Environmental drilling is a specialized process of penetrating the earth’s subsurface to investigate, monitor, and manage potential environmental hazards. Unlike drilling for construction or resource extraction, this technique focuses on understanding the environmental profile of a site, particularly the presence and movement of contaminants in soil and groundwater. It employs unique equipment and methodologies designed to gather accurate, uncontaminated samples. The primary goal is to assess the geological and hydrogeological conditions beneath a site to ensure regulatory compliance and safeguard public health.
Why We Drill the Subsurface
Environmental drilling is primarily driven by the need to conduct comprehensive site investigations, often referred to as Phase II Environmental Site Assessments. This work determines if previous industrial activities, accidental spills, or leaky underground storage tanks have resulted in subsurface contamination. The information gathered protects human health and ecosystems by identifying and characterizing threats.
A main objective is delineating a contaminant plume—the three-dimensional zone of affected soil, groundwater, or soil vapor. By drilling multiple boreholes in a grid pattern, geologists map the extent and concentration of pollutants to understand how they are migrating. The data collected is used to assess the potential risk to nearby receptors, such as drinking water sources or residential homes. Regulatory compliance mandates that land owners and developers must prove a site is safe for its intended use, making the subsurface investigation an obligatory step.
Specialized Techniques Used
The choice of drilling technique is determined by the site’s geology, the depth of investigation, and the specific sampling requirements. Specialized techniques are chosen specifically to minimize the risk of cross-contamination, where the drilling process inadvertently moves contaminants from a shallower zone to a deeper, clean zone.
Direct Push Technology (DPT)
DPT, often using a Geoprobe, utilizes a hydraulic hammer and static weight to drive sampling rods into the ground without rotating a cutting bit. This method is highly efficient for shallow investigations in soft, unconsolidated soils. DPT produces minimal soil cuttings, which reduces the amount of contaminated waste generated.
Hollow Stem Auger (HSA) Drilling
HSA drilling uses a screw-like mechanism to bore a hole. The auger flights bring the soil cuttings to the surface, but the central hollow stem remains open, allowing professionals to collect soil samples or install monitoring equipment. This method is well-suited for unstable formations like sand or gravel and is preferred for installing groundwater monitoring wells.
Rotary Drilling
Rotary drilling, including air and mud rotary techniques, is reserved for deeper investigations or sites with hard rock or bedrock formations. This method uses a rotating drill bit to cut through dense material, with circulating air or drilling fluid carrying the cuttings back to the surface.
Data Collection and Monitoring Installation
Once the borehole is created, the next step involves collecting representative samples from the soil, groundwater, and sometimes soil vapor for laboratory analysis. Specialized tools, such as split-spoon samplers or temporary well points, are used to ensure sample integrity for accurate contaminant concentration readings. The lab results confirm the presence and concentration of specific pollutants, providing the quantitative data needed for decision-making.
A frequent outcome of the drilling process is the installation of permanent infrastructure, most commonly groundwater monitoring wells. These wells consist of a filtered screen and protective casing placed within the borehole, providing a secure access point to the subsurface. Monitoring wells allow for the long-term collection of groundwater samples and measurement of water levels, which is crucial for tracking the movement of a contaminant plume over time.
Beyond sampling and monitoring, boreholes can also be converted into remediation infrastructure. This may include injection wells, used to introduce chemical oxidants or microbes to actively break down contaminants (in-situ remediation). Conversely, some wells are installed as extraction points to actively pump out contaminated groundwater or soil vapor as part of a clean-up strategy.