Published in J. Environ. Qual. 34:156-163 (2005).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA
Analysis of Soils to Demonstrate Sustained Organic Carbon Removal during Soil Aquifer Treatment
Peter Foxa,*,
Waleed Aboshanpa and
Bashar Alsamadib
a Arizona State University, Department of Civil and Environmental Engineering, Tempe, AZ 85287-5306
b Department of Water and Environment, Institute of Earth, Water and Environment, The Hashemite University, Zarka, Jordan
* Corresponding author (Peter.Fox{at}asu.edu)
Received for publication January 30, 2004.
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ABSTRACT
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Soil samples from column studies using five soil types and from a field site were analyzed to assess the ability of soil aquifer treatment to sustain removal of organic carbon. The soil types used in the column studies were chosen to represent a wide range of soil properties that might be used for soil aquifer treatment. Soil samples were analyzed for total organic matter, and a subset of samples was sequentially extracted to determine the effects of soil aquifer treatment. For both column studies and the field site, no accumulation of organic matter was observed below a depth of 8 cm. Near the surface, biological activity at the soil–water interface resulted in an accumulation of biomass and associated organic matter. For the column studies, the accumulation of organic matter in the top 8 cm of soil was <20% of the total organic matter applied to the columns. Soils at depths greater than 8 cm had total organic matter levels less than the original soils before soil aquifer treatment. Significant changes in extractable iron and manganese oxides were observed at the field site, which had been in operation for >10 yr with extended periods of low redox conditions. However, these changes had no apparent effect on the removal of organic carbon in the system. This study provides evidence that soil aquifer treatment can remove organic carbon without accumulation from adsorption that might eventually lead to breakthrough.
Abbreviations: DOC, dissolved organic carbon • OC, organic carbon • SAT, soil aquifer treatment • TOC, total organic carbon • WWTP, wastewater treatment plant
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INTRODUCTION
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GROUND WATER RECHARGE with reclaimed wastewater and other water sources is an increasingly valued practice for replenishing aquifers used for domestic supply, especially in the arid parts of the world. A widely used recharge method is rapid infiltration from spreading basins (Bouwer et al., 1980), although direct injection of reclaimed water is possible when land is not available for spreading basins. Percolation through the unsaturated zone and subsequent ground water transport and storage provide final treatment of the reclaimed water such that the extracted water can be used for nonpotable and potable purposes. Collectively, the water quality improvements that arise from percolation and ground water transport and storage process are termed soil aquifer treatment (SAT). A schematic representation of SAT is provided in Fig. 1
.
Soil percolation encompasses several processes that occur during downward transport in the unsaturated zone. At the basin–soil interface, the combined effects of sedimentation, filtration, aeration, and microbial growth lead to the formation of a biologically active zone that may become impermeable (Bouwer and Rice, 1984). While the time scale for SAT may be on the order of years, a large fraction of dissolved organic carbon (DOC) is removed near the infiltration interface within a depth of 1 m and with a time scale of days (Wilson, 1995). In the soil percolation zone, oxygen is supplied continuously from the surrounding unsaturated zone. Nevertheless, dissolved oxygen concentrations can be <1 mg L–1 during infiltration periods. When infiltration is stopped, the upper soil layer is re-aerated resulting in cyclic aerobic–anoxic conditions in the vadose zone.
Previous work has demonstrated that the DOC present in reclaimed waters is composed of easily biodegradable organic carbon, natural organic matter, soluble microbial products, and anthropogenic compounds (Drewes and Fox, 1999). Large variations in the DOC present in reclaimed waters are primarily due to variations in the easily biodegradable organic carbon concentrations, which in turn are a function of wastewater treatment efficiency. Because easily biodegradable organic carbon is removed during SAT, the composition of the organic carbon present in product waters is primarily natural organic matter and soluble microbial products (Drewes and Fox, 2000). Spectroscopic characterization of DOC following SAT found that the structure and functional groups of the organic compounds were similar to natural organic matter (Fox and Drewes, 2001). Anthropogenic compounds are a small but important component of the DOC and the majority of these compounds are removed during SAT (Fox, 2002).
