Sampling Strategy for Environmental Site Investigations

Sampling strategy is a critical component of environmental site investigations, particularly when dealing with potentially contaminated land. For Avada Environmental Ltd (Avada), a robust sampling strategy not only ensures compliance with regulatory standards but also provides a solid foundation for risk assessment and remediation planning. This blog post will explore the detailed processes and considerations involved in designing and implementing an effective sampling strategy.

Soil Sampling Strategy

Purpose and Design of Sampling Strategy

Every site investigation must begin with a clearly defined sampling strategy. This strategy should articulate specific reasons for each sample collected, aligning with the overall objectives of the investigation and addressing uncertainties identified in the initial conceptual model (CM). The primary goals are to determine the presence of significant pollutant linkages and to gather data for subsequent risk assessments and remediation planning.

Targeted vs. Non-Targeted Sampling

There are two main approaches to soil sampling:

  • Targeted Sampling: This approach relies on professional judgment to select sample locations based on existing information. It aims to verify whether suspected contaminated areas are indeed contaminated. Factors influencing the number of samples include the level of confidence required, nature of contamination, site size, and investigation stages.
  • Non-Targeted Sampling: This method uses a systematic grid pattern to cover an area comprehensively. It is particularly useful for detecting contamination hotspots and understanding the spatial distribution of contaminants. Non-targeted sampling objectives often include eliminating hotspots to a certain confidence level, determining average concentrations and their distribution, and generating variograms for geostatistical analysis.
Determining the Number of Samples

The number of soil samples needed depends on various factors such as the investigation stage, the degree of site heterogeneity, and the required confidence level. For exploratory investigations, sampling grids with 50-100m centers are suggested, while main investigations may require grids with 20-25m centers. In areas with heterogeneous conditions or where high confidence is needed, even denser grids (e.g., 10m centers) might be necessary.

Taken from CLR4 – 1994, Environment Agency
Calculating Sample Spacing and Hotspot Detection

The spacing of samples is a crucial element in the sampling strategy, impacting the ability to detect hotspots and characterize the site accurately. The calculation of sample spacing involves statistical considerations and a balance between coverage and resource constraints.

  1. Grid Spacing Calculation:
    • Exploratory Stage: A wider grid spacing of 50-100m is often sufficient to identify general contamination trends.
    • Detailed Investigation: Tighter grid spacing of 20-25m helps in pinpointing contamination hotspots.
    • High-Confidence Areas: In areas where contamination is suspected to be highly variable, grid spacing of 10m or less may be necessary.
  2. Hotspot Size and Detection:
    • The size of the hotspot that needs to be detected dictates the grid spacing. For instance, if the aim is to detect hotspots with a radius of 5m, the grid spacing should be smaller than the hotspot size, typically around 2.5m to ensure the hotspot is within one or two grid cells.
    • Confidence Levels: The confidence level desired for hotspot detection impacts the number of samples. Higher confidence levels require more samples and closer spacing.
Statistical Methods:
  • Kriging: A geostatistical method used to predict the spatial distribution of contamination and optimize sampling locations.
  • Monte Carlo Simulations: These can be employed to assess the probability of hotspot detection given various grid spacings and contamination scenarios.
Soil Characterisation and BS5930

BS5930, the British Standard for the “Code of practice for site investigations,” provides comprehensive guidelines for soil sampling and characterization. It is a crucial reference for ensuring that soil investigations are carried out systematically and accurately. Key aspects of BS5930 include:

