Theme: 01. Remediation Technology Innovations

Topic Area: 01j. Injectable Activated Carbon Amendments: Lessons Learned and Best Practices

Formatted Title:

Optimization of In Situ Remediation Using Activated Carbon after a Failed Remedy

Background/Objectives:

At a former UST tank site in central New Jersey (NJ), petroleum impact remained in groundwater after a 300-gallon UST was removed and impacted soil was remediated. Benzene and 1,2,4-trimethylbenzene (TMB) exceeded NJ Groundwater Standards (NJGWS) and LNAPL was observed at the site. A combination of in situ chemical oxidation (ISCO) using RegenOx® and in situ enhanced bioremediation (ISEB) using ORC-A® was conducted in November 2020. The post-injection monitoring indicated that a statistically significant decreasing trend can not be extrapolated for all wells. Additional remediation was required to address the remaining concentrations within the regulatory timeframe. Therefore, a second in situ remediation was conducted in October 2022 and the results to date indicate that contaminant concentration reductions were achieved in both source (upgradient) and downgradient areas.

Approach/Activities:

Before the site remediation in November 2020, a MIP-HPT investigation was conducted to reveal that a low permeability silty layer with various thicknesses and permeability separates the upper and lower more permeable sand layers. The upgradient area has a thicker silty layer with lower permeability than that of the downgradient area. A review of previous remediation appears to find that the 2020 remediation did not fully consider the subsurface geology characteristics and challenges to remediation. RegenOx® and ORC-A® were injected as a slurry at the upgradient and the downgradient areas, respectively. However, the reagent slurries can not be fully delivered into the subsurface as indicated by significant daylighting, especially in the upgradient area.

Before implementing the second remedial action in 2022, the subsurface geological and hydrological conditions were evaluated to inform the selection of reagents and the design of remedial action. Petrofix®, a colloidal activated carbon product, was injected along with nutrient and buffer reagents. This product was selected based on the following:

The colloidal carbon was suitable for the varying permeability identified in the site.
The average groundwater velocity is slow enough so that the colloidal carbon would not migrate quickly out of the treatment area.
The low pH in the aquifer (near 4.5) likely prevents effective biodegradation so diammonium phosphate (DAP, as buffer and nutrient) and sodium bicarbonate (as buffer) were included in the reagent mixture.

Results/Lessons Learned:

The contaminant concentration reductions have been observed site-wide. The key lessons learned from this implementation are as below:

Colloidal carbon migrated significantly faster in the permeable zone than in the silty layer. The radius of influence during injection and post-injection was larger than expected thereby expanding the effective remediation area that was originally anticipated.
The duration to settle the suspended colloidal carbon in the upgradient area was significantly longer than that of the downgradient area. The difference could be caused by the higher dosage of colloidal carbon at the upgradient area.
Although high dosages of buffer and nutrients were injected along with the carbon, the optimal pH near 7 could not be maintained for 5 months. It is proven to be challenging to maintain an optimal pH range in this region, which is known for its acidic aquifer.
Due to the presence of the residual product at the source area, a higher dosage of colloidal carbon or a combination of powder and colloidal carbon could further improve the remediation outcomes.

Thomas Geiger

Affiliation

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(Group 2, Poster Board #209) Optimization of In Situ Remediation Using Activated Carbon after a Failed Remedy