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3.1.1
Soil Sampling Strategies
OUTLINE There should always be a written sampling strategy for a Phase 2 site investigation stating the purpose(s) of the sampling. The strategy should be linked to the uncertainties identified in the Phase 1 conceptual model and the data quality objectives set by the intended risk assessment. The strategy should include: • Sample locations 1. Approach: use of professional judgement (targeted sampling); non-targeted sampling; combined approach 2. Sampling pattern 3. Sampling frequency • Sampling depth The purpose of each sampling location and depth should be stated. Sampling strategy can include collecting many more samples than are finally analysed, in which case the reason for selecting samples for analysis and the criteria for deciding when extra analysis is needed should be stated. APPROACH: PROFESSIONAL JUDGEMENT VERSUS NON-TARGETED SAMPLES • Professional judgement – uses available information to investigate areas suspected to be contaminated • Non-targeted samples – sets out a defined sampling pattern and spacing to investigate an area • May use a combination eg: 1. Professional judgement for obvious areas of potential contamination and non-targeted samples over the rest of the site 2. Non-targeted samples across the site, but with professional judgement to decide on sampling densities SAMPLING BASED ON PROFESSIONAL JUDGEMENT When to use sampling based on professional judgement When there is enough information to indicate where contamination is likely to be found. This includes: • • • • •
Former potentially contaminating activities or structures (eg underground storage tank) Infilled areas. (If large may need non-targeted sampling to characterise materials) Areas of stressed vegetation or stained ground Probable migration routes of mobile contaminants Very sensitive areas, to prove they are not contaminated (eg dedicated vegetable plots)
Sampling pattern Dictated by arrangement of potential sources of contamination to be investigated Sampling frequency This depends on the aim of the investigation. There should be sufficient samples to answer the site investigation objectives. Factors affecting sampling frequency include degree of confidence required, nature of contamination, number of stages of investigation. Fewer samples are required to identify presence of contamination compared to demonstrating absence of contamination or delineating extent of contamination.
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SAMPLING BASED ON NON-TARGETED SAMPLING When to use sampling based on non-targeted sampling
Examples
Very sensitive areas where there is a need to • Dedicated vegetable plots have a high degree of certainty that they are not • Land being returned to another party contaminated. When there is insufficient information on the • Airfield likely locations of the contamination and/or • Cleared industrial site insufficient information to suggest one part of an • Contamination associated with underlying area is more likely to be contaminated than geology another. Areas of a potentially contaminated site which • Due to undocumented activities do not appear to have potentially contaminative • Due to migration from elsewhere uses but, because they are part of the site, there may be some localised hotspots. Areas where the distribution of contamination is • Landfill sites expected to be random. • Made ground • Validation of remedial works Areas where the distribution of contamination is • Agricultural sites (but local hotspots are expected to be homogeneous. possible, eg sheep dip disposal or diseased stock burial pits) • Contamination associated with underlying geology Sampling objectives The objectives for the non-targeted sampling should be clearly stated, eg: • Eliminate hotspots of given size to given confidence level (CLR4 approach) • Determine average concentration and standard deviation and 95%ile upper confidence level for mean (enough to show 95% UCL < Guideline Value) • Determine spatial distribution and confidence (geostatistical tools; minimum 30 sample points to allow variogram generation) Sampling pattern CLR4 (DOE 1994) identifies the following patterns • • • •
Simple random Stratified random Square grid Herringbone
Herringbone is the most efficient, especially for elongate hotspots. Sampling frequency Based on statistical techniques which determine how many sampling points are needed for a given hot-spot of a defined shape and size within a site of a given area. Table 3.1.2 shows the equations and example calculations for sampling frequency based on statistical technique. Table 3.1.3 is a ready reckoner for a number of predetermined scenarios. NSW EP (1995) provides additional statistically justifiable procedure. Choice of size and shape of hotspot and probability should take into account the likely shape of potential contaminated areas based on former use, end use of site and level of confidence required from risk assessment. BS 10175:2001 suggests sampling grids with 50m to 100m centres for exploratory investigations and 20m to 25m centres for main investigations but comments that higher density grid, eg 10m centres, may be necessary, eg in heterogeneous conditions or if a high level of confidence is required.
