Soil sampling is the foundation of every environmental site investigation. Whether you are characterizing contamination for a Phase II ESA, monitoring remediation progress or confirming regulatory closure, the quality of your conclusions depends entirely on the quality of your samples. Poor sampling design, improper handling or inconsistent protocols can invalidate an entire investigation and cost tens of thousands of dollars in rework.
This field guide covers sampling strategies, equipment selection, sample handling, field QA/QC and decontamination procedures. It is written for environmental professionals working under Canadian (CSA Z769) and American (ASTM) standards, though the core principles apply in any jurisdiction.
Sampling Strategy Selection
The sampling strategy you choose determines whether your data can answer the questions driving the investigation. There is no single correct approach. The right strategy depends on the site history, contaminants of concern, data quality objectives and the regulatory framework you are working under.
Systematic Grid Sampling
Systematic grid sampling divides the site into a uniform grid and collects samples at each node or at random locations within each cell. This approach provides even spatial coverage and is well-suited for sites where contamination distribution is unknown or where you need to estimate the spatial extent of impacts.
Grid spacing is typically determined by the size of the area under investigation and the detection limits of the contaminants of concern. For volatile organic compounds in soil, 15-metre grids are common. For metals at industrial sites, 30-metre grids may be sufficient for initial characterization.
The main advantage of systematic grids is reproducibility. A second investigator can replicate your sampling plan exactly. The main disadvantage is cost - grids generate a high sample count and may over-sample clean areas while under-sampling hot spots.
Judgmental Sampling
Judgmental sampling places sample locations based on professional knowledge of the site. You target areas most likely to be contaminated: former tank locations, loading docks, drainage pathways, stained soil and areas identified through historical records or site reconnaissance.
This approach is efficient and cost-effective for sites with well-documented histories. It is the standard approach for Phase II ESAs where you are testing specific areas of potential environmental concern (APECs). However, judgmental sampling introduces bias and cannot be used to make statistical statements about the entire site.
Random Sampling
Simple random sampling assigns sample locations using a random number generator. Every point on the site has an equal probability of selection. This approach supports statistical analysis and can be used to estimate population parameters like mean concentration.
Stratified random sampling combines the benefits of random selection with targeted coverage. You divide the site into strata (based on land use, soil type or suspected contamination zones) and randomly sample within each stratum. This approach is more statistically efficient than simple random sampling for heterogeneous sites.
Composite vs. Discrete Samples
A discrete sample is collected from a single point. It represents conditions at that exact location and depth. Discrete samples are required when you need to identify hot spots, delineate contamination boundaries or compare results to point-of-compliance standards.
A composite sample blends soil from multiple locations into a single sample. Compositing reduces analytical costs by reducing the number of samples sent to the laboratory. It provides an estimate of the average concentration across the composited area. However, compositing masks spatial variability and dilutes hot spots. It is generally inappropriate for volatile organic compounds because the blending process causes volatilization losses.
CSA Z769 and most provincial guidance documents specify when compositing is acceptable. As a rule, use discrete samples for delineation and compliance and composite samples only for screening-level assessments of non-volatile parameters.
Sampling Depths and Intervals
Sampling depth depends on the contaminants of concern, their transport mechanisms and the subsurface geology. Surface soil samples (0 to 0.15 metres) assess exposure risk for human receptors. Shallow samples (0.15 to 1.5 metres) capture the zone most affected by surface spills and leaks. Deep samples (below 1.5 metres) are necessary when contaminants have migrated downward through permeable soils or when you are investigating buried infrastructure like underground storage tanks.
Standard practice for Phase II investigations is to sample at 0.5-metre intervals through the zone of interest. At a minimum, collect samples at the surface, at the water table interface and at one interval below the water table. Adjust your intervals based on visual or olfactory evidence of contamination encountered during drilling.
Always log the soil type, colour, moisture content, odour and any staining or sheens at each sample interval. Field observations are critical context for interpreting laboratory results.
Sampling Equipment
Hand Auger
Hand augers are suitable for shallow sampling (typically less than 3 metres) in soft, unconsolidated soils. They are inexpensive, portable and require no mechanical support. Common types include the Dutch auger (for cohesive soils) and the sand auger (for non-cohesive soils).
Limitations include inability to penetrate hard or gravelly soils, limited depth capability and potential for cross-contamination between intervals if the auger is not properly decontaminated.
Direct Push Technology (DPT)
Direct push rigs (such as Geoprobe systems) advance sampling tools into the ground using hydraulic percussion or static force. DPT is the workhorse of environmental site investigations. It is faster than conventional drilling, generates minimal investigation-derived waste and can collect continuous soil cores.
ASTM D6282 provides the standard practice for direct push soil sampling for environmental site characterizations. DPT can typically reach depths of 15 to 30 metres depending on soil conditions. Macro-core samplers (large-bore sampling tools) provide higher-quality samples with less disturbance than standard DPT samplers.
Hollow-Stem Auger and Drill Rigs
Conventional drill rigs using hollow-stem augers are necessary for deep investigations, hard ground conditions or when you need to install monitoring wells. Split-spoon sampling per ASTM D1586 (the Standard Penetration Test) provides both a soil sample and a measure of soil density.
Sonic drilling is increasingly common for environmental work. It produces continuous core samples with minimal disturbance and can penetrate difficult formations including cobbles and boulders. The trade-off is higher mobilization cost.
Sample Handling: Containers, Preservation and Chain of Custody
Proper sample handling begins the moment soil leaves the ground. Errors in containerization, preservation or documentation can render samples invalid regardless of how carefully they were collected.
Container Selection
Use laboratory-supplied, pre-cleaned containers appropriate for the target analytes. Volatile organic compounds require sealed vials (such as EnCore samplers or 40-mL VOA vials with methanol preservation) with zero headspace. Metals and inorganics typically use wide-mouth glass jars. Semi-volatile organics use glass jars with PTFE-lined lids.
