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a Biological and Agricultural Engineering, Kansas State University, 147 Seaton Hall, Manhattan, KS 66506
b Agronomy Department, Purdue Univ., 1150 Lily Hall of Life Sciences, West Lafayette, IN 47907-1150
c Civil Engineering, Purdue Univ., 1284 Civil Engineering Building, West Lafayette, IN 47907
* Corresponding author (pschwab{at}purdue.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: GC, gas chromatography • TPH, total petroleum hydrocarbons
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The water content in soils is continually changing. Fluctuations in soil moisture can affect microbial populations, nutrient availability, and soil physical structure. Cycling of the water content can liberate occluded organic matter resulting in increased microbial activity (Lundquist et al., 1999; Tate, 1995). Drying and rewetting of soils enhances carbon and nitrogen mineralization (Van Gestel et al., 1993), the magnitude of which is controlled by soil texture and total carbon content. Using four different soil types, West et al. (1992) found that rewetting of dried soils resulted in increased respiration. The similarity of microbial response in contrasting soils suggests that microbial communities have similar survival strategies to resist desiccation. Jager and Bruins (1975) concluded that the loss of carbon because of microbial respiration in soils with cycling water contents was proportional to the temperature at which the soils were dried, suggesting that more severe treatments resulted in increased availability of organic matter for microbial growth.
Changes in soil water content occur naturally through precipitation, flooding, and evaporation. Soil water can be controlled with proper irrigation and drainage. Five basic methods can be used to apply irrigation water: flooding, furrows, sprinklers, trickle, and subsurface irrigation (Linsley and Franzini, 1979). Although sprinkler irrigation is the most common method for turf grasses, new subsurface irrigation methods are gaining popularity in some areas (Barth, 1999).
Subsurface irrigation offers several benefits including uniform application, reduced evaporative losses, and the ability to apply water and chemicals directly to the root zone (Schwankl et al., 1990). Studies on wheat and barley have shown that root biomass increases with subsurface application of N, P, and K (Jackson et al., 1992; Kushnak et al., 1992; Jackson and Caldwell, 1989; Drew, 1975). Murphy and Zaurov (1994) found that subsurface application of fertilizer increased root biomass with depth in perennial ryegrass whereas broadcasting fertilizer on turf may stimulate aggressive rooting in the surface soil while limiting deeper root development.
Available water and nutrients are important factors for phytoremediation of soils contaminated with hazardous organic compounds. Degradation of petroleum organic compounds is greater in the presence of roots because of enhanced microbial metabolism or cometabolism of the contaminants in the rhizosphere (Cunningham et al., 1996). Therefore, soil moisture should be optimal for both plant root growth and soil microbial activity. Due to the physical characteristics of these soils, maintaining optimal water content for these aerobic processes can be difficult. In unvegetated areas, infiltration is slow because of the hydrophobic nature of the contaminants, and water is tightly held in the soil matrix, generally resulting in unfavorable conditions for aerobic microbial activity. After vegetation is established, evapotranspiration accelerates the removal of water from the soil profile, and the water must be replenished. Understanding the effect of different irrigation methods on phytoremediation of aged contaminated soil can lead to improved site design and enhanced soil remediation potential.
One of the limitations of phytoremediation is the depth of effectiveness. Metabolically active plant roots generally extend to depths of less than 1 m, and management techniques are in demand that can increase the rooting depth resulting in increased phytoremediation efficiency. A greenhouse study was conducted to evaluate the effect of water placement and nutrients on root growth and the remediation of total petroleum hydrocarbons (TPH) in an aged contaminated soil, a petroleum sludge from a Chevron oil refinery in Richmond, California. The primary objective was to determine if subsurface irrigation–fertilization affects the zone of remediation by increasing root growth and/or microbial activity with depth.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Contaminated Soil
The growth medium was a petroleum sludge generated during the refining process at a Chevron oil refinery in Richmond, CA. Gas chromatographic analysis (see below) suggested that the sludge contained carbon chains from C10 to C35, with the largest fraction falling between C21 and C35. The petroleum sludge had been held in clay-lined oxidation ponds for more than 40 yr. Results from a soil analysis conducted by the Kansas State University Soil Testing Laboratory are shown in Table 1. All analyses were performed according to standard soil testing procedures (Brown, 1997). The pH was measured by glass electrode in a 1:1 water suspension; available P by the Bray method; exchangeable cations (Ca, K, Mg, Na) by ammonium acetate extraction; organic matter by Walkley–Black method; Cu, Mn, Fe, and Zn by DTPA extraction; Al by KCl; cation exchange capacity (CEC) by NH+4 saturation; texture by hydrometer; NH+4 and NO-3 by KCl extraction; total P by peroxide digest; and total N by dry combustion.
