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TECHNICAL REPORTS

Heavy Metals in the Environment

Distribution of Copper, Zinc, and Phosphorus in Coastal Plain Soils Receiving Repeated Liquid Biosolids Applications

Virginia Tech Univ., Blacksburg, VA 24061

^* Corresponding author (bsukkari{at}vt.edu).

Received for publication December 22, 2006.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Continuous N-based application of biosolids contributes to a gradual increase of trace elements and P in soils. The objectives of this study were to assess the accumulation and vertical transport of Cu, Zn, C, N, and P within the profile of two coastal plain soils. Liquid (6–8% total solids) biosolids were applied to an Acredale silt loam (fine silty, mixed, thermic typic Ochraqualfs) and Bojac loamy sand (coarse loamy, mixed, thermic typic Hapludult) annually from 1984 to 1998. The repeated applications supplied 70, 204, and 3823 kg ha^–1 of Cu, Zn, and P, respectively, to the Acredale and 81, 225, and 4265 kg ha^–1 of Cu, Zn, and P, respectively, to the Bojac. The total C and N contents were not different than background levels in the Bojac soil and were slightly higher in the Acredale soil 7 years after cessation of biosolids application. Phosphorus, Cu and Zn are still concentrated in the top 0.25 m of the Acredale soil. Enrichment of P, Cu, and Zn were detected to the deepest soil increment in the coarse-textured Bojac soil. Approximately 20 to 40% of the Cu and Zn applied in the biosolids could not be accounted, which was likely due to a combination of leaching and incomplete extraction. Excessive Mehlich 1-P concentrations and a high degree of P saturation were found in amended soil, raising the potential for P release to runoff or leaching water.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
BIOSOLIDS are recognized as sources of beneficial nutrients (e.g., N and P) and organic matter, and their application to agricultural land can improve soil properties. Biosolids also contain potentially deleterious trace elements that can affect humans (e.g., Cd, Pb, Zn), plants (i.e., Cu, Ni, and Zn), or animals (Mo) (Page and Chang, 1994).

The land application of biosolids is regulated by the U.S. federal government by the 40 CFR Part 503 Regulations "Standards for the Use and Disposal of Sewage Sludge" (USEPA, 1993). The 503 Rule permits long-term application of biosolids to agricultural land under the assumption that soil accumulation of nine trace elements in biosolids meeting ceiling concentration limits (CCL) will not cause environmental or health problems before the applications must cease. The long-term application of biosolids whose trace elements meet pollutant concentration limit (PCL) standards is assumed not to increase solubility of metals due to the adsorptive properties of the biosolids matrix. Biosolids meeting the PCL standards can be applied to agricultural land without monitoring the cumulative soil loading limits. The Part 503 Rule does not require monitoring trace elements and nutrients below the incorporation zone based on the assumption that nearly all of the trace elements will be immobilized in the zone of incorporation and N and P will not be applied at rates exceeding crop requirements.

Selective adsorption and precipitation of metals on oxide surfaces provide the basis of adsorption in biosolids-amended soils. Iron, Al, and Mn oxyhydroxides and organic matter added to soil with biosolids increase the soil's capacity to adsorb and bind trace elements (Corey et al., 1987). Sorption capacity and properties of both the biosolids and the amended soil will affect metals solubility with the soil-biosolids mix having intermediate properties (Basta et al., 2005). Basta et al. (2005) demonstrated that the inorganic components of biosolids can dominate the binding chemistry of the soil-biosolids mixture when the residuals are applied in sufficient amounts.

The environmental health and safety of land-applied biosolids has been subject to debate despite the widespread use of the practice. The protectiveness of the Part 503 Rule has been questioned on the basis of assumptions regarding trace metal immobilization made in the underlying risk assessment (McBride, 1995; Harrison et al., 1997). McBride (1995) cited unexplained metal losses from experimental sites that have received biosolids as empirical evidence for biosolids metal leaching. Calculations of mass balances in a number of land application studies were unable to account for up to half of the biosolids-applied metals, which challenged the concept of long-term metal immobilization in soil (Chang et al., 1984; Williams et al., 1987; Alloway and Jackson, 1991; Dowdy et al., 1991). Researchers reporting such discrepancies have attempted to explain the apparent metal loss by lateral distribution and dilution by tillage (Williams et al., 1987; McGrath and Lane, 1989) or incomplete extraction (Chang et al., 1984; Dowdy et al., 1991). Downward translocation of metals has been discounted in most cases because no increases in metal concentrations with depth have been observed in the soil profile (Chang et al., 1984; Dowdy et al., 1991) and in conventional soil columns (Emmerich et al., 1982).

