CH4 emissions from the summit area of the landfill, where active waste deposition was going on, were 1.7 times as high as from the slope area with a better surface cover. The variation in emissions over the source area of the measurement was high. Significant seasonal variation, linked to air and soil temperature, was only seen in the CO2 release rates. Results obtained with the EC method were comparable to those measured with closed static chambers. According to the EC measurements, the gas recovery system decreased CH4 fluxes by 69-79%. The ratio of the measured CH4 and CO2 emissions roughly indicated the route of the landfill gas emission, resembling the ratio of the gases measured in the gas wells (1.24) when the emission originated from the area with no oxidizing cover layer and being smaller when CH4 oxidation had taken place.
Introduction Emission of methane (CH4) into the atmosphere is one of the most serious environmental problems associated with the landfilling of municipal waste. CH4 is an effective greenhouse gas with a Global Warming Potential (GWP) 23 times as high as that of carbon dioxide (CO2) in a 100-year time horizon. Of the global anthropogenic CH4 emissions, more than 10% originates from landfills (1). Landfill gas (LFG), consisting of about 30-60% CH4 and about 20-50% CO2, is produced in the anaerobic conditions in the waste by microbial degradation of organic matter. The proportion of raw LFG released into the atmosphere varies between different landfill sites and is affected by, for instance, the thickness and compactness of the landfill cover, the gas recovery system, meteorological conditions, and microbial CH4 oxidation. * Corresponding author phone: + 358-9-1929 5498; fax: + 3589-1929 3503; e-mail:
[email protected]. † Finnish Meteorological Institute. ‡ University of Helsinki. 10.1021/es061631h CCC: $37.00 Published on Web 03/15/2007
2007 American Chemical Society
Traditionally, LFG emissions have been estimated based on the amounts, quality, and decay rates of the annually deposited waste and on the rates of gas recovery and microbial CH4 oxidation (2). Due to the several assumptions behind these estimates and the potentially poor information on the quality and amount of the deposited waste, uncertainties in the emission rates are high. In practice, the only way to obtain landfill-level information on the LFG effluxes and, for example, on the gas recovery efficiency is to measure the emissions directly. However, direct measurements are scarce, which is at least partly explained by the problems in the LFG measurement methodologies. Landfills are typically characterized by a large spatial heterogeneity in surface fluxes (3-6). This leads to specific problems in estimating LFG fluxes representative of the whole landfill by means of enclosure methods, which require a large amount of single measurements, supported by geospatial modeling [e.g., ref 7]. In addition, CH4 may escape directly into the atmosphere through cracks, old pipes, and wells, thus bypassing the oxidizing microbial influence (4). With the chamber measurement technique these emissions may remain undetected, requiring rather the use of a measurement technique of a spatially integrating nature, such as the tracer (8, 9) or micrometeorological (10) method. Micrometeorological methods rely on the fact that within the surface layer, typically during the daytime the lowest 0-50 m of the atmosphere, vertical mixing induced by turbulent eddies carries gas down the concentration gradient (11). Here we have used the eddy covariance (EC) technique, in which the three wind components and a scalar, for example temperature or gas concentration, is measured at a high frequency, typically ten times a second. The covariance between the vertical wind speed and the scalar, calculated over an averaging period of usually 30 min, gives the surface flux from an area upwind of the flux mast. The measurement height, the horizontal wind speed, atmospheric stability, and the roughness of the surface determine the magnitude of the area sampled with this technique. In this paper we present the results of the first continuous long-term EC measurements, spanning over 6 months, of CH4 and CO2 in a municipal landfill. Earlier, the EC measurement instrumentation at the same landfill site has been presented, and the applicability of the measurement method in the existing conditions has been demonstrated (10). The nitrous oxide (N2O) emissions have also been measured during a 1-week period (12). The aim of the present study was to examine the factors controlling the landfill gas emission and to observe the possible daily and seasonal variation in the emission rates. We also wanted to find out whether the EC method is suitable for monitoring the effects of landfill gas recovery on CH4 and CO2 emission. In addition, emission rates measured with the EC and chamber methods were compared.
