Heavy Metal Analysis in Soil Frank Sikora, David Hardy, and Rao Mylavarapu




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Heavy Metal Analysis in Soil


Frank Sikora, David Hardy, and Rao Mylavarapu


Application and Principle


Elevated levels of heavy metals in soil can be a concern to human health. Soil test methods using Mehlich-1 and -3 extractions in agricultural soil laboratories can provide a rapid screening of soil samples for heavy metals to assess the potential for adverse human health effects.
Heavy metals may be harmful to humans through ingestion of edible plants containing metals through root uptake, ingestion of plants splashed with contaminated soil, or by accidental direct ingestion of soil usually by children. Inhalation of contaminated dust is also a potential health risk. Metals of concern are arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), nickel (Ni), selenium (Se), and zinc (Zn). Although some metals such as Cu and Zn are essential plant nutrients (see Chapter 6), high concentrations can be a concern. High levels of soil Pb can be present due to paint from older homes and gasoline emissions prior to 1980. Arsenic may be present in soil from use of arsenic-bearing pesticides. Chromium, Cu, and As may leach from pressure treated lumber. Soils can have elevated levels of metals due to industrial activity or land-application of biosolids from wastewater treatment or various by-products (animal/poultry wastes, fly-ash, etc) that are valued as sources of nitrogen, phosphorus, and potassium. The United States Environmental Protection Agency (US EPA) set regulatory limits for metal application rates from biosolids through Code of Federal Regulations Title 40, Part 503 (US EPA, 2008). These federal regulations are usually administrated through state environmental departments to meet the federal guidelines

The US EPA set levels of metals in soil, called soil screening levels (SSL), to determine if further evaluation is required for cleanup efforts (US EPA, 1996). The New York State Department of Environmental Conservation (NYS DEC) also set state-wide limits for evaluating whether Brownfield and Supefund sites need further evaluation for cleanup efforts (NYS DEC, 2006). The limits for heavy metals in soil are based on total concentrations of metals in soil determined by digesting soil with acid according to EPA standard methods 3050B or 3051A (US EPA, 1996; US EPA, 2007). These methods are time consuming and labor intensive. Quicker methods for assessing heavy metals in soil can occur with traditional soil testing using extractants such as Mehlich-1 and Mehlich-3. A disadvantage of using these soil test extractants is difficulty in interpreting results since only a fraction of the metal is determined in soil and potential health hazards set by US EPA and NYS DEC are based on total soil concentrations using the EPA standard methods. Advantages of using Mehlich-1 or -3 extractants are rapidity of the test and no special sample preparation or extraction is required beyond what is routinely conducted in soil test laboratories to determine nutrient status of soils. This chapter outlines the procedure of determining heavy metal concentration in soil with Mehlich-1 and -3 with guidelines on interpreting values that are obtained.


Equipment and Apparatus




  1. If soil is measured using a soil scoop, use a scoop calibrated to 5.0 g for Mehlich-1 or 2.0 g for Mehlich-3. (The original Mehlich-1 method was developed with measurement of 4 cm3 of soil to approximate 5 g. The original Mehlich-3 method was developed to measure 2.5 cm3 extracted with 25 mL of extractant with results reported on a volume basis of soil.)



  2. 50 mL extraction container that can be plastic or glass



  3. Automatic solution dispenser



  4. Reciprocating shaker




  1. Whatman #1 or equivalent filter paper




  1. Inductively coupled plasma spectrophometer (ICP)


Reagents



Solutions are made with reagent grade chemicals and distilled water.

Chemicals for Mehlich-1


  1. Hydrochloric acid (HCl), 36-37%, fw = 36.46




  1. Sulfuric acid (H2SO4), 96-98%, fw = 98.08



Chemicals for Mehlich-3


  1. Ammonium nitrate (NH4NO3), fw = 80.05




  1. Ammonium fluoride (NH4F), fw = 37.04




  1. Nitric acid (HNO3), 68-70%, fw = 63.02




  1. Ethylenediamine tetraacetic acid (EDTA), (HOOCCH2)2NCH2CH2N(CH2COOH)2, fw = 292.24




  1. Acetic acid, glacial [CH3COOH] fw = 60.05


Solutions


  1. NH4F-EDTA stock solution (3.75 M NH4F and 0.25 M EDTA): Dissolve 138.9 g of NH4F in 600 mL of deionized water and add 73.06 g EDTA (or 93.06 g of Na2-EDTA.2H2O). Dissolve and dilute to 1000 mL.