The mechanisms of removal of organic carbon during SAT have been characterized as a combination of biodegradation and adsorption (Bouwer and Rice, 1984). Because the soil matrix is complex and the DOC is a complex mixture of organic compounds, the use of radiolabeled compounds cannot be used to reliably differentiate between biodegradation and adsorption. The ability to sustain removal of organic carbon in a soil system depends on biological removal. If adsorption was a major removal mechanism, breakthrough of organics could eventually occur and the system would not be considered sustainable. Some SAT systems have been in operation for >40 yr, such as the Montebello Forebay project located in Los Angeles County, California, USA (Asano, 1998). Analogous to SAT systems, bank filtration where water flows through a saturated zone has been used as a soil filtration system to treat surface waters influenced by wastewaters for >100 yr (Drewes and Jekel, 1998). The length of operation time for these systems provides strong empirical evidence for sustained biological removal or transformations of organic carbon. While data has been reported on the removal of organic carbon through soil systems (Bouwer et al., 1980; Kopchynski et al., 1996; Wilson, 1995; Quanrud et al., 2003), only limited data have been published on the changes in soil composition during soil aquifer treatment. Over a period of 10 yr, a series of soil column tests and field studies were conducted to evaluate the sustainability of SAT (American Water Works Association Research Foundation, 2001). During this time period, analysis of soils for total organic carbon or by sequential extraction was performed. Results are presented herein to demonstrate the sustainability of SAT for the removal of organic carbon by demonstrating that (i) organic carbon is not accumulating in the soil matrix and (ii) soil composition is not changing in a way that will adversely affect the attachment of microbes that support biological removal.
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MATERIALS AND METHODS
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Five soils were used for SAT in soil columns using several different sources of reclaimed water. All five soils used are Typic Torrifluvents. Four soils were obtained from a proposed SAT site near the city of Phoenix, AZ, USA. They were labeled as follows: Agua Fria sand, North Pond silt, South Pond silt, and agricultural field clay. The fifth soil was derived from the Sweetwater SAT site location in Tucson, AZ. The soils were selected to represent a wide range of soil properties (Table 1) that might be used during SAT.
Soil Column Studies
Apparatus
Two sets of soil columns were operated during this study. One set of pilot-scale columns was operated at the 91st Avenue wastewater treatment plant (WWTP) in Phoenix using three different effluents (Table 2) available at the plant. A second set of bench-scale columns was operated at the University of Arizona and used secondary effluent from the Roger Road WWTP. During column operation, the influent and effluent organic carbon concentrations for the columns were monitored. After operation of the systems was terminated, soil samples were obtained from all columns studied. All columns were operated for a minimum of one year.
The pilot-scale columns were 2.4 m in height with a 30.5 cm inside diameter and a soil depth of 2 m. The columns were equipped with sampling ports to measure matric potential and to obtain liquid samples. The columns had effluent weirs to control water levels, and vacuum pumps were attached to the effluent lines to promote unsaturated flow conditions. Additional details of soil column construction may be found in American Water Works Association Research Foundation (1998). The experimental matrix of soils and effluents used to simulate SAT in the columns is summarized in Table 2. The columns were operated using different wetting and drying periods to determine the effect of operational variables on the removal of organic carbon and infiltration rates. The range of wet–dry cycle times is presented in Table 2, along with the total period of operation. Pilot-scale column operating conditions were monitored daily. Infiltration rates, pond depths, soil temperature, soil suction (matric potential), and dissolved oxygen concentrations were measured.
The columns used at the University of Arizona to simulate SAT had an 8.6-cm inner diameter with a total length of 1.3 m and a soil depth of 1.0 m. The specific column that soil samples were obtained from was packed with Sweetwater silty sand (Table 1) and received secondary effluent from the Roger Road WWTP. The column was operated in 2-wk cycles consisting of 7d of wetting and 7 d of drying. Additional information on these columns is available in Quanrud et al. (1996a).
Soil samples were obtained from all columns by taking 10-cm-diameter cores to a depth of 51 cm. The soil cores were extruded and divided at incremental depths of 5.1 cm. Samples of the original homogenized soils used to pack the columns were also analyzed to determine the effects of SAT. Samples were placed in airtight bags and stored at 4°C before analysis. The majority of samples were analyzed for total organic content and a subset of these samples was analyzed by sequential extraction.
