Enhanced reductive dechlorination of trichloroethene in an acidic DNAPL impacted aquifer

A field pilot test was conducted using an emulsified vegetable oil (EVO) and colloidal magnesium hydroxide [Mg(OH)₂] formulation to enhance reductive dechlorination of dense non-aqueous phase liquid (DNAPL) trichloroethene (TCE) in an acidic (pH < 4), heterogeneous aquifer. The field test consisted of i) a single well injection test to evaluate Mg(OH)₂ distribution and ii) installation of two EVO-Mg(OH)₂ permeable reactive barriers (PRBs; PRB-1 & PRB-2) at varying distances downgradient of the DNAPL source area. Distribution of Mg(OH)₂ was observed up to 2.3 m away from the injection point within a permeable coarse sand layer; however, Mg(OH)₂ transport in the overlying clayey-silty sand was minimal. Downgradient of the PRBs, colloidal Mg(OH)₂ increased the pH of the coarse sand to levels appropriate for biological reductive dechlorination (pH >∼5); however, some settling of Mg(OH)₂ in the injection wells generated persistent high pH (∼9-10) within the PRBs. A redesigned suspension of colloidal Mg(OH)₂ was tested and proved to be more effective at raising aquifer pH without an excessive rise in pH within the PRBs. At PRB-1 (located closest to the DNAPL source area), limited TCE biodegradation was observed due to the influx of high TCE concentrations (up to 400 mg/L) and inhibition of dechlorinating bacteria. At PRB-2 (located 25 m downgradient of the DNAPL source area), TCE concentrations were much lower (13-26 mg/L) and production of cis-1,2-dichloroethene (cDCE) and some vinyl chloride (VC) was observed. Subsequent bioaugmentation with a commercial dechlorinating culture at PRB-2 improved conversion of cDCE to VC and ethene at downgradient monitoring wells over the duration of the study. These results emphasize the importance of PRB location (relative to the DNAPL source), base selection for pH adjustment, source strength, and local heterogeneities for the design and long-term performance of ERD in acidic DNAPL-impacted aquifers.

Enhanced reductive dechlorination of tetrachloroethene dense nonaqueous phase liquid with EVO and Mg(OH)2

In situ treatment of dense nonaqueous phase liquids (DNAPL) by enhanced reductive dechlorination (ERD) can be limited by contaminant toxicity, low pH, and challenges in effectively delivering electron donor. Flushing emulsified vegetable oil (EVO), colloidal Mg(OH)2 buffer, and a bioaugmentation culture (BC) through a zone containing neat tetrachloroethene (PCE) was effective in reducing contaminant toxicity, limiting pH declines, and accelerating bioenhanced dissolution of the DNAPL. In the effluent of porous media columns with little fine material, PCE concentrations reached a maximum of 40-50 times PCE aqueous solubility in water, demonstrating NAPL PCE was distributed throughout the 1.5 m column length. In a column treated with only EVO+BC, reductive dechlorination was limited. However, a single injection of EVO+Mg(OH)2+BC was effective in reducing PCE to below detection for over 400 days with a large increase in Cl(-) and dichloroethene (DCE), accelerating bioenhanced DNAPL dissolution. Dechlorination rates gradually increased over time with the rate of total ethene (TE) release from the Mg(OH)2+EVO+BC column reaching 5-6 times the TE release rate from the EVO+BC column. The accelerated dechlorination was likely due to both Mg(OH)2 addition which limited pH declines from HCl, volatile fatty acids (VFAs), and inorganic carbon (IC) production, and formation of a mixed PCE-vegetable oil NAPL which provided a readily accessible electron donor, resulting in rapid PCE degradation with reduced PCE toxicity.

Chlorinated ethene source remediation: lessons learned

Chlorinated solvents such as trichloroethene (TCE) and tetrachloroethene (PCE) are widespread groundwater contaminants often released as dense nonaqueous phase liquids (DNAPLs). These contaminants are difficult to remediate, particularly their source zones. This review summarizes the progress made in improving DNAPL source zone remediation over the past decade, and is structured to highlight the important practical lessons learned for improving DNAPL source zone remediation. Experience has shown that complete restoration is rare, and alternative metrics such as mass discharge are often useful for assessing the performance of partial restoration efforts. Experience also has shown that different technologies are needed for different times and locations, and that deliberately combining technologies may improve overall remedy performance. Several injection-based technologies are capable of removing a large fraction of the total contaminant mass, and reducing groundwater concentrations and mass discharge by 1 to 2 orders of magnitude. Thermal treatment can remove even more mass, but even these technologies generally leave some contamination in place. Research on better delivery techniques and characterization technologies will likely improve treatment, but managers should anticipate that source treatment will leave some contamination in place that will require future management.

Numerical modeling of emulsified oil distribution in heterogeneous aquifers

In situ anaerobic bioremediation using edible oil emulsions will be most effective if the oil droplets can be brought into close contact with the contaminant to be treated. However, uniformly distributing oil in heterogeneous aquifers can be difficult. The impact of injection conditions on emulsion distribution in a three-dimensional heterogeneous aquifer is examined using MODFLOW and RT3D. Emulsion retention is simulated using a rate-limited Langmuir isotherm. Volume and flow contact efficiency are shown to be functions of mass of oil injected, injection fluid volume, well spacing, and injection sequence. Regression equations are developed relating dimensionless scaling factors to expected contact efficiency for area treatment and barriers. Cleanup time for uncontacted zones is estimated using a mobile-immobile zone modeling approach.