GHD's vital role in RCRA corrective action: environmental excellence at former automotive plant

The site had been used for manufacturing automotive components and included chrome plating leading to widespread impacts to soil and groundwater that were required to be addressed under RCRA. 

Major initial tasks completed by GHD included:

• Preparation of a Current Conditions Report 
• Implementation of a comprehensive phased RCRA Facility Investigation (RFI) 
• Preparation of Environmental Indicator Reports 
• Completion of interim measures. 

The interim measures included the operation of an existing groundwater collection and treatment system, excavation of impacted soils, installation of a soil bentonite barrier wall, preparation of a Corrective Measures Proposal, and implementation of a long-term groundwater monitoring plan. Subsequently, GHD developed a 3D groundwater flow and metals transport model for the site. The 3D model (MODFLOW 2000/MT3D) was used to assist in the design of an alternative remedial measure including the addition of permeable reactive gates to treat groundwater discharge from the enclosed area. The simulation results were used to determine an optimal design for the number and size of reactive gates. 

Relevancy to L& RR 

• Treatability study with in-house laboratory evaluating remedial alternatives
• Recalcitrant contaminants of concern, including emerging contaminants 
• Pum and treat interim remedy at pilot scale 
• Groundwater modeling for the design of remedial measures 

A 12-acre portion of the site has soil and groundwater impacted with hexavalent chromium and nickel. A French drain collection system was installed to collect groundwater that is treated in a treatment plant (GWTP). The treatment process involves pH adjustment, flocculation, precipitation, polishing, and discharge to the sanitary sewer. GHD has been responsible for the operation and maintenance of the GWPT since October 2006. GHD also designed and installed a soil bentonite slurry wall around the area impacted with hexavalent chromium and nickel to prevent off-site migration. 

In 2010, GHD conducted a field study evaluating the effectiveness of in situ precipitation of impacted groundwater. The field pilot test included injecting sodium dithionite carbon source and a tracer into each of the two injection wells. The injected material was effective in reducing groundwater concentrations of chromium and nickel. In order to deliver more reducing agents to the area. The results showed that sodium dithionite reduced concentrations of hexavalent chromium but was less effective in reducing the concentration of dissolved nickel. Sodium sulfide was shown to be an effective treatment for nickel in the treatability study. 

A third, larger-scale pilot study was performed in May and June 2015. Eight hundred gallons of a 29 percent sodium sulfide solution were injected at 54 locations where dissolved nickel was the primary compound of concern; 800 gallons of a 20 percent sodium dithionite solution were injected at 18 locations where dissolved chromium was the primary compound of concern, and both sodium sulfide and sodium dithionite were injected at 12 locations where both dissolved nickel and dissolved chromium were present. The pilot study showed that sodium sulfide and sodium dithionite/sodium sulfide mixtures were effective in reducing concentrations of dissolved chromium and dissolved nickel and the sodium dithionite injections were effective in reducing concentrations of dissolved chromium. Soil cores were collected from the treatment areas and sent to the GHD laboratory for testing to determine whether chromium and nickel could be re-mobilized from the cores. The results of the testing showed that chromium could not be re-mobilized by low pH, however, exposure to a strong oxidant could remobilize the chromium. Nickel could be solubilized by low pH (pH < 3) but was not solubilized by strong oxidants. 

Based on the data from the pilot studies a full-scale implementation which involved the injection of 120,000 lb of sodium dithionite and 250,000 lb of sodium sulfide has been performed. The data shows that chromium and nickel have been reduced to the below criteria everywhere that the sodium dithionite and sodium sulfate were injected. There are some isolated areas where the injections could not be performed due to lack of access and the metals persist in this area. As a result of the full-scale injections, the collected groundwater no longer requires treatment before discharge to the sanitary sewer. 

A laboratory study was performed to test the effectiveness of using a zero-valent iron (ZVI) permeable reactive barrier installed in a downgradient portion of the soil-bentonite barrier wall to allow the natural flow of the groundwater to passively treat any residual hexavalent chromium or nickel remaining in groundwater following the full-scale on-site in-situ chemical reduction injections. The ZVI permeable reactive barrier (s) would eliminate the need for ongoing pump and treatment operations at the Site. The study showed that the ZVI barrier would be effective for the treatment of any residual metals in the groundwater and groundwater modelling was performed to determine the design parameters for the barrier. 

Recently, the presence of Per- and Polyfluoroalkyl Substances (PFAS) within the former plating areas has complicated the planned implementation of the ZVI permeable reactive barrier(s). Additional characterization, laboratory treatability studies, and modeling activities have been conducted to evaluate supplemental remedial alternatives that would allow the installation of the ZVI permeable reactive barrier(s), and/or eliminate the ongoing requirement to pump groundwater from within the contained area.

GHD has been successful in meeting various regulatory agency requirements throughout the complex site investigation and remediation process. Through the work of our ITG GHD was able to eliminate the need for active groundwater treatment that provides significant long-term cost savings.