Never reuse sample containers. Never substitute containers unless approved by your laboratory and documented in your field notes.
Preservation
Chemical preservation prevents degradation of target analytes between collection and analysis. Common preservation methods include cooling to 4°C (for most parameters), acidification (for metals), methanol preservation (for VOCs in soil) and sodium bisulfate (for VOCs in water).
Place samples on ice immediately after collection. Maintain the cold chain through transport and delivery to the laboratory. Record cooler temperatures at both ends of the journey.
Holding Times
Every analytical parameter has a maximum holding time - the period between sample collection and analysis during which results are considered valid. Exceeding holding times can result in biased results (typically low-biased due to analyte degradation) and may trigger data qualification or rejection.
Critical holding times to remember: VOCs in soil are 48 hours (unpreserved) or 14 days (methanol-preserved). Metals are 6 months. Petroleum hydrocarbons vary by fraction but are generally 7 to 14 days. Plan your sampling schedule around laboratory turnaround times to avoid holding time exceedances.
Chain of Custody
The chain of custody (COC) document tracks sample possession from collection through analysis. Every person who handles the samples must sign the COC. It must include the sample ID, date and time of collection, matrix, number of containers, requested analyses, preservation method and any special instructions.
Seal coolers with custody tape. Photograph the COC and sealed cooler before handing off to the courier. A broken chain of custody can invalidate results in regulatory or legal proceedings.
Field QA/QC
Field quality assurance and quality control samples quantify the precision and accuracy of your sampling program. Without QA/QC data, you cannot assess whether your results are reliable.
Field Duplicates
Field duplicates are two samples collected from the same location at the same time using the same methods. They measure the combined variability of sampling and analysis. Collect field duplicates at a rate of 1 per 10 samples (10%) or as specified by your project quality assurance plan.
Relative percent difference (RPD) between duplicates should generally be less than 30% for soil samples, though acceptable RPD depends on the concentration range and the analyte.
Field Blanks
Trip blanks are sealed vials of laboratory-grade water that travel with your VOC samples from the laboratory to the field and back. They detect contamination introduced during transport and storage. Every cooler containing VOC samples should include a trip blank.
Equipment blanks (also called rinsate blanks) are collected by pouring analyte-free water over decontaminated sampling equipment and collecting the rinse water. They verify that your decontamination procedure is effective. Collect equipment blanks at a rate of 1 per 20 samples or once per day, whichever is more frequent.
Spike Samples
Matrix spike and matrix spike duplicate (MS/MSD) samples are created by the laboratory by adding known concentrations of target analytes to a field sample. They measure matrix effects and laboratory precision. Your laboratory will request additional sample volume for MS/MSD analysis - ensure you collect enough material.
Decontamination Procedures
All non-dedicated sampling equipment must be decontaminated between sample locations to prevent cross-contamination. The standard decontamination sequence is:
- Remove gross contamination with a brush or scraper
- Wash with a non-phosphate detergent (such as Alconox) and potable water
- Rinse with potable water
- Rinse with deionized or distilled water
- For organic contaminants, add a solvent rinse (isopropanol or hexane) before the final deionized water rinse
- Air dry or pat dry with clean paper towels
Set up a dedicated decontamination station with wash tubs, rinse buckets and a waste collection container. All decontamination fluids are investigation-derived waste and must be managed according to applicable regulations.
Whenever possible, use dedicated or single-use sampling equipment to eliminate the decontamination step entirely. Disposable acetate liners, single-use bailers and pre-cleaned sampling syringes reduce both cross-contamination risk and field time.
Health and Safety During Sampling
Soil sampling at contaminated sites presents chemical, physical and biological hazards. Every field program must have a site-specific health and safety plan (HASP) that addresses the contaminants of concern and the activities planned.
At a minimum, the HASP should address:
- Personal protective equipment requirements for each task (typically Level D modified: steel-toe boots, hard hat, safety glasses, nitrile gloves, high-visibility vest)
- Air monitoring protocols and action levels for upgrading PPE
- Underground utility locates and clearance procedures before any intrusive work
- Drill rig safety zones and communication protocols
- Emergency contacts and hospital routes
- Heat stress and cold stress prevention measures
- Tailgate safety meetings before each shift
Never work alone at a contaminated site. Maintain a buddy system and ensure at least one person on site holds current first aid certification.
Documentation and Digital Tools
Thorough field documentation is as important as the samples themselves. Record everything: borehole logs, sample IDs, GPS coordinates, field measurements (PID readings, pH, temperature), weather conditions, deviations from the sampling plan and any unexpected observations.
Traditional paper field books work but introduce risks: illegible handwriting, lost notebooks, delayed data entry and transcription errors. Digital field data collection tools eliminate these risks while adding GPS tagging, photo documentation and automatic data validation.
EnviroLog by North Van Environmental provides digital chain of custody tracking, real-time field data entry and automatic QA/QC calculations. Field data syncs to your project database immediately, reducing turnaround time and ensuring nothing falls through the cracks. Learn how EnviroLog can streamline your next sampling program.
Summary
Quality soil sampling requires careful planning, proper execution and meticulous documentation. Choose your sampling strategy based on your data quality objectives. Use appropriate equipment for the depth and soil conditions. Handle samples according to laboratory specifications and regulatory requirements. Implement a QA/QC program that quantifies the reliability of your data. Decontaminate thoroughly and work safely.
The standards and protocols described here - ASTM D1586, ASTM D6282, CSA Z769 and applicable regulatory guidance - represent minimum requirements. Your project-specific sampling and analysis plan should build on these foundations with site-specific details that address the unique conditions at each site.