3.1 Sampling objectives
The first stage in soil investigations is to establish clear sampling objectives. These must define why and how samples are being collected, and lead to the formulation of the sampling strategy (eg, where to collect the samples). The sampling objectives will be site specific and depend on the purpose of the investigation (as defined by the DQOs). Common sampling objectives include:
- to establish the type and location of sources of contamination
- to establish the nature, degree and extent of contaminant distribution (both vertically and laterally)
- to verify that the contamination on site has been reduced to below an established value (eg, following clean-up of a chemical spill)
- to determine the nature of material for waste characterisation.
In some instances it could be appropriate to establish different sampling objectives for different areas within a site. This is often done when stratified sampling is used at a site (section 3.4.3).
3.2 Preparing for fieldwork
Before commencing fieldwork you should:
- obtain the necessary permits from the regional council or territorial authority for undertaking the works (eg, land-use consents for borehole installation)
obtain permission for access to the site and individual sampling locations, which may include access to neighbouring properties, and notify the relevant authorities and neighbours (eg, permission from the New Zealand Transport Agency to sample below a state highway)
- check clearance of underground and above-ground services
- ensure the availability of suitably trained and qualified site personnel
- review the sampling and analysis plan and obtain the appropriate sampling equipment, including containers from the analytical laboratory and storage containers, and make sample transport arrangements
- check and calibrate field instruments, as necessary
- arrange for sampling equipment decontamination
- arrange for the suitable disposal of excess soil, wash water and any contaminated materials (such as gloves) generated during the works
- ensure the availability and suitability of the required contractors
- prepare a health, safety and environment plan (HSEP), 3 which should include:
- an assessment of the on-site hazards
- measures to eliminate, isolate or minimise these hazards for the tasks proposed
- emergency response measures
- site-specific training needs
- protective equipment.
An example of a job safety analysis form within an HSEP is presented in Appendix A. Adequate preparation beforehand should ensure that on-site work is carried out safely and minimises unnecessary delays in the field.
3.3 Sampling and analysis plans
A sampling and analysis plan should be prepared as part of the process of establishing DQOs. It should be a working document that is utilised by field staff undertaking the sampling. As a minimum, the following items should be included in the plan:
- purpose of the investigation
- sampling objectives
- information about the site (location, history and conceptual site model with contaminants identified)
- sampling pattern and strategy to be used
- field screening or on-site testing requirements
- location, depth, type and number of samples to collect
- sampling method(s) to be used
- order of sample collection (where practical, sampling should start at the part of the site suspected to be least contaminated to minimise the possibility of any cross-contamination)
- quality assurance / quality control requirements
- decontamination procedures
- handling and sample preservation requirements
- sample transport and holding times
- laboratory contact details.
The form, content and level of detail documented in the sampling and analysis plan will be site specific.
3.4 Sample pattern selection
The soil-sampling strategy should be consistent with the sampling objectives, and the rationale for the sample pattern chosen must be based on the DQOs. There are three types of sampling patterns commonly used (see Figure 4):
Although judgemental sampling is inherently biased and limits the usefulness of the data for statistical interpretation, it is routinely used when sufficient knowledge of site history and activities is available. Statistically sound sample patterns include systematic and stratified sampling, which are designed to minimise bias in the sample collection. Random sampling may also be used in some cases, although this may be of limited value because the sampling points can, by chance, cluster together. Depending on the number of sample locations, random sampling also can be deficient for detecting hot spots and for giving an overall picture of the spatial distribution of site contamination.
In practice, an investigation of a site for the presence of hazardous substances normally involves the use of more than one sampling pattern. If a predetermined sampling point needs to be relocated (eg, due to a physical obstruction), then the deviation from the sampling pattern should be documented.
Further information on sampling patterns can be found in the following references:
- US EPA (2002) Guidance on Choosing a Sampling Design for Environmental Data Collection for Use in Developing a Quality Assurance Project Plan, EPA QA/G-5S
- The Centre for Research into the Built Environment (1994) UK Contaminated Land Research Report CLR 4: Sampling strategies for contaminated land. The Centre for Research into the Built Environment, Nottingham Trent University, Nottingham.
3.4.1 Judgemental sampling
Judgemental sampling is also referred to as targeted, selective, strategic or model-based sampling. Sample locations are selected based on prior knowledge of contaminant distribution established from the site history, evidence of staining, and professional judgement. Only use judgemental sampling when there is reliable information about the site (eg, site history, the location of specific areas of concern is known).
Judgemental sampling can be used to:
- provide insight into what chemicals may be present in relation to particular activities that have occurred
- confirm the presence or level of contamination at a specific location (eg, a 'worst case' location)
- provide screening information to assist the scoping of subsequent investigation phases.
The advantages of judgmental sampling are that it is less expensive than statistical sample designs and can be efficient and easy to implement. One major limitation, however, is the introduction of bias due to the sample pattern. However, this approach is often used for sites where reliable site history data identify areas of possible contaminants on an otherwise 'clean' site. Care must be taken when interpreting the results of judgmental sampling, because the validity of the data is dependent on knowledge of the site and professional judgement.
Owing to the bias introduced, judgemental sampling should not be used for validation sampling, and a statistically designed investigation is recommended for the collection of validation samples.
Example: A timber treatment site is being investigated using judgemental sampling. Based on the preliminary site inspection, aerial photographs and discussions with workers, samples are located around the edge of the drip pad, along vehicle tracks where treated timber is carried, under an above-ground diesel tank, and under the piles of treated timber stored in the yard. Samples are also taken at the bulk Copper Chromium Arsenic solution delivery point, the point where the PCP antisapstain bath used to be located, and the area where sludge from the treatment operations used to be dumped.
