Orion Performed In Situ Chemical Reduction for Treating Heavy Metals in Groundwater

Orion was retained to design and implement an in situ chemical reduction (ISCR) treatment program at an active manufacturing facility in New England.  Historical operations at the site included rotogravure printing.  The heavy metal cadmium was released from multiple sources including a former process waste pipeline and stormwater management system.  Groundwater at the site is naturally acidic and oxidizing, which results in cadmium speciating as a soluble cation (Cd²⁺).  Because of relatively high groundwater flux and low organic carbon content in the aquifer matrix, soluble cadmium impacts have migrated downgradient towards a gaining river.  Results of hydrologic modeling indicated that cadmium-impacted groundwater discharges to the river through riverbed sediments and anoxic wetland hydric soil.  Precipitation of soluble cadmium from discharging groundwater has contributed to impacts in wetland hydric soil and riverbed sediments, which present a chronic risk to potential ecological receptors.  Phased remediation is required to remove cadmium-impacted hydric soil and river sediments, while reducing the potential for recontamination by soluble cadmium impacts.

Orion teamed with ISOTEC of Lawrenceville, New Jersey, to evaluate potential ISCR reagents.  Bench- and field-scale testing were conducted and indicated that calcium polysulfide (CPS) was a viable technology for treating cadmium-impacted groundwater at the site.  Orion and ISOTEC designed and implemented a CPS injection program to treat the downgradient portion of the soluble cadmium plume.  The injection program focused on an approximately 28,600-square-foot area with treatment zone thicknesses ranging from 12 to 24 feet.  ISOTEC advanced 92 temporary injection points using direct-push technology and injected CPS in 4-foot intervals across the treatment zone.  ISOTEC injected approximately 146,000 gallons of dilute CPS solution in the treatment area over 19 days.  Orion provided field oversight during the injection and monitored key performance indicators including average daily injection productivity to manage potential risks to the project baseline.

Orion and ISOTEC conducted intensive field monitoring of geochemical parameters to evaluate reagent distribution during injection.  Groundwater samples were collected throughout the program with expedited analysis of dissolved metals and other constituents to evaluate short-term treatment and allow for near real-time optimization of injection activities.  Orion and ISOTEC responded to field and laboratory results by installing supplemental injection points near compliance wells with limited or no geochemical response to injections.  Groundwater samples were collected from compliance wells within the treatment area approximately 1 week after completing the injection program and analyzed for dissolved metals.  Results indicated that dissolved cadmium concentrations in all 8 compliance wells within the treatment area were below the site-specific remediation goal.  Data collected during and after injection activities will be used to design a full-scale treatment program for the remaining upgradient soluble cadmium plume.