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Column Design
Columns (Fig. 1) were constructed of 15-cm-diameter PVC pipe and were 1 m in height. Subsurface water inlets were located 30 cm below the surface. Water potential was measured using microtensiometers placed at 30 and 70 cm from the surface. A 1-bar porous plate was placed in the bottom of each column, and a slight vacuum was imposed on the porous plate to ensure unsaturated conditions and facilitate downward water movement through the column.
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Irrigation
Peristaltic pumps were used to apply water to the soil surface and at 30 cm below the surface. The two depths simulated the use of different irrigation systems and tested the effect of subsurface water and nutrient application on the remediation zone. Two different irrigation schedules also were tested: daily addition of water to maintain the soil at or near optimum moisture conditions, and weekly applications of water that allowed the soil to dry near the wilting point before rewetting the soil. The optimal irrigation rate was a daily application of water at 30 mL h-1 for 5 h per day. The remaining columns received the cycled treatment that was one application per week of 30 mL h-1 for 35 h.
The columns supported plants for 7 mo with different irrigation techniques and schedules running for the final 5 mo of the experiment. Fertilizer was added continuously with the irrigation water at a rate of 850 mg N L-1 and 85 mg P L-1. This rate was selected based on an examination of fertilizer treatments from a previous study (Hutchinson et al., 2001).
Extraction and Analysis
At the end of the experiment, all soil columns were cut into three sections: 0 to 15 cm, 15 to 45 cm, and 45 to 75 cm, for contaminant analysis. The sludge in these sections was homogenized and subsampled for analysis.
Extraction of Total Petroleum Hydrocarbons from Sludge
Total petroleum hydrocarbons (TPH) was extracted from the sludge using a sequential shaking extraction method (Schwab et al., 1999). Recovery of TPH from this soil by this method was found to be identical to soxhlet extraction. The sludge was air-dried and mechanically ground to pass through a 0.25-mm mesh sieve. One gram of petroleum sludge was spiked with 100 µL of 100 mg L-1 tetracosane and shaken on a reciprocal shaker with 10 mL Optima grade dichloromethane (DCM; Fisher Scientific, St. Louis, MO) for 30 min, then centrifuged at 2000 rpm for 10 min to enhance solid–liquid separation. The supernatant was collected in 60-mL glass jars. This procedure was repeated two more times, and the mass of the final extractant was recorded. A 1.5-mL sample was transferred into a 2-mL gas chromatography (GC) vial, spiked with 10 µL of 1000 mg L-1 
-androstane, capped with a Teflon-lined septa, and stored at 4°C until analysis by GC.
Leachate Extraction
Leachate was collected in 120-mL glass bottles attached to the bottom of the column. Leachate pH and electrical conductivity (EC) were monitored as indicators of changing chemical conditions in the soil. The electrical conductivity was measured using a Model 31 conductivity bridge (Yellow Springs Instruments, Yellow Springs, OH). Total petroleum hydrocarbons was extracted from selected leachate samples using solid-phase C18 cartridges (J&W Scientific, Folsom, CA). The cartridge was conditioned with 5 mL dichloromethane, 5 mL methanol, and 5 mL deionized water distilled in the presence potassium permanganate. Ten milliliters of leachate was loaded onto the cartridge, TPH was eluted with 3 mL dichloromethane, and the eluent was dried with anhydrous sodium sulfate. A 1-mL aliquot of the eluent was transferred to GC vials, spiked with 10 µL androstane, capped with a Teflon-lined septa, and stored at 4°C until analysis.