3.4.2 Systematic sampling
Systematic sampling, also referred to as non-targeted or grid sampling, is a statistically based sampling strategy. Sample locations are selected at regular intervals throughout the site area on a grid pattern, with the first sampling location chosen at random to lessen bias.
Grid patterns include square grid, triangular, radial and herringbone, and are selected on the basis of factors such as the size and geometry of contamination hotspots and the overall site size. Situations appropriate to the use of systematic sampling include:
- site validation of both residual soil and backfill material
- to detect hotspots (see Appendix D)
- estimating the mean concentrations of contaminants
- general site characterisation in the absence of adequate site history information.
Systematic sampling has the advantage of being practical and convenient to use in the field. In areas where contamination is suspected to exhibit periodic spatial variations (eg, grape vines spaced at regular intervals), locations must be designed in a way to avoid introducing a bias to the samples; for example, by choosing appropriate grid spacing. A disadvantage of systematic sampling is that the number of sampling locations could be large and it may not be as cost effective as other designs if prior information on site use is available.
Example: An automotive dismantler's yard is to be redeveloped for residential use. Over its 20-year life, car parts and machinery have been stored all over the site. An appropriately sized grid is used over the entire site because contamination could be present at any location on the site.
3.4.3 Stratified sampling
In stratified sampling the site is divided into non-overlapping sub-areas, with differing sampling densities and patterns. The sub-areas are identified as regions of the site that are expected to be uniform in character, and sampling points within these areas are selected systematically or judgementally. Prior knowledge of the sampling sub-areas is combined with likely contaminant behaviour to determine where to sample and to reduce the number of samples. The number of samples within each sub-area is proportional to the relative size of the site and sub-area. The basis for the selection of sub-areas may include:
- geological features
- the layout of current process or storage facilities
- site history
- lateral and vertical distribution of contamination
- intended future use of the sub-area.
Stratified sampling is used for investigating large and complex sites or when an area can be subdivided on the basis of anticipated contamination levels (eg, based on knowledge of site history).
Example: A known chemical manufacturing company owns a two-hectare parcel of land. A detailed site history (affidavits from site owners, aerial photograph search, etc.) reveals that before ownership by the chemical company the site was used as pasture. Since purchase, only the front hectare has been used by the chemical company - the rear hectare has been leased out and retained in pastoral use. A stratified sampling pattern would split the site into the two one-hectare blocks. Judgemental sampling would be used at locations around the manufacturing plant, material storage areas, etc., with a grid (systematic sampling) over the rest of the front hectare. The rear hectare could be sampled using six composites each made up from four sub-samples (section 3.6.4).
3.5 Field-screening techniques
Field-screening techniques can be used before a detailed site investigation, or as part of the investigation strategy. Field-screening methods are used to:
- define the soil contamination cost-effectively and assist in limiting the extent of an investigation
- refine the sampling locations
- identify samples to be analysed.
Any field-screening technique must be appropriately validated. The use of a field-screening technique may not remove the requirement for intrusive ground investigation and laboratory analysis. These techniques require expertise to use: all equipment must be appropriately calibrated, and the work undertaken by trained staff. The limitations of the field-screening techniques should be specified in the reporting stage (eg, instrumental interference, depth limitations).
3.5.1 Non-intrusive techniques
Geophysical surveys are non-intrusive techniques used to identify irregularities or hidden features in the subsurface (eg, the edge of a landfill, buried objects and the location of foundations). Geophysical surveys involve taking measurements of the subsurface properties such as conductivity and electrical resistivity. The work should be performed and the results interpreted by qualified specialists. The choice of geophysical techniques used will depend on the site-specific conditions, such as the purpose of the survey, ground conditions, depth to the water table, etc.
Another example of a non-intrusive technique is the use of infra-red photography to determine the areas of contaminated ground, landfill gases and stressed vegetation.
3.5.2 Soil-screening techniques
Soil-screening techniques are field measurements taken to identify contamination, and they can be used to determine which samples to analyse, or where to position a sample borehole or test pit. Soil-screening techniques are used to detect the soil concentrations in the ground by taking in-situ or ex-situ measurements (eg, before excavations). Field soil-screening techniques are constantly being developed. The following discussion refers to the most common techniques, but you should identify any other techniques that might be more appropriate to the site under investigation and/or their DQOs.
In situ field screening involves the use of instruments such as portable photoionisation detectors (PID), flame ionisation detectors (FID) or X-ray fluorescence (XRF) detectors, to take measurements across the site. For soil gases, measurements are taken by either drilling a narrow-diameter probe hole into the soil, or inserting metal probes into the ground to measure the vapour concentration. The absence of vapour readings in the soil does not necessarily indicate the absence of contamination, and confirmatory soil sampling should also be undertaken.
Ex-situ field-screening techniques use portable field instruments to measure the concentrations in samples of soil collected from the ground. Headspace analysis for soil vapours is a widely used technique in which a soil sample is collected in a bag/container. The headspace in the bag/container is measured after a set time and the results used to determine which samples to analyse. Field screening for volatiles should not be undertaken on the same sample that is submitted for analysis, and duplicate samples must be collected.
Other examples of soil-screening equipment include portable gas chromatography instruments for hydrocarbons, immunoassay kits to measure hydrocarbons, and hand-held Geiger counters to measure radiation.
3.6 Collecting a representative soil sample
A representative soil sample is one that represents the actual environmental conditions. It is dependent on good sample design, the method used to collect the sample and how it is handled.
The sampling and analysis plan must set out the minimum number of samples to be taken and specify sampling depths, in line with the sampling objectives. There should be sufficient flexibility in the sampling and analysis plan to enable additional samples to be collected as a result of on-site observations, which may differ from the conceptual site model. Professional experience and judgement should be used.
