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a Imperial Oil Resources, Research Centre, 3535 Research Rd. N.W., Calgary, AB, Canada T2L 2K8
b College of Science and Management, Univ. of Northern British Columbia, 3333 University Way, Prince George, BC, Canada V2N 4Z9
c Matrix Solutions Inc., 230, 319-2 Avenue S.W., Calgary, AB, Canada T2P 0C5
d Alberta Research Council, P.O. Box 4000, Vegreville, AB, Canada T9C 1T4
* Corresponding author (julie.roy{at}esso.ca)
Received for publication November 20, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DCM, dichloromethane • DEO, dichloromethane-extractable organics • MED, molarity of ethanol droplet • OC, organic carbon • TOC, total organic carbon
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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A common means of assessing for the presence of petroleum compounds in soil is to extract them with dichloromethane (DCM). (The Canadian Council of Ministers of the Environment (CCME) recommends using an equal mixture of n-hexane and acetone to extract petroleum hydrocarbons from soil since 2001 [Canadian Council of Ministers of the Environment, 2001].) The gravimetric content of dichloromethane-extractable organics (DEO) is used as an estimate of the content of petroleum compounds in soil. Because DCM also extracts components of native organic material from soil (e.g., biogenic lipids), the estimate is typically corrected for local background DEO. Some background DEO may also originate from atmospheric deposition of petroleum-based aerosols and particulates (Craul, 1985). Control adjacent soil in the present report refers to soil that does not contain petroleum-derived compounds other than from atmospheric deposition. Its DEO content is within the natural variability expected for local background DEO. For analysis purposes, we define it quantitatively as soil that has an MED = 0 M and DEO content < 1000 mg kg-1.
In 1998, representatives of the provincial government and oil industry concerted to fund an investigation to address the question of the extent and potential causes of soil water repellency in Alberta. In addition to characterizing poorly known water-repellent sites from across the province, the investigation had two main objectives. The first was to determine if 26 water-repellent sites have common landscape-scale characteristics, such as topography, hydrology, vegetation, or soil properties, that may have predisposed them to accumulate hydrophobic material. The second was to compare the spatial distribution of soil water repellency and residual oil contamination in soil at 12 sites to determine if there is a positive correlation between DEO content in soil and severity of soil water repellency. Lowen (2001) describes the protocol followed and the results achieved in completing the first objective. The present report describes the protocol followed and the results achieved in completing the second objective.
Crude oil in soil can be separated into a DCM-soluble and a DCM-insoluble fraction. The relative size of each of these fractions depends mainly on crude oil composition, the soil capacity for sorption of crude oil compounds, and the extent of weathering that crude oil has undergone in soil. In Roy et al. (1999), we tested and rejected the hypothesis that DCM-soluble petroleum compounds cause soil water repellency because exhaustive extraction with DCM barely altered the degree of water repellency displayed by soil. In the present work, we tested the hypothesis that soil water repellency increases with increasing soil DEO content across a variety of sites. We proceeded in this direction with the expectation that the size of above-background DEO and DCM-insoluble fractions in petroleum-contaminated water-repellent soils may bear a positive correlation to one another.
In parallel, we studied how control adjacent and water-repellent soils differ in terms of DEO and total organic carbon (TOC) content. First, we tested whether DEO content alone could explain most of the difference in TOC content between water-repellent and corresponding control adjacent soils. Second, we tested if background DEO could be predicted from TOC in control adjacent soil.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The 26 sites were inspected a first time (Phase 1) to delimit and measure the size of water-repellent area(s), assess the severity of water repellency, and collect information from land users about site management history. A soil pit (60 cm deep) was excavated inside a water-repellent patch to describe the subsurface soil and record, if present, the form of detectable residual contamination (e.g., petroleum odor, oily stains, or aggregates cemented by asphalt-like material). Surface and subsurface soil samples were collected for assessment of water repellency under laboratory conditions. Soil texture, surrounding vegetation, and site topography were described.
The database built with the results of Phase 1 inspections at 26 water-repellent sites was used to select 12 sites for intensive Phase 2 soil sampling and analysis. The latter sites were selected for their diversity in soil texture, soil classification, and dominant vegetation (Table 1).
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One-liter samples were collected from the entire depth of each horizon, including oily spots when they were present. Soil profiles were described according to the Canadian System of Soil Classification (Soil Classification Working Group, 1998). Soil samples were transported to the University of Alberta in Edmonton, AB, and placed in cold storage at 4°C for as long as two months before analysis.
