Real-Time Data through Horizontal Soil Sampling

By Kyle Carlton, PG, Senior Geologist

Horizontal soil sampling and immediate analysis of the samples were used to determine the optimal placement of horizontal wells; both the soil sampling and the horizontal well installation were completed within hours of each other.

Directional Technologies recently accomplished this task on a project in Massachusetts where the overall objective of the horizontal soil sampling was to provide immediate field data to delineate the extent of chlorinated solvent impact beneath the building.  The data was then used to determine the optimal locations for installation of horizontal vapor extraction wells, thus completing assessment and installation in one mobilization. 

Background

Further assessment and vapor mitigation were required at an industrial facility in the Northeast where subsurface impacts from perchloroethylene (PCE) and trichloroethylene (TCE) resulted in conditions for potentially harmful vapor intrusion.  The surrounding areas of the building had been fully assessed with vertical soil sampling methods; however, the area beneath the northern portion of the building had little assessment data due to the difficulties involved with accessing the sub-slab soil.  Vertical borings through the floor of the facility were logistically impossible, and therefore horizontal drilling methods were selected for additional site characterization.  In addition, vapor extraction beneath the building was required to prevent vapor intrusion into the building.  Our client selected horizontal soil vapor extraction (SVE) wells as the method to assess and remediate underneath the building.

Horizontal Soil Sampling Project Considerations

Collecting soil samples through directional drilling is not as simple as collecting soil samples through vertical drilling techniques.  Similarly, to vertical soil sampling, the ground conditions of the target soils play a major factor in whether sample recovery is possible.  The horizontal soil sampling tool is comparable to a 2-foot split spoon sampling device.  As with vertical soil sampling, horizontal soil recovery can be difficult in formations which are too hard, too soft, or very saturated.  Directional Technologies has developed a proprietary soil sampling tool to allow for the maximum soil recovery within difficult ground conditions.

Real-Time Data Analysis

Being able to collect data real-time is crucial to the success of environmental assessment and remediation projects.  It is standard procedure to collect soil samples when installing vertical monitoring wells.  Real-time field analysis of soil samples helps to determine how to construct a well with regard to the screened interval.  Directional Technologies has successfully implemented real-time soil sampling for horizontal wells on multiple projects.  The field analysis of the horizontal soil samples enables horizontal wells to be accurately placed into the optimal locations for soil and groundwater for remediation.

Approach and Activities

Directional Technologies was contracted to collect horizontal soil samples beneath the manufacturing facility and install horizontal SVE wells.  The horizontal wells were installed blind (single-entry) method, so that exit locations were not required.  The building footer extended five feet below ground surface (bgs) in some locations.  A steep entry angle of the horizontal boring was needed so that the directional drill bit could safely pass beneath the concrete footer.

The exact length and screened intervals of the horizontal SVE wells were determined based on the field screening of the horizontal soil samples, which were collected every 5-feet during soil boring advancement.  Previous logging of vertical soil borings at the site indicated the expected ground conditions were loose to dense clayey sand, and silty sand, with some gravel present.  Collecting soil samples in this type of formation requires extensive experience and expertise as gravel could block the sampling device, preventing sample recovery. 

Despite the challenges, horizontal soil samples were successfully collected once every 5-feet along the bore path and then screened for presence volatile organic compounds (VOCs) to determine the extent of the horizontal wells.  If the soil sample indicated presence of VOCs above a predetermined threshold, the horizontal boring would be extended until the vapor concentrations reduced, indicating the extent of the contamination was defined.  Based on the screening of the soil samples, the final target locations and screened intervals of the horizontal SVE wells were determined and 3-inch diameter horizontal SVE wells were installed.

Results

Because the client elected for real-time horizontal soil sample analysis, the two horizontal SVE wells were installed at the optimal locations to target the chlorinated solvents.  Two horizontal wells were installed beneath this portion of the facility.  The first horizontal SVE well was 75 feet in total length, installed to a final depth of approximately 22 feet BGS.  The second horizontal SVE well was installed in an area of more significant chlorinated solvent impact; therefore, the second horizontal well was 90 feet in total length and extended to approximately 25 feet BGS.  

Without the ability to collect the horizontal soil samples, the optimal locations for the horizontal vapor extraction well screens may have been missed, resulting in an under-performing vapor extraction system with continued risk to the air quality within the building.