There are many areas of soil testing whereby site staff and consultants may not be aware of ways in which samples may be compromised, or the most appropriate testing may not be selected. This article discusses some of the common areas which could be improved upon in order to maximise the value of your data.

Sampling

This is an area which may be considered well documented, but laboratories still receive samples which have not been taken or stored correctly, and this may result in samples being classified as deviating. A comment on the report issued by the laboratory will then state ‘the integrity of these samples may be compromised due to incorrect procedures during sampling or storage and the data may therefore be affected’. This could result in the end client refusing to pay for the cost of the analysis, which would therefore be a significant waste of money.

Examples of deviating samples include:

  • Volatile samples not taken in separate jars
  • Headspace in a volatile sample
  • Samples for organic testing taken in plastic containers, rather than glass
  • Samples not sent to the laboratory quickly so holding times are exceeded
  • Samples not stored at correct temperatures
  • Insufficient sample available for testing

Defining your objectives

The aims and objectives of the sampling need to be clearly understood in order to schedule the most effective analysis for each particular site. For example, is there previous data on the site? Are you verifying previous results or is this a new site investigation? Do you need an in depth comprehensive survey across a wide range of analytes or do you need to concentrate on a small list of known contaminants? Are there hotspot areas on the site which need to be sampled in greater intensity? Are you just delineating an area of a spill or perhaps just monitoring the progress of some on-site remediation? What is the end use of the site? – levels of investigation will be different for a housing development than somewhere which is intended as a car park.

Organics in soils

The behaviour of organic compounds in soil is a very complex topic, but it is essential that site staff understand this, and can try to predict what compounds are likely to be present, where they are most likely to be found, how they behave in the soil, and what is the likely direction of movement due to groundwater flow and dispersion. It is therefore of paramount importance that a comprehensive desk study is performed and a Conceptual Site Model (CSM) produced before any boreholes/ trial pits are dug or samples taken.

Organics can be LNAPLs or DNAPLS (light or dense non-aqueous phase liquids) and their behaviour and distribution will vary depending upon their solubility, density and partition coefficients with the soil.

They can break down or be removed by a variety of mechanisms:

  • Sorption
  • Degradation
  • Dispersion
  • Volatilisation
  • Advective flow – carried by the groundwater
  • Diffusive flow – movement along a concentration gradient

Some of these mechanisms depend upon the soil matrix itself:

  • Particle size
  • Organic content
  • Moisture content
  • pH
  • Microbial content
  • Chemical composition of the soil
  • Homogeneity of soil across the site

Common issues with organic analysis

In groundwater, do you mean ‘total’ in the whole sample, or ‘total dissolved’?

Groundwater samples often contain sediment, and laboratories can analyse the whole sample, or just the aqueous layer, or filter the water to analyse only the truly dissolved organics. This last category is likely to provide a low result, as many organic compounds are immiscible with water and therefore do not exist as truly dissolved, but may be present as a colloidal suspension (tiny droplets suspended in the water) or as a free product. All forms can be carried in the groundwater, so it is important that site staff decide exactly what information is required, as the sample pre-treatment will give widely differing results.

What level of certainty and/or detection limits do you require?

This is basically a decision as to whether an analysis should be performed by GCFID or by GCMS. Gas Chromatography (GC) is performed to separate the sample organics into separate compounds, but detection can be either by Flame Ionising Detection (FID) or by Mass Spectroscopy (MS). For the analysis of petroleum based compounds, GCFID is usually the best choice, as the response factors for hydrocarbons are very good, and the chromatography spectra are well understood. The only exception to this is if MTBE is required, as this is best performed by GCMS.

The benefits of GCMS include lower limits of detection, greater certainty of identification, library search facility and the choice of Selected Ion Monitoring (SIM) or Full Scan. SIM allows the system to look just for selected target ions and the instrument ignores everything else, which allows lower limits of detection to be achieved. GCMS SIM would be the best choice for pesticides or polyaromatic hydrocarbons, for example, where EQS values are often very low, and it would be difficult to identify the compounds by FID. A broad screen for Volatile Organic Compounds (VOCs) or Semi Volatile Organic Compounds (SVOCs) would be run in full scan mode.

The downside for choosing GCMS analysis is that the cost will be higher than GCFID, so it is a choice of what is most appropriate for the site.

What information do you require from Total Petroleum Hydrocarbons (TPH) analysis?

TPH consists of two main groups – Volatile Petroleum Hydrocarbons (VPH) and Extractable Petroleum Hydrocarbons (EPH), but TPH is sometimes considered to be synonymous with EPH, which is not the case. VPH cover the carbon range C5 – C10 (the petrol or gasoline range), and EPH cover >C10 – C40 (which includes kerosene, diesel and lube oil).