Gas Chromatographic Analysis
All extracted samples were analyzed on a Hewlett–Packard (HP) 5890A gas chromatograph equipped with a flame ionization detector (FID), HP Chemstation integration software (Hewlett Packard, 1997), and an HP7673A autosampler using a DB-TPH column (Hewlett–Packard, Wilmington, DE). The DB-TPH column, designed specifically for analysis of TPH, had an inside diameter of 0.32 mm, a length of 30 m, and a 0.25-µm film thickness. The carrier gas (H2) was delivered at 67 µL s-1 and the fuel gas (H2) at 670 µL s-1. Nitrogen was used as the make-up gas at a flow rate of 530 µL s-1. Air was supplied as the oxidant at a rate of 7.0 µL s-1. The initial oven temperature was maintained at 40°C for 2 min, then increased at 0.2°C s-1 to 320°C and maintained at 320°C for 1 min for leachate samples and 10 min for petroleum sludge samples. The temperature of the injection port was 250°C and the detector temperature was 350°C. The injection volume was 2 µL. Total petroleum hydrocarbon concentrations were measured using integrated GC areas and then converted to concentrations using external standard calibration curves.
Biomass and Root Parameters
Plant biomass was collected at the end of the study and reported as total dry weight. Root physical parameters were analyzed using a HP 4C flatbed scanner. In preparation for analysis, roots were extracted from the soil using repeated water rinses and hand agitation. The slurry was passed through a 2-mm sieve for root recovery, and the clean roots stained with methyl violet. Stained roots were spread on clear transparency film and covered to form a slide; multiple slides were prepared for large root quantities. Images were scanned in black and white at a resolution of 300 dots per inch and analyzed for root length estimation, distribution of root diameter, root length density, and surface area density using Delta-T image processing software (Delta-T Devices, 2001). Scanned roots were then dried and weighed to measure root biomass.
Statistical Analyses
All data were subject to analysis of variance using COSTAT (Cohort Software, 1994). Mean separations were determined using least significant difference. In all instances, P < 0.05 was used for comparisons, but P < 0.10 also was used for some comparisons.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Biomass
Plants in all treatments showed no signs of stress and produced abundant biomass. Over the course of the experiment, the mean, total shoot biomass was 25 g per column (Fig. 2). Although the surface-irrigated plants had slightly higher mean shoot biomass than subsurface-irrigated plants, the differences were not statistically significant (P < 0.05). Therefore, it appears that water was not limiting for any of the irrigation treatments.
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Total Petroleum Hydrocarbons Degradation
Degradation of TPH was significantly (P < 0.05) affected by all the variables tested in this experiment: irrigation location, irrigation frequency, vegetation, and depth of soil (P < 0.05). When averaged over the other treatment variables, change in TPH concentration was significantly greater in the vegetated columns (35% decrease) than in the unvegetated columns (18%). In terms of the main effects, the TPH reduction was greater for the surface soil than other depths; greater with subsurface irrigation than surface irrigation; and greater when water and nutrients were applied daily rather than weekly. The positive effect of vegetation on petroleum degradation has been observed previously (Banks et al., 1998; Hutchinson et al., 2001; Wiltse et al., 1998; Reilley et al., 1996) and is believed to be related to the stimulation of microbial activity in the rhizosphere and an improved degradation environment due to the physical and chemical effects of roots.
Because one of our objectives was to determine the effect of irrigation frequency, irrigation placement, and vegetation on TPH reduction as a function of depth, all main effects and interactions were examined. The effect of irrigation placement was not statistically significant, so degradation was averaged over this variable. Data for this three-way interaction (soil depth x irrigation cycle x vegetation) are plotted in Fig. 4. It should be noted that within the vegetated columns, TPH reduction was significantly greater for subsurface-optimal irrigation than surface-optimal regardless of soil depth. The difference between these treatments averaged 30%. No other differences due to irrigation placement were significant. For all increments of soil depth, TPH degradation was significantly greater for optimal irrigation in vegetated columns than for all other treatments (Fig. 4). The effect of vegetation was not significant when a weekly irrigation cycle was used.
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Enhanced petroleum contaminant dissipation in the rhizosphere has been attributed to stimulation of microbial activity because plant roots have little capacity for direct degradation (Cunningham et al., 1996). Microbial stimulation in the presence of plant roots may be the result of root exudation of soluble carbon and nutrients, enhanced transport of water and oxygen through root channels, and physical exploration into sites otherwise not accessible to microbes (Cunningham et al., 1996). Trends in colony forming units (determined by plate counts) were not similar to the trends in root parameters or TPH degradation (data not shown). The microbial numbers did not reflect the effects of depth or irrigation treatment on contaminant dissipation. Plate counts enumerate only a small fraction of the microorganisms present in the soil and may have no relevance to degradation potential. More appropriate microbial assessments might be most probable numbers of petroleum degraders, quantification of known degraders, or more sophisticated techniques such as denaturing gradient gel electrophoresis (Nakatsu et al., 2000).