3.6.1 Number of samples
The number of samples collected is determined by the intended use of the data, the level of confidence required for the investigation, the area of the site, site-specific constraints/limitations, and budget. The sampling and analysis plan should specify the number of samples to be collected, and the vertical and lateral locations. A staged approach to investigations is used, in which a greater number of samples are collected in the field, with selected analyses undertaken on selected samples. Provided the samples have been stored correctly and holding times for the samples are sufficient, further analyses on samples can be subsequently undertaken once the initial results are received and the conceptual site model has been updated.
The number of samples may be weighted towards near-surface sampling for assessing health and ecological risks from exposure to soil contaminants (eg, via skin contact for human health risk). If groundwater is considered to be a potential pathway or receptor, then an increased number of samples collected from near the water table may be selected for analysis.
A method for calculating the minimum number of sample locations based on the design of an investigation using statistical methods is provided in Appendix B. The appropriateness of the sampling rationale must be justified in the context of each individual site investigation. Table A1 in Appendix B summarises the minimum sampling points required for detecting circular hot spots with 95% confidence using a systematic sampling pattern, based on site area and grid size. This guideline sets out the minimum number of sampling points required, and any variations to the minimum requirements should be justified.
Specific guidance on the minimum number of samples required for the investigation of oil storage tanks and lines is provided in Guidelines for Assessing and Managing Petroleum Hydrocarbon Contaminated Sites in New Zealand (Revised 2011) (Ministry for the Environment, 1999).
We must emphasise that these are guidance numbers and represent the minimum number of sampling locations, and that further samples may be required to produce representative data for contamination at a site (eg, several tanks may be included in one tank pit, so the guidance on the minimum number of samples of five per tank pit may need to be increased).
Example: A paddock on a farm contains a buried offal pit, including bags of pesticides. Anecdotal information collected from local residents suggests that the pit lies within a one-hectare corner of the paddock and that the original dimensions of the pit were approximately 20 metres in diameter. Using the formulas given in Appendix B, it is determined that to locate the pit (based on a circular hot spot of radius 10 metres), a square grid size of 17 metres should be used. For a one-hectare site, this equates to a minimum of 35 sample locations. Additional samples at various depths may also be required.
3.6.2 Sampling depth
Sampling depths must be based on known site conditions and the likely distribution of the contaminants of interest, and should be defined in the sampling and analysis plan. If there is uncertainty about the probable vertical behaviour of a given contaminant in a particular soil, collect soil samples at various depths. Soil samples should be collected at two or more depths to establish the vertical extent of contamination. The sample depth and the soil profile (eg, fill material, topsoil, humus/leaf litter) from which the sample was collected must be recorded and considered as part of the data interpretation (section 5.2.1).
Soil samples can be collected from throughout the soil profile, from the surface (0-15 cm), at regular intervals (say every 1.0 m), at any change in strata, and at the depth at which contamination is anticipated or observed. Samples should generally not be collected from across different strata (for example across the boundary between natural ground and fill).
Surface samples are defined as no deeper than 15 cm, and are typically collected from 0-7.5 cm. The collection of surface soil samples deeper than 15 cm increases the possibility of dilution of the surface soil sample by mixing with less contaminated subsurface soils. Depths of surface soil samples will be dependent on the DQOs, and 0-7.5 cm is commonly used to represent the direct human exposure pathway, whereas 0-15 cm is commonly used to represent the home produce exposure pathway, because the latter covers the significant root zone.
When sampling for volatiles, be aware that volatiles are readily lost from the surface layers of soil so they are not normally collected from surface layers unless investigating a spill of chemicals that has just occurred (depending on site-specific DQOs).
Example: A petrol loss from the base of an underground storage tank has caused contamination below the base of the tank pit only, with little or no impact on shallower soils. The top 1 m of the ground surrounding the tank is made up of boiler ash containing arsenic and residual diesel from an earlier surface spill. The sampling objectives are directed at assessing the impact of benzene on groundwater, which lies several metres below the base of the tank. No sampling of the fill materials is undertaken during the site investigation, and soil samples need to be collected at the depth of the base of the tank, and down to the groundwater table.
3.6.3 Soil-sampling techniques
There are a number of different soil-sampling techniques available, and the actual method used will depend on a variety of factors, including the objectives of the investigation, cost, access, degree of disturbance, and reinstatement. Often a variety of methods are used as part of an investigation, but whichever technique is used, the soil sampling must be undertaken in a manner that retains the sample integrity.
The following techniques can be considered when undertaking soil sampling:
- surface and shallow subsurface grab sampling
- hand auger sampling
- test pit sampling
- borehole sampling.
Table 2 (below) summarises the main techniques, and some of their advantages and disadvantages.