“it is important that site staff decide exactly what information is required, as the sample pre-treatment will give widely differing results”

Analytically, laboratories can offer the following GCFID options:

  • TPH screen – one extraction covering the C6 – 40 range, but with no speciation or identification
  • VPH by headspace – a detailed volatile C5 – C10 range including BTEX
  • EPH – the diesel/lube oil range >C10 – C40, plus indigenous organics
  • Cleaned up EPH – the sample is passed through a clean up column to remove most of the polar indigenous compounds, so will be a mix of the aliphatic and aromatic petroleum hydrocarbons
  • Mineral oil – the clean up procedure as above, but a non-polar solvent is used so only the aliphatics will be included, not the aromatics
  • TPHCWG – TPH by the Criteria Working Group method which covers the full C5 – C40 range, and splits the samples into aliphatic and aromatic compounds to allow full speciation and banding. This is most suitable for risk assessment purposes

Obviously, there will be increasing costs with the increased preparation for each level of definition and information, but if you are only monitoring the progress of remediation or delineating the area of a spill, then the TPH screen may be perfectly adequate

Inorganics in soil

Metals

A wide range of metals are present in soils, with suites of metals selected for analysis – usually by Inductively Coupled Plasma Emission (ICP), which provides multielement analysis in a few minutes per sample, following an acidic digestion preparative stage.

The first question to consider is:

Which metals do you need?

A common suite is:

As, Cu, Cr, Cd, Pb, Ni, Zn, Hg, Se, B, but the CLEA suite will include Be, Co, Mn

However, there may be others if the site was used for an unusual purpose, and if groundwaters are to be analysed, then the alkaline metals (Na, Mg, Ca and K) may be required. If you know which metals are likely to be a problem, then it is cheaper just to request these rather than an extensive suite.

Secondly, the question of what is meant by ‘total’ should be considered with respect to including sediment in groundwaters, or just analysing the filtered water (dissolved metals).

Total concentrations may be required for discharge consents, where the total loading is required.

Total or speciated?

The other use of ‘total’ for metals relates to speciation. Some metals can exist in different forms – chromium is an example. Laboratories can analyse for total chromium by ICP, or for hexavalent chromium using a spectrophotometric method. This is often required, as hexavalent chromium is much more toxic than the more common trivalent form. The other metal commonly requested for speciation is mercury, which can exist as elemental, inorganic or organic forms. These have very differing toxicities and can be measured using cold vapour atomic absorption. Again, it will depend on the original site risk assessment as to which of these analyses are most appropriate, but you can potentially save on remediation costs by proving that metals are not present in their most toxic form.

“laboratories can analyse for total chromium by ICP, or for hexavalent chromium using a spectrophotometric method”

Nitrogenous compounds

Which nitrogen compounds are most appropriate to the site?

This is a complex topic, as there are many interrelated nitrogen compounds and they are very important in soil and water chemistry. They can be broken down into the following groups:

Nitrogen gas – a major part of the atmosphere – not usually measured in the lab

  • Nitrite (NO2) – an oxidised form
  • Nitrate (NO3) – another oxidised form

Together they are called Total Oxidised Nitrogen (TON), and the units expressed as mg/kg N, rather than the individual components.

  • Free ammonia (NH3) – the un-ionised form, fairly volatile
  • Ammonium (NH4) – the ionised form

Together they are called Ammoniacal Nitrogen, and can be expressed as mg/kg of N, NH3 or NH4. In groundwaters, the free ammonia can be performed by calculation from the ammoniacal nitrogen, as long as the pH of the water is also known.

  • Organic nitrogen – organically bound nitrogen from plant and animal breakdown
  • Kjeldahl nitrogen – organic nitrogen plus ammoniacal nitrogen

To obtain a result for organic nitrogen, you need to schedule both Kjeldahl nitrogen and ammoniacal nitrogen and subtract one from the other

  • Total nitrogen – the sum of all the above, excluding nitrogen gas

Sulphate compounds

The sulphate content of the soil is of high importance to developers, as some sulphates can attack cement. Most laboratories will offer total (acid soluble) sulphate and/or water soluble sulphate.

Which sulphates provide the correct information for your site?

The following levels of total sulphate provide an indication of the risk of sulphate attack:

< 0.1% sulphate Negligable risk
0.1 – < 0.2 % Moderate
0.2 – < 2 % Severe
> 2 % Very severe

However, this relates to total (acid soluble) sulphate content and BS1377 quotes a figure of 0.24% above which it is necessary to measure the water soluble sulphate, as if most of the sulphate is insoluble in water, then it will not be a risk to the cement.

Therefore, it would not be necessary to schedule all samples for both total and water soluble sulphate, but only the samples with total values above 0.24% will require testing for water soluble sulphate.

Dependent options

This is a useful method for automatically scheduling follow on analysis once preliminary testing is completed.

Using the example of sulphate as discussed above, the Laboratory Information Management System (LIMS) can be programmed to automatically schedule water soluble sulphate whenever a result for total sulphate is > 0.24%, cutting out the need for human intervention and thereby reducing any delays.

“correct sample containers and cold storage are essential to preserve sample integrity”

For organics, samples can be scheduled for an SVOC full scan, and then only those samples which show positive values for the analytes of interest can be scheduled for more detailed analysis using specific target ion analysis to reach lower detection limits.

Similarly, all samples can be scheduled for a TPH screen, and then only those samples giving a value of, for example, > 1000 mg/kg can be scheduled for more detailed analysis such as TPHCWG.

General comments

There are some parameters for which it is preferable to perform the analyses on site, within a few minutes of taking the samples. This is particularly true for groundwater samples, and so pH, dissolved oxygen, electrical conductivity and redox potential should all be performed on site. The results from these examples would all change during the storage and transportation back to the laboratory, and data is therefore more meaningful if carried out as soon as possible after sampling.

Correct sample containers and cold storage are essential to preserve sample integrity.

And above all, communication with your laboratory is vital to ensure you schedule the correct testing to meet your objectives and to obtain the best value in testing.