Materials and Methods Experimental Site. The A¨ mma¨ssuo municipal landfill has been in use since 1987, receiving during recent years annually about 350 000 tons of mixed municipal waste. In 2003, the height of the waste tip was 20 m, and the LFG extraction system, which was partly under construction, recovered 46.3 × 106 m3. During the study period, the average CH4 recovery rate at gas pumping station 2, which collected gas from the 8 ha area surrounding our measurements, was 60 m3 ha-1 h-1. Within this area, there were on average 2.8 vertical wells per hectare, comprising 11 new wells, which had a diameter of 1.2 m, penetrating to the base of the landfill, and 9 narrower VOL. 41, NO. 8, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. A map of the measurement site showing the summit area of the landfill and the eastern slope. The location of the flux mast is indicated with the star. The rectangle denotes the measurement cabin. The contours indicate height above sea level. The circle denotes the 150 m radius from the mast, from which area about 90% of the flux originates in neutral conditions. The gray triangles indicate the area of chamber measurements in August 7 (A) and 13 (B) and in October (C). older wells, that were shallower, but each built on a heap of rocks. Additionally, in the western sector there were older horizontal underground trenches, lined with blocks of rock, for gas collection. Measurements of the pressures in the individual wells showed that the underpressure in the older wells was higher (mean -1300 Pa) than in the new wells (-56 Pa), providing more power (mean 430 kW) than the new ones (110 kW). The average CH4 and CO2 concentrations in the recovered gas were 52 and 42%, respectively, corresponding to a CH4/CO2 ratio of 1.24. The flux measurement mast was located in the northeastern part of the landfill (Figure 1). To the west, the landscape was slightly undulating, whereas to the east (45160°) there was a slope of 7°. The northern direction (35020°) was excluded from the analysis due to a cabin at a distance of 30 m from the flux mast, which was assumed to interfere with the turbulence measurements. Earlier, Rinne et al. (12) estimated that under neutral conditions, 90% of the flux measured by the EC method originated from within an area 150 m from the measurement mast, which represents an area of about 7 ha area around the mast. Hence, our main flux source area was of approximately the same size as that of pumping station 2. On the summit of the landfill, waste deposition continued throughout the study period. The new waste was compacted daily and covered with a thin soil layer consisting mostly of construction and demolition waste rejects. On the slope, where no waste deposition was going on, the preliminary cover consisted of a 0.2-0.5 m surface compost soil layer on top of a 0.5-2 m thick diamicton and clay layer. The EC Instrumentation. The CH4 flux instrumentation included an SATI-3SX (Applied Technologies, Inc.) threeaxis sonic anemometer and a Flame Ionization Detector (FID) in a Hewlett-Packard 5890 gas chromatograph chassis for fast-response total hydrocarbon concentration measurements. Dried air samples were fed directly into the FID, from which the column had been removed. Simultaneously with the FID measurements, the CH4 flux was measured with a Tunable Diode Laser (TDL) instrument (TGA-100, Campbell Sci.) (10, 12). We assume that in landfill conditions the total hydrocarbon concentration variations represent those of CH4 well, which was confirmed in a comparison between the FID and the CH4-specific TDL (10). The flow rate of the FID sample 2718
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gas in the tubing was 6 L min-1. The measurement height was 2.5 m. The length of the Bev-A-Line tubing (inner diameter 3.1 mm) for the FID was 9 m. CO2 and water vapor (H2O) fluxes were measured with a LI-7500 (Li-Cor, Inc.) open path analyzer mounted in the vicinity of the anemometer and the sample inlets for FID and TDL. Concentration and anemometer data were sampled at a frequency of 10 Hz, and the fluxes were calculated as 30-min averages. The measurements and the associated corrections and data postprocessing methods have been explained in more detail by Laurila et al. (10), who also demonstrated the applicability of the EC method at this micrometeorologically nonideal site. The FID was calibrated daily against the data measured with the TDL during June-August, after which gases with known concentrations (zero and ambient) were used for the calibration conducted approximately once a week. The LI7500 was periodically calibrated against known concentrations of CO2 and H2O. The CO2 data presented here covered the time periods from February to December 2003. The CH4 flux measurements were initiated in June 2003 with both analyzers and were then extended to August and December with the TDL and FID, respectively. The CH4 flux data used in this paper were mainly measured with the FID; however, there was a 1-week period in July when there were no FID data