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Types of grid
Simple random
Square grid
Stratified random
Herringbone
SAMPLING DEPTH Sample depth(s) may be based on judgement or regular depths or a combination. It should always be appropriate for the intended risk assessment and reflect the final site levels. In most scenarios 3 or 4 samples should be taken through the soil profile with the deepest sample being natural strata. If contamination has penetrated the natural strata sampling should continue to depths where contamination is at background concentrations or it is not physically possible to sample. Depths should reflect the likely source(s) of contamination eg: • Near surface • Sub-surface eg underground tanks, pipes Depths should reflect receptors of concern and be driven by the exposure scenario of concern eg: Receptor: human health • Surface samples in top 0.15m for direct ingestion and inhalation • At 0.30m to 0.50m for vegetable uptake • Greater depths, equivalent to 1m below final level, should be sampled in proposed garden areas to allow assessment of risks from activities such as pond digging, foundations to extensions etc. and from volatile contaminants
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Receptor: groundwater • Samples from made ground at fixed depth intervals, (often every 0.5m); • Additional samples from fill to reflect changes in appearance and any identifiable changes in strata or material • Samples of natural ground close to the boundary with the fill (approximately 0.25m to 0.5m into natural ground) • At one or more greater depths (often at fixed intervals of 0.5m or 1.0m) to prove extent of contamination • At changes of geological strata • Immediately above the water table (as slightly soluble compounds tend to concentrate in the capillary zone) • Usually there should be sufficient sampling to prove the depth where the material is uncontaminated Depths should also take into account proposed activities on site eg: • Removal of topsoil (may need to test for waste disposal purposes, but not relevant for exposure scenario) • Services – BS10175 2001 suggest excavations to at least 1.5m are likely to be required. Deeper excavations may be required for the installation of main sewers • Foundations – BS10175 2001 suggest excavations to at least 1.5m are likely to be required. Pile foundations (from which there may be waste for disposal or which could act as a migration pathway) go much deeper PURPOSE The purpose of each sampling location (or group of sampling locations) should be clearly stated eg: Aerial distribution TP 1–18 are to investigate oil contamination in the vicinity of the goods yard. TP 19–25 are to investigate the nature of and contamination in the filled quarry. TP 26–40 are placed at 50m centres in a herringbone pattern across the site to investigate for the presence of contamination in other areas of the site. Vertical distribution Within each trial pit samples are to be collected: • Within the fill • Immediately beneath the fill (to find out whether contamination has migrated into natural ground) • At 0.5m and 1.0m below the fill (to find out what depth contamination has migrated into natural ground) • At changes in geology In some cases it may be appropriate to discuss sampling strategy in relation to lines or clusters of sample locations eg. to delineate the extent of contamination or to trace the boundary of made ground. PLANNED ANALYSIS OF A SUBSET OF SAMPLES For both judgemental and non-targeted sampling collecting more samples than those to be analysed in the first instance is highly recommended. Justification of the samples analysed should be provided. DISCRETION OF FIELDWORKER The sampling strategy should be set prior to going on site. Fieldworkers should have some discretion on where samples are collected, taking into account the overall sampling strategy and objectives of the site investigation and information revealed as sampling proceeds. Fieldworkers should also assist in selection of which samples to analyse. Release 1: June 2002 Contaminated Land Management: Ready Reference 3 – 5
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REFERENCES BSI (1999) BS 5930. Code of Practice for Site Investigations. BSI, London BSI (2001) Investigation of Potentially Contaminated Sites. Code of practice 10175. BSI, London DEPARTMENT OF THE ENVIRONMENT (1994) CONTAMINATED LAND RESEARCH REPORT 4. A Framework for Assessing the Impact of Contaminated Land on Groundwater and Surface Water. DoE (London) DEPARTMENT OF THE ENVIRONMENT (1994) Sampling Strategies for Contaminated Land. Department of the Environment CLR Report No 4 FERGUSON, C.C., TUCKER, P. and NATHANAIL, C.P. (1998) Site-ASSESS, A Decision Support System for Designing Sampling Strategies. ConSoil 1998, pp. 801-802. Thomas Telford, London MONITOR ENVIRONMENTAL CONSULTANTS (2001) Secondary Model Procedure for the Development of Appropriate Soil Sampling Strategies. WRC, PS-0661 TR. ISBN 1857055772 NSW EPA (1995) Contaminated Sites: Sampling Design Guidelines. NSW Environment Protection Agency. ISBN 0 7310 3756 1
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