Contaminated Site Investigation Guide

A contaminated site investigation is the intrusive field investigation phase of environmental site assessment where soil, groundwater, soil vapour and sediment samples are collected and analyzed to determine whether contamination exists, identify the contaminants of concern, quantify their concentrations and delineate the horizontal and vertical extent of impact. It follows the identification of recognized environmental conditions during a Phase I site assessment and provides the data needed for risk evaluation, remediation planning and regulatory decision-making. In British Columbia, investigations follow the Contaminated Sites Regulation under the Environmental Management Act. In the US, EPA guidance documents and state-specific standards govern investigation procedures.

This guide covers the technical aspects of contaminated site investigations - from developing a conceptual site model and sampling strategy through field investigation methods, data evaluation, reporting requirements and the digital tools that modern environmental professionals use to manage investigation data efficiently.

The Conceptual Site Model

Every contaminated site investigation begins with a conceptual site model (CSM) - a working hypothesis about contamination sources, release mechanisms, transport pathways and potential receptors. The CSM drives the investigation design and is refined as new data becomes available.

CSM Components

• Sources: Where did contamination originate? Underground storage tanks, chemical storage areas, disposal pits, spill locations, atmospheric deposition, fill material.
• Release mechanisms: How was contamination released to the environment? Leaks, spills, improper disposal, infiltration, migration from adjacent properties.
• Transport pathways: How is contamination moving? Groundwater flow, soil vapour migration, surface water runoff, preferential flow paths (utility trenches, fractured bedrock).
• Affected media: Which environmental media are impacted? Soil, groundwater, soil vapour, surface water, sediment, indoor air.
• Receptors: Who or what is potentially exposed? Human receptors (residents, workers, construction workers), ecological receptors (aquatic life, terrestrial wildlife), water supply wells.
• Exposure routes: How could receptors be exposed? Ingestion, dermal contact, inhalation of vapours or dust, consumption of contaminated water or food.

Building the CSM

The initial CSM is developed from Phase I ESA findings, historical records, geological mapping and any available analytical data. As investigation data is collected, the CSM is updated and refined. A well-maintained CSM prevents unnecessary sampling, focuses resources on the highest-risk areas and provides a clear framework for communicating findings to stakeholders.

Investigation Planning

Sampling and Analysis Plan

The Sampling and Analysis Plan (SAP) is the blueprint for the field investigation. A well-designed SAP ensures that data collected will meet the investigation objectives and satisfy regulatory requirements.

Key SAP Components

Utility Clearance

Before any drilling, all underground utilities must be located and cleared. This typically involves calling the local one-call service (BC One Call, Ontario One Call, 811 in the US), reviewing as-built drawings and performing a private utility locate using ground-penetrating radar (GPR) or electromagnetic locators. Striking an underground utility during drilling can cause serious injury, property damage, environmental releases and project delays.

Field Investigation Methods

Drilling and Soil Sampling

The choice of drilling method depends on geology, target depth, analytical requirements and site access constraints.

Groundwater Investigation

Groundwater investigation involves installing monitoring wells, measuring water levels to determine flow direction and collecting groundwater samples for laboratory analysis.

Monitoring Well Design

• Screen interval positioned across the water table or target aquifer zone
• Filter pack surrounding the screen to prevent fine material intrusion
• Bentonite seal above the filter pack to prevent surface water infiltration
• Grouted annular space from the seal to surface
• Protective casing (flush-mount or stick-up) with locking cap
• Well development (surging and purging) to remove drilling fines before sampling

Groundwater Sampling

Low-flow purging with a peristaltic pump, bladder pump or inertial pump is the preferred sampling technique. Field parameters (pH, conductivity, dissolved oxygen, ORP, temperature and turbidity) are monitored during purging until stabilization criteria are met, confirming that the sample represents formation water rather than stagnant well water.

Soil Vapour Investigation

When volatile organic compounds (VOCs) are a concern, soil vapour probes are installed to assess the potential for vapour intrusion into overlying buildings. Investigation methods include:

• Temporary vapour probes: Driven or drilled to the target depth, purged and sampled in a single mobilization
• Permanent vapour probes: Installed with dedicated tubing for repeated sampling over time
• Sub-slab vapour sampling: Probes installed through building floor slabs to assess vapour concentrations beneath occupied spaces
• Indoor air sampling: Summa canisters placed inside buildings to measure actual indoor air quality for comparison against health-based criteria

Data Evaluation and Reporting

Regulatory Standard Comparison

Laboratory results are compared against applicable regulatory standards to determine whether contamination exceeds acceptable levels. In BC, the Contaminated Sites Regulation provides generic numerical soil, groundwater and vapour standards for different land use scenarios (agricultural, urban park, residential, commercial, industrial).

Investigation Reports

A contaminated site investigation report must clearly communicate the investigation findings, data quality assessment and recommendations. Standard report contents include:

• Site description, history and regulatory context
• Conceptual site model (updated based on investigation findings)
• Investigation scope and methodology
• Geological and hydrogeological characterization
• Analytical results presented in tables and on site maps
• Regulatory standard comparison with exceedance tables
• Data quality assessment (QA/QC evaluation)
• Conclusions regarding the nature and extent of contamination
• Recommendations for further investigation, risk assessment or remediation
• Appendices with borehole logs, laboratory certificates, chain of custody forms and raw data

Digital Tools for Site Investigations

Modern contaminated site investigations generate large volumes of spatial and analytical data. Managing this data effectively requires digital tools that go beyond spreadsheets and word processors.

Field Data Collection

Tablet-based field logging applications allow technicians to record borehole descriptions, PID readings, sample information and photographs in real time. GPS coordinates are captured automatically, eliminating the need for separate survey data. NVES EnviroLog's mobile field tools support offline data collection for sites without cellular coverage.

Chain of Custody Management

Digital chain of custody tracking eliminates handwriting errors, ensures consistency between sample labels and COC forms and provides real-time visibility into sample status from field to laboratory.

Laboratory Data Integration

Electronic data deliverables (EDDs) from accredited laboratories can be imported directly into compliance management platforms, eliminating manual data entry and enabling automated comparison against regulatory standards. NVES EnviroLog flags exceedances automatically and generates comparison tables for report inclusion.

Hazard Mapping and Visualization

GIS-based visualization tools plot analytical results on site maps, showing concentration contours, exceedance zones, monitoring networks and geological cross-sections. This spatial understanding is essential for delineation planning and communicating findings to non-technical stakeholders.

Next Steps After Investigation

Investigation findings determine the path forward. The options depend on whether contamination was found, its extent and concentration relative to regulatory standards and the planned use of the site.

Investigation Outcomes

• No contamination found: Issue a negative determination or no further action letter. Document and archive investigation records.
• Contamination below standards: Site may qualify for closure without remediation, depending on jurisdiction. In BC, a Certificate of Compliance may be issued for sites meeting generic numerical standards.
• Contamination above standards - limited extent: Proceed to remediation planning. Excavation and off-site disposal is often the most cost-effective option for small volumes of impacted soil.
• Contamination above standards - extensive: Detailed delineation investigation, risk assessment and remediation options analysis. In-situ treatment, monitored natural attenuation or risk management with institutional controls may be appropriate.
• Groundwater plume identified: Plume delineation, receptor survey and migration pathway assessment. Long-term monitoring program design. Potential pump-and-treat or in-situ treatment.

For detailed guidance on the full site assessment lifecycle, see our contaminated site assessment guide. For information on regulatory compliance frameworks, visit our environmental compliance guide.