Horizontal Remediation Wells
Overview
Horizontal remediation wells (HRWs) are installed below ground parallel to the ground surface or at a shallow angle using horizontal directional drilling (HDD). HRWs can reach areas that may not be directly accessible with vertical wells due to surface obstructions. HRWs can bisect or follow the long axis of a contaminant plume or geologic feature to maximize contact with subsurface contamination. HRWs may be used exclusively or in conjunction with vertical wells, depending on site needs.
Horizontal wells are drilled at a shallow angle, using specialized HDD equipment to create a gently curved borehole to the desired depth. Once the target depth has been reached, the drill bit can be steered along a straight or curved path to target a specific feature or avoid obstructions. It can also be steered to follow the downward trajectory of a groundwater contaminant plume. Wells are completed by pushing or pulling casing and screen into the curved borehole.
HRW Installation Showing Relationship of Depth and Setback Distance. (Courtesy of Ellingson-DTD)
The geometry of a HRW requires a setback distance approximately three to five times the well's target depth. Because of this, the staging area and entry hole for the well may need to be off-site. HRWs can be thousands of feet long with hundreds of feet of well screen, potentially providing greater contact with contamination than vertical wells and making them especially useful in treating elongated, relatively shallow groundwater contaminant plumes.
HRWs offer an alternative to the installation of conventional vertical wells or trenching, particularly where access to soil and groundwater contamination is impeded by surface or near-surface obstructions. HRWs have been used to sample and treat contamination beneath buildings, utility lines, roads, railyards, airport runways, landfills, tank farms, surface water bodies, and sensitive ecological areas. Because of the setback distance, drilling and well installation can occur without interrupting activities at active facilities.
Example of HDD Rig.
(Courtesy of Ellingson-DTD)
HRWs also can facilitate treatment of elongated groundwater plumes by increasing the area of contact between well screens and contamination. A single, long-screened HRW oriented along the axis of a dissolved-phase or NAPL plume might achieve a zone of influence (ZOI) comparable to that of multiple vertical wells. The ZOI of HRWs takes on an elliptical shape across the length of the screen during extraction or injection and is widest toward the center. The radius of the ZOI is smallest at the ends of the horizontal screen and is similar to that of a vertical well in the same formation. Thus, the ZOI of the HRW is larger than for a vertical well. The larger, elliptical ZOI of HRWs is attributed to the longer screen length and more efficient flow when a screen is placed within a single sedimentary unit.
Like vertical wells, HRWs can be nested, i.e., multiple wells bundled together in one boring, with discrete target depth intervals. Reducing the number of vertical wells drilled by installing one or more HRWs can reduce impacts not only on facility operations but on sensitive surface habitats. For additional protection, the entry and exit holes for HRWs can be placed outside the boundaries of these habitats.
The ZOI of a single HRW may be comparable to that of several smaller wells.
Plus, HRWs can extend beneath buildings and with less interruption of facility activities.
Considerations for Selecting HRWs
- Fewer drilling contractors have the specialized training experience needed for installing HRWs.
- Drilling beneath rail lines and other infrastructure may require special permits and fees.
- Although costs can be mitigated by the need for fewer wells, the costs of HDD mobilization, drilling, and well materials are usually greater per foot than vertical wells.
- Due to their length, HRWs can generate more soil cuttings and drilling fluids than HRWs. This means higher costs for waste handling, characterization, and disposal.
- Planning the bore path for a HRW is more complicated than for a conventional vertical well and requires rigorous site characterization. The HRW driller must consider the three-dimensional subsurface geology, the location of underground utilities, and where to locate the entry and exit points.
- HRWs avoid costly excavation of trenches required for piping that interconnects vertical air sparge and soil vapor excavation (SVE) wells. In addition, HRW installation avoids the interruption of normal business operations.
