It is an unfortunate consequence of the clustering of commercial businesses such as gas stations, automotive service shops, and dry cleaners at busy city intersections that groundwater contaminant plumes often migrate across such intersections, just below the water table.  An automotive service shop once operated at a street corner just half a block away from the Governor’s mansion in Tallahassee, Florida. As luck would have it, the plume migrated diagonally across the intersection, making it virtually impossible to gain access to the plume through vertical wells.  If the plume was allowed to continue to migrate further, it might threaten indoor air quality in several commercial buildings on the opposite side of the street.  The consultant met a similar challenge on the outskirts of town a few years earlier using two pairs of horizontal air sparging (AS) and soil vapor extraction (SVE) wells that reached from a corner retail gas station diagonally across the intersection to a fast-food restaurant, placing Horizontal Remediation Well screens continuously under the intersection.

The plume near the Governor’s mansion, however, was wider and required four sets of Air Sparing – Soil Vapor Extraction (AS-SVE) pairs.  The plume spread out, getting wider down-gradient as it crossed under the intersection. Curved pathways were necessary for the Horizontal Remediation Well AS-SVE pair to achieve coverage of the plume.  Multiple buried utility lines interfered with the locating equipment, but Directional Technologies, Inc. (DTI) was able to accurately follow the compound curves of the well paths by combining downhole sonde locating equipment with the continuous calculation of wellbore depths and topographic survey points of the uneven ground surface.  DTI applied 20 years of environmental Horizontal Remediation Well directional drilling experience and successfully placed portions of the four AS horizontal wells along the top of a clay aquitard unit.  Among the many useful skills DTI has developed over the years is the mastery of the logistics of crossing intersections at night with locating equipment to follow the drill bit and ensure accurate well placement.  DTI’s environmental directional driller advanced each horizontal SVE well after its companion AS well so that the horizontal SVE well would follow the detailed path of the AS well, thereby satisfying Florida Department of Environmental Protection (FDEP) requirements for AS-SVE pairs.

The most challenging aspect of this project, however, was the resistance of commercial property owners across the intersection to having exit pits excavated in their front or even back lawns.  Directional Technologies had the perfect solution for this problem:  Install the wells blind, without exit points at the far side of the intersection.  Installing a blind horizontal well was typically a routine operation for DTI, which pioneered blind Horizontal Remediation Well installation techniques in the past decade. However, this proved more complicated at this site because of the completion material, which was high-density polyethylene (HDPE) and its lack of stiffness that allows stainless steel or even fiberglass-reinforced epoxy (FRE) screen and riser to be pushed directly into the wellbore.  But DTI was not about to let the softness and flexible nature of HDPE keep them from completing the blind Horizontal Remediation Wells.  DTI devised a method whereby the HDPE could essentially be pulled into the blind wellbore.  Directional Technologies installed 1,800 feet of Horizontal Remediation Well completion pipe, roughly half of which consisted of slotted well screen, the rest being riser pipe.