  1. Site Investigation Planning:
    • Preliminary Investigations: Initial desk studies and site reconnaissance to gather existing information and plan the main investigation.
    • Investigation Phases: Dividing the investigation into phases (preliminary, main, supplementary) to progressively refine the understanding of site conditions.
  2. Sampling Methods:
    • Sampling Techniques: Various techniques such as trial pits, boreholes, and auger drilling are recommended based on site conditions and investigation objectives.
    • Sample Types: Guidelines for collecting different types of samples (disturbed, undisturbed, bulk) to meet specific analytical and engineering needs.
  3. Soil Description and Classification:
    • Soil Properties: Detailed procedures for describing soil properties, including texture, color, consistency, structure, and presence of organic material or contaminants.
    • Classification Systems: Use of standard classification systems like the Unified Soil Classification System (USCS) or the British Soil Classification System (BSCS) to ensure consistency in reporting.
  4. Field Testing and In-Situ Measurements:
    • In-Situ Testing: Conducting field tests such as screening with a Photo Ionisation Detector (PID) or Laser-induced fluorescence (LIF).
    • Groundwater Measurements: Monitoring groundwater levels and pressures to understand hydrogeological conditions.
  5. Quality Assurance and Control:
    • Sampling Protocols: Ensuring that samples are collected, handled, and stored according to strict protocols to maintain their integrity.
    • Laboratory Testing: Guidelines for selecting accredited laboratories and conducting reliable soil analyses.
Sampling Density and Variability

The density of sampling should account for the expected variability of soil conditions and contamination levels. BS5930 provides guidance on determining appropriate sampling densities, emphasizing the need to adapt strategies to site-specific conditions:

  1. Homogeneous Sites: For relatively uniform sites, a lower sampling density may suffice to provide an accurate representation of conditions.
  2. Heterogeneous Sites: Sites with complex geology or varying contamination levels require higher sampling densities to capture spatial variations accurately.
  3. Hotspot Identification: Dense sampling grids in suspected hotspot areas ensure that small-scale contaminant distributions are detected and characterised effectively.

Groundwater Sampling and Monitoring Strategy

Objectives and Strategy

Groundwater sampling aims to understand the groundwater regime, detect contamination, and delineate its extent. Key considerations include selecting appropriate sampling depths, locations, and monitoring durations to capture the necessary data.

Sampling Locations and Depths

Groundwater monitoring typically requires fewer locations than soil sampling. A minimum of three wells, arranged in a triangular pattern, is recommended to determine groundwater flow direction. Additional wells may be necessary for larger sites or areas with varied geology or contamination levels. Locations should be chosen to monitor upstream and downstream boundaries, near potential sources of contamination, and at receptors or sensitive areas.

Frequency and Duration of Monitoring

Monitoring should be conducted over varying intervals to capture seasonal variations and migration patterns. Initially, samples might be collected frequently (e.g., three sets over a few weeks), followed by less frequent sampling (e.g., quarterly). Long-term monitoring may be required for remediation and natural attenuation assessments.

BS EN ISO 5667-1:2023

BS EN ISO 5667-1:2023 provides comprehensive guidelines for the design and implementation of water sampling programs, ensuring that samples are representative and reliable. Key points from this standard that must be adhered to include:

  1. Purging Requirements: Wells should be purged before sampling to remove stagnant water and ensure that samples are representative of the aquifer. This involves removing three to five well volumes of water or until parameters such as pH, temperature, and electrical conductivity stabilize.
  2. Sample Collection Methods: Proper techniques should be employed to avoid contamination during sample collection. This includes using clean, decontaminated equipment and following specific procedures for different types of water bodies.
  3. Sample Preservation and Handling: Samples must be preserved and transported according to standard protocols to prevent changes in their composition. This often involves cooling samples to 4°C and adding preservatives when necessary.
  4. Avoiding Sample Contamination: Sampling from trial pits is generally not recommended due to the higher risk of sample contamination and the potential for inaccurate representation of groundwater conditions. Boreholes are preferred as they provide more reliable and consistent samples.
Monitoring Locations and Frequency

Groundwater monitoring wells should be strategically placed to capture key data points across the site:

  1. Proximity to Potential Contamination Sources: Wells should be located near known or suspected contamination sources to detect the presence and concentration of contaminants.
  2. Pathways and Receptors: Additional wells should be placed along potential migration pathways and near sensitive receptors, such as drinking water wells or surface water bodies.
Sampling Frequency and Atmospheric Conditions

A robust sampling program includes multiple rounds of sampling to capture temporal variations and environmental influences:

  1. Four Rounds of Sampling: Conducting four sampling events within a month helps identify both short-term and long-term trends in groundwater quality.
  2. Sampling During Falling Atmospheric Pressure: At least two sampling events should occur during periods of falling atmospheric pressure, as this condition can cause changes in groundwater levels and contaminant concentrations.

