HORIZONTAL WELLS CAN LOWER COSTS OF REMEDIATING SOIL, GROUNDWATER

Conventional approaches to soil and groundwater remediation make extensive use of vertical wells that penetrate the various contamination phases-liquid, adsorbed, dissolved, and vapor. But advances in horizontal drilling have added a new dimension to the remediation of hazardous soils and groundwater.

Nov. 29, 1993

5 min read

Whereas conventionally drilled wells are perpendicular to the central axis of hazardous waste, horizontal wells can travel parallel to the axis. Dual wells can flank entire plumes for aggressive treatment, and sparge points can become sparge barriers - boundaries against migration of the contaminants.

In an unpublished report, Richard W. Lewis of Groundwater Technology Inc., Norwood, Mass., and Louis B. Fournier, president of STAR Environmental Inc., West Chester, Pa., describe the benefits of using this technology for remediation applications.

Under the right conditions, says Lewis, senior vice-president and chief hydrogeologist for Groundwater Technology, a single horizontal well can treat areas that previously required as many as 10 vertical wells. This not only reduces drilling costs, but also eliminates redundant hardware for groundwater pumping or soil vapor extraction.

Horizontal wells make it possible to address contamination without disturbing buildings, normal operations, or activities on the surface. They also provide a cost-effective alternative to trenches, say Lewis and Fournier.

APPLICATIONS

At present, about 12 horizontal well installations are addressing soil and groundwater contaminations in the U.S. Lewis and Fournier give a brief description of each application to illustrate situations to which horizontal drilling can be applied:

Refinery landfarm-biobarrier
A regulatory agency instructed a refinery to either close its landfarm or excavate it and install a finer, making remediation costs prohibitive. Vertical-well samples indicated that little if any contamination was reaching the groundwater, at a depth of 25 ft.
A series of horizontal wells was installed 10 ft below grade. They comprise an aeration system, which creates an aerobic biobarrier in which hazardous waste is degraded before it moves downward. Regulatory requirements were met at a fraction of the initially anticipated costs, said the report.
Aquifer-sparging barrier
In another instance, an aquifer-sparging system using a single horizontal well was installed within the water table, 10-20 ft from the boundary of neighboring property. This created an in situ aerobic barrier in which any hazardous waste in the area would be rapidly volatilized or degraded by naturally occurring bacteria.
DOE trial
For aggressive treatment of hazardous waste dispersed over a wide area in both saturated and unsaturated soil, a dual horizontal-well, vapor-extraction/sparge system can be used. This technology, developed and patented by the U.S. Department of Energy, involves drilling two parallel wells, one above the water table, the other below. The lower system is used for aquifer sparging, the upper one for vapor extraction.
In one test, the technology completely detoxified trichloroethylene (TCE). Groundwater concentrations of TCE were reduced rapidly to 5 ppb, and eventually to 2 ppb, even in areas impacted with liquid-phase TCE. Costs were projected to be 40% of conventional groundwater extraction and treatment methods.
Leaking pipeline
After a pipeline leak threatened four homes, the pipeline company installed a horizontal well system, thus eliminating the need to purchase the homes and remediate the property. To remedy the situation, three wells (75 ft x 4 in-) were drilled to serve as a flexible, multipurpose system.
The system drained liquid-phase contaminants to a sandy zone, where they were recovered by one vertical well; and it was used for subsurface dewatering, air injection, vapor-gradient control, and soil vapor extraction. Total installed cost was $65,000 vs. an estimated $865,000 for the original approach.
Refinery groundwater
During the rainy season, rising groundwater created a nuisance in the basements of about 36 homes near a refinery. The original $1 million remediation plan called for digging trenches, installing perforated piping, and pumping the water to a treatment station.
Instead, the refiner installed a series of four horizontal wells, totalling 2,100 ft. The project provided gravity drainage at less than half the estimated cost of the original plan, and caused minimal disruption to the neighborhood.

LIMITATIONS

Lewis and Fournier offered a few precautions regarding the use of this technology. For example, if subsurface obstacles are suspected, it is advisable to scan the subsurface using geophysical techniques such as ground-penetrating radar.

The effectiveness of treatment by this method frequently depends on navigating the drill bit along a fairly precise path, through or parallel to be waste plume or groundwater. This calls for comprehensive subsurface mapping using monitor wells, geoprobes, cone penetrometers, or other techniques. It is also important to know if underground utilities are nearby so that the driller can compensate for potential distortion and interference.

In addition, some treatment plans using horizontal wells can be adversely affected by seasonal changes in the water table. Vapor extraction is impossible if the water table rises above the well. And if the water level drops below the well, a groundwater extraction and treatment system will be defeated.

It is also important to know if an aquifer is 'layered." A horizontal well drilled along the bottom of a thick aquifer may not be able to draw from some of the upper layers. If the contamination is distributed throughout, an aquifer vertical well may be required.

In economic terms, Lewis and Fournier suggest that if a horizontal well can replace three or four vertical wells, the costs are typically near break-even. If five or more vertical wells can be replaced, however, horizontal can reduce costs significantly. The availability and mobilization costs of the drilling equipment and services must, of course, be factored into the decision.