Practical 3 – Analysis and Interpretation of Metals in Soils
Assessment: Full lab report (i.e. with Introduction, Methods, Results and Discussion). The report should be no more than 4 pages, including Figures and Tables. The lab report is worth 10%. The brief introduction should include information about metal levels in soils and refer to the Healthbased investigation levels (HILs) from the NEPM Guidelines on Contaminated Sites.
Part A – Collection of a Soil Sample
To become familiar with the apparatus and approaches used to collect representative soil samples
To collect a soil sample for analysis of extractable-metals
Practical 3 has both an on-campus field and a laboratory component in the same class. After or during the demonstration of particular soil sampling apparatus and considerations your group should collect a single soil sample in the container provided. This can be a grab sample or an integrated sample of surface soil or soil from depth. In the Methods section of the lab report briefly describe what apparatus was used to collect the soil, the soil sampling plan and the sample type and location.
What to bring:
Sturdy footwear for walking through the remnant woodland between the Gold Coast campus and Smith Street Motorway. Hat, sunscreen and drinking water are recommended. Also bring a pen and notebook.
Meet in the courtyard outside of G12 (Science 1) 10 minutes before your normal start time to assist us with carrying equipment to the site.
Part B – Characterizing Soil Properties
a)Gain experience in characterizing important chemical properties of soils with tests that can be used in the field
Soils are made up of minerals and ions derived from parent rock material, which are deposited by erosion and/or are modified by human activities. Soils have physical and chemical properties that are influenced by the origin and composition of the minerals present. Metal ions associated with soils will be influenced by these features – there may be significant relationships with soil pH or with aluminium, iron or calcium due to the predominance of certain minerals within the soil.
Some members of the group should start this activity, while others should start Part C. In this partyou will measure the Soil Reaction (pH and conductivity) of four soils. Some of these soils are natural and some have been modified for particular purposes or by anthropogenic contamination.
You have collected a woodland soil sample during the soil field trip in the previous practical. There are three other soils available in the lab – an industrial soil, a gardening soil and a natural basaltic soil. Use each of these soils for this practical.
If the soil that you collected earlier today contained moisture (which is likely for most field samples) use the mortar and pestle to break up any large peds in the soil and dry about 20 g of the soil in the oven at 110°C for 20 minutes. If the soil is very wet, such as due to recent rain, then it may require longer for the soil to dry out. The group members that are undertaking Part C should look after the drying soil now, while the remaining group members continue with Step 3 below.
For each of the four soils, obtain ≈10 g using the top-loading balance in the plastic container provided– these weights do not need to be recorded. Note: this test can be done using soil samples that have not been dried beforehand and therefore this test can be completed in the field.
Add 100 mL of deionised water to each container and place the lids on firmly. Shake the soil solution frequently over approximately 20 minutes.
Remove the containers and carefully manipulate the sample so that soil particles are removed from the upper walls of the container. Allow the soil to settle for several minutes.
Measure the pH and the conductivity of the solutions for each soil and record in the Results section of the Practical 3 lab report.
Part C – Weak Acid Extraction of Metals from Soils
Use a weak-acid extraction to solubilize the most reactive fraction of metals associated with the soils
Use Excel or another spreadsheet to make multiple calculations using a calibration curve.
Liquid-solid extractions are routinely done on soil samples to test for the metal concentration. Some metals are essential elements for the growth of plants and others are toxic to plants and the people or animals that consume them. Metals that are essential can also become toxic at high concentrations. A series of extractions can be done on soils to predict the likely uptake of metals in plants. Weak acid HNO3 extractions will solubilize the most reactive fraction of metals in soils.
Continue on from Step 2 in Part C in which the sampled woodland soil was placed in the oven to dry. While you are waiting for this soil to dry and cool down you should commence the following activities for the other soils from Step 2 below.
After the woodland soil has dried allow it to cool down on the lab bench for several minutes.