available, and the TDL data were used instead. Depending on the application, the fluxes have been expressed in this paper either in mg m-2 s-1 or m3 ha-1 h-1 (STP) of CO2, CH4, or N2O. Data Screening. Turbulent flow is not well developed under nearly calm periods or when the temperature stratification is very stable. In these conditions the EC method tends to underestimate the surface fluxes (11). Based on the relationship between the flux and the friction velocity (u*), we omitted all observations with u* lower than 0.1 m s-1. In addition, we omitted the data, when the variance of the measured quantity was abnormally high, or when there were more than 20 obvious spikes in the data during the 30-min period, indicating heavy rain or technical problems affecting the functioning of the instruments. Flux Measurement with the Closed Chamber Method. CH4, CO2, and N2O fluxes were measured four times in total using the closed static enclosure technique (13). On August 7 and 13, the measurements were conducted northwest and southeast of the EC flux mast, respectively, and on October 23 and 31 in a sector of east from the mast. A few days before the measurements, 7-10 steel collars (0.6 m × 0.6 m × 0.2 m) were inserted to a depth of 0.10 m in the soil. An aluminum chamber (height 0.2 m) equipped with a pressureequilibrium tube (length 100 mm, diameter 3 mm) was placed on the collar successively at each of the measurement points. To ensure an airtight connection, the chamber was compressed against a foam rubber insulated groove with four clamps. Four air samples at even intervals were drawn from the chamber during the 4-7-min closure time. Air samples were analyzed with a gas chromatograph as described in ref 13.
Results and Discussion CH4 and CO2 Emissions Measured with the EC Method. The 30-min averaged CO2 flux measured from February to December (n ) 6038) ranged from 0 to 110 m3 ha-1 h-1 (equal to 0-6 mg m-2 s-1), the average flux being 32.6 ( 20.5 (SD) m3 ha-1 h-1 (1.78 mg m-2 s-1). The corresponding CH4 emission rate measured from June to December (n ) 2920) ranged from 0 to 151 m3 ha-1 h-1 (0-3.0 mg m-2 s-1) with an average flux of 26.7 ( 24.2 m3 ha-1 h-1 (0.53 mg m-2 s-1). The day-to-day variation in the emissions of both gases was large and strongly related to the flux source area, mainly determined by the wind direction (Figure 2). The highest
FIGURE 2. CH4 and CO2 fluxes measured with the micrometeorological method (circles) in August 1-14 plotted against the wind direction, together with the fluxes measured with the chamber technique (triangles with error bars) on August 7 in the direction of 315° from the flux mast and on August 13 (155°). The micrometeorological data are shown as half-hourly values; the chamber data are shown as the mean ((SD) of the 7-10 chamber plots. fluxes of CH4 and CO2 were typically observed from the southwestern and western sectors, representing the summit area. Possibly due to the strong wind-direction dependency and high spatial variation of the emissions, no correlation between the short-term LFG emission and the meteorological conditions was found. It has been proposed that changing ambient air pressure (14, 15) or diurnal temperature variation in the surface soil affecting the CH4 oxidation (16) could be responsible for the short-term dynamics in landfill CO2 and CH4 fluxes. To examine closer the spatial and seasonal variation in the LFG emission, we calculated 2-week mean emissions for two sectors: the slope in the east (45-160°) and the summit of the landfill in the west (200-280°) with active waste deposition, daily covering, and less efficient gas recovery. Throughout the whole measurement period the LFG emissions from the summit were higher than those from the slope area (Figure 3); the mean CH4 emissions were 39.8 and 22.7 m3 ha-1 h-1, respectively, while the CO2 emissions averaged 39.2 and 27.9 m3 ha-1 h-1, respectively. We suggest that the higher emissions from the summit area are due to the uncovered surface and the daily deposited new waste. In the slope area there was no recent waste disposal activity, and the old waste was better covered, resulting in more efficient microbial CH4 oxidation (17). The seasonal variation in the CO2 emission was highest in the summit area. The maximum emissions occurred in July-August and the minimum in February (Figure 3), and the seasonal behavior of the summit CO2 flux seemed to follow that of the air temperature. Most of the measured CO2 originates from the deep waste LFG and is independent of the cover soil temperature and thus on seasonally changing factors. However, CH4 oxidation and the aerobic decay of organic matter (i.e., microbial respiration) also produce CO2. The temperature dependence of these aerobic processes taking place in the surface layers at least partly explains the higher CO2 emission in the summer. In the summit, the waste deposition also probably contributed to the increased emission, since in July and August the site of the active waste deposition located very close to our measurement mast.