Surface Water Sampling and Monitoring Strategy

Purpose and Sampling Points

Surface water sampling focuses on identifying contamination and determining whether it originates from the site or upstream sources. Sampling points are typically placed at upstream and downstream boundaries and at intermediate points to provide a comprehensive assessment of the water body’s quality.

BS EN ISO 5667-1:2023

BS EN ISO 5667-1:2023 provides essential guidelines for designing and implementing water sampling programs to ensure that samples are representative and reliable. The key points from this standard that must be adhered to include:

  1. Sampling Locations:
    • Upstream and Downstream Points: Place sampling points upstream to establish baseline conditions and downstream to capture the influence of potential contamination sources.
    • Intermediate Points: Additional sampling points within the site area to provide a detailed understanding of contamination dispersion.
  2. Sampling Techniques:
    • Direct Sampling: Use appropriate equipment to collect samples directly from the water body. Ensure the equipment is clean and free from contaminants.
    • Avoiding Contamination: Use non-reactive materials such as stainless steel or plastic for sampling containers. Rinse containers with the water to be sampled before collection.
  3. Purging and Stabilization:
    • While purging is more relevant to groundwater, it is crucial in surface water sampling to ensure the sample represents the water body accurately. Allow the water to flow for a few minutes before collecting samples if taken from a point source like a tap or outlet.
  4. Sample Preservation and Handling:
    • Immediate Preservation: Preserve samples immediately after collection by cooling to 4°C and, if necessary, adding preservatives to prevent changes in sample composition.
    • Transport: Transport samples in insulated containers to the laboratory as quickly as possible to maintain their integrity.
Timing and Frequency

Sampling must consider external influences such as tides, diurnal and seasonal changes, temperature variations, and contaminant discharges. More variable conditions necessitate more frequent sampling to ensure representative data. A minimum of three sampling events is recommended:

  1. Diurnal and Seasonal Variations: Conduct sampling at different times of the day and during different seasons to capture variations in water quality.
  2. Event-Based Sampling: Include additional sampling during significant events such as heavy rainfall, which can affect contaminant levels and water flow.
Monitoring Parameters

Key parameters to monitor in surface water sampling include:

  1. Physical Parameters: Temperature, pH, turbidity, and electrical conductivity.
  2. Chemical Parameters: Concentrations of contaminants such as heavy metals, hydrocarbons, and nutrients.
  3. Biological Parameters: Presence of pathogens, biological oxygen demand (BOD), and chemical oxygen demand (COD).

Sediment Sampling Strategy


Sediment sampling aims to detect contamination, trace its source (site, upstream, or groundwater), and assess the potential release of contaminants into surface water. Sampling points are generally located at upstream and downstream ends of the site, with possible additional points in between.

Sample Collection and Analysis

Sediment samples should be collected using appropriate methods to avoid cross-contamination. Common techniques include grab sampling and core sampling. The analysis should include physical characteristics, chemical composition, and potential bioavailability of contaminants.

Gas Sampling and Monitoring Strategy

Purpose and Methods

Gas sampling strategies focus on understanding the gas regime (composition, concentration, pressure, flow), delineating gas sources, and evaluating migration pathways. This is crucial for assessing risks to human health and the environment, especially in areas prone to gas accumulation, such as landfills and contaminated industrial sites.