  1. Mehlich-3 extracting solution (0.2 N CH3COOH, 0.25 N NH4NO3, 0.015 N NH4F, 0.013 N HNO3, and 0.001 M EDTA at pH 2.5 ± 0.1). Dissolve 80.0 g NH4NO3 in about 3,000 mL of deionized water. Add 16.0 mL of NH4F-EDTA stock solution and mix well. Add 46 mL of concentrated glacial CH3COOH and 3.3 mL of concentrated HNO3 and bring to 4,000 mL final volume. The final pH should be 2.5 ± 0.1.




  1. Mehlich-1 extracting solution (0.05 N HCl and 0.025 N H2SO4). Dilute 16.6 mL of concentrated HCl and 2.8 mL concentrated H2SO4 to 4 L with deionized water.



Calibration Standards


  1. From a standard solution containing 1,000 mg L-1 of the analyte, prepare 1 L of standard in Mehlich-1 or Mehlich-3 extracting solution containing the highest concentrations of each element. Then, use this standard to prepare 250 mL of other calibration standards using dilution with the respective extractant. Recommended ranges of final calibration standards are shown below for concentrations in mg L-1.


As

Cd

Cr

Cu

Ni

Pb

Se

Zn


0

0

0

0

0

0

0

0

0.5

0.5

0.5

1

0.5

0.5

0.5

1

1

1

1

2

1

1

1

2

5

5

5

10

5

5

5

10

Procedure



Mehlich-1 Extraction

  1. Scoop or weigh 5.0 g of air-dried soil pulverized to pass 10 mesh sieve (< 2.0 mm) in a 50-mL extraction container.




  1. Add 25 mL of Mehlich-1 extraction solution to soil. Include a method blank where the method is performed without soil. Include check samples as dictated by the lab’s QA/QC protocol.



  2. Place extraction flask(s) on reciprocating mechanical shaker (200, 4-cm reciprocations/ minute) for five (5) minutes.



  3. Filter suspension immediately and collect the extract in plastic vials. Refilter if filtrate is cloudy.


Mehlich-3 Extraction

  1. Scoop or weigh 2.0 g of air-dried soil pulverized to pass a 10 mesh sieve (< 2.0 mm) in a 50-mL extraction container.



  2. Add 20 mL of Mehlich-3 extracting solution to soil. Include a method blank where the method is performed without soil. Include check samples as dictated by the lab’s QA/QC protocol.



  3. Place extraction flask(s) on reciprocating mechanical shaker (200, 4-cm reciprocations/ minute) for five (5) minutes.



  4. Filter suspension immediately and collect the extract in plastic vials. Refilter if filtrate is cloudy.



Analysis



  1. Calibrate the ICP using calibration standards following manufacturer’s recommendations in the operation and calibration of the instrument. Wavelengths with the greatest sensitivity for the metals are usually best because concentrations are normally low. Since concentrations of Cu and Zn are not as low as the other metals, using the most sensitive wavelength is not as critical. Relative emission intensity or sensitivity may not always be the best guideline for wavelength selection on some instruments. For example, beware of high ultraviolet wavelengths (280-400 nm) on dual-view instruments that view these wavelengths with radial plasma orientation. Several wavelengths should be viewed and optimized during method development to select the most appropriate wavelengths for the instrumental configuration used.

Suggested wavelengths (nm) are shown in the table below.

As

Cd

Cr

Cu

Ni

Pb

Se

Zn


188.980

226.502

267.716

324.754

231.604

220.353

196.026

213.857



  1. Dilution should be made if a sample has concentrations above the highest standard.




  1. Since the concentrations may be at or below the limit of quantification (LOQ) of the ICP, it is helpful to define these limits. Run the blank calibration standard ten times at the end of the analysis. The variations in intensity signals from the repeated analyses of the blank are used to determine LOQs.

Calculations



  1. To report metal concentration as mg kg-1 in soil use the following calculation.

Mehlich-1 extraction:

Metal concentration in soil (mg kg-1) = (mg L-1 in extract) × 5


Mehlich-3 extraction:

Metal concentration in soil (mg kg-1) = (mg L-1 in extract) × 10

If the method blank contains detectable concentration of a metal, this concentration will need to be subtracted from the concentration in the soil extract before multiplying by 5 or 10.

The mg kg-1 unit is equivalent to parts per million (ppm).



