Formatted Title

GWQS Achieved in Fractured Bedrock at a TCE Release Site in the Passaic Formation

Background/Objectives

Trap & Treat® BOS 100® was injected in fractured, shale bedrock at a corporate site in New Jersey in 2017. An unknown TCE discharge was discovered in groundwater testing in 1992, and over the course of 20 years several remedial actions were completed including soil removal. In 2016, a Remedial Investigation Report (RIR) was completed and the Trap & Treat® Approach was recommended to update the existing conceptual site model (CSM) before applying a focused BOS 100® injection program in 2018 with post-injection monitoring conducted quarterly. The Classification Exception Area /Well Restriction Area (CEA/WRA) was lifted in 2020 and the site was granted a No Further Action (NFA) status, after achieving New Jersey Ground Water Quality Standards (GWQS) only 25 months post-injection.

Approach/Activities

The site was in a grassy area abutted by woods at a corporate campus. Previously, three other areas of concern (AOCs) onsite were managed and closed using excavation and removal of overburden soils; afterwards, chlorinated solvent impacts were not above limits in overburden soils. This injection effort focused on treating groundwater in bedrock. The Area of Interest encompassed ~3,500 ft²and was situated in the Passaic Formation, which consists of reddish-brown argillaceous shale with localized sandstone/siltstone interbedding. Depth to water occurs at ~10 ft below ground surface (ft bgs), and the impacted groundwater interval was from ~10 to 60 ft bgs. The contaminants of concern (COCs) were TCE and 1,1-DCE, and the project objective for both constituents in groundwater was 1 µg/L.

A key to bedrock remediation is to not focus solely on highly transmissive zones. The smaller aperture fracture networks and overlying weathered bedrock habitually contain more residual contaminant mass than more transmissive features. Being able to access, isolate, and treat these zones is key to success at difficult fractured rock sites, and many sites require a combination of methods due to dissimilar properties between the less consolidated and consolidated units.   

A Transition Zone (TZ) of partially weathered rock was present from 10 to 25 ft bgs that prevented the use of standard direct push equipment (DPT) to reach and isolate this interval for assessment and treatment. TZ remediation consisted of utilizing RPI’s GeoTAP™ Method; this methodology was developed to gain access to challenging geologies and first involves pre-drilling to the desired depth using sonic, air rotary, or hollow-stem augers (HSA). The evacuated borehole is then backfilled with hydrated bentonite chips or pellets to seal the bore wall, and a DPT rig can then push through the bentonite column to reach the desired injection depth intervals without compromising the bore seal. Remediation slurries or fluids are then injected through the bentonite. This technique has been used successfully on more than 30 project sites across the country, accessing depths as great as 180 ft bgs.

Bedrock characterization and injection points (BC/IPs) were installed with air rotary/HSA in the competent lithology. Casing was installed from ground surface to 15 ft bgs and the vertical interval of interest was left uncased so borehole geophysical tooling and discrete interval (18 in.) sampling with a straddle packer could be conducted. Finally, the bedrock zone remediation implementation used a custom straddle packer (18 in. interval) to deliver BOS 100® slurries pumped from a unique bedrock injection unit with accessible flow rates ranging from 50 to 250 gallons per minute and pressure up to 3,000 psi.

Results/Lessons Learned

The focus of this presentation will be to showcase recent improvements to techniques and approaches to access contaminant impacts in transition zones/saprolite/epikarst and consolidated lithologies for characterization in situ remediation. This site-specific case study will illustrate the development of quantified, high-density CSMs using high-density, discrete groundwater sampling for quantitative lab analysis, which provides for surgical and aggressive in situ remediation techniques that install requisite in situ treatment product precisely where it is needed.