In addition to the soil samples obtained from the column studies, one set of samples was obtained from a full-scale operational SAT site. The Mesa Northwest Water Reclamation Plant has been recharging reclaimed water since 1990 and soil samples were obtained after 12 yr of operation at depths of 0 to 5 and 60 to 65 cm. Intact soil cores could not be obtained because of cobbles in the basins; therefore, the soil samples were manually excavated. Two control samples (0–5 cm) from areas adjacent to the basin were collected representing soil that was not subjected to SAT. Extensive long-term water quality monitoring of this field SAT site was reported by Fox et al. (2001).
Analytical Methods
Liquid Samples
Column influent and effluent was monitored for pH, alkalinity, total organic carbon (TOC), DOC, ultraviolet absorbance (UV) nitrate, ammonia, nitrite, and phosphorous in some instances. Organic nitrogen was monitored periodically. Nitrate, ammonia, and nitrite were determined according to Sections 4500-NO3– D, 4500-NH3 H, and 4500-NO2– B in American Public Health Association (1989). Organic nitrogen and TOC and DOC were determined by Methods 4500-Norg B and by 5310 C in American Public Health Association (1989).
Soil Samples
The majority of samples were analyzed for total organic content by oven-drying the samples and then placing them in a muffle furnace at 550°C to volatilize all organic matter (Carter, 1993). Samples were weighed before and after volatization to determine total combustible organic matter. All analyses were done in triplicate.
A multistep extraction process was used to sequentially remove individual soil coating layers and associated materials (Carter, 1993). Originally, all samples were analyzed in triplicate to verify the accuracy and precision of the method. All subsequent analyses were done in duplicate after verification. The principal of the extraction process was to convert each layer into a soluble form using extractant(s). Extractants were analyzed for different materials associated with the layer. Table 3 lists the extractants used in each step and the extraction phases during the sequential procedures. After each step, the dry soil weight was determined. The difference in soil mass before and after each extraction step was the mass of the soil layer extracted during this step. The extracts and washes were filtered through 0.45-µm fiberglass filter paper, and then analyzed for cations using a PerkinElmer (Wellesley, MA) Model 3100 atomic absorption spectrometer (Method 3111; American Public Health Association, 1989). Because the extracted materials do not represent distinct layers or coatings, the term fraction is used to described the extractable materials.
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RESULTS AND DISCUSSION
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Column Studies
The water quality characteristics of the reclaimed waters applied to the soil columns and the field site are presented in Table 4. The mass of DOC applied to the columns was a function of the influent organic carbon concentration and the hydraulic loading rate. The Agua Fria sand and Sweetwater soils had the greatest hydraulic conductivities, and the organic loading rate on these columns ranged from 1.5 to 2.5 kg organic carbon (OC) m–2 yr–1. The North and South Pond silts had an organic loading factor of 0.5 to 1.0 kg OC m–2 yr–1, whereas the agricultural field clay had an organic loading factor of less than 0.1 kg OC m–2 yr–1. The potential for OC to accumulate in the columns was a function of the removal of OC. Autotrophic biological activity from algae growth on the surface and from nitrifying bacteria may also contribute to the organic carbon pool. At low infiltration rates, the relative contribution of DOC from autotrophic biological activity from algae periodically caused effluent DOC concentrations to exceed influent DOC concentrations (American Water Works Association Research Foundation, 1998).
In the soil column study, effluent OC concentrations from the Agua Fria sand, Sweetwater, and South Pond silt columns ranged from 4 to 8 mg L–1. Figure 2
presents long-term effluent data from a column with Agua Fria sand. The average removal efficiency for the column was 37 ± 14%. The initial effluent TOC concentrations were higher than the influent TOC concentrations as soil organic matter was dissolved from the system. As the effluent concentration decreased, biological removal in the system stabilized the removal of organic carbon. Similar trends were observed for the Sweetwater and South Pond silt columns. The contribution of algae caused significant variations in the effluent of the agricultural field columns and the North Pond silt columns (Kopchynski et al., 1996).
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Fig. 2. Total organic carbon (TOC) vs. time for Pilot Column 9 with Agua Fria sand fed chlorinated denitrified effluent.
