This project demonstrated and validated polymer-amended in situ chemical oxidation (PA-ISCO), the use of water-soluble polymers to enhance ISCO using permanganate. The two main goals of the project were to: (1) reduce the effects of site heterogeneities on oxidant delivery; and (2) manage the deposition of MnO2 (permanganate oxidation byproduct) to improve the delivery of oxidant. Experiments were carried out using two polymers. Xanthan gum was used to improve sweep efficiency of the oxidant through lower permeability areas. Sodium hexametaphosphate (SHMP) was used to reduce the instance of MnO2 precipitation and build-up. Through previous laboratory development funded by the Strategic Environmental Research and Development Program (SERDP), the effectiveness of both polymers was tested, and both were deemed viable candidates for field study.
The demonstration was conducted at Marine Corps Base Camp Lejeune’s Site 88, at which chlorinated solvents (trichloroethene [TCE] and tetrachloroethene [PCE]) are found in the subsurface. The test site consisted of two injection wells. One injection well received only permanganate (control plot); the other injection well received a mixture of oxidant and polymers (test plot). Multi-level wells were used to monitor groundwater prior to treatment, during treatment, and post-treatment. Real-time groundwater quality measurements included specific conductivity, oxidation-reduction potential (ORP), permanganate, and viscosity. Groundwater samples were collected from the multi-level monitoring wells for laboratory measurements of pH, ORP, solids concentration, cations, anions, cation exchange capacity (CEC), and anion exchange capacity (AEC) toward understanding how polymer addition affects groundwater quality. Twelve to fifteen soil cores per injection well were extracted at various distances from the injection site immediately following treatment. One year post-treatment, groundwater and soil cores were collected to investigate long-term effects of treatment on soil and groundwater chemistry and the fate of the polymer.
Comparisons between test and control plots were used to assess treatment performance. Key indicators of performance for the polymer included: (1) movement of permanganate into lower permeability layers or strata; (2) preferential flow around areas of high contaminant mass due to MnO2 deposition; (3) potential for rebound; (4) treatment effectiveness; (5) injection pressure; and (6) groundwater quality.
The addition of polymer improved oxidant movement into lower permeability strata, reduced preferential flow, and greatly improved the overall sweep efficiency of the PA-ISCO fluid compared to the permanganate control plot where polymer was not used. As a result, a greater percentage of the aquifer was contacted by the oxidant. Specifically, one pore volume of solution was introduced to each plot; the use of polymer resulted in a test plot sweep efficiency of 67%, double that of the control plot (sweep efficiency = 33%). Because a greater portion of the aquifer was swept, overall treatment was more effective and there was lower rebound. It is important to note that even with polymer, the oxidant solution did not contact 100% of the contaminated media in one pore volume. Rebound could still be expected if an insufficient volume of oxidant is applied. The volume appropriate for treatment should be determined on a site-by-site basis. However, based on this demonstration, the use of polymer can reduce the necessary volume of solution by more than an order of magnitude. Importantly, the polymer-amended solution did not negatively affect injection pressure during this demonstration.
Groundwater quality data collected included parameters associated with the soil that can affect groundwater concentrations of contaminant and key cations and anions. Most geochemical outcomes using polymer were similar to those without polymer; however, changes due to ISCO were slower to appear downgradient of the polymer-treated plot. Two possible reasons for this observation include retention within, and groundwater flow around, the more viscous polymer solution. Polymer was still present at the site one year post-treatment. Geochemical analyses indicated the polymer was “retreating” from its leading edge and releasing the geochemical signature of the amended oxidant solution rather than being flushed through the treated zone by upgradient solution as was measured in the control plot. This has important implications with respect to appropriate monitoring of treatment performance. Byproducts of permanganate oxidation (e.g., MnO2 solids) and short-term mobilization of metals (e.g., chromium) observed at some ISCO sites may be sequestered within the viscous PA-ISCO solution during treatment and held within the treated zone post-treatment until viscosity dissipates. Long-term post-treatment viscosity could also be leveraged in future PA-ISCO applications to sequester permanganate within the treatment zone providing longer-term treatment to address contaminant rebound from low permeability media (e.g., fine silts or clays) that are not swept during the initial PA-ISCO treatment.
Lessons learned beyond those tied to specific performance objectives included:
- Results of the control plot exemplified the surprising extent to which preferential flow occurs in the subsurface, even within lithology that appears the same or similar.
- Site characterization and monitoring efforts must be designed based on site-specific factors. In order to prepare an appropriate treatment system design and capture treatment outcomes effectively, operational and performance monitoring strategies must match the operational goals and the physico-chemical characteristics of the amendments applied.
- Soil core data were more valuable than groundwater data with respect to making accurate pre- and post-treatment subsurface evaluations of treatment performance for PA-ISCO.
- Simple geochemical field measurements such as conductivity and ORP provided valuable, real-time information with respect to operational performance. High data density using real-time monitoring approaches added significant value with respect to making real-time, cost-saving decisions in the field.
- Automated amendment delivery schemes, while costly, will add value to field operations by allowing for round-the-clock, unmanned injection. This will be particularly valuable at sites where unescorted activity is not possible.
Key differences in the cost of PA-ISCO relative to traditional ISCO include the cost of the polymers, the on-site polymer mixing equipment, and the associated shipping of these items to the field demonstration site. Further, an automated process control system was employed to allow 24-hr injection, which was important for maintaining the viscosity of the injection solution at design levels during injection. Key drivers of total project cost included the nature and extent of contamination, degree of heterogeneity, and rate of injection achievable (average hydraulic conductivity).
Technical and/or implementation issues associated with PA-ISCO included:
- Uncertainty regarding the influence of polymer on hydraulic conductivity over the long-term within the treated area.
- Uncertainty regarding the degree of heterogeneity the polymer can overcome.
- Concern over the potential for daylighting evoked by the use of a higher viscosity solution.
- Improved sweep efficiency could be achieved by increasing injection volume for any delivered amendment, particularly for more reactive (short-lived) amendments. Polymer can significantly reduce required injection volumes; however, low volume delivery is not appropriate for most remediation approaches that involve the delivery of aqueous phase amendments, including ISCO. This translates to longer duration delivery and increased field time per injection event.
- The dry powdered form of xanthan gum and SHMP used in this demonstration required expensive (~20% of total equipment cost) hydrodynamic mixing equipment. Procuring polymer in liquid form that could be trucked to the site in tanker trailers may be more cost-effective, particularly for larger-scale operations.
The benefits of PA-ISCO over traditional ISCO include: (1) greater movement of contaminant into areas of lower hydraulic conductivity – less preferential flow and contaminant bypass; (2) improved sweep efficiency; (3) lower injection volume requirement; (4) lower field time requirement; (5) greater applicability of ISCO to moderately heterogeneous sites; and (6) lower probability of contaminant rebound.