The presence of plants had a strong effect on the physical characteristics of the petroleum sludge. The sludge in vegetated columns had assumed many soil-like characteristics including structure, aggregation, and enhanced drainage compared with the sludge in unvegetated columns, which kept its original massive structure and gelatinous texture. The superior physical properties of the vegetated soils led to enhanced water infiltration, increased diffusion of oxygen, and an environment more generally conducive to contaminant degradation.
The slight vacuum at the bottom of the columns maintained unsaturated conditions throughout the experiment and enhanced the effect of the optimal irrigation–fertilization treatments. One hundred fifty milliliters of irrigation water containing 850 mg N L-1 and 85 mg P L-1 were delivered every day over a 5-h period. By drawing the vacuum at the bottom of the column, the downward movement of the water, nutrients, oxygen, and perhaps microbes was promoted, particularly in the vegetated columns. This created an ideal environment for the degradation of the petroleum hydrocarbons by the soil microbes. In the columns with a weekly water cycle, the highly active roots and microbial populations may have quickly depleted the available nitrogen from the irrigation water for use functions not related to contaminant degradation (e.g., increasing biomass). Also, adding approximately 1 L of water and nutrients continuously over a 30-h period may have induced zones of saturation, and petroleum hydrocarbons are degraded inefficiently in reducing environments (Atlas and Bartha, 1992). Zones of saturation also give rise to denitrification, which can be a major sink for the fertilizer N. Under these circumstances, the microbial populations may have become N-limited within a few days after an irrigation event.
In the columns without vegetation, there was no effect of irrigation location or schedule, and no degradation was observed in the 45- to 75-cm increment (Fig. 4). As mentioned previously, the soils in the unvegetated columns had very poor physical structure and infiltration of water was quite slow. There was no significant difference (P < 0.05) between surface and subsurface application of irrigation nor between the 0- to 15-cm depth and the 15- to 45-cm depth. The low rates of degradation may be the result of limited infiltration of air into the columns resulting in the establishment of highly reduced zones.
Degradation of TPH in the unvegetated columns was significantly less than in the vegetated columns only for the optimal irrigation treatments. Significant degradation in unvegetated treatments has been observed previously (Banks et al., 1998; Hutchinson et al., 2001; Wiltse et al., 1998; Reilley et al., 1996) and has been attributed to biodegradation. Enhanced degradation in rhizosphere soils, particularly in the 45- to 75-cm depth, illustrates the importance of the presence of roots.
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The combined treatments of vegetation and optimal irrigation provided the desired result of increasing the effective depth of remediation. This appears to be the result of (i) deeper penetration of the roots and their associated rhizosphere (Cunningham et al., 1996); (ii) improved physical structure of the soil allowing more rapid and deeper penetration of water, nutrients, and microbes; and (iii) greater diffusion of oxygen into the soil through root channels and aggregates, thus providing the necessary oxidizing environment for hydrocarbon degradation (Li et al., 2000; Sepic et al., 1995).
Further research on varying depths of irrigation–fertilization would help determine the optimal depth for nutrient and water application. This will depend on the type of soil, the ability of water to move through the soil profile, and the height of capillary rise. In this experiment, the sludge in vegetated treatments maintained a relatively open structure due to the formation of aggregates. Capillary rise was observed to be only about 15 cm during column takedown, which is low for the amount of clay-sized particles present in the sludge. In the field, placement of subsurface irrigation lines at deeper levels may be desirable because of increased capillary rise due to smaller pore sizes. Understanding and predicting water movement through the soil profile will enhance the potential success of phytoremediation at a given field site.
With time and increased plant growth, the depth of significant remediation should increase. As plant roots develop, they create soil structure that increases the depth of conditions conducive to TPH degradation. Remediation depths of 2 to 3 m may be possible with proper plant selection and water and nutrient management.
One of the advantages of phytoremediation over other cleanup alternatives is the cost; phytoremediation will cost at most half of other remediation options (Cunningham et al., 1996). Although the subsurface irrigation systems described in this paper have the potential to improve remediation efficiency, they will add to the total cost of the process. Increased rates of degradation and lower final contaminant concentrations are benefits of subsurface irrigation and may be considered to be worth the added investment.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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