Wilson & Card Methodology

The Wilson & Card methodology is a well-established approach for soil gas monitoring, particularly in assessing methane and carbon dioxide levels in the subsurface. This method is designed to provide a comprehensive understanding of gas production, migration, and potential accumulation in the subsurface environment. It involves the following key steps:

  1. Installation of Gas Monitoring Wells: These wells are strategically placed to intercept potential gas pathways and areas of concern. The placement is based on site-specific conditions, such as geology, contamination sources, and potential receptors.
  2. Sampling Protocol: Gas samples are collected at regular intervals to monitor variations in gas concentrations over time. The methodology typically includes both shallow and deep sampling points to capture vertical gas migration patterns.
  3. Data Analysis: The collected data are analyzed to identify trends in gas production and migration. This includes assessing the influence of environmental factors such as temperature, pressure, and moisture content on gas behavior.
Sampling Frequency and Atmospheric Conditions

A robust gas sampling program should include multiple rounds of sampling to capture temporal variations and environmental influences on gas migration. The recommended approach involves four rounds of sampling over the course of a month, with specific attention to atmospheric pressure changes:

  1. Four Rounds of Sampling: Conducting four sampling events within a month allows for the detection of both short-term and longer-term trends in gas concentrations. This frequency helps to identify any periodic fluctuations or consistent patterns in gas production and migration.
  2. Sampling During Falling Atmospheric Pressure: At least two of these sampling events should occur during periods of falling atmospheric pressure. Falling pressure can cause gas to migrate from the subsurface to the surface, thereby providing critical data on potential gas release scenarios. This condition is particularly relevant for assessing the risk of gas accumulation in confined spaces, which can pose significant hazards.

The CIRIA C665 document, “Assessing Risks Posed by Hazardous Ground Gases to Buildings,” provides comprehensive guidance on the assessment and mitigation of risks associated with hazardous ground gases. It is a crucial reference for gas monitoring strategies, offering detailed methodologies and best practices for evaluating gas risks. Key aspects of CIRIA C665 include:

  1. Risk Assessment Framework: CIRIA C665 outlines a systematic approach for assessing gas risks, including the identification of potential gas sources, pathways, and receptors. It emphasizes the importance of developing a robust conceptual model to guide the investigation.
  2. Gas Monitoring Techniques: The document details various gas monitoring techniques, including continuous monitoring, spot sampling, and flux measurements. It provides guidance on selecting appropriate methods based on site-specific conditions and risk factors.
  3. Mitigation Measures: CIRIA C665 offers recommendations for mitigating gas risks, such as the installation of gas protection measures in buildings and the design of effective ventilation systems. These measures are crucial for protecting human health and ensuring the safety of built environments.
Monitoring Locations and Frequency

Gas monitoring wells should be placed to intercept potential sources, pathways, and receptors. The number and pattern of wells depend on site-specific factors such as contamination levels, geological variations, and potential risk areas. Key considerations for monitoring locations include:

  1. Proximity to Potential Gas Sources: Wells should be located near known or suspected gas sources, such as landfills, buried waste, or contaminated soil. This helps to capture data on gas production and migration directly from the source.
  2. Pathways and Receptors: Monitoring wells should also be placed along potential migration pathways, such as fractures or permeable soil layers, and near sensitive receptors, such as buildings or water sources.
Monitoring Frequency and Data Interpretation

Initial monitoring should be frequent, with data collected over short intervals (e.g., weekly or bi-weekly) to capture early trends and variations. Over time, the frequency can be adjusted based on observed data trends and the stability of gas concentrations. Data interpretation should consider:

  1. Temporal Variations: Analyzing data over time helps to identify patterns and correlations with environmental factors such as atmospheric pressure and temperature.
  2. Spatial Distribution: Mapping gas concentrations across the site provides insights into the extent and pathways of gas migration, which is crucial for effective risk assessment and remediation planning.


Developing a comprehensive sampling strategy is essential for effective site investigation and environmental management. By following detailed guidelines and adapting strategies to site-specific conditions, Avada Environmental Ltd can ensure accurate risk assessments and informed decision-making for remediation efforts. Each type of sampling—soil, groundwater, surface water, sediment, and gas—requires careful planning and execution to gather reliable data, ultimately contributing to the successful reclamation of contaminated land.

Published: 21 May 2024