Theme: 01. Remediation Technology Innovations

Topic Area: 01i. In Situ Chemical Oxidation: Optimized Design Approaches and Lessons Learned

Formatted Title:

Using “Stacked Fractures” to Increase Oxidant Loading to Subsurface for VOC and 1,4-Dioxane Treatment

Background/Objectives:

Although ISCO can be effective for in situ treatment of many contaminants, its success is often limited because oxidants are 1) short-lived and 2) only oxidize contaminants that are in direct contact with the oxidant. Standard liquid oxidants combined with injections into relatively high permeability portions of the aquifer often lead to limited effectiveness, especially when contaminants are diffused into lower permeability materials.

This presentation will focus on a pilot study involving a state-led Superfund site in Colorado is impacted with tetrachloroethene (PCE) and 1,4-dioxane (1,4-D) in groundwater with concentrations up to 3,500 micrograms per liter (µg/L) and 140 µg/L, respectively. Previously, nine in situ chemical oxidation (ISCO) events were performed with modified Fenton’s reagant to treat PCE and 1,4-D but concentrations continued to rebound limiting long-term effectiveness. A subsequent pilot study was performed to evaluate the effectiveness of a slow-release oxidant using more aggressive amendment implementation measures to overcome the issues causing rebound. This study is intended to evaluate if more sustainable treatment can be applied by delivering oxidants to the low permeability units via fracturing and allowing treatment to continue for extended periods (years) using slow-release oxidants. However, applying the proper loading of oxidant required for effective PCE and 1,4-D destruction was challenging due to loading limitations with the injection slurry.

Approach/Activities:

A pilot study was implemented using five fracture boring locations, with fractures implemented every four feet between 38-52 feet bgs with a 20-foot target fracture delivery radius. Potassium persulfate activated with hydrated lime was selected as the slow-release ISCO reagent. A pre-design bench study was performed to evaluate both the natural oxidant demand and pH buffer capacity of the aquifer. Based on the bench study, the required oxidant loading was slightly higher than the loading that could be blended for fracturing. In order to increase the mass of oxidant (and buffer) added to the treatment area, some fracture intervals were re-used to inject additional amendment. Previous studies by FRx suggest that this second injection will be distributed immediately above or below the original fracture and will not push the original fracture further, thus doubling the fracture thickness and loading. These “stacked fractures” were critical to achieving target delivery radius without performing additional fractures. All fractures were successfully applied, including all “stacked fractures”. Following fracturing, soil sampling and high-resolution OIHPT screening.

Results/Lessons Learned:

A discussion of results will include the vertical and lateral distribution of oxidant using a combination of the DPT soil sampling and OIHPT results. Groundwater monitoring data and MiHPT data (still being collected) will also be used to discuss the long-term effectiveness of the ISCO pilot study, both within and downgradient of the treatment area. Lessons learned will include overall effectiveness of PCE and 1,4-dioxane using slow-release persulfate and what adjustments can be made to increase both short-term and long-term treatment. Immediate monitoring results indicated fractures extended at least 16 feet in all vertical intervals. Groundwater monitoring results to date have indicated that the perfulfate remains active and continues to treat PCE (after 3 months) but 1,4-dioxane results have been mixed. Based on groundwater velocity estimates, impacts in downgradient wells are not expected after 3 months.

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Shaun Cwick