Grid Samples
Soil samples were collected at each site with a flexible grid sampling protocol. The area of water-repellent patches was determined at each site with the water droplet penetration test (Letey, 1969). The most severely affected areas within each patch were identified with the density of plant cover as an index of the degree of soil water repellency. A rectangular grid with 24 intersecting points was laid out. Grid cell size was adjusted at each site so that approximately half of the sampling points would fall in water-repellent soil and half would fall in control adjacent soil. A full 500-mL jar of soil was collected from the 0- to 10- and 20- to 30-cm layers at the 24 grid sampling points. A spoon was used to sample the 0- to 10-cm layer; a hand auger was used to sample the 20- to 30-cm layer. Additional samples of adjacent control soil were collected at four locations 
25 m away from sampling grid borders at each site. Soil samples were transported to the Research Centre (Imperial Oil Resources) in Calgary, AB, and placed in cold storage at 4°C for as long as two months before analysis.
Laboratory Analyses
Soil Preparation
All profile and grid samples were spread in drying dishes and air-dried for more than seven days. They were then sieved to <2 mm, homogenized, and transferred back into their original sampling jars.
Total Organic Carbon Content
Total organic carbon (TOC) was obtained with a Carlo Erba (Milan, Italy) NA-1500 Series II CN elemental analyzer following the method of Ellert and Janzen (1996). Briefly, soil samples were air-dried, ground to a maximum particle size diameter of 150 µm, oven-dried at 60°C overnight, and cooled to room temperature in a desiccator. Total carbon was measured on whole soil samples by high temperature oxidation, gas chromatographic separation, followed by thermal conductivity detection. Carbonates were removed with 6 M HCl and total carbon was measured again, yielding TOC.
Molarity of Ethanol Droplet Assessment
The molarity of ethanol droplet (MED) test was used to assess the severity of soil water repellency (Watson and Letey, 1970; King, 1981). Soil preparation before MED assessment consisted of further sieving to <1 mm and then <85 mm, oven-drying at 105°C for 24 h, and cooling to room temperature in a desiccator. These steps were followed to standardize test conditions and reduce variability in MED test results (Roy and McGill, 2002). In our experience, reducing the amount of small rocks and plant fragments and air- and oven-drying soil improves the precision of the test without significantly altering the mean MED value of the soil (Roy and McGill, 1998, 2000a). Gentle grinding was used to detach soil particles adhering to plant and rock fragments. The final <85-mm mesh size was considered acceptable because >95% v/v of the 1-mm sieved soil passed through with gentle shaking (Roy and McGill, 2002). Remaining rock and plant fragments were picked out with tweezers.
Thirty-one solutions of different ethanol concentrations were prepared to include all 0.2 M increments in the 0 to 6 M range. Soil samples (approximately 20 g) were poured into aluminum dishes (70-mL capacity) so that thickness of the samples in the dish was 0.75 mm. The MED test was performed by placing droplets (2- to 3-mm diameter) of increasing ethanol solution concentration on the leveled soil surface and by assessing the time required for their complete absorption by the soil. (This work was completed before the theory presented in Roy and McGill [2002] was fully developed and published.) The MED index is the molarity of the least concentrated ethanol solution that is absorbed by soil in a mean time of 
10 s. The reported MED values were derived by applying 10 to 20 droplets of increasing ethanol concentration to the soil until the MED value was narrowed down to one aqueous ethanol concentration (Roy and McGill, 2002). The tests were duplicated and mean MED indices were used in the analyses.
Dichloromethane-Extractable Organics Content
The content of dichloromethane-extractable organics (DEO) in soil was determined by Soxhlet extraction using glass-distilled DCM (McGill and Rowell, 1980). Extractions were performed with approximately 20-g samples of air-dried soil thoroughly mixed with a comparable volume of granular anhydrous MgSO4. Another 20-g air-dried soil sample was used to determine soil moisture content. Soxhlet extraction was performed for a minimum of 6 h at the rate of 10 cycles h-1. Few samples required more than 6 h before solvent in the extraction chamber returned to its clear and colorless, glass-distilled appearance.
The DCM extracts were concentrated with a rotary evaporator, transferred into preweighed aluminum dishes, and allowed to dry to constant mass in a fume hood. The DEO content in soil was calculated as follows (Eq. [1]):
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Thirteen of the 119 DEO content determinations on profile soil samples and 46 of the 224 DEO content determinations on grid soil samples were replicated (Table 2). We used the mean of replicated DEO determinations in all analyses reported in this paper.
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Summary of Analyses Performed
All profile samples were analyzed for MED, DEO, and TOC (n = 119). Grid samples from the 12 sites were analyzed for MED (n = 672), but only those from four of the 12 sites were analyzed for DEO (n = 224). The four sites were chosen because they presented the sharpest lateral gradients in MED and contained 8 sampling points with MED = 0 M throughout the 0- to 30-cm depth (data not shown). Total organic C was not determined for any of the grid samples (Table 2).
