Four Offsite Properties Accessed with Horizontal Remediation Wells

By: Kyle Carlton, PG, Senior Geologist

Even a small plume can have a large effect. A total of four properties and a state right-of-way were impacted due to subsurface migration of the petroleum plume. To access these areas for in-situ remediation, our long-time client selected horizontal air sparge and soil vapor extraction wells to extend the coverage of the remediation system.

To achieve the remedial goals for the consultants and their client, horizontal wells were used as the solution. They provided an alternative to typical remedial technologies with less of an impact to both the site and the general public. 

Background:
A relatively small contamination source area (approximately 40 feet long and 15 feet wide) resulting from a former gasoline station contributed to dissolved benzene, ethylbenzene, and methyl-tertiary butyl ether (MTBE) concentrations in soil and groundwater above regulatory limits on-site and beneath several surrounding properties. The gasoline station ceased operations in the mid 2000s; however, fluctuating groundwater flow directions spread the contamination beneath several adjacent properties. The site has since been redeveloped as a restaurant and coffee shop. Vertical soil vapor extraction wells were utilized to remediate the source zone. However, residual concentrations remained beneath the adjacent properties and under a state highway creating concerns over vapor migration and indoor air quality. To access beneath multiple properties and the under-state highway, horizontal air sparge/soil vapor extraction wells were selected as the preferred remedial technology.

Horizontal Well Pilot Testing:
To determine the effectiveness of the proposed horizontal remediation wells, our client first conducted a pilot test with one horizontal air sparge well and two horizontal soil vapor extraction (SVE) wells extending from the source property to the adjacent property to the east.
The horizontal SVE wells were installed at depths ranging from 3 feet below ground surface (bgs) to 5 feet bgs. The horizontal air sparge well was installed with horizontal sparging screen ranging at depths from 7 feet bgs to 13 feet bgs, below the groundwater table in the saturated zone.

Pilot Test Results:
Results of the pilot testing indicating significant vacuum influence with estimated radius of influence of 15 feet to 33 feet from the horizontal SVE wells. The horizontal radius of influence from the horizontal air sparge well was approximately 17 feet with applied pressures of approximately 30 pounds per square inch (psi). Horizontal wells were constructed of 3-inch diameter schedule 80 PVC and air sparge wells were constructed of HDPE. Horizontal well screen was custom machine cut with a screen design produced by Directional Technologies’ proprietary software to help ensure even fluid flow over the entirety of the well screen.

Full Scale Horizontal AS/SVE Well Installation:
In the summer of 2018, Directional Technologies mobilized back to the site for full scale implementation of horizontal AS/SVE well installation. Five additional horizontal SVE wells and three additional horizontal AS wells were installed to access beneath the state highway, and to the offsite properties beyond the highway. Horizontal SVE wells were extended beneath two buildings to provide vadose zone remediation and mitigate indoor vapor intrusion concerns. Directional Technologies’ experienced directional drillers successfully navigated the pilot bores past multiple utilities and through a challenging subsurface geology with shallow bedrock. Shallow 3-inch diameter conduits were also installed with directional drilling methods to connect the horizontal well heads to the remediation system. After several horizontal wells were installed, a mobile piloting unit performed pilot testing to determine the radius of influence of the newly installed horizontal well and verify the spacing required for the next horizontal wells.

Without horizontal AS/SVE wells, the impacted areas could not have been as easily accessed for remediation. Horizontal wells successfully minimized disruption to the businesses and traffic patterns within a congested urban setting.