Weigh out ≈5 g of each of the four dried soils in an appropriately labelled 50 mL centrifuge tube using an analytical balance. Record the exact masses in Table 2. Note: we are only using a single sample here for each soil (not duplicate samples as in Practical 2).
Add 20.00 mL of 0.25 M HNO3 to each centrifuge tube and place on the shaker for 15 minutes. Assist with or observe the Part B activities while you are waiting.
Remove the tubes – try to manipulate the tubes so that minimal particulate matter is stuck to the upper part of the centrifuge tube - and place in a centrifuge. Centrifuge for 5 minutes at 4000 rpm. Remove the tubes carefully, so that you do not resuspend any sediment and stand upright in a rack.
Rinse the syringe once with a few mL 0.25 M HNO3 before using to collect the extract sample.
Remove about 10 mL of solution from each tube and filter into a 10 mL centrifuge tube. Repeat for each of the soil extracts. Note that we don’t need to measure exactly 10 mL at this stage as the concentration does not change with volume.
Place your soil extract samples in the auto-sampler for the inductively coupled plasma optical emission spectrometer (ICP-OES) and type a code that relates to each sample into the open screen on the computer that controls the ICP-OES. The ICP-OES can make measurements over many orders of magnitude and therefore a series of dilutions is not required for these extractions. Your samples will be analysed for you and the various metal concentrations will be below
Calculate the concentrations of Fe, Al, Mn, Ca and two other metals (your choice from the metal concentrations provided for the extraction solution) in the soils using the same stepsin which you calculated extractable P in the Practical 2 Template, but using the volume and mass from above. Your final concentrations should have the units mg/kg dry weight (wt). Note: It will be good practice for you to set these calculations up in a spreadsheet in Excel – check that you have the same result as for your worked example and use excel to complete the remaining calculations. Show the working out for a single metal for a single soil in the lab report in the Results section, with the other soil metal concentrations reported in a Table in the Results section.
Part D – Analysis and Interpretation of Soils Data
To gain experience in data analysis and interpretation (Step 5 in the Environmental
To gain experience in presenting data in a clear and informative manner
To gain experience in drawing conclusions from data analysis
Step 5 in the Environmental Monitoring Framework requires actual data to be experienced fully, which is something won’t be able to do in the assignment. There are three main ways in which data can be evaluated and interpreted from the Monitoring Framework:
analyse changes in time and space (not available here as you have only a single sample only)
explore relationships between measurement parameters (this can be done using basic statistics such as correlations between two parameters and/or metals, which can be done with this data
compare data (e.g. contaminant concentrations or other parameters) with relevant guidelines. I have included an extract from the NEPM Guidelines on Contaminated Sites in Appendix 1 that you can use to compare with the concentrations measured.
You have obtained the following data for four soils:
pH and conductivity from a Soil Reaction measurement.
0.25 M HNO3-extractable metal concentrations for Fe, Al, Mn and Ca – these metals may represent dominant minerals within the soil.
0.25 M HNO3-extractable metal concentrations for two other metals of your choice. It is recommended that you select trace metals that were measured in each of the four soils and/or present in the NEPM guidelines.
Analyse your data (e.g. by plotting appropriate graphs which demonstrate how metal concentrations vary with changes in another parameter - perhaps another metal concentration, pH, or conductivity or compared with the guidelines). Select three figuresthat display data of interest to you.
Interpret and discuss the results that you have selected. Use statistics if you have completed relevant courses, but this will not be the major basis on which your data or interpretation is evaluated.
Write the Results and Discussion section (including figures) of your lab report. This exercise is to give you complete freedom to explore Analysis and Interpretation of the data obtained. You be marked based on the presentation of your data in the figures and the quality of your interpretation.
Appendix – HILs for metals from NEPM - Contaminated sites
From page 48 and 49 of the NEPM document. Health-based Investigation Levels (HILs) are determined from several methods relevant to environmental health. They are applied to different land uses, as listed below. Other measurements are also used to assist with interpretation of the HIL values. Note that conductivity is somewhat indicative of cation exchange capacity (CEC).