  1. The LOQ is generally not important for assessing health hazards since any value near the LOQ is well below hazardous levels for all of the metals except As. Determining LOQs is more important for research projects since erroneous values reported below the LOQs would confound interpretation of treatment effects.

To determine the LOQ, first obtain the average (xb) and standard deviation (sb) of 10 intensity readings of the blank calibration standard. Next, obtain the slope (m) and intercept (b) of the calibration curve. Enter the parameters into the following equation to obtain the LOQ.

LOQ = (xb – b + 10 sbi) / m

The LOQ is a concentration in the extract in mg L-1.

Example LOQ values for metals in the Mehlich-3 extract using a Varian Vista-Pro are shown below with respective wavelengths in nm.


As

Cd

Cr

Cu

Ni

Pb

Zn


Wavelength =

188.980

226.502

267.716

324.754

231.604

220.353

213.857

LOQ (mg L-1) =

0.01

0.001

0.002

0.002

0.008

0.01

0.001

Multiply the LOQ in mg L-1 by the appropriate factor (5 for Mehlich-1 or 10 for Mehlich-3) to obtain LOQ as mg kg-1 concentration in soil. If the calculated concentration of metal in soil as determined in step 1 is less than the LOQ, report < LOQ for the soil metal concentration. For example, if the LOQ for Pb was 0.01 mg L-1 in Mehlich-3 extract, values for Pb below the LOQ in soil would be reported as <0.1 mg kg-1.

The LOQ should not be confused with the limit of detection (LOD). The LOD is the lowest concentration that produces a signal detected by the instrument. Just because the metal can be detected does not guarantee the metal concentration can be quantified. The LOQ is about 3.3 times the LOD. Thomsen et al. (2003) provides a good description between LOQ and LOD for spectroscopic analysis.



Analytical Performance

Range and Sensitivity


  1. Concentrations are usually low. Therefore, wavelengths with the greatest sensitivity are normally used but exceptions can occur as mentioned in the Analysis section. Ex ample slopes of calibration curves for metal analysis in Mehlich-3 extracts using Varian Vista-Pro ICP are shown below with intensity of ICP signal (I = counts sec-1) plotted versus concentration (mg L-1)



As

Cd

Cr

Cu

Ni

Pb

Zn


Wavelength =

188.980

226.502

267.716

324.754

231.604

220.353

213.857

I / conc. =

714

96,200

55,600

74,100

6,630

2,990

41,200



Precision and Accuracy


  1. Precision suffers since concentrations analyzed are often near the LOQ. The Horowitz equation (Horowitz and Albert, 2006) predicts interlaboratory precision, measured as percent relative standard deviation, to be 32% for 0.01 mg L-1 metal concentration in the Mehlich-1 or Mehlich-3 extract. This corresponds to 25% and 23% relative standard deviation for corresponding mg kg-1 concentrations in soil for Mehlich-1 and -3, respectively. Intralaboratory precision is approximately one-half to two-thirds the interlaboratory precision.




  1. Accuracy is difficult to assess because reference samples with known Mehlich-1 or Mehlich-3 concentrations are not readily available. Analysis by several cooperating laboratories can assist in determining accuracy.


Interferences


  1. Consult instrument software for potential interferences. The very fine resolution of current ICPs to one thousandth of a nm helps minimize peak overlap. Elevated baselines may occur in some extracts. Interference from elevated baselines is eliminated with baseline correction. If interferences are suspected, a standard-additions technique (Skoog et al., 1999) can be used for more accurate measurements.

Interpretation

  1. The US EPA and NYS DEC have set guidelines for determining safety of various land uses based on total soil metal concentrations using standard EPA methods. The table below shows these limits.

Total metal concentrations in soil (mg kg-1) to guide cleanup efforts

US EPA

NYS DEC

soil cleanup objectives ‡


Metal

Soil screening level †

Unrestricted use


Residential use



As

0.4

13

16

Cd

70

2.5

2.5

Cr (hexavalent)

230

1

22

Cr (trivalent)

120,000

30

36

Cu




50

270

Pb

400

63

400

Ni

1600

30

140

Zn

23,000

109

2200

† US EPA (2002)

‡ NYS DEC (2007)

The US EPA soil screening levels are set as a guide to determine when cleanup efforts may be needed. Values exceeding these concentrations need further study for cleanup efforts. The NYS DEC soil cleanup objective values are based on removing health risks for unrestricted or residential use. Unrestricted use refers to land without imposed restrictions such as environmental easements. Residential use is for land with single-family housing with no livestock raised for human consumption.