Statistical Analyses
Analysis of DEO, TOC, and MED data revealed dependence of the standard deviation on the mean and significant nonnormality in the data. The nonparametric Kruskal–Wallis test was used to test the null hypothesis that the different horizon–soil combinations have the same DEO, MED, and TOC median (95% confidence level). Comparison of median notches in box-and-whisker plots was used to determine which sample medians did not differ significantly from each other. The median notches in box-and-whisker plots represent an approximate 95% confidence interval for the median (Statistical Graphics Corporation, 2000). They are the error bars depicted in Fig. 1 through 3
. Comparison of medians and regression analyses were performed with Statgraphics Plus Version 5.0 for Windows 95/98/NT (Statistical Graphics Corporation, 2000). Figures were graphed with Microcal Origin Version 6.0 (Microcal Software, 1999). Medians with a common letter are not significantly different at the 95% confidence level. The numbers in brackets next to the letters represent sample size (n).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Vertical Distribution of Total Organic Carbon in Control Adjacent and Water-Repellent Soil
The vertical distribution of TOC in control adjacent and water-repellent soil was determined with the profile sample data set. Control adjacent soil was defined as having MED = 0 M and DEO < 1000 mg kg-1. Such criteria were chosen because, in our experience, they reflect MED and DEO values commonly found in uncontaminated and unamended mineral soil. Water-repellent soil was defined as having MED > 0 M and DEO < 30 000 mg kg-1. Samples containing 30 000 mg kg-1 DEO were excluded from the analysis because they were visibly intermixed with tarry or asphalt-like material, which severely complicated interpretation of MED test results. Five samples with MED = 0 M were thus excluded from the analysis because they had a DEO content ranging from 1000 to 3500 mg kg-1. Of these, three were from the A horizon and two were from the B horizon. Eleven samples with MED > 0 M were similarly excluded because they had a DEO content ranging from 30 000 to 91 100 mg kg-1. Of these, seven were from the A horizon and four were from the B horizon.
We found a median TOC content of 22 100, 5100, and 3410 mg kg-1 in control adjacent soil from the A, B, and C horizon, respectively, and of 36 900, 9300, and 11 500 mg kg-1 in water-repellent soil from the A, B, and C horizon, respectively (Fig. 2).
Vertical Distribution of Dichloromethane-Extractable Organics in Control Adjacent and Water-Repellent Soil
The vertical distribution of DEO in control adjacent and water-repellent soil was determined with the combined profile and grid sample data set. Grid samples from the 0- to 10-cm layer were grouped with profile samples from the A horizon and grid samples from the 20- to 30-cm layer were grouped with profile samples from the B horizon. Control adjacent soil and water-repellent soil were defined as described above.
In all, eight samples with MED = 0 M were excluded from the analysis because they had a DEO content ranging from 1000 to 3500 mg kg-1. Of these, five were from the A horizon, one was from the B horizon, and two were from the C horizon. Nineteen samples with MED > 0 M were also excluded from the analysis because they had a DEO content ranging from 30 000 to 91 100 mg kg-1. Of these, ten were from the A horizon and nine were from the B horizon.
We found a median DEO content of 383, 216, and 44 mg kg-1 in control adjacent soil from the A, B, and C horizon, respectively, and of 4950, 3900, and 10 500 mg kg-1 in water-repellent soil from the A, B, and C horizon, respectively (Fig. 3).
Spatial Distribution of Soil Water Repellency at the Twelve Study Sites
All samples collected in this study were assessed for soil water repellency with the laboratory MED test protocol described in the Materials and Methods section. In all, we collected 791 soil samples from two or more depths at 336 grid sampling points and 24 profile sampling points at 12 sites. Of those, 398 were from A horizons (or the 0- to 10-cm layer), 369 from B horizons (or the 20- to 30-cm layer), and 24 from C horizons. The proportion of samples with MED > 0 M amounted to 76.4% of those from A horizons, 30.1% of those from B horizons, and 12.5% of those from C horizons (data not shown). This simple analysis demonstrates that soil water repellency was encountered more frequently in samples from A horizons, or the 0- to 10-cm layer, than in samples from underlying soil. From this, we conclude that soil water repellency is predominantly a surface phenomenon at the study sites.