Grab sampling (trowel, push tubes, shovel or scoop – plastic or stainless steel)
No access restrictions
Minimal soil disturbance
Depth limit: surface - 0.5 m
Impractical in difficult soil conditions
Care is required to ensure the quality of sample recovered
Hand auger, split-barrel devices
No access restrictions
Minimal soil disturbance
Depth limit: 2-3 m (with ease)
Impractical in difficult soil conditions
Care is required to ensure the quality of sample recovered
Limited ability to observe the nature of the material
Test pits (machine dug)
Lower cost than boreholes
Ability to make detailed observations of the strata
Ability to recover samples
Extent of soil disturbance, occupational exposure, compaction
Depth limit is 3-5 m depending on excavator
Impractical in unstable soil conditions and hard rock
Not suitable for installing monitoring bores due to disturbance
Boreholes (drilling rigs – hollow-stem auger, air rotary drilling, shell and auger)
Minor disturbance of soils
Limited occupational exposure
Accurate recovery of samples
Ability to sample at depth
Suitable for most ground conditions
Can be used for installing groundwater and gas monitoring wells
More expensive than other techniques
Limited ability to observe materials
Air rotary rigs not suitable for volatiles
Can cause preferential pathways for contaminant migration, if not appropriately constructed
Surface and shallow subsurface grab sampling
Soil samples can be collected using an appropriate hand trowel, push tube, plastic scoop or shovel. This is a quick and efficient method to collect shallow surface and subsurface samples. A push tube, or soil corer, is a stainless-steel tube pushed into the ground by hand to a set depth (typically 7.5-10 cm depths) to collect soil. All hand tools must be appropriately decontaminated between samples and sample locations (refer to section 3.8). The main disadvantage of the surface grab sampling is the depth restriction.
Hand auger sampling
A hand auger is a sampling device manually or mechanically driven into the soil, with typical dimensions excavated of between 6 cm and 15 cm in diameter. Sampling depths up to 2-3 m can be easily achieved, depending on soil type, and greater depths are sometimes possible. Soil samples can be collected from the auger head or from an auger fitted with a split-spoon-type sampler. Augers may be used to sample locations with restricted access, and a monitoring well may also be installed in the hole excavated. Disadvantages of auger sampling are the limited sample size, depth restrictions and the potential for cross-contamination with depth if the sample is collected off the auger flight. Some loss of volatiles can occur from samples collected from the auger head or flight.
Test pit sampling
Test pits are excavated using a backhoe excavator, but may also be hand dug. Typical dimensions are rectangular pits of around 3 m length, 1 m breadth and 3-4 m depth. The test pit size will depend on the stability of the pit, strata, bucket size, and reach of the backhoe. Collect soil samples from the centre of the excavator bucket. Take care to avoid cross-contamination. Test pit excavations may be hazardous due to the possibility of slumping, or a build up of hazardous gases. No person should enter a pit if the depth is greater than 1.5 m, and assess shallower pits for stability and the potential for hazardous gases to be present if a person is to enter the excavation. Further safety guidance is provided in Approved Code of Practice for Safety in Excavation and Shafts for Foundations (OSH, 1995).
Test pits enable visual inspection of the shallow strata and can be extended into trenches to observe the extent of strata or visible contamination. A disadvantage of test pits is the disturbance caused to the ground, and for this reason they are not suitable for collecting undisturbed soil samples or for installing wells for groundwater or soil gas monitoring. When excavating test pits, the excavated material should be laid out at the side of the pit in the order of excavation. When reinstating test pits, the spoil excavated must be replaced in the same order that it was excavated (material from the base of the pit is returned to the base, and so on).
Boreholes can be drilled using different types of drilling rigs and are suitable for soil sampling and for installing soil gas and groundwater monitoring devices. Boreholes are typically 150 mm to 200 mm in diameter and extend to many metres in depth. The type of drilling rig will depend on the depth of the bore, geology, and the nature of the proposed scope of works. Drilling rigs commonly used for soil sampling include a continuous-flight auger, hollow-stem auger and air rotary. Split-spoon or push-tube-type samplers can be fitted for collecting soil samples. A disadvantage is the possibility of introducing preferential pathways for the migration of contaminants, so appropriate drilling and borehole construction techniques must be used to minimise this.
Do not use air rotary drilling for collecting samples to be analysed for volatile contaminants, as the air affects the integrity of the sample collected. Drill in a manner to avoid introducing contaminants, with minimal water added during drilling. Use non-hydrocarbon-based oil on the rig and casing if sampling for organic compounds to avoid interference with analytical parameters to be tested. The advantages of boreholes include the collection of undisturbed samples at depth, and the option to install correctly constructed groundwater or soil-gas monitoring wells.
Factors to take into account when selecting the sampling technique should include:
- the DQOs
- target analytes
- sampling depth
- physical constraints at the site (height and access obstructions, topography)
- ground conditions (ground cover, soil type, stability, groundwater depth)
- reinstatement requirements
- health, safety and environmental implications associated with the sampling techniques.
3.6.4 Composite sampling
Composite sampling consists of collecting individual samples from different locations and bulking and mixing an equal mass of the samples (called sub-samples) together to form one composite sample. A composite sample can then be analysed, and represents the average of the constituent sub-samples. The use of composite sampling should only be undertaken by experienced site investigators after full consideration of the site history. Use sample compositing with caution because high contaminant concentrations in one or more of the samples making up the composite can be masked by a dilution effect of the other samples. The decision to use composite sampling must be made with reference to the site DQOs.
The investigation of horticultural land and broad-scale contamination typically uses composite techniques, often with more than four sub-samples per composite. This method is appropriate where low-concentration, uniform contamination is present and can be confirmed by site history.
Composite sampling can be cost effective because the number of samples to be analysed is reduced. However, costs are incurred at the laboratory for preparing the composites and you may need to retest individual sub-samples. For this reason compositing should be done in the laboratory and individual samples retained. Additional information on compositing in the laboratory is provided in section 4.4.3.
Compositing can be used to characterise a stockpile of material; for example, to determine an acceptable disposal location, or for characterising sites with similar contaminant levels (such as horticultural sites). Soils containing or suspected of containing volatile organic compounds are not suitable for compositing.
The following guidelines apply to the use of composites.
- A reliable and comprehensive site history has been compiled for the site, so areas of hot spots or broad-scale contamination are known.
- All composite samples are made up of the same number and weight of sub-samples.