The concentrations of metals determined from Mehlich-1 or -3 extraction are only a fraction of the total metal concentration in soil. Therefore, concentrations from Mehlich-1 or -3 extraction have to be related to total concentrations for assessing health safety risks using the above table. Hamel et al. (2003) found a relationship between Mehlich-1 and -3 extractable Pb and EPA 3050B method (EPA 3050B Pb) using the equations below.

EPA 3050B Pb = 1.3 x (M3 Pb) + 39 r2 = 0.60

EPA 3050B Pb = 5.3 x (M1 Pb) + 96 r2 = 0.49

The total Pb concentration calculated in the above equations can be compared to the limits set by US EPA and NYS DEC in the table above. The equations must be used with caution since there is a high variation in the regression with low r2 values and the comparisons were only performed for Alfisols and Ultisols in New Jersey. Therefore, the calculated values should only be considered an initial screening method to provide an approximation of total Pb concentration. A follow up analysis using EPA method 3050B or 3051A would provide a more definitive value to compare to the safety limits.

Equations for calculating total metal concentration from Mehlich-1 and -3 extractable metals are only shown for Pb. Relationships for total concentration versus Mehlich-1 or -3 extractable metal concentration for other metals from future research can be used in a similar fashion.



  1. Another method to assess potential hazards from metals in soil is to compare Mehlich-1 or -3 extractable metal concentrations to an average background level obtained from several soils in a particular region. The table below shows average concentrations for metals determined with Mehlich-3 extraction on 3286 samples in North Carolina. If a Mehlich-3 extractable metal concentration is near the average background level, no hazard is anticipated since the metal concentration is at an expected level.

Average concentrations of Mehlich-3

extractable metals in 3286 soil samples

from North Carolina (Hardy et al., 2008).


Metal

Conc. (mg dm-3)†

As

4.5

Cd

0.1

Cr

0.2

Cu

9.2

Pb

4.2

Ni

0.8

Se

0.2

Zn

27.2

† mg dm-3 equals mg kg-1 if processed soil

density is 1 g cm-3.


The table above should not be used universally since the soils surveyed in the table are from North Carolina. A survey of Mehlich-1 or -3 extractable metals from a region of interest would help determine natural background levels in soils in that region.

Evaluating background levels of Mehlich-1 or -3 extractable metals from soils in a particular region provides a general guideline to assess potential hazards. Since regulatory agencies set guidelines for risk assessment based on total metal concentrations (see Interpretation-1), just assessing risk on Mehlich-1 or -3 extractable metal concentrations may be misleading if there is no consistent relationship between Mehlich-1 or-3 extractable and total metal concentrations.



  1. There are no “safe” regulatory limits on soil metal concentrations for vegetable gardens. However, the scientific information on concentrations for cleanup efforts by USEPA and NYC DEC have been used to develop contaminant levels and actions to take for ranges of total soil Pb concentrations. The ranges from Pennsylvania State University and Rutgers University are shown below. Pennsylvania State University performs the EPA 3050A method. Rutgers University performs a Mehlich-3 extractable Pb analysis and calculates the total Pb concentration using the observed relationship between EPA 3050 Pb and Mehlich-3 Pb (Hamel et al., 2003). The differences in the two tables also highlight the lack of uniformity in interpreting total soil Pb content for vegetable gardening.

Pb ranges and guidelines from Pennsylvania State University (Stehouwer and Macneal, 1999)

Total Pb conc.

(mg kg-1)



Contamination level

Action


<150

None to very low

No need to be concerned about Pb exposure.

150 to 400

Low

Conduct best management practices (BMPs) to minimize Pb exposure from vegetable gardens.

400 to 1000

Medium

Apply phosphate fertilizer and maintain high pH for fruiting vegetables. Do not grow leafy vegetables.

1000 to 2000

High

Do not grow a vegetable garden.


>2000

Very high

Contact local health department for Pb abatement measures.

Pb ranges and guidelines from Rutgers University (Hamel et al., 2010)



Total Pb conc.

(mg kg-1)



Contamination level

Action


0 to 100

None

No need to be concerned about Pb exposure.

101 to 300

Elevated

Conduct BMPs to minimize Pb exposure from vegetable gardens.

300 to 400

Significant

Conduct BMPs to minimize Pb exposure from vegetable gardens.

>400

Cleanup

Do not grow a vegetable garden.