Biosolids application rates have typically been based on N requirements of crops. Such a long-term, continuing basis for application rate will supply P in excess of crop needs (Smith, 1996; Stehouwer et al., 2000), which will likely result in a sizeable increase in soil P concentrations (Peterson et al., 1994). Phosphorus transport to surface water bodies is of considerable concern since P is typically the limiting nutrient in fresh water systems (Sharpley and Beegle, 1999). Leaching of P from biosolids application sites has received limited investigation (Sui et al., 1999) because P contamination of subsurface water has not been considered to be a problem. Potentially significant P leaching may take place in coarse-textured soil with low P sorbing capacity (Lu and O'Connor, 2001). This might be of particular concern in the Atlantic coastal plain where shallow groundwater can be hydrologically connected to surface water (He et al., 1999; Novak et al., 2000) via shallow subsurface lateral flow.

Long-term agronomic N-based application of biosolids will cause a gradual increase in the concentration of trace elements and P in amended soil. It is important to increase our knowledge regarding solubility and mobility of trace metals and P in receiving fields with marginal adsorptive capacity. The Hampton Roads Sanitation District (HRSD) Progress Farm (Virginia Beach, VA) provided us with an opportunity to investigate the long-term vertical transport of P and trace metals from biosolids-amended soils of varying sorption capacities. The site received annual liquid biosolids applications from 1984 to 1998. The soils are coarse-textured with low cation exchange capacity and low oxide and organic matter concentrations. The site is located in the lower Coastal Plain of Virginia, receives considerable rainfall amounts, has a shallow groundwater (<1 m) table, and is drained via open ditches. The objective of this study was to assess the effects of long-term biosolids application to fine- and coarse-textured lower coastal plain soils on vertical transport of trace metals and P. In this article, we report results on Cu, Zn, and P mobility in coarse- and fine-textured coastal plain soils 7 yr after cessation of biosolids application.

	Materials and Methods

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Background
The study was conducted on two lower coastal plain soils varying in surface soil texture at the HRSD Progress Farm in Virginia Beach, VA. The average annual precipitation is 120 cm, and the average annual low and high temperatures are 10.8 and 20.3°C, respectively. The two soils series are classified as an Acredale silt loam (fine-silty, mixed, active, thermic Typic Endoaqualfs), which is a deep and slowly permeable soil, and a Bojac sandy loam (coarse-loamy, mixed, semiactive, thermic Typic Hapludults), which is deep and moderately rapidly permeable soil. Both soils have a 2 to 3% slope. The soil chemical and physical properties at the initiation of the biosolids applications are presented in Table 1 . The soils have low clay, hydrous oxide and organic matter contents, low cation exchange capacity, and are underlain by a shallow groundwater (<1 m) table.

View this table: [in this window] [in a new window]   Table 1. Chemical and physical properties of the Ap horizon of the Acredale silt loam and Bojac loamy sand.

Biosolids from the HRSD Atlantic wastewater treatment plant (Virginia Beach, VA) are stabilized via anaerobic digestion and thickened by centrifugation to an average of 6 to 8% total solids. Ferric salts were used occasionally in the treatment process during the 15-yr application period. The liquid biosolids slurry was surface applied to land using Terragator applicator (AGCO Corporation, Duluth, GA) equipment and disked into the top 15 to 20 cm. Biosolids were applied annually during September to October for wheat (Triticum aestivum L.) or March to April for corn (Zea mays L.) or soybean (Glycine max L.), depending on which crop in the corn–wheat–soybean rotation was being fertilized. The biosolids were of high quality (Table 2 ) and met the USEPA Part 503 PCL, which precluded the use of cumulative metal loading limits.

Deep (15 m) and shallow (10.5 m) monitoring wells were installed at the edges of the Progress Farm (Fig. 1 ) to assess groundwater levels and direction of flow and to monitor water for constituents of concern. The groundwater flow is WSW (i.e., from the treatment plant toward monitoring well site #5 and Scopus Creek). The wells were sampled every July for Zn and P analysis and every February, May, July, and October for Cu and nitrate analysis.

View larger version (49K): [in this window] [in a new window]   Fig. 1. Map of biosolids application sites illustrating soil treatment types and locations of monitoring wells.

