Theme: 08. Metals

Topic Area: 8c. Precipitation and Stabilization of Metals

Formatted Title:

Halting Migration through Coprecipitation: A Nontraditional Approach to Treatment of Dissolved Arsenic in Groundwater

Background/Objectives:

Arsenic (As) and other geogenic metals can remain in solution in groundwater (GW) for years to decades following both active treatment and natural attenuation (NA) of other primary contaminants at remediation sites. Landfills can also have high concentrations due to the presence of organic carbon and reducing conditions caused by capping. In cases where no risk to receptors exists, the most pragmatic practical solution is monitored NA or compliance monitoring. When active remediation is required due to regulatory requirements or receptor exposure, GW extraction and treatment (GWET) is common, but is costly and without a clear endpoint. Iron-based reagent injection treatment methods are currently being explored, and environments where dissolved iron (Fe) co-occurs with As present an opportunity for effective in-situ treatment.

Two case study sites will be presented, both at which in situ air sparging (IAS) has been used to provide oxygen to facilitate reoxidation of dissolved (ferrous) Fe, resulting in coprecipitation of As with ferric Fe. IAS in these instances was implemented in Fe rich GW zones with co-occurring dissolved As. This technology has the potential to significantly reduce As concentrations in GW within a short period. IAS systems are robust, relatively inexpensive to operate, and have a much smaller carbon footprint than other technologies. Site-specific factors must be considered to ensure effectiveness and minimize fouling of injection points and reducing aquifer permeability.

Approach/Activities:

At the Vineland Chemical (VC) site in New Jersey, IAS to treat dissolved As in situ began as a pilot study following GWET for 22 years. After the successful implementation of the pilot, stability of the iron precipitates is being evaluated after system shutdown. At the Shepley’s Hill Landfill (SHL) site in Massachusetts, a GWET system has been operating since 2006. Capture is adequate, but As concentrations remain elevated. Because of the positive results at the VC site and similarities in the aquifer conditions at VC and SHL, an IAS pilot study was performed at SHL to assess the efficacy of this approach.

Results/Lessons Learned:

A summary of remedial performance and lessons learned will be presented. At the VC site, IAS was piloted where As concentrations were greater than 1 mg/L. Concentrations declined significantly within the first two months, and after over five years of operation were reduced from up to 15 mg/L to less than 0.05 mg/L within 30 feet of IAS points (with DO at the solubility limit at this distance), 75 feet further downgradient from the IAS points, As was reduced to less than 0.35 mg/L and DO was at 5 mg/L. No sign of fouling was observed. Similar results were observed at SHL during a three-month pilot test. The keys to success are: 1) Maintaining GW pH less than 7 SU, which minimizes the kinetics of Fe precipitation, resulting in slow Fe oxidation rates. Rather than occurring in the IAS well pack, precipitation happens hydraulically downgradient. 2) Sufficient dissolved Fe must be present relative to dissolved oxygen (DO) to achieve As concentrations below 0.05 mg/L. 3) IAS is effective in soils where sufficient air distribution can be achieved. At the VC site, the source of Fe and As is within the effective sparging zone. At SHL, the greatest Fe and As concentrations are just above bedrock, necessitating an alternative oxygen delivery strategy that has yet to be determined.

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Janet Mathews-Flynn