FIGURE 3. Two-week averages of the CH4 (white bars) and CO2 (gray bars) flux from the summit (200-280°) and from the slope (45-160°) area. Also shown are daily precipitation and half-hourly air temperature at the site. The asterisks denote the periods with missing data, either CO2 or CH4, or both. There was no marked seasonal variation in the CH4 release (Figure 3). The highest fluxes were observed in August both on the slope and on the summit. On the summit, the peak emission could be explained by the waste deposition near to the flux mast. However, as there was no waste deposition activity on the slope, this does not explain the increased emissions from there. A rain period in the mid-August may have been responsible for this. Excess water may have, e.g., decreased the CH4 oxidation of the cover soil or increased the CH4 production. Although our measurements only extended into the late autumn, it can be concluded that the decrease in air temperatures close to or slightly below zero did not affect the CH4 release rates significantly. Previously, higher CH4 emissions during the cold season have been observed [e.g., refs 3 and 16]. This phenomenon has been generally connected to the inhibition of microbial CH4 oxidation at low temperatures. It is worth noticing that winter conditions are very challenging for making chamber measurements due to frozen soil, snow layer, and ice and water lenses at the soil surface (18). Interestingly, in a Swedish study, in which an area-integrating measurement method was used, no significant seasonal variation was found (8). Measurements during the snow-covered time of the year with the EC method, which does not interfere with the soil structure or the snow cover, would be useful for assessing the seasonal variation in LFG emissions. In Table 1, we have collected data from various studies conducted at landfills with an area-integrating emission measurement method. The emissions ranged from 0.05 to 1.5 mg CH4 m-2 s-1. The lowest emission was measured in The Netherlands at a site (Nauerna) having gas recovery, while the highest emissions took place at sites with no gas recovery (PLF-A and Nashua), highlighting the reducing effect of gas recovery on the atmospheric fluxes. At Nauerna, containing industrial, construction, and demolition waste, very different results were obtained in consecutive years. This was explained by the different time of the year and the freshly dumped waste (9). The highest CH4 emission (0.18 mg m-2 s-1) was observed in November with low temperatures, which may have decreased the CH4 oxidation. The VOL. 41, NO. 8, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. CH4 Emissions from Different Landfills Measured by Area-Integrating Methods site PLF-A (U.S.A.) PLF-D (U.S.A.) Nashua (U.S.A.) Nauerna (NL) Nauerna (NL) Nauerna (NL) Falko¨ ping (SWE) Landfill 4 (NL) Amma¨ ssuo (FIN), summit A ¨ mma¨ ssuo (FIN), slope
waste typea
covering
gas recovery
not reported not reported, FDW MSW, C&D, FDW IND, C&D, PS, SL, FDW IND, C&D, PS, SL, FDW IND, C&D, PS, SL, FDW MSW, C&D, IND, SL MSW, FDW
partial partial partial preliminary
no yes no no
preliminary
measurement method
mean CH4 emissionb
time of measurements
tracer tracer tracer tracer
1.5 0.54 0.90 0.10
Oct-Nov 1994 Oct-Nov 1994 Oct-Nov 1994 spring 1997
19 19 15, 19 9
yes
tracer
0.05
spring 1998
9
preliminary
yes
tracer
0.18
autumn 1999
9
preliminary partial
yes yes
tracer isotopic tracer
0.2-0.5c 0.19
8, 20 16
MSW, C&D, FDW
daily
yes
EC
0.79
all seasons all seasons but winter Jun-Dec 2003
MSW, C&D
preliminary
yes
EC
0.45
Jun-Dec 2003
this study
ref
this study
a C&D ) construction and demolition waste; IND ) industrial waste; MSW ) municipal solid waste; PS ) polluted soil; SL ) sludge; FDW ) freshly dumped waste. b mg CH4 m-2 s-1. c Only annual min. and max. reported.
