A 21-acre parcel in the Mid-Atlantic United States hosted several industrial operations from 1907 to 1982. Groundwater at a pH of 5 SU and containing as much as 30 mg/L of zinc discharges to a small stream on one edge of the facility. The site surface was remediated and redeveloped into an apartment complex. Groundwater remediation to a goal of 2.0 mg/L zinc was deferred until after the apartment complex was built. In-situ stabilization technologies that could be applied with minimal interference with site use were evaluated and implemented to achieve the remediation goal.
The remediation had to reduce aqueous zinc concentrations using reagents that were amenable to injection. Two approaches were considered:
1. Zinc Hydroxide: Zinc sequestration as a hydroxide [Zn(OH)2] can achieve the necessary concentration reductions with an increase of pH from the acidic site conditions to pH values in the range of 8 to 10 SU. EnviroBlend AQ was selected as a pH buffer for zinc stabilization.
2. Zinc Sulfide: Zinc can also be sequestered as a zinc sulfide mineral [ZnS] to low aqueous concentrations. Sulfide application is commonly used in wastewater treatment systems as calcium polysulfides [CaSx]. The material has a very high pH and has the potential to create hydrogen sulfide odors. A slurry consisting of native sulfur and a pH buffer that would produce polysulfides after injection (and thereby mitigate potential odor issues) was also evaluated.
A pilot test was completed to demonstrate the in-field application of the injection process and the efficacy of the remedy. Zinc concentrations in groundwater downgradient of the injections fell from 21.2 mg/L to less than 1.0 mg/L within 7 months following the injections.
A total of 45,000 gallons of EnviroBlend AQ was injected into 63 borings to form treatment barriers along the downgradient side of the site. Approximately 3 weeks were required to complete the injections. Downgradient monitoring wells will be sampled periodically to assess the full-scale performance of the remedy.