HDD Used to Install Horizontal Environmental Wells in Florida

CA activities were conducted continuously through 1997. In 1998, an air stripper remedial system initiated groundwater remediation using a combination of eight groundwater recovery wells. The operation of the air stripper system continued through 2011 with minimal contaminant plume and concentration level reduction. Additional soil and groundwater sampling confirmed that the soil beneath the facility building was a potential source of VOH contamination. A Limited Scope Remedial Action Plan (LSRAP) for corrective action of source soil was generated and submitted to the state agency for approval.

Technical Approach

After reviewing all the options, the proposal recommended the installation of horizontal wells to be installed under the building, using the continuous borehole technique. The horizontal directional drilling (HDD) method was chosen because drilling under the building would not affect the business, which operated on a 24-hour basis. A dual-phase extraction system consisting of three horizontal Soil Vapor Extraction (SVE) wells at approximate depths of 7 ft below land surface, with lengths of approximately 300 ft were to be installed. A deeper horizontal groundwater recovery well was also installed for the purpose of dewatering the treatment area to lower the static water level in the area of the SVE screens, resulting in increased contaminated soil exposed to the soil vapor extraction wells.

Trenchless Specialties, a Florida-based horizontal well contractor was chosen by Bock Environmental Services to assist in the design and complete the installation of the wells. A Central Mine & Equipment (CME) 50-DD was mobilized to the site to complete the installation of the wells. Due to a large magnetic field that was being generated by the equipment inside the building, a Digital Control (DigiTrak) Mark IV locator and cable sonde was used. The high-powered sonde provided enough signal strength to overcome the interference and allow the pilot bore to be tracked through the maze of equipment and obstructions located inside the building.

Upon completion of the pilot bore, a 5-in. reamer was used to ream the hole and install the well screen and riser. The custom-slotted, 3-in. diameter SVE wells and a 4-in. diameter dewatering well were installed. All wells were installed using a biodegradable drilling fluid. Using the biodegradable fluid helped ensure proper well performance after development. Biodegradable fluids break down easier during well development and do not leave a “wall cake” like bentonite-based fluids. A 15-ft grout seal was placed on each of the well risers to ensure that short circuiting of flow would not be encountered during system operation. Surface completion included the installation of 12-in. vaults on each end to allow for access to each end of the well riser.

Well installations were successfully completed March 2011. The treatment system installation was completed in August 2011 and began operation on Sept. 2, 2011. The SVE system currently operates all four horizontal wells using a total air flow of approximately 130 Scfm with an inlet pressure of 16 Hg and a pump vacuum of 22 Hg.

Results

System influent and effluent samples have been collected quarterly since system startup and have resulted in significant VOH reductions in both soil vapor and groundwater concentrations. In 10 months of operation, the dual-phase extraction system, employing the three horizontal SVE wells and one horizontal groundwater recovery well, has reduced TVOH vapor concentrations from 3,665 mg/m3 to 206.14 mg/m3 (94.4 percent) and TVOH groundwater concentrations from 1,705 mg/L to 47.6 mg/L (97.2 percent). A significant reduction in the contaminant plume extent and overall concentration levels has been observed at the site as a result of the system’s performance. The attached tables list production and sample analytical data collected since system start-up.

Conclusions

The use of properly designed and installed horizontal wells have provided the desired results with minimal disruption to the business and surrounding area.

The Dual Phase Extraction remedial system has been activated and put on-line. All samples collected during system operation indicate that the system is performing according to system specifications and reducing onsite contamination within the soil and groundwater media.

Randy Bratcher is Florida district manager at Trenchless Specialties, based in Orlando, Fla.