- No more than four sub-samples should be used to make up the composite, although the number is governed by the analytical detection limits.
- Sub-samples are usually taken from adjacent locations and from similar depths (from the same soil/fill horizon), and must not be heterogeneous (eg, one sub-sample a gravel, the other a clay).
- Sub-samples should be taken from areas with similar history (similar contaminants and contaminant distribution).
- Compositing must be undertaken in the laboratory, and original samples retained for possible retesting.
- Compositing is not suitable for volatile substances, because the mixing procedure results in loss of volatiles.
- Compositing is not suitable for soils that are not easily mixed (eg, clay) or for soils with different moisture contents.
- When comparing composite results against guideline values, the guideline value must be adjusted by dividing the value by the number of sub-samples in the composite:
3.6.5 Background samples
Background samples are collected in the area near the site that is not affected by the contaminant sources on the site. If required by the DQOs, at least one background sample should be collected. Background samples are used as a reference point to represent undisturbed natural soil at or near the surface. In practice, obtaining true background samples can be difficult owing to general anthropogenic sources of contamination in the areas surrounding most sites where hazardous substances are present or suspected.
Background samples can help to show whether contaminants present on a site are due to wider area effects, either natural or artificial. Suitable locations for background samples should be chosen based on the:
- site geology (background concentrations of metals are related to the parent rock types)
- site history (should indicate no disturbance at the location)
- topography (sample collection should not be from any low-lying areas, such as ditches, but from areas of raised ground).
Some regional councils have information on background levels of common contaminants (usually metals) in the main types of natural soils within their region.
3.7 Sample handling and transport
3.7.1 Sample logging
All soil samples collected must be inspected and the soil profile logged using a consistent method and format for soil descriptions. Record any general observations on the soil-sampling locations, weather conditions, ground surface, topography, and preferential pathways for contaminant migration. Identify the location and depth of samples collected on a location plan. The recommended method for logging soil samples is the New Zealand Geomechanics Society terminology for description of soils in the field, as presented in Appendix C. The soil description should include the general appearance, colour, soil type, strength, moisture content and particle shape. For environmental investigations, record the evidence of contamination (visual signs, obvious odours) and specific information on the nature of any fill materials. Also record any obvious odours, but for health reasons do not undertake any direct smelling of samples. Avoid directly handling the soil with bare hands on suspected contaminated sites by wearing appropriate gloves.
3.7.2 Sample locations and labels
Once a soil sample is collected it should be clearly and uniquely labelled. Records kept for each sample should include:
- a unique sample reference number (avoid numbers and letters that are easily confused, such as 1 and l, or O and 0))
- date, time, depth and location collected
- sampler's name
- any site observations and weather conditions.
Keep the sampling records in a field notebook, which must identify the site, exact sampling location and any observations or measurements that could influence the interpretation of the results. The sample locations can be documented by photographs with a reference location marked on a board. The sampling records should be taken with a waterproof pen or pencil, and dated and signed.
3.7.3 Sample handling
Sample containers should be supplied by the analytical laboratory and must be clean and of an appropriate size for the analyses to be undertaken. Recommended sample containers and guideline sample holding times before analysis are presented in Appendix D. The sample containers should be handled so as to ensure the integrity of the sample is not compromised during storage. Keep samples in sealed containers away from sources of heat and protected from light, and deliver to the laboratory for analysis. Recommended holding times are used as a guide to the length of time samples may be held prior to analysis, and will vary depending on the parameters to be analysed.
3.7.4 Sampling for volatiles
Soil samples collected for volatile parameters (eg, solvents, benzene) must be collected quickly, with as little disturbance as possible. Collect the samples using the appropriate soil-sampling equipment. Be careful if taking samples using other equipment (eg, backhoe excavator, air rotary) because there is potential for loss of volatiles. The limitations of the method must be identified in the reporting stage. Table 3 lists recommended equipment for sampling soils for volatiles.
Table 3: Sampling methods for volatiles
Zero headspace samplers
In all cases the sample should be taken so as to minimise loss of volatile compounds. This may involve using:
- a zero headspace sampler, which is sealed and transported to the laboratory, where it can be interfaced directly to the analytical instrumentation (this is an expensive technique and requires special laboratory set-up)
- solvent extraction sampling with a coring device and transfer to a pre-weighed vial containing methanol
- direct fillof a glass container filled with no headspace (this will be sub-sampled in the laboratory using a corer).
For practical reasons the third method is most often used, because there is no need for pre-weighed vials, or handling and transport of methanol.
Samples must be collected, sealed and placed in a chilled container as soon as practicable and kept chilled by storing on ice in a cool container (eg, chilly-bin). Samples should not be frozen because glass sample jars can crack or break. Where any field screening is required (eg, head-space testing), a separate sample must be collected. All samples for volatiles should be delivered to the laboratory as soon as practicable after sampling.
3.7.5 Chain of custody procedures
Chain of custody documentation is prepared to document sample handling and transport procedures from the point of collection at the site to the laboratory, and can include instructions for the laboratory analysis. The chain of custody can include transfer of samples within the investigation team and transport by courier. A typical chain of custody form is presented in Appendix E. Further details on the information that should appear on a chain of custody and the procedure for receipt of samples in the laboratory are provided in section 4.2.
Decontamination procedures include the process of washing, rinsing and removing material from exposed surfaces of equipment and clothing that can, or has, come into contact with the sample. Any decontamination must be undertaken in a manner that avoids contaminating areas to be sampled, or the spread of contamination around or off the site. Take care to ensure vehicles do not become contaminated, and avoid future cross-contamination. Collect all decontamination waste and wash water for proper disposal. Rinsate blanks (see section 3.9) can be collected to assess the effectiveness of the decontamination process. The level of decontamination adopted should be practical and commensurate with the DQOs. It will not be necessary to observe the same level of decontamination in every case.