Plot Management
The cropping sequence consisted of a corn–wheat–soybean rotation, with corn (Zea mays L.) planted during the sampling year of 2005. The amounts of biosolids applied were based on N assimilative capacity of the crops for the soil productivity classes for the specific crop yield potential (Simpson et al., 1993). Based on annual biosolids test results the rates of biosolids applied P was then estimated. The buffer (control) areas were fertilized annually with inorganic commercial fertilizer sources of N and P at rates recommended by routine soil testing procedures (Donohue and Heckendorn, 1994).

Soil Sampling and Processing
An area adjacent to the buffer strip within each field was selected as the sampling unit. Soil cores were randomly sampled from each of two fields differing by soil type and biosolids treatments (Fig. 1). Soil cores measuring 5 cm in diameter were collected in plastic sleeves to a depth of 0.75 m using a Giddings hydraulic soil probe. Four cores were collected from each soil series by biosolids application history treatment in April, 2005 yielding a total of 16 cores (2 soil types*2 treatments per soil type*4 cores per treatment).

The cores were kept intact in plastic tubes until they were sectioned by depth in the laboratory. The cores were sectioned in 0–15 (Ap horizon), 15–20, 20–25, 25–35, 35–45, 45–55, 55–65, and 65–75 cm increments. Samples were air-dried and subsequently ground with a glass mortar and pestle to pass a 2-mm (10 mesh) stainless steel sieve in preparation for laboratory analysis. Soil samples were digested for total elements analysis following USEPA 3051 method (USEPA, 1994). The method uses a mixture of nitric and hydrochloric acids to digest the samples. Elements bound in the silicate structures are not normally dissolved by this procedure. Samples digests were analyzed for total Cu, Zn, and P with a Thermo Jarrell Ash (Fitchburg, MA) inductively coupled argon plasma-atomic emission spectrometer (ICAP-AES).

Soil samples collected from the Ap horizon (0–15 cm) were additionally extracted with a Mehlich-I (Mehlich, 1953) solution employing Virginia Cooperative Extension Soil Test methodology (Donohue, 1992). The Mehlich-I method (Mehlich, 1953) uses a solution of 0.05 mol L^–1 HCl and 0.0125 mol L^–1 H₂SO₄ to extract a fraction that is an indicator of plant-available soil trace metals. The extracts were analyzed for Cu, Fe, Zn, and P by ICAP-AES. Soil pH was determined in a 1:1 soil/water ratio after 30 min equilibration. Soil organic matter (modified Walkley-Black) and cation exchange capacity of the soil were estimated according to the soil test recommendations for Virginia (Donohue and Heckendorn, 1994). Total carbon and N were analyzed by combustion using a CNS analyzer (Elementar America, Inc., Mt. Laurel, NJ).

Density and volume measurements were used to calculate total soil mass for each sectioned depth increment, from which elemental mass balance was calculated. Total soil mass of each section multiplied by its respective metal concentration yielded the total Cu and Zn loading. The control (baseline) metal content for each depth was subtracted from treated soil to yield the net metal accumulation in each section. Concentrations of the depth increments were added. This total concentration was divided by the total metal loading in the sewage sludge to estimate recovery of the biosolids applied metals.

Whole corn samples were collected from each plot at physiological maturity (i.e., black layer or R6 reproductive stage; http://www.agronext.iastate.edu/corn/production/management/growth/yield.html) and separated into vegetative and reproductive parts. Plant tissue was dried in a forced air oven at 68°C for 72 h, ground in a Wiley mill to pass 0.5 mm stainless steel sieve, and digested on a heating block using nitric acid and hydrogen peroxide. Samples were analyzed for Cu, Zn, and P via ICAP-AES.

Statistical Analysis
Data for the control and biosolids treatments for each soil type was statistically evaluated using the student t test for pair wise mean comparison at the 0.05 level of significance (Steel and Torrie, 1980).

