Formatted Title
Combining Nature-Based and In Situ Technologies to Develop a Full-Scale Remedy to Address a Dissolved Arsenic Plume at an Industrial Facility
Background/Objectives
To address arsenic impacts (up to 9,100 µg/L) in shallow and intermediate groundwater zones, in situ remedial approaches for source and margin control were evaluated at an active industrial facility in California. A comprehensive treatability study was conducted followed by two pilot studies to assess remedial effectiveness of in situ chemical oxidation (ISCO) and air sparge technologies. In addition, a tidal influence study and geochemical assessment was conducted to evaluate natural tide-cycle influenced oxygenation at the site margin.
Arsenic mobility in groundwater is a function of three factors: pH, Eh (ORP), and adsorption sites (primarily ferric iron). Generally, arsenic is least soluble under oxidizing conditions, at a neutral pH, and in the presence of ferric iron (iron [III]). Under oxidizing conditions, arsenic is expected to precipitate out and form stable and generally immobile compounds such as ferric arsenate (FeAsO4).
The treatability study included seven treatment schemes including iron oxyhydroxide (FeOOH), GeoBind, FerroBlack, Persulfate/FeOOH, ZVI/gypsum, EHC-M, and green tea extract. Of those, persulfate/FeOOH was found to be the most effect approach for arsenic treatment. The initial pilot test consisted of injection of sodium persulfate/FeOOH activator into permanent injection wells in both groundwater zones. The ISCO pilot study indicated that oxidation of arsenic and co-precipitation is a potentially viable remedy; however, the reagent delivery, distribution and longevity limitations determined that ISCO was a less cost-effective approach at the site.
Based on the results of the geochemical assessment at the site, which indicated increased oxygen is addressing mobility of arsenic, air sparging, was pilot tested to evaluate subsurface oxygen distribution at the site.
Approach/Activities
The air sparge pilot study consisted of installing six air sparge wells, three each in shallow zone (0 to 35 feet below ground surface (bgs)) and intermediate zone (35 to 75 feet bgs), and portable test equipment. The pilot study was conducted over 8 weeks, followed by an 8-week rebound monitoring period. Following initial testing for two weeks to collect baseline information on radius of influence (ROI), a pulsed operation was implemented over a six week period.
Results/Lessons Learned
Distribution of sparged air within each treatment zone showed effective coverage to the extent of the pilot study area in all directions. This was evidenced by helium tracer gas detections, water level response, visual observation, geochemical response, and dissolved arsenic concentration reductions (up to 99%) at the performance monitoring wells. Results of the pilot study were used to develop a full-scale design consisting of 6,000 linear feet of horizontal wells to be used for a pulsed air delivery system coupled with natural tide-cycle influenced oxygenation, to address source and margin control of arsenic impacted groundwater at the site. This presentation details that a synergetic approach combining nature based and active remedies can often provide for a cost-effective and non-intrusive remedial solution for in situ treatment of arsenic at active sites.