Decontamination procedures may include the following.
- Personnel handling soil samples should replace gloves between each sample.
- Scrape or brush off any soil adhering to the sampling equipment, clothing or boots.
- Wash equipment in detergent (phosphate-free, where required).
- Rinse with tap water, followed by a rinse in high-purity analytical-grade deionised water.
- For some equipment, the following additional measures may be required:
- for metal analysis, rinse in dilute nitric acid then rinse in high-purity analytical-grade deionised water
- for gross organic contamination, rinse with water, then acetone followed (in some cases) by hexane (acetone and hexane solvents should not be used if sampling for volatile organics).
- Store tools so as to prevent recontamination (eg, wrap in clean aluminium foil when sampling for organics).
Large sampling equipment, such as the backhoe bucket and drill casings that come into contact with the soil, should be cleaned between sampling locations on a dedicated area as follows.
- Remove any loose soil by brushing, scraping or wiping.
- Steam clean or wash with a high-pressure washer.
- Rinse with potable water.
3.9 Field quality assurance (QA) / quality control (QC)
Soil samples collected during an investigation should be as fully representative as possible of the area to be characterised and the location sampled. Common sources of errors and uncertainty that can arise in the sampling and analysis of soils include:
- poor sampling design
- inappropriate sampling procedure
- improper labelling of samples (eg, illegible or missing)
- improper handling and storage of samples
- laboratory errors.
In general, the bias associated with field sampling is usually more significant than the errors associated with analytical methods. Common situations that give rise to errors and uncertainty in soil sampling include:
- samples are not collected from the correct depths or locations
- samples are contaminated using contaminated probes, utensils or other instruments when making field measurements
- decontamination is not undertaken between samples, leading to cross-contamination between samples
- the parameter of interest is volatile, and samples are exposed to air for a prolonged period
- samples are exposed to vehicle exhaust fumes, lubricants and other external sources of contaminants.
Quality assurance (QA) and quality control (QC) programmes for field sampling are required to control sampling errors to an acceptable level, as set out in the DQOs. The QA/QC procedures should consider all the stages of the investigation, including the:
- qualifications and experience of staff carrying out the work, particularly the field staff
- qualification, accreditation and experience of sub-contractors (including the laboratory)
- appropriate sample collection methods, cleaning and calibration of equipment, collection of field QC samples
- accurate recording of the work carried out and data collection, including any observations or conditions at the time of sampling that may assist in interpreting the data
- chain of custody procedures and sample storage
- reviews and audits of the work being carried out, including data reporting and interpretation.
The two data quality indicators most often used in field sampling to measure compliance with DQOs are bias and precision. Precision is defined as a measure of random variation in data and is a measure of reproducibility. Bias is defined as a systematic deviation (error) in data, and it affects accuracy (ie, proximity to the true value). Precision is typically estimated using duplicate samples, and bias can be assessed using blank sample types (see sections 3.9.1 and 5.4.3).
3.9.1 Field QC
Field QC procedures should be in place to manage sampling errors and should be documented in the sampling and analysis plan. Procedures should include the collection of field QC samples and technical review steps during the data collection process. The number of QC samples taken will be dependent on the DQOs and the type of investigation undertaken. Greater confidence in site assessments can be achieved by taking QC samples and by increasing the number of samples taken.
Field QC procedures are used to measure the uncertainty in the data from sampling, handling and laboratory errors. Table 4 summarises the recommended number of field QC samples that should be included when collecting soil samples.
|Quality control sample||Recommended number of samples||Use|
Blind replicate sample
1 for every 10 samples collected
1 for every 20 samples collected
Site validation / possible problem identified
1 per analysis matrix per piece of equipment per day
1 per consignment** of samples for organics or volatiles
Generally used for water sampling only – DQO dependent
1 per consignment of samples for organics or volatiles
* See also sections 4.5.3 and 5.4.3.
** A consignment is a group of samples transported to the laboratory at the same time.
Blind replicate samples
A blind replicate sample, also referred to as a field duplicate or replicate, involves collecting two separate (replicate) samples from a single sample location, storing in separate containers and submitting them for analysis to the laboratory as two separate samples. Samples should be given separate sample numbers and labelled so that the laboratory does not know the sample is a duplicate. The blind replicate can provide information on the overall variability or precision of both the sampling technique and the analytical laboratory. As a minimum, one blind replicate should be collected up to the first 10 samples, and an additional replicate taken for every 10 samples thereafter, although this will be dependent on the specific DQOs.
The analytical results for the primary and replicate samples should be compared. A typical sampling DQO would be for a sample to be acceptable if the relative percent difference 4 for blind replicates of less than 30-50% is achieved, depending on the analyte. The relative percent difference acceptance should be established at the outset of the investigation and included in the sampling plan along with the DQOs. Further information on interpreting replicate results is provided in section 5.8.
Split samples are used to check on the analytical proficiency and provide information on the overall variability or precision of the analytical laboratory. Do not prepare split samples in the field. A split sample is prepared by requesting the primary laboratory to prepare a sample by thorough homogenisation and sending a portion to a second independent laboratory for analysis. The results from the second laboratory should not be reported back to the primary laboratory, but directly to the field investigator. Split samples are not used routinely during detailed or supplementary site investigations, and are more commonly used during site validation investigations, or when a problem is suspected with the analysis. Split samples are not applicable for volatiles. As a recommendation, one split sample should be collected up to the first 20 samples, and additional split samples collected for every 20 samples thereafter, although this will be dependent on the specific DQOs.