Best management practices for gardening are shown below for minimizing plant uptake and exposure when total Pb concentration in soil is elevated above background levels and below 400 mg kg-1 (Univ. Mass, 2004).

1. Locate gardens away from old painted structures and heavily travelled roads.

2. Give planting preferences to fruiting crops (tomatoes, squash, peas, sunflowers, corn, etc.).

3. Incorporate organic materials such as finished compost, humus, and peat moss.

4. Lime soil as recommended by soil test (pH 6.5 minimizes lead availability).

5. Discard old and outer leaves before eating leafy vegetables. Peel root crops.

6. Wash all produce.

7. Keep dust to a minimum by maintaining a mulched and/or moist soil surface.



  1. Although total metal concentrations are used to assess potential health hazards, Mehlich-1 or -3 extractable metals may provide values better related to plant uptake and a more accurate assessment of the risk for metals entering the food chain via plant uptake from soil. Further research is needed to assess toxic levels of metals in soil by relating Mehlich-1, Mehlich-3, or some other extractant to plant uptake.

Effects of Storage

  1. Air-dried soils may be stored several months without affecting measurements.

  2. The Mehlich-1 and -3 extraction solutions are stable and can be stored for several weeks due to their acidic nature. A specific shelf life is not known.


Safety and disposal

  1. The chemicals should be stored and disposed of according to routine laboratory procedures. Some labs may require neutralization before pouring extracted Mehlich-3 or Mehlich-1 solution into the sink due to low pH.


References

  1. Hamel, S.C., J.R. Heckman, K.L. Shilke-Gartley, and B. Hoskins. 2003. Lead extraction using three soil fertility tests and Environmental Protection Agency Method 3050. Comm. in Soil Sci. and Plant Anal. 34: 2853-2873.




  1. Hamel, S., J. Heckman, and S. Murphy. 2010. Lead contaminated soil: minimizing health risks. Fact sheet FS336. Rutgers, The State University of New Jersey, New Jersey Agricultural Experiment Station.




  1. Hardy, D. H., J. Myers, and C. Stokes. 2008. Heavy metals in North Carolina soils - significance and occurrence. http://www.ncagr.gov/agronomi/pdffiles/hmetals.pdf; accessed 19 Aug 2010.




  1. Horowitz, W. and R. Albert. 2006. The Horwitz Ratio (HorRat): A useful index of method performance with respect to precision. J. of AOAC International 89:1095-1109.




  1. NYS DEC. 2006. New York State Brownfield Cleanup Program Development of Soil Cleanup Objectives Technical Support Document. New York State Department of Environmental Conservation and New York State Department of Health, Albany, NY. http://www.dec.ny.gov/chemical/34189.html; accessed 19 Aug 2010.




  1. Skoog, D.A., D.M. West, F.J. Holler, and S.R. Crouch. 1999. Analytical chemistry: an introduction, 7th ed. Cengage Learning.




  1. Stehouwer and Macneal, 1999. Lead in residential soils: sources, testing, and reducing exposure. Pennsylvania State University, College of Agricultural Sciences, Cooperative Extension.




  1. Thomsen, V., D. Schatzlein, and D. Mercuro. 2003. Limits of detection in spectroscopy. Spectroscopy 18(12): 112-114.




  1. US EPA. 1996. Method 3050B. Acid digestion of sediments, sludges, and soils, revision 2. http://www.epa.gov/wastes/hazard/testmethods/sw846/online/3_series.htm; accessed 19 Aug 2010.




  1. US EPA. 1996. Soil screening guidance: technical background document, 2nd ed. EPA/540/R95/128. Office of Solid Waste and Emergency Response, Washington, D.C. http://www.epa.gov/superfund/health/conmedia/soil/index.htm; accessed 19 Aug 2010.




  1. USEPA. 2007. Method 3051A. Microwave assisted acid digestion of sediments, sludges, soils, and oil, revision 1. http://www.epa.gov/wastes/hazard/testmethods/sw846/online/3_series.htm; accessed 19 Aug 2010.



  1. USEPA. 2008. A plain English guide to the EPA Part 503 Biosolids Rule. http://water.epa.gov/ scitech/wastetech/biosolids/503pe_index.cfm; accessed 19 Aug 2010.





  1. University of Masschusetts –Amherst. 2004. Soil lead levels. http://www.umass.edu/plsoils/ soiltest/lead1.htm; accessed 19 Aug 2010.


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