The overall objective of this project was to evaluate if inexpensive flow reduction agents delivered via permeation grouting technology could help manage difficult-to-treat chlorinated solvent source zones. This approach aimed to provide two benefits for improving groundwater quality at chlorinated volatile organic carbon (CVOC) sites by:
- physically reducing the mass flux of contaminants leaving the source zone by using permeation grouting (Figure ES-1), thereby reducing risk and making the downgradient plume more amenable for management by natural attenuation processes; and
- increasing the Natural Source Zone Depletion (NSZD) rate within the source by diverting competing electron acceptors (e.g., dissolved oxygen, nitrate, and sulfate) around the source zone to create an enhanced reductive dechlorination zone (ERDZ) (Figure ES-2).
In addition to the objectives highlighted above, the demonstration included the following tasks:
- : Several novel silica gel/vegetable oil-formulations were developed and tested in lab-scale batch and column studies by project team member Solutions-IES. In a parallel effort, the technical literature regarding properties and field injection protocols of conventional silica gel was reviewed and supplemented with confirmation lab tests at GSI to select the most cost-effective silica gel material and the specific silica gel hardening reagent necessary for subsurface gelling. The results of this evaluation were used to select one type of silica gel and a vegetable-oil formulation for the Small-Scale field demonstration (Task 2).
- Test cells were constructed in an unimpacted zone at the demonstration site. Two cells were constructed with the selected silica gel solution and two cells were constructed with the vegetable-oil formulation developed by Solutions IES. The main goal of the Small-Scale demonstration was show positive performance of a small barrier test cell, and to demonstrate how commonly used remediation equipment (direct push rigs, injection skids) can be adapted to make permeation grouting barriers.
- The results of Task 2 (Small-Scale Field Demonstration) were designed to make a go / no-go decision for a larger-scale technology demonstration. Key performance metrics involved the measurement of the change in mass flux, hydraulic gradient and geochemical parameters. Because the design work on Task 3 was conducted partly in parallel to the other Tasks, a site had been selected, a conceptual design completed, and some detailed design work was performed. However, the results of the Small-Scale Field Demonstration did not reach the pre-established performance goals and therefore the Large-Scale Field Demonstration was not performed.
The project demonstration had these results:
- Two grout mixtures were selected based on gel tests and a treatability study by Solutions-IES:
- A Silica Gel Grout: 10 vol-% of sodium silicate (NaSi), 5 vol-% of dibasic ester (DBE) hardener, and 85 vol-% of water. This formulation had a gel time of approximately 4 hours and had an estimated viscosity of 3-4 centipoise (cP).
- Solutions-IES Novel Silica Gel/Veg-Oil Grout: 5 percentage by weight (wt-%) of emulsified vegetable oil (EVO), 10 wt-% of NaSi, 1.8 wt-% of DBE, and 83 wt-% of water. This formulation provided a 3-4 orders of magnitude reduction in lab permeability tests, and a gel time of 18 hours.
- A description of a Small-Scale Demonstration that achieved an average 64% reduction in flow through three small barriers. This was lower than the performance objective of a 90% reduction in flow and was likely caused by the low permeability of the silty sands in the test area.
- A Large-Scale Demonstration was not performed due to the low permeability of the planned test area. However, based on standard geotechnical practice, 90% groundwater flow reduction with silica gel permeation grouting is likely achievable at sites with the main transmissive units having hydraulic conductivity closer to the optimal range (from 5x10-4 to 10-2 centimeter per second [cm/sec]).
- Performance of 90% groundwater flow reduction with silica gel grouting is likely achievable at sites with the main transmissive units having hydraulic conductivity closer to the optimal range (from 5x10-4 to 10-2 cm/sec).
- Applications of one acre in area or more are significantly less costly than conventional in-situ remediation technologies ($996K per acre and $21 per cubic yard for a one acre site).
- This Environmental Security Technology Certification Program (ESTCP) demonstration was able to use existing remediation technology (direct push rigs and injection skids) to build four small barriers for the Small-Scale Demonstration.
- The mixing process is generally more complex than standard injection-based remediation projects because the injection skid needs to mix three fluids, delivery multiple locations simultaneously, let operators see pressure, flowrate, and have contingency for grout set-up in the injection manifolds. The design described in the Final Technical Report worked well.
- The hydraulic conductivities were relatively low (0.63 feet [ft] per day [2x10-4 cm/sec]) resulting in low pumping rates (< 0.1 gallons per minute [gpm]) and low volumes of extracted groundwater during the before- and after-tests (< 20 gallons [gal]);
- Potential construction problems associated with the multi-level injection wells in a very fine-grained heterogeneous unit as one injection well had to be abandoned.
- The “donut” configuration (Section 5.1.2) may have not been efficient at testing the permeation grouting process; a larger demonstration area may have resulted to better test data. However, using constant head injection tests, an average of 64% reduction in flow resulted, which is significant but below the 90% reduction performance goal. This result, and relatively low hydraulic conductivities in the planned Northern Plume test area, led to the decision not to perform the Large-Scale Demonstration.
- Applications for the flux reduction technology are likely to have better performance at sites with higher permeability and higher groundwater velocity than at the site used for the demonstration, both for demonstrating the hydraulic effect of the barrier and the benefits from electron acceptor diversion.