A typical data quality objective would be for a sample to be acceptable if the relative percent difference for split samples is less than 30-50%, depending on the analyte.
Equipment rinsate blank samples
An equipment rinsate blank is collected after equipment decontamination and is obtained by running deionised water through the sampling equipment and collecting the water. The blank is tested for any residual contamination, which assesses the potential for cross-contamination between samples as a result of poor decontamination procedures. Rinsate blanks for soil sampling are collected from equipment that comes in direct contact with the samples (eg, auger head, trowel), and where cross-contamination of samples is likely to affect the validity of the sampling and assessment process. The recommended practice is to collect one rinsate blank per day, per sampling technique/team, although again this is dependent on the site investigation DQOs. The sample should be analysed if there are indications of cross-contamination or field contamination.
Trip and field blank samples
Field blanks, in conjunction with trip blanks, are normally used when sampling volatile organics and undertaking baseline studies. Section 5.8 provides further information on interpreting field blank results. One trip blank and one field blank are typically collected per consignment of samples, depending on the DQOs. A consignment is a sample group (usually 20-30 samples) that is transported to the laboratory at the same time.
Trip blanks are sample bottles filled with deionised water, and originate in the laboratory with the sample containers. They are kept with the soil samples, remain unopened in the field, and are returned to the laboratory. The trip blank is used to identify compounds that may have been introduced into the soil samples during transport or storage. Field blanks are sample bottles filled with deionised water in the field and kept open during the duration of the sampling, then returned with the soil samples to the laboratory. Analysis of the field blank is used to identify any compounds that may have been introduced to the sample during sample collection (eg, from air deposition or vapours).
3.10 Choice of analytes
The choice of analytes must be consistent with the findings of the preliminary site inspection, the DQOs and any field investigation observations and measurements. Information on the current and historical activities at the site - in particular any known incidents, leaks or spills of chemicals - should provide the basis for determining the contaminants of interest. The conceptual site model for the site should be used to help determine where contaminants are likely to be in the soil profile. Contaminated Land Management Guideline Schedule B (Ministry for the Environment, 2004b) identifies contaminants commonly associated with certain activities or industrial processes.
The analytical techniques available for soil samples range from broad-screening techniques, to detailed analysis for individual parameters. The choice of analytical method will be determined by the DQOs.
Discuss analysis requirements with the laboratory to ensure the detection limits for the tests requested will be appropriate for the purposes of the investigation. Typically, when comparing results to guideline values, the detection limit should be at least 1/10th of the guideline value, and detection limits must always be below the guideline. Further guidance on method detection limits is provided in Appendix F.
The choice of analytes will influence the amount of sample to be collected, and appropriate sample containers and preservation requirements must be decided before the investigation commences. For volatile soil analyses, the main preservation requirements are keeping samples chilled and out of direct sunlight.
3.11 Health, safety and environmental considerations
Health, safety and environmental considerations are an important part of any contaminated site investigation, because there are risks including toxic effects, physical injury and harm to workers and the environment which must be assessed and managed. Occupational Health and Safety requirements for fieldwork are covered under the Health and Safety in Employment Act 1992 (as amended), which places an emphasis on employees at work to take responsibility for the wellbeing of themselves and others at work. In addition, Section 17 of the Resource Management Act 1991 places a duty on each person to avoid, remedy or mitigate any adverse effect on the environment arising from an activity. Refer to the relevant legislation, documents and approved codes of practices. A list of commonly used material is included in the References and Additional Information sections.
A health, safety and environment plan (HSEP) should be prepared as part of the planning for site work. This should identify all potential hazards and steps that should be taken to eliminate, isolate or minimise these hazards. The site HSEP is used to inform workers of potential physical and chemical hazards, health and safety responsibilities, normal work precautions, monitoring requirements, and action levels and emergency provisions. A job safety analysis is a recommended method to document the different tasks undertaken as part of the site investigation. Site-specific training needs should also be identified to ensure all site workers know how to carry out the work safely.
Table 5 is a checklist for identifying the hazards encountered during a site investigation. These can be categorised as:
- chemical, biological and radiological hazards
- physical hazards
- environmental hazards.
An example table of contents for an HSEP and an example of a job safety analysis form within an HSEP are included in Appendix A.
Table 5: Hazard identification checklist
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3.11.1 Chemical, biological and radiological hazards
The nature of the likely contaminants must be identified, any associated chemical, biological or radiological hazards assessed and appropriate precautions taken during the site investigation, sample handling and disposal. The Occupational Safety and Health (OSH) workplace exposure standards provide a guide for acceptable worker exposure levels for various chemicals. Site workers can be harmed by inhaling vapours or dusts, ingestion and skin contact, and the appropriate procedures to protect the site workers, all staff handling the material (eg, laboratory staff) and the general public must be in place.
A detailed site investigation can be managed by establishing work areas, including an exclusion zone around the contaminated area, a decontamination zone for site workers and equipment, and a support zone. These work areas should be controlled, with only essential site workers allowed access to the contaminated work area. Site work zones may be marked in many ways, ranging from tape, cones and signage, to full barriers and fencing. The activities undertaken in each zone can be defined in the HSEP and the level of monitoring and personal protective equipment required for each zone established. Monitoring requirements should be assessed and action levels defined in the HSEP. Monitoring can include use of hand-held gas monitors, personal monitors, automatic gas detectors, environmental monitors and radiation dosimeters. All monitoring equipment should be appropriately calibrated and checked before and during use and clear instructions given to all personnel on the use of equipment and any emergency evacuation procedures. The use of personal protective equipment and the level of equipment required should be specified in the HSEP. It typically includes chemical-resistant gloves, boots, overalls, hard hat, ear defenders, eye protection and high-visibility vests. Respiratory protection and further skin protection may also be necessary depending on the site hazards.
