Theme: 01. Remediation Technology Innovations

Topic Area: 01h. Horizontal Wells: Applications and Lessons Learned in Site Characterization and Remediation

Formatted Title:

Evaluation and Implementation of Horizontal Soil Vapor Extraction Wells for Remediation of Large-Scale Vadose Zone Contamination at a Redevelopment Site

Background/Objectives:

Both LNAPL and DNAPL had been detected at high concentrations in the vadose zone at a former industrial site. The site was scheduled for redevelopment consisting of the construction of a large institutional building. A large area within the site boundary was impacted with high vapor concentrations of VOCs in the soil at various depths, exceeding regulatory thresholds for commercial and residential levels. Contaminants of concern at the site included benzene, ethylbenzene, tetrachloroethene (PCE), and trichloroethene (TCE). Maximum soil gas concentrations of PCE of up to 210,000 µg/m³ had been identified, which exceeded the applicable screening levels of 460 µg/m³. Not only was the impacted area over 10 acres in size, but the impacts to the soil vapor had been found to extend to depths as great as 70 feet bgs. Prior to initiation of redevelopment activities, the consultant overseeing the project needed to develop a remediation plan to address the soil vapor exceedances. The remediation system would need to be designed to accommodate the proposed new building, which would occupy the majority of the site. After evaluating a variety of options, horizontal wells were selected as the best way to implement an SVE system to remediate the VOCs in the vadose zone.

The objectives of the presentation are as follows: review design constraints due to the planned redevelopment efforts and explain why horizontal wells were selected as the ideal way to implement the remediation, discuss why SVE was selected as the remedy, explore how horizontal remediation well design principles were used to optimize removal of soil vapors, identify the various design challenges due to site conditions, and go over the process of implementing the large scale horizontal remediation system.

Approach/Activities:

The basis for design of the horizontal SVE wells was to maximize removal of soil vapors in the shallow and deeper portions of the vadose zone without adversely impacting redevelopment plans for the site.

Twelve horizontal SVE wells with screen lengths between 100 feet to 420 feet were installed with horizontal directional drilling methods at target depths ranging from 15 feet bgs to 50 feet bgs. Over 5,800 linear feet of wells were installed, with the longest wells being over 700 feet. The wells were installed using a combination of entry-exit and single-entry configurations to accommodate site limitations. The horizontal well screens were engineered using proprietary software to produce a site-specific custom-engineered design, providing the desired air flow rates and air distribution. All twelve wells were installed from a common entry point to allow for consolidation of wellheads and simplification of O&M activities. Despite the depth of the deeper SVE wells, tracking of the bore path was able to be accomplished using state-of-the-art walkover locating equipment, without the need to use magnetic or gyroscopic instruments.

Results/Lessons Learned:

The 12 wells were successfully installed, albeit with various minor field adjustments due to ground conditions and previously unidentified subsurface utilities. While various boring logs were available during the design phase to document ground conditions, few boring log records were available for depths deeper than 20 feet bgs. Unexpected bedrock was encountered in various areas along the bore path for the four deeper wells. Due to the rocky conditions, steering the drill bit became a significant challenge. A different drill rig with dual-rod capabilities ultimately needed to be mobilized to the project in order to complete the bores according to the final design. The initial scope of work for the project called for a magnetic locating system due to the depths of the deeper wells and the presence of a thick concrete slab densely reinforced with rebar that remained from the previous site building which extended over most of the site. As a major cost-saving measure, the consultant and driller prepared an alternative plan to use the latest advancements in walkover locating technology to attempt to provide the necessary means for accurate locating. Findings from the field indicated that the walkover equipment was able to provide accuracy to within an acceptable margin of error, precluding the need for magnetic locating and resulting in significant cost savings as well as an acceleration of the project schedule.

Meg Sequino

Affiliation

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(Group 2, Poster Board #230) Evaluation and Implementation of Horizontal Soil Vapor Extraction Wells for Remediation of Large-Scale Vadose Zone Contamination at a Redevelopment Site