	Results and Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Surface Soil Chemical Properties
The repeated applications of biosolids resulted in significant increases in Mehlich-I–extractable P, Cu, and Zn concentrations (Table 3 ) in the 0 to 15 cm soil horizon. Mehlich-I–extractable Zn and Cu constituted 40 and 31% of the total extractable (EPA3051) metals in the Bojac soil. These figures were 18 and 12% respectively for the Acredale soils. The soil test P concentrations were 5 to 6 times greater than the maximum amount of P required for corn, wheat, and soybean (Donohue and Heckendorn, 1994). Soil texture and pH play a crucial role in controlling mobility with highest mobility of most metals being in acidic, coarse-textured soils. Soil pH (1:1 soil/deionized [DI] water ratio) was 6 and 5.8 for the Acredale and Bojac soils, respectively. Soil pH in both fields was maintained above 5.5 as per Part 503 regulations with the addition of lime to keep metal solubility low. The average soil pH (from 1984–2000) was 6.11 ± 0.56 for the Bojac soil and 6.23 ± 0.55 for the Acredale soil (n = 51).

View this table: [in this window] [in a new window]   Table 3. Mehlich-I-extractable Cu, Zn, P, and Fe and Virginia Cooperative Extension Soil Test Laboratory estimated CEC of the Ap horizon in the control and biosolids receiving fields.

View larger version (28K): [in this window] [in a new window]   Fig. 2. Distribution of Total (EPA 3051) extractable Cu with soil depth in the Acredale and Bojac soils 7 yr after cessation of biosolids application. Bars represent standard errors.

View larger version (26K): [in this window] [in a new window]   Fig. 3. Distribution of total (EPA 3051) extractable Zn with soil depth in the Acredale and Bojac soils 7 yr after cessation of biosolids application. Bars represent standard errors.

Metal Recovery
Heavy metals recovery was based on metal concentrations in the upper 25 cm of the Acredale soil—since no significant metal transport was detected below this level—and the whole profile for the Bojac soil. The net total crop removal was assumed to be no more than 1% (McGrath, 1987; Sukkariyah et al., 2005). Elements removed through plant harvest are not considered a major factor affecting recovery calculation. Soil bulk density was calculated for each soil section by weighing the soil mass in each known volume increment. Our analysis showed incomplete recoveries of Cu and Zn at both fields. We accounted for 80% of the applied Cu and 56% of the applied Zn in the Bojac and 66% of the Cu and 81% of the Zn in the Acredale soil (Table 6 ).

Total Cu and Zn concentrations in the control fields were similar to the background levels before biosolids application (Table 6, Fig. 2 and 3)—note difference in units. Data analysis and comparison between 1998 and 2005 indicates that metal loss (leaching) from the Ap horizon in both soils has continued after cessation of biosolids application (1998–2005) (Table 6); however, more metals were lost from the Ap horizon of the Bojac soil during the years of biosolids application (1984–1998). Although metal losses occurred in the coarse-textured Bojac soil, the largely undetectable concentrations of Cu and Zn in the monitoring wells (Table 5) indicates that such transport was not a significant contributor to groundwater quality. Detailed analysis of the monitoring wells data before, during, and after biosolids application revealed no change in the quality of groundwater (Table 5).

Phosphorus
Since biosolids were applied to these fields based on crop N requirements, P has been supplied in excess of plant removal. This application rate basis resulted in a dramatic increase in total P in the Ap horizon (0–15 cm) of the treated fields. The increase in biosolids treatment soil P above the control was fivefold for the Acredale and twofold for the Bojac. Phosphorus transport through the profile followed similar patterns to that of Cu and Zn (Fig. 4 ). Significant enrichment of P was observed down to 75 cm in the Bojac, while no increase occurred beyond the top 20 cm in the Acredale soil. Phosphorus leached in the Bojac soil might reach surface water through lateral subsurface flow and drainage water. Elevated subsoil P levels from biosolids addition in the mid-Atlantic region were also documented by Maguire et al. (2000). They reported P enrichment in amended soils down to 60 cm in 6 out of 11 soils studied. Elliott et al. (2002) reported minimal leaching of P in two P-deficient Florida sands with low to moderate P-sorbing capacities for most biosolids except biosolids produced in biological nutrient removal facilities where no Al and Fe salts are added. There was no detectable increase in the concentrations of total P in monitoring well water samples (Table 5).

View larger version (26K): [in this window] [in a new window]   Fig. 4. Distribution of total (EPA3051) extractable P with depth in the Acredale and Bojac soils 7 yr after cessation of biosolids application. Bars represent standard errors.