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The OC content as a function of column soil depth over the first 12 cm is presented in Table 5. With the exception of the agricultural field clay, soil OC content decreased with depth. The surface soil (0–2 cm) OC contents were greater than the OC contents before SAT (Table 1), and the OC contents at soil depths greater than 8 cm were less than the OC contents before SAT. The accumulation of OC near the surface was expected due to biological activity and algal growth. The decrease in OC content at a >8-cm depth was not expected. During initial operation of all columns, the effluent OC concentration exceeded the influent OC concentration, providing evidence for the dissolution of soil organic carbon. Several soil samples from greater depths had similar OC content to samples obtained at a depth of 10 to 12 cm. The total mass of OC removed from the liquid phase by each soil column was calculated based on the influent and effluent OC loadings. Assuming the OC content of soils deeper in the columns is the same as OC content measurements at 8 to 12 cm, there was no net accumulation of OC below a depth of 8 cm. The total accumulation of OC near the surface of the system was calculated and was <20% of the OC mass removed from the liquid phase.
Sequential extractions of surface soils (0–2 cm) and of soils at a depth of 39 to 41 cm were performed using soils obtained from the Agua Fria sand and North Pond silt columns fed chlorinated denitrified effluents. The greatest difference in soil properties from the different depths was the oxidizable organic matter fraction (Fig. 3 and 4)
. This fraction represents the portion of the total soil organic matter that was either susceptible to oxidation with hydrogen peroxide or was solubilized during the extraction step. Acid extraction did not remove combustible organic matter content (Fig. 4). Because acid extraction will remove carbonates that are known to volatize during the measurement of combustible organic matter, the acid-extractable fraction of carbonates did not affect the combustible organic matter measurement.
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Fig. 3. Extractable organic matter for soil samples from the North Pond silt column (referred to as silt) and the Agua Fria sand column (referred to as sand).
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Fig. 4. Combustible organic matter for soil samples before and after soil aquifer treatment (SAT). Measurements were done after each sequential extraction step carbonates by acid extraction.
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For pre-SAT soil, the majority of combustible organic matter was removed during the metal oxide extraction step for both the sand and silt samples. The combustible organic matter associated with the metal oxide layers was not susceptible to oxidation during the organic matter extraction step. After SAT, only the surface silt sample had a major increase in total combustible organic matter and this was commensurate with a large increase in the mass removed during organic matter extraction. Therefore, the combustible organic matter associated with the organic matter extraction step can be assumed to be particulate organic matter, such as microbial cells and organic compounds adsorbed to the particulate organic matter (Hamaker and Thompson, 1972). If this is true, it appears that negligible accumulation of organic matter from the reclaimed water is occurring in the soils and the majority of accumulation is associated with biological productivity in the upper layer of the soils.
Another factor that may affect the sustainability of microbial activity is the stability of the metal oxide coatings, which are known to affect both the adhesion of microorganisms and the adsorption of organic compounds (Dunnivant et al., 1992). Figure 5
presents the results for the extraction of the iron and manganese oxides. For the North Pond silt, SAT resulted in an increase in the mass of materials extracted during the metal oxide extraction step. Extractable iron and manganese in the North Pond silt actually decreased by 10 to 20% after SAT. Cyclic changes in redox potential during wetting and drying cycles may be responsible for decreases in extracted iron and manganese; however, these changes did not alter the efficiency of SAT. The increase in extracted oxide material was not due to the accumulation of combustible organic matter (Fig. 4). Minor decreases (<5%) in metal oxide–extractable materials after SAT were also observed with Agua Fria sand.
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Fig. 5. Extractable metal oxides and associated materials for soils before and after soil aquifer treatment (SAT).
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Field Site Soil Aquifer Treatment
The removal of OC during percolation at the Mesa Northwest Water Reclamation Plant has remained steady for >10 yr (Fox et al., 2001). The influent DOC concentrations (6–10 mg L–1) decrease to less than 3 mg L–1 within the top 1.5 m of soil (Fig. 6)
. Soil samples were obtained from the top 1.5 m of basin soil, and analyzed for total combustible organic matter (Fig. 7)
. The two control surface soil samples from outside the basin both had more total combustible organic matter than the sample obtained at depth in the basin. The soil sample from the basin surface (0–2 cm) accumulated organic matter, the majority of which was extractable, similar to the soil samples collected from the column studies. This accumulated material probably consists of microbial cells and associated organic matter. In comparison with the soils outside the basin, there was no accumulation of organic matter in any soil fraction for the sample at a depth of 30 to 32 cm in the basin.
