Directional Sampling Beneath Landfill Assists Site Characterization – Superfund Site – Eastern Washington

Directional drilling has been widely adopted for installation of remediation wells and as an access technology for the injection of various chemical and biological amendments to clean up contaminated sites. The technology is particularly well suited for areas where surface access is blocked due to infrastructure or other obstructions, ongoing site operations, or sensitive environments.

Another application for HDD is for characterization of contaminated sites. Just as vertical wells are not appropriate for all remediation and treatment programs, vertical sampling operations are also not well suited for all site conditions. Although the technology is not in common use, for some projects it is the only practical method to obtain representative samples of a suspected contaminant zone.

Recently, DTD accomplished what is likely to be the most extensive sampling program yet attempted using directional drilling. A closed, mixed-waste landfill in eastern Washington — now a Superfund site — required additional characterization to support remediation or mitigation measures that were being contemplated. The landfill is capped with a geosynthetic membrane and soil and contains several layers of stacked drums known to contain hazardous waste. To perform additional sampling with vertical drilling would require that the cap be breached, and would also encounter drums of waste, with the risk of puncturing the containers and dragging the waste downhole — complicating the collection of representative samples.

DTD was contacted nearly a year in advance of the project and began planning the extensive drilling endeavor. Several horizontal bores were initially planned to be drilled up to 300 ft from either end of the elongated landfill footprint, collecting multiple samples from each bore at a 30-ft spacing. Surface casing would be set to a distance of 70 ft, to prevent cross contamination by drilling fluid from the potentially contaminated zone into “clean” soil outside the landfill footprint. A traditional walkover navigation/steering system was selected to keep costs in check.

Soil sampling with a directional rig is a prolonged operation. For each sample, the bore must be advanced to the targeted interval, the bit must be tripped out, and then the sampler is tripped in, the sample collected, and then retrieved. Finally, the bit is tripped in again to drill to the next sampling interval. The use of a walkover navigation system with a battery-powered downhole sonde eliminates the need to connect a wireline through the rods, which would add more time and complexity to an already lengthy process. By the conclusion of the project, more than 91,000 ft of rods had been tripped into and out of the bore — the equivalent of installing more than 75 wells, 600 ft in length.

DTD lead driller for the project Tim Jones noted, “The drilling wasn’t that hard, but the constant rod-handling put a lot of extra wear-and-tear on the machine. By the end of the project, the carriage and rod loader were ready for overhaul.”

Since this work was for site characterization on a Superfund site, all workers had to be current with OSHA training for hazardous waste operations (HAZWOPER, 29 CFR 1910.120). Work at the site included the use of exclusion zones, decontamination of equipment and samplers, and other health standards common to the environmental industry.

Drilling was accomplished in several bores and at two depths — at approximately 15 and 25 ft beneath the stacked drums. The drilling itself was relatively easy, in a silt formation and a deeper, coarse sand formation. Due to the relative ease of advancement, the initial design to drill from both ends of the landfill was revised, and single bores were advanced to more than 600 ft in length from one end, with more than a dozen samples taken in each boring. As the bores were advanced, the amount of time spent tripping rods increased significantly — at 600 ft it could take nearly four hours to collect a sample. The drilling schedule was optimized with the consultant to assure that the collected samples would reach the laboratory without exceeding hold times.

The primary challenge faced during the project was steering and location. DTD used the most advanced walkover system available from Digital Control Inc. (DCI), the F5, equipped with the DCI high-powered downhole sonde. In the deep bore, DTD used a custom-, $10,000-sonde, built to DTD’s specifications by DCI, to achieve higher signal strength to punch through the passive interference created by the stacks of steel drums. Even with this advanced equipment, steering and navigation required a high level of skill and deductive reasoning to determine the course to steer and to provide location information. The high level of interference resulted in final reference locations for the samples that described a “radius of probability,” generally 8 to 10 ft across, with a high level of confidence for the actual X and Y sample location within that zone. Since the bores were drilled on pitch data to achieve the vertical profile, the actual elevation of the samples was always accurate within a couple of feet. This uncertainty should be contrasted with vertical drilling — in an unguided auger boring of 60 to 70 ft depth, the drill string can easily deviate more than 20 ft from the surface entry, with no way to tell where the deviation occurs unless the bore is surveyed; this is seldom done.

In one boring, the drilling encountered what appeared to be landfill debris at an unexpected depth. Working with the client to revise the profile for this bore, the DTD team was able to pull back several rods and redirect the bore to a new elevation — as a result, samples were taken from two depths at the same X-Y location, in a single directional bore. DTD was able to redirect this particular bore twice, while collecting viable samples in all three forks of the boring.

Another challenge surmounted by the DTD team was the actual process of collecting samples. DTD initially used a sampler obtained from Ditch Witch. This sampler, no longer in production, uses a sliding sampler body, which contains a piston to close off the open end. The sampler is pushed into the soil and retracted to expose the open bore, then pushed again to collect the sample. DTD had Ditch Witch custom-machine a spare sampler of the same design for this project.

In practice, the piston sampler worked as designed in silty soils, but did not operate effectively in the coarse sands encountered. The DTD senior geologist and crew redesigned and modified the sampler in the field to enhance the collection of the deeper samples, with excellent results. The experience gained in this field experimentation was applied to designing and building a new horizontal sampler, which has recently been tested for use on future DTD sampling projects.

In the end, DTD and the client overcame a variety of difficult challenges to complete this sampling project. From equipment selection to a liberal application of field savvy and experience, the project resulted in the collection of nearly three dozen samples, which will enable the consultant to plan additional controls at the site.

Michael Lubrecht, LG, is senior geologist at Directed Technologies Drilling Inc.