3.11.2 Physical hazards
The physical hazards at a site should be assessed and can include the following.
Underground and above-ground services
The presence of underground and above-ground services can pose a hazard during contaminated site investigations. Underground services, including electricity and gas, present a very serious and potentially fatal hazard if damaged. Above-ground power lines can cause electrical shocks through contact and arcing with conductors (eg, a drill rig or surveyor's pole). Procedures must be in place to identify and manage these hazards, including marking out no-go areas within the minimum clearance distances from overhead power lines and obtaining information on the location of underground services from service plans from site owners, occupiers and utility companies, tracing out the location of the underground services using detection equipment, and hand-digging to a depth that should be clear of the services.
Excavations deeper than 1.5 m are considered a confined space, and should not be entered. Appropriate training of personnel is required for confined space entry, and excavations should then be appropriately battered or shored. There are hazards associated with slips, trips and falls into pits, and they should never be left unguarded. There is the potential for gases to be released from open pits and gases can accumulate in trenches. All pits and bores should be backfilled and capped or appropriately fenced and labelled. The reinstatement of pits should be done in an appropriate manner, as outlined in section 3.6.3.
Hazards associated with using machinery and equipment include the use of equipment on soft, sloping or unstable ground, vibrations, noise generation, restricted vision during moving operations (lifting, reversing, swing), rotating equipment on drill rigs, bursting hoses, and equipment breakage. All equipment should be operated by appropriately trained operatives and should be inspected and maintained in safe working order. Any fieldwork around machinery should be undertaken with clear communication signals agreed between the machine operator and the field investigator on sampling and emergency procedures.
Dust and odour
Dust and odour are potentially hazardous to site workers through contaminant exposure and can pose a hazard to the occupiers of surrounding land. Procedures to minimise dust include restricting certain activities on the site during dry and windy conditions, damping down the work area, limitations to vehicle access, speed control measures, and installing windbreaks.
3.11.3 Environmental hazards
Any hazards to the environment posed by the contaminants, or by disturbance of the contaminants by the nature of the works should be assessed. Environmental risks include damaging rare habitats or endangered species, creating contamination pathways to groundwater, introducing contaminants into previously uncontaminated strata, uncontrolled run-off water, and inappropriate disposal of waste spoil. Potential environmental hazards such as animal bites, insect stings, sunburn, heat and cold stress should be considered. Procedures to manage environmental hazards should be identified as part of the planning for the site investigation works, and appropriate consents obtained before undertaking the works.
If groundwater is assessed to be sensitive in the preliminary site study, then steps must be taken to protect it during the works if there is potential for disturbance of the ground to affect groundwater quality. Steps to avoid contaminating groundwater include the use of appropriate bore construction techniques to isolate aquifers when drilling, and restricting the depth of excavations if contaminants are present at or near the water table, or are likely to be mobilised as a result of the intrusive investigations.
Take care to avoid spreading contamination into previously uncontaminated areas of the site. Controls on the movement of equipment and vehicles from contaminated to uncontaminated areas should be in place. During excavation works, contaminated spoil material should be handled appropriately. Remove contaminated spoil for off-site treatment and/or disposal. If returned to the ground (eg, when backfilling a test pit), the spoil should be returned to the pit in the order in which it was excavated (see section 3.6.3).
Take special care when undertaking works in areas where plants and animals are protected. Identify the effects of the site activity and undertake appropriate measures to minimise any adverse effects. Measures could include timing the schedule of the works at certain times (eg, not during breeding season), using investigation techniques with minimal disturbance (eg, hand auger rather than machine-dug test pits), and relocating sampling locations to areas with less impact on habitat.
3.11.4 Waste handling
Investigations on sites where hazardous substances are present can lead to the production of range of wastes, including:
- wash water and solid residues from decontamination procedures
- waste gloves, cloths and plastic sheeting from handling and cleaning tools
- excess excavated soil from sampling locations.
Each of these wastes could be contaminated and must be handled so as to minimise the risks associated with the hazardous substances. Take care to prevent contamination from spreading onto neighbouring properties or roads. Contaminated wastewaters may be disposed of via the site wastewater treatment system, if available, subject to the necessary approvals. Waste spoil and contaminated field-sampling equipment such as gloves, overalls and plastic sheeting are usually stored temporarily, before off-site treatment or disposal.
Store the waste securely in a labelled container (eg, a drum or skip), and review and assess the results of the analysis to determine the waste classification (hazardous or non-hazardous). Consult the New Zealand Waste List (L-Code) to determine if the waste is listed as hazardous, or assess the material to determine if it contains hazardous substances, characteristics or generates hazardous leachate. Examples of hazardous characteristics include flammability, toxicity and corrosiveness. Thresholds for hazard characteristics are provided in the User Guide to the HSNO Thresholds and Classification (2001). Guidance on identifying hazardous waste is available from the Ministry for the Environment website (www.mfe.govt.nz).
Once classified, the waste material should be treated and disposed of in accordance with the local treatment or disposal facility consent conditions. It may require stabilisation or treatment if the concentrations in the waste exceed landfill waste acceptance criteria. Guidelines for the management of hazardous wastes require appropriate records on hazardous waste management to be kept, including the type and quantity of waste generated, transported and disposed of. If hazardous waste is stored, treated or disposed of on-site, then records must also be kept for future reference.
Back to footnote reference 3 Also refer to any appropriate Occupational Safety and Health guidance; eg, Health and Safety Guidelines on the Clean-up of Contaminated Sites (OSH, 1994).