The degree of phosphorus saturation (DPS) is a commonly used parameter for assessing P loss from soil (erosion, leaching, and runoff) (Sims et al., 2000). The likelihood of P release to runoff or leaching water is higher in soils with DPS > 40% (Maguire et al., 2000). Biosolids applications have increased DPS from 17 and 41% in the Acredale and Bojac topsoil, respectively, to 79% in both soils (Table 7); therefore, there is a higher likelihood of loss from these soils. Phosphorus leaching was observed in the Bojac soil but not in the Acredale. This could be due to higher DPS in the subsurface layers of the Bojac soil making it saturated with respect to P sorption capacity and likely susceptible to P leaching.

Carbon and Nitrogen
Total organic carbon (C) and total Kjeldahl N (TKN) concentrations were significantly different between the control and biosolids-amended soils in the Acredale, but not in the Bojac (Fig. 5 ). Total organic C and TKN in the Acredale was significantly increased by biosolids to a depth of 25 cm. The soil organic C concentration had been expected to increase in these fields following 15 yr of applying organic matter-supplying biosolids. This was not the case, as our results showed a slight increase in the Acredale soil and no increase in the Bojac. Organic matter was likely rapidly mineralized under the climatic conditions (high temperature, high rainfall) prevalent at the site and the intensive tillage management practice implemented. Rapid mineralization was not unexpected in the well-aerated, coarse-textured Bojac sandy loam. Some mineralized N likely leached, but well monitoring data showed no significant increase in N concentration in the water.

View larger version (32K): [in this window] [in a new window]   Fig. 5. Distribution of total carbon and nitrogen in the Acredale and Bojac soils 7 yr after cessation of biosolids application. Bars represent standard errors.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Fourteen years of biosolids liquid slurry application (1994 to 1998) increased total and Mehlich-I-extractable Cu, Zn, and P in two coastal plain soils differing in texture. The finer-textured Acredale silt loam exhibited no subsurface horizon accumulation of either metals or P. There was no evidence of leaching as measured by accumulation of metals below the surface horizon. In the coarser-textured Bojac loamy sand, significant enrichment of Cu, Zn, and P were found down to 0.75 m; however, this was not reflected by a change in quality of monitored groundwater, which showed no change in Cu, Zn, or P concentrations. The Bojac and Acredale soils have similar clay mineralogy and content but differ in amounts of sand and silt. The two soils exhibited different profile distribution concentrations which lead us to speculate that soil physical as well as chemical properties had an important role in retaining biosolids constituents in the Acredale soil.

Approximately 60 to 80% of the metals were accounted for by mass balance calculations in both soils. Leaching losses, subsurface lateral flow, and incomplete metal extraction are speculated to be the mechanisms accounting for the majority of our inability to account for the trace metals.

Fourteen years of N-based biosolids application resulted in excessive Mehlich-I-P levels, a high degree of P saturation in the top 15 cm of both soils, and significant P leaching in the amended Bojac. With no observed increase in oxalate-extractable (Al + Fe) to mitigate P accumulation and the possible P saturation of the soil column (15–75 cm), further P release may be expected at this site. This could increase the likelihood of P movement to surface water via leaching and/or subsurface lateral flow to drainage ditches.

The leaching of metals in biosolids-amended soils has been extensively investigated with most research showing little potential for mobility. The coarse-textured Bojac soil, which has low retention capacity, may pose a trace metal water quality risk if long-term application of biosolids of lower quality (i.e., higher pollutant trace metal concentration) were practiced. Repeated applications (up to 154 dry Mg ha^–1) of the low Fe- and Al-containing biosolids did not increase soil metal-binding oxalate-extractable Al and Fe. Sufficient Al and Fe should be present in applied biosolids to ensure an increase in metal and P soil retention capacity to mitigate the effect of the biosolids-added metals. Applications of biosolids with low Fe/Al content (generated from BNR or where no salts are added during the treatment process) to sandy coastal plain soils might present leaching risk. The increase in total carbon and oxide contents in the Acredale topsoil might partly explain the greater immobilization of Cu, Zn, and P in the zone of incorporation. It is likely that the finer texture in the Acredale contributed to topsoil accumulation of both biosolids-binding constituents (i.e., Al, Fe, and organic matter) and potential biosolids pollutants, while the coarser Bojac failed to physically retain the biosolids constituents that are important for immobilization. Soils like the Bojac series might not be good candidate for biosolids applications. Use of dewatered cake and/or the addition of reactive Fe and Al materials could present a better option and is worthy of investigating.

	ACKNOWLEDGMENTS

 
The authors are grateful to the Hampton Road Sanitation District for funding this research.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
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