Source Area MIP Investigation and Pilot-Scale Groundwater Remediation Using Activated Persulfate
Background/Objectives. AECOM performed a pilot test at a site in Southern California to assess the applicability and effectiveness of in situ chemical oxidation (ISCO) using activated persulfate for treating residual gasoline free product and high concentrations of dissolved-phase petroleum hydrocarbons as gasoline, BTEX, and MTBE. A former leaking UST, which was removed in 1995, was the apparent source of soil and groundwater impacts at the site. Several site investigations were performed in the late 1990s and interim remedial actions were performed at the source area in the early 2000s.
Approach/Procedures. The pilot test was performed in 2010, starting with a membrane interface probe (MIP) investigation to characterize the lithology and total VOCs in the source area in high-resolution. The MIP results were then used to design the ISCO injection grid. Soil and groundwater samples were collected for a bench-scale study to evaluate the total oxidant demand (TOD) of three persulfate activation methods (iron, alkaline, and peroxide). The TOD results were then used to choose the most cost-effective activator and calculate reagent mass and volume.
6,600 lbs of sodium persulfate were mixed with 660 pounds of iron EDTA and water before being injected across the top 10 feet of the saturated zone using direct-push methods.
Five groundwater monitoring events were performed: baseline and 1, 2, 3, and 6 months following the injection, to track changes in geochemical conditions, assess the distribution of injected reagent, and evaluate changes in VOC and metal concentrations.
Results/Conclusions. The MIP results indicated the source in the unsaturated zone is beneath the former UST and sorbed onto tight silty soils. There is also a significant mass in the saturated zone set in more permeable/sandy soils. The bench-scale study indicated high TODs for alkaline and peroxide activation (approximately 20 grams of oxidant per kilogram of saturated soil [g/kg]) and a lower TOD for iron activation (4.8 g/kg).
Persulfate distribution and oxidation were achieved within the injection zone and as far as 20 feet cross- and 35 feet down-gradient of the injection zone. Iron-activated persulfate was effective in reducing concentrations of dissolved-phase VOCs and enhancing free-phase desorption and dissolution. Concentration rebound was observed indicating that repeated injections would be required to achieve a sustained cleanup of the source area saturated zone.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California