Relationship between Dichloromethane-Extractable Organics and Total Organic Carbon in Control Adjacent Soil
We correlated DEO and TOC content in control adjacent soil using profile samples with MED = 0 M and DEO < 1000 mg kg-1. The <1000 mg kg-1 limit on DEO content considerably improved the correlation coefficient of the relationship between DEO and TOC. Without it, the r2 was less than 0.10 (data not shown). The equation is: DEO (mg kg-1) = 0.02 (TOC, mg kg-1) - 0.14; r2 = 0.85; n = 40 (Fig. 4)
. It indicates that approximately 85% of the variation in DEO can be predicted from TOC in control adjacent soil. It also indicates that approximately 2% of TOC in control adjacent soil is extractable by DCM. If this relationship holds more widely, it may be useful in assessing remediation endpoints and in correcting DEO values for the contribution by native organic material in soil.
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Relationship between Dichloromethane-Extractable Organics and Molarity of Ethanol Droplet in Water-Repellent Soil
The correlation between MED and DEO was analyzed with the profile, grid, and combined sample data sets (Fig. 5)
. To analyze data from profile and grid samples in combination, we grouped grid samples from the 0- to 10-cm layer with profile samples from A horizons, and grid samples from the 20- to 30-cm layer with profile samples from B horizons. We limited the analyses to samples with MED > 0 M and DEO < 30 000 mg kg-1 because samples with DEO contents 30 000 mg kg-1 contained too much free product to enable a reliable assessment of soil water repellency.
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The analyses indicate that there is a strong positive correlation between soil DEO content and severity of soil water repellency. This correlation, however, is not taken as proof of causality because Roy et al. (1999) showed that DEO extraction at best only slightly reduces soil water repellency. Instead, we propose that DEO is positively correlated to MED in water-repellent soil because DCM-soluble OC of petroleum origin is positively correlated to DCM-insoluble OC of petroleum origin in water-repellent soil. Concurring evidence presented in Roy et al. (1999) and Litvina (2001) supports the hypothesis that the latter fraction contains most of the compounds imparting water repellency to soil at old crude oil spill sites. This hypothesis is further supported by the analysis presented in Table 3, which indicates that water-repellent, A-horizon soil contains about 29% more TOC than can be explained by accounting for by background TOC and DCM-soluble OC of petroleum origin.
Most crude oil mixtures naturally contain a small fraction of DCM-insoluble material. This fraction, however, tends to increase substantially when crude oil is released into soil due to irreversible sorption to mineral and organic soil colloids over time (Connaughton et al., 1993; MacLeod and Semple, 2000; Reid et al., 2000). As a result, the DCM-insoluble fraction tends to become a greater proportion of the OC of petroleum origin remaining in soil over time. Soils that developed soil water repellency several years or decades following oil contamination were typically left untreated following the contamination event. Work reported in Toogood (1977) suggests that rapid intervention (i.e., cultivation, fertilization, and seeding) following oil contamination might have prevented the buildup of DCM-insoluble OC of petroleum origin and the development of soil water repellency at some sites.
The 12 sites included in this study were chosen because they were known to display soil water repellency. They were otherwise poorly characterized with respect to native chemical and physical soil properties and presence of petroleum-derived OC in soil. The fact that petroleum residues were found at the 12 sites was in itself an important finding of the study. The literature contains hundreds of reports on soil water repellency and petroleum decomposition in soil, but comparatively few that discuss a potential association between the two. While we are confident that above-background DEO in the water-repellent soils under study was of petroleum origin, we know that DCM extracts nonpolar and weakly polar compounds of any parental origin. Because plant residues and soil microorganisms contribute DEO compounds to soil, the positive correlation between MED and DEO may not apply exclusively to petroleum-contaminated, water-repellent soils.
This study began with a province-wide search for as many water-repellent sites as could be found. It was conducted by interviewing several dozen provincial government and oil company employees over a one-year period. The search produced 35 locations. A systematic inspection of oil-contaminated sites in the province might have produced a greater number of water-repellent sites, but it was not a practical option. Based on the small number of sites we were able to find, we conclude that development of soil water repellency following oil contamination is rare and probably a product of an unusual combination of circumstances. We can only speculate that possible contributing factors may involve: properties of the crude oil spilled, dryness of the soil at the time of the spill, and prolonged exposure to hot and dry weather conditions following oil contamination.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Measured TOC is about 29% higher in water-repellent, A-horizon soil and approximately 10% higher in water-repellent, B-horizon soil than can be explained by accounting for background TOC and DCM-soluble OC of petroleum origin. We think that this TOC fraction consists of DCM-insoluble OC of petroleum origin occurring in soil as intact and weathered petroleum residues. Because it is consistent with soil water repellency developing following oil contamination and with DCM extraction barely altering the MED index of water-repellent soils (Roy et al., 1999), we propose that this fraction contains most of the causative agents of soil water repellency at the studied sites.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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