Partitioning electron donors (PEDs) are electron donors that partition directly into a target dense nonaqueous phase liquid (DNAPL). PEDs are water soluble, hence are easily transported to a DNAPL source zone. This property aids in their mixing throughout the source zone and maximizes contact with the DNAPL. Even at high dose rates, PEDs are slowly metabolized, which facilitates delivery without significant loss and allows efficient distribution throughout the source zone. The objectives of this field demonstration/validation (DEM/VAL) included:
- Demonstrate application of the PED technology at field scale, assess the ability to distribute PED within the source area, and enhance biodegradation.
- Validate the enhanced performance and efficiency of DNAPL dissolution and dechlorination following the injection of a PED.
- Collect cost and performance data for the application of PEDs for source zone bioremediation.
Based on treatability studies to evaluate candidate PEDs (Caprio et al., 2011), n-butyl acetate was selected for the DEM/VAL. The technology field demonstration was conducted at a source zone (Hot Spot 1) at the National Aeronautics and Space Administration (NASA) Launch Complex 34 (LC34). At this site, trichloroethene (TCE) DNAPL was associated with a silty sand/silty clay horizon at about 42 to 48 feet below land surface (ft BLS) and TCE concentrations up to 141,000 µg/L had been reported.
Two sweep zones, one above and one below the clay horizon, were separately instrumented and operated, providing two data sets with which to evaluate the performance of the PED technology. Each sweep zone was instrumented with a single central extraction well, from which integrated groundwater samples were collected routinely to monitor the average concentration of various dissolved constituents over time. Extracted groundwater was returned to the aquifer through a set of ten groundwater injection wells on the perimeter of the TCE plume. At each of five injection locations, a pair of injection wells was installed above and below the clay horizon to help create an inward hydraulic gradient and promote horizontal flow across the top and base of the clay horizon.
Routine groundwater samples were collected during recirculation to assess the concentrations and flux of various compounds. Comparison of concentrations (volatile organic compounds [VOCs], PED, tracers) in groundwater initially and over time extracted from the central wells was used to assess the “disturbance effect” of direct injection and evaluate the quantity of n-butyl acetate that was taken up by the DNAPL. Soil sampling was conducted before (baseline delineation) and after the demonstration area was amended, to establish mass distribution within the plots, and again after operation was halted, to assess changes over the DEM/VAL operation and correlate these results with the observed trends in groundwater concentrations.
Cápiro, N.L., E.K. Granbery, C.A. Lebrón, D.W. Major, M.L. McMaster, M.J. Pound, F.E. Löffler, and K.D. Pennell. 2011. Liquid-Liquid Mass Transfer of Partitioning Electron Donors in Chlorinated Solvent Source Zones. Environ. Sci. Technol., 2011, 45 (4), pp 1547–1554.
The PED was able to promote biodegradation and achieved sustained production of dechlorination products. Donor longevity was assessed and donor was present up to 1 year following PED injection. Tests confirmed that the PED was capable of partitioning into a TCE DNAPL.
The use of PEDs for source zone bioremediation is expected to be cost-equivalent to emulsified vegetable oil applications. Donor longevity of n-butyl acetate was at least equivalent to emulsified vegetable oil applications for similar applications. Time-trend data for the electron donor concentrations also was monitored and compared. Results using n-butyl acetate were compared to those from tests that used soluble donors, such as lactate; the n-butyl acetate provided a longer period of activity, since it partitioned into residual NAPL initially and then gradually became re-supplied to groundwater, whereas any unused soluble donor would have migrated away from the NAPL source area.
As with other source application-based technologies, understanding and identifying the extent of the source zone (i.e., site characterization) to estimate the mass of DNAPL present should be completed. Such an effort would require capital cost expenditures, but should reduce application costs. Another limitation of using the PED technology would include the prerequisite of suitable geochemical conditions to promote biodegradation through reductive dechlorination.
Delivering the PED into the source area is critical. This project showed that the selected PED, n-butyl acetate, can:
- achieve high rates of biologically-enhanced DNAPL dissolution.
- be easily and effectively delivered.
- sustain donor supply at an effective concentration at the DNAPL:water interface.
The PED was water soluble, easily transported to a DNAPL source zone, and less expensive to deliver than other commercial products.