lowest emission rate (0.05 mg m-2 s-1) was measured in spring. Although there was partial gas recovery at our site, the fluxes measured at A¨ mma¨ssuo appear relative high compared to Nauerna. At the Falko¨ping landfill (SWE), the measurements covered the whole year, showing quite constant emissions over the whole period. The emissions were similar or only slightly lower than at our site. Comparison of the Chamber and Micrometeorological Methods. The fluxes measured with the chamber and micrometeorological methods agreed reasonably well, although the number of chamber plots was relatively small (Figure 2). The spatial variation in the chamber fluxes was very large on August 13 in the direction of 155° from the flux mast, when the mean emissions ((SD) of CH4 and CO2 were 0.87 ( 2.2 and 5.8 ( 8.4 mg m-2 s-1, respectively. On August 7, both the spatial variation and the mean chamber fluxes were smaller, 0.07 ( 0.13 and 2.2 ( 2.4 mg m-2 s-1 for CH4 and CO2, respectively. The relative variation of the chamber CO2 fluxes was larger than that of CH4. Relationship between the Emissions of Different Gases. CH4 and CO2 emissions were strongly correlated with each other. The volume ratio of emitted CH4 and CO2 may be regarded as a rough indicator of the degree of CH4 oxidation. If the CH4-CO2 ratio of the emission is smaller than that of the LFG measured in the gas recovery wells (1.24 at our site), CH4 oxidation has presumably occurred. Here we show an example from a 1-week period of EC measurements in August (Figure 4). With low emissions (40 m3 CH4 ha-1 h-1), the ratio of CO2 and CH4 emissions (1.26) was practically identical to that of the raw LFG (Figure 4). This indicates that, at that time, most of the LFG emission from the summit area had avoided CH4 oxidation. The CH4-CO2 volume ratio in the measured emissions always stayed at or below that in the raw LFG, which serves as an indication of the reliability of our measurements. The long-term volume ratios of CH4 and CO2 indicate that with one exception in August on the summit, the volume of emitted CO2 exceeded that of CH4 during each 2-week period (Figure 3), the CH4-CO2 ratios being on average 0.77 and 0.59 on the summit and slope, respectively. The results imply that also in the long term, more oxidation took place on the slope with better covering than on the summit area. 2720
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FIGURE 4. CH4 vs CO2 emission (triangles) measured with the eddy covariance method on August 1-8. Also shown is the ratio of CH4 to CO2 concentrations in the landfill gas deep in the waste (solid line) and the observed ratio of CH4 to CO2 fluxes for low fluxes (40 m3 CH4 ha-1 h-1; dash-dot line). Effects of Gas Recovery on the Landfill Gas Emissions. The short-term effect of the gas recovery system on the atmospheric fluxes was tested twice, in August and October, by stopping it temporarily. The test area was located in the eastern sector, between approximately 45° and 140° from the flux mast. The gas collection was stopped between August 15 and 22, when the wind direction was favorable for several days for making EC measurements from the recovery area. The CH4 flux decreased by 79% when the gas collection was on (mean flux 0.37 mg m-2 s-1), as compared to the situation in which the gas recovery was turned off (1.79 mg m-2 s-1) (Figure 5). At the same time, the CO2 emission decreased by 62% (results not shown). The gas collection was again stopped on October 20-24. On this occasion, there were no EC observations from the test area due to unfavorable wind directions. However, a chamber measurement campaign was conducted before and during that period. The gas recovery decreased the CH4 flux on average by 39% as compared to the emission when the recovery was off (0.45 vs 0.75 mg m-2 s-1) (Figure 6). The average CO2 and N2O fluxes also decreased but much less than that of CH4 (on average 12 and 18%, respectively). Surprisingly, in some chamber plots all gas emissions increased with the turning-on of the gas recovery system. Again, the spatial variability was rather high, some of the plots showing CH4 and nitrous oxide uptake (Figure 6).
