In this expert guide the environmental specialists at WTS consider the most common techniques used in soil testing and the analysis of contaminated land and brownfield sites. The guide identifies the most common test parameters including VOCs, phenols, sulphides and cyanides; outlines the importance of sample preparation, and then examines a number of popular techniques used in the testing of soil samples.

Soil testing and analysis of contaminated land

Analysing soil samples can be an extremely complex, but important process especially for land owners and those responsible for the management of contaminated land and brownfield sites.

The problem facing those analysing soil samples is often even more complex on sites which have been contaminated with various chemical substances.

The soil itself will usually be mostly silica (sand), calcium carbonate (limestone), clay or a mixture of lots of different minerals.

However, in brownfield or contaminated sites, there could also be different contamination problems caused by old building materials, through to the most dangerous contaminants caused by toxic waste, pharmaceutical waste, explosive compounds, mercury or more.

The main difficulty in performing chemical analysis of soil is that whatever method you choose for the analysis it has to take into account this very wide range of possible contamination.

Companies working on sites with soil contamination are regulated under the MCERTS system, which approves people, testing equipment and laboratories carrying out soil testing and analysis procedures.

How to prepare soil samples for testing

Although this step in the soil testing process is often overlooked, if the samples are taken incorrectly, this can seriously affect the end results.

It is important that staff taking soil samples should be trained to take, store and process samples in the correct manner.

Samples which are crushed or dried out cannot be tested properly.

Therefore, soil should be mixed and then tested when wet.

Common parameters when testing soil samples

Standard analysis of soil samples often includes tests for the following parameters:

Volatile organic compounds (VOCs)
Phenols
Leachates
Nitrogen
Sulphides
Cyanides
Hexavalent chromium

Preparing soil samples for analysis

The most common technique for preparing soil samples for analysis in the laboratory is the classic cone and quartering technique.

Alternatively, you could use a crusher to break up larger clumps of soil.

Kneading can also work when the soil is mainly clay based.

If there is a lot of fibrous material in your soil sample, then this might need to be cut or shredded before analysis.

If you are drying out soil samples before analysis, this should be done at temperatures of between 35 ^oC and 40^oC.

Most reporting standards require that samples be dried, and the weight of moisture removed recorded.

Dealing with contaminated soil samples

If you are dealing with samples where you suspect contamination, it’s best not to remove anything from the sample itself.

There may be hazardous materials such as paint on the surface of lumps only, or the entire lump might be a contaminant.

If you do remove any part of the sample, it should be recorded on the final report as this is a MCERTS requirement.

After the subsample has been weighed out, most sampling methods then involve a liquid being added to the soil, depending on the type of contaminant you are trying to extract.

For example, you’d use an acidic digest for metals or alkaline extraction for cyanides.

Samples are then shaken, filtered or spun in a centrifuge.

Common techniques used in soil testing

There are several techniques used for soil testing and analysis, and they are usually selected based on the type of contamination. Here we explore some of the most common:

Calibration

Calibration works by running a series of samples at known concentrations through a special analysis machine to create a calibration standard curve.

Then the samples you have taken are run through the machine again, and the results can be compared against the graph.

Colourimetric spectroscopy

One of the longest- established methods for analysing soil samples is colourimetric spectroscopy which has been around for centuries.

Although the instruments that are now used have improved over time, the method is broadly the same.

Colourimetric spectroscopy is most suitable for negatively charged compounds such as chloride, sulphate, phosphate, nitrite, nitrate or ammoniacal nitrogen.

Analysis using an automated spectrophotometer

Modern testing laboratories often use an automated spectrophotometer which can look at several elements at the same time.

There will also be pre-determined systems for adding chemicals to develop the colour.

The solution then passes into the spectrophotometer.

The general principle behind this technique is that the more intense the colour that the machine produces, the higher the concentration of the given chemical in the sample.

This type of soil testing machine is very efficient, and can easily process as many as 200 samples in an hour.

Analysis of metals in soil using ICP-OES

The most common soil tests carried out for environmental contamination is analysis for the presence of metals.

These tests are usually run on a machine called an Inductively Coupled Plasma Optical Emission Spectroscopy, or ICP-OES for short.

The metals analysis is done by measuring out a sample of the dried and crushed soil.

This sample, usually weighing 5g, is then left on a hot block in aqua regia (a mixed of hydrochloric and nitric acid) for two hours.