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Fig. 6. Soil depth profile for infiltrating water at the Mesa Northwest Water Reclamation Plant Recharge Facility.
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Fig. 7. Total combustible organic matter from soils at the Mesa Northwest Water Reclamation Plant Recharge Facility.
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Iron and manganese concentrations in the extracts of the soils obtained from the Northwest Water Reclamation Plant Recharge Facility are presented in Fig. 8 and 9
, respectively. In all samples, the extracted iron was associated with the metal oxides fraction for all four samples obtained (Fig. 8). The in-basin surface sample had iron levels similar to the background samples, whereas iron in the 30- to 32-cm depth decreased by approximately 30%. The operation of the basins could result in extended periods of low redox potential at depth creating conditions for the dissolution of iron oxides.
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Fig. 8. Total iron extracted from soils at the Mesa Northwest Water Reclamation Plant Recharge Facility.
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Fig. 9. Total manganese extracted from soils at the Mesa Northwest Water Reclamation Plant Recharge Facility.
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The two background samples have a similar distribution of manganese, whereas both basin samples show a 70% decrease in the manganese associated with the metal oxides (Fig. 9). The surface sample had accumulated manganese associated with the carbonate fraction. The low redox potentials may have had a major effect on the manganese in association with the metal oxides.
Organic Carbon Adsorption Studies
Several other studies on the adsorption of organic carbon in reclaimed waters to soils demonstrate that the fraction of organic carbon is <20%. Adsorption of wastewater organic matter on soils has been studied in soil columns that were inhibited with sodium azide by Quanrud et al. (1996b). During these studies, the removal of 11% of the wastewater organic matter was sustained under inhibited conditions. During batch adsorption studies using soil and wastewater (Cha et al., 2004), rapid adsorption of 13 to 18% of wastewater OC was observed while the remaining OC did not adsorb. These observations indicate that the majority of wastewater organics could not be removed effectively by adsorption and only a small fraction of the wastewater organics could accumulate.
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CONCLUSIONS
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The data collected from both soil columns simulating SAT and an actual field SAT site demonstrate that sustained removal of organic carbon is possible. Although accumulation of organic carbon at the surface from biological activity occurs, however, there was no evidence of organic carbon accumulation in soils below a depth of 8 cm. These results are consistent with previous studies where organic carbon accumulation was determined during SAT (Bouwer et al., 1980; Quanrud et al., 1996b). This study demonstrated that organic carbon accumulation on soils is similar for many different soils and operational conditions. In addition, mass balances revealed that the total OC accumulation at the soil surface of columns was <20% of the total OC mass applied to the soil systems. The accumulated OC at the surface was extractable and probably represented biological activity near the soil–water interface rather than adsorbed OC. Adsorption is a nonsustainable removal mechanism that could result in eventual breakthrough of OC after long-term operation of SAT systems. The data provided suggest that OC removal is sustainable because adsorbed organics are subsequently transformed or degraded and do not accumulate in the soils.
Changes in the quantities of iron and manganese oxide fractions were observed at the SAT field site and soil columns. The changes in iron and manganese oxide fractions during the column studies suggest minimal effect on the ability of SAT systems to remove organic carbon. Low redox conditions in the field may have promoted dissolution of metal oxides. The majority of manganese was removed from the metal oxide fraction. The field site had been operated for >10 yr and redox effects might have been more pronounced in the field as compared with columns because of the differences in time of operation. Nevertheless, the changes in metal oxide composition have had no deleterious effects on the long-term performance of SAT to remove organics at the field site.
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ACKNOWLEDGMENTS
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The authors wish to thank the many graduate students at Arizona State University and the University of Arizona that operated soil columns and monitored SAT systems. Special thanks are warranted for David Quanrud and Teresa Kopchynski. Our gratitude is also extended to the many municipal agencies that supported this research including the Cities of Mesa, Phoenix, and Tucson, AZ, and the County Sanitation Districts of Los Angeles County. Principal funding for the majority of research presented was provided by the American Water Works Association Research Foundation (AWWARF) and the USEPA.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