FIGURE 5. CH4 emission in different wind directions when the gas recovery was off (August 15-22; open circles) and on (combined data from August 1-14 and August 23-27; closed circles). The curves show the smoothed data based on a polynomial regression and weights computed from the Gaussian density function (sampling proportion ) 0.1; polynomial degree ) 1). The sector representing the effective gas recovery experiment area (45°-140°) is also indicated.
the gas collection system was able to recover 69% of the CH4 formed. The CH4 recovery efficiencies estimated here (79 and 69%) are slightly lower than that of 90% estimated by Mosher et al. (19) in the United States. The lower emission reduction (39%) found in October with the enclosure technique may be explained by the fact that part of the observed CH4 flux had escaped through different structures and remained thus undetected by the chambers. The larger uncertainty associated with the chamber method and the small number of measurement points also partly explains the difference between the methods. In some chamber plots the efflux actually decreased when the gas recovery was off, suggesting the area-integrating EC method to be more appropriate for monitoring such large-scale effects. The recovery efficiencies estimated here may be biased due to uncertainties in the gas collection and in the CH4 emission data. Since the reported gas collection rates were relatively similar at different pumping stations, the error related to the different areas of EC measurement and gas recovery should be relatively small. Here, collection efficiencies of 69 and 79% were obtained based on the gas volume collected during the measurement period and the shortterm gas recovery experiment, respectively. Assuming that the variability of fluxes in the direction of the gas recovery experiment (45-140°, Figure 5) represents measurement uncertainty, we obtain standard errors of 0.13 mg m-2 s-1 (mean 1.79 mg m-2 s-1) and 0.03 mg m-2 s-1 (mean 0.37 mg m-2 s-1) when the gas recovery was off and on, respectively. From these we obtain a combined uncertainty of (19% (95% confidence interval) for the collection efficiency of 79%. Even though the micrometeorological EC method is subject to some restricting assumptions (10), it is capable of producing continuous, area-averaged estimates of the landfill CH4 emission. The EC technique causes no disturbance to the surface under examination, a problem which is very difficult to avoid with the chamber technique. The simultaneous CO2 efflux measurement may be used in evaluating the effectiveness of the oxidizing surface cover and finding possible leakage points. Although not shown here, the water vapor flux from the landfill surface is measured concurrently with the CO2 flux, enabling the estimation of evaporation, an essential part of the water balance of the landfill. One problem with the EC method in heterogeneous environments such as landfills is the dependence of the measurement on the wind direction. For example, monitoring of day-to-day variation in emissions is difficult with constantly changing wind directions. A solution for this might be having more than one flux mast at the same landfill.
Acknowledgments
FIGURE 6. Emissions of CH4, CO2, and N2O from ten chamber plots in the east sector when the gas recovery was off (October 21; white bars) and on (October 31; black bars). In addition to the interruption experiment, the long-term effect of gas recovery could be estimated by comparing the average fluxes to the amount of collected gas during the halfyear measurement period. The average CH4 emission from the site was 27 m3 ha-1 h-1. At the same time, approximately 60 m3 CH4 ha-1 h-1 was collected at gas pumping station 2. Assuming that the total CH4 production was a sum of these,
We thank Reetta Anderson, Matti Ettala, Kirsi Karhu, Sauli Kopalainen, and Vesa Nyka¨nen for information on landfill processes and gas collection data and YTV Ja¨tehuolto for the measurement site and help with the measurements. We acknowledge financial support by the National Technology Agency of Finland (TEKES), the Academy of Finland, YTV Ja¨tehuolto, the Rebecca project (Helsinki University Environment Centre), and the Nordic Council of Ministers through the Nordic Centre for Studies of Ecosystem Carbon Exchange and its Interactions with the Climate System (NECC).
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Received for review July 10, 2006. Revised manuscript received November 7, 2006. Accepted February 2, 2007. ES061631H