After this time, the extract is put into the ICP-OES machine which pumps it through superheated gas.

This causes the electrons within the soil sample to emit energy as the sample passes back out.

Each metal contained within the soil will emit energy at different wavelengths.

The machine detects this energy using its signature wavelength, and can produce a report to give concentrations of each metal found in the sample.

The main advantage of this technique is that the machine can look at several wavelengths at once, analysing more than 20 metals in less than 4 minutes.

The atomic absorption method can also be used but can only look at one element at a time.

Testing soil containing sulphur based compounds

The total concentration of sulphur in a soil sample is easily measured using an induction furnace.

However, given that sulphur takes many forms, this analysis won’t help much with identifying risks.

The best way of analysing elemental or free sulphur in soil samples is using solvent extraction.

This type of sulphur is common on many sites which used to be gasworks and it can cause an itchy skin reaction when contact is made.

If you are more interested in measuring total sulphides, this is most commonly done using acid digestion and ICP.

Most sulphides are relatively inert and insoluble and therefore don’t pose a major risk to health.

More of a risk is acid soluble sulphide, which can mix with rainwater and groundwater and become very toxic.

The best way to test for acid soluble sulphide is using acid digestion and colourimetric analysis, or by ion selective electrode.

Acid soluble sulphate, also known as total sulphate, can be analysed by acid digestion and an ICP-OES machine.

This analysis is important when dealing with concrete as relatively low concentration solutions can attack concrete so foundations and other underground concrete structures become at risk.

Testing soil containing nitrogen compounds

The type of soil analysis method used to detect nitrogen compounds will typically depend on the compound.

Nitrate and nitrite samples are analysed using colourmetric spectroscopy using a 2:1 water extract.

Testing for ammoniacal nitrogen again uses water extract, or is distilled to extract the ammonia.

A soil testing method involving distillation and titration can also be used to look at the levels of Kjeldahl nitrogen (KTN).

If you are looking to test for total nitrogen (TN), a stronger acid digestion can measure all of these variables at once.

Soil testing for cyanide compounds

Potassium cyanide is a type of simple cyanide salt which is highly toxic.

Free cyanide also easily dissolves in water.

The best soil analysis method when trying to identify cyanide compounds is water extraction, followed by further analysis using distillation or colourimetric analysis.

If you are interested in measuring total cyanide, then the required digestion is more aggressive and can be done using the Skalar system.

A separate colourimetric analysis can also be done to pick up any thiocyanate in the soil sample.

Using chromatography when testing soil samples

Chromatography involves unravelling the different components in a soil mixture by moving them from the different states of solid, liquid and gas.

The technique uses the fact that individual chemicals have different properties such as larger molecules, or their electric charge.

Liquid chromatography is typically used to analyse speciated phenols.

The technique can be used to detect a range of compounds within this group, so knowing what you want from your soil analysis investigations is very important.

The other type of chromatography used in the testing of soils is gas chromatography, which is usually used to pick up petroleum hydrocarbons.

The pattern which the machine produces allows the skilled analyst to identify the range of compounds contained in the soil sample.

Gas chromatography and mass spectroscopy

Gas chromatography with mass spectroscopy is used to identify a wide range of compounds such as:

- Volatile organic compounds (VOCs),
- Polyaromatic hydrocarbons (PAHs)
- Pesticides

The mass spectroscopy analyser can be set in two different ways.

It can either be set to look for every peak on the chart, or to target specific compounds and ignore anything else in the sample.

What soil tests do you need?

If you need your soil testing but are unsure what type of tests are most appropriate, then it’s usually best to best to start by analysing for VOCs, semi-volatile organic compounds and Tentatively Identified Compounds (TICs).

These tests will give a good indication as to the levels of contamination and can be used to develop more rigorous soil testing protocols where required.

However, if you are still unsure of what tests to carry out then please contact us to discuss your project with one of our specialists.

Laboratory soil testing and analysis solutions

WTS offer a comprehensive range of soil testing and laboratory analysis solutions for the detection of a range of parameters to support organisations involved in the management of contaminated land and brownfield sites.

With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained technicians and consultants we can offer professional, cost effective soil testing and laboratory analysis solutions across the whole of the UK.

Contact us today to learn how our professional soil testing and analysis services can help you deal more effectively with contaminated land and brownfield sites.