Overview

A Loureiro client site in Clinton, CT had high levels of zinc detected in some of the groundwater locations. A treatment mechanism needed to be identified and instituted to rectify the problem and decrease the levels of zinc. A permeable reactive barrier (PRB), or other approach using similar geochemical treatment, was identified as the most likely remedy for the removal of zinc and other constituents of concern mobilized in groundwater at the Site.

Approach

Apatite II and zeolite were identified as the two most appropriate reactive media for zinc remediation given the geochemistry of groundwater at the site. A column study was needed to test the different reactive materials and determine optimal composition prior to design of the remedial system.

Due to the complex groundwater geochemistry, groundwater from the site needed to be used for the column study in order to more accurately access the efficacy of the reactive media. Therefore, a groundwater stability evaluation was also necessary to evaluate whether groundwater collected from the remedial area would maintain specific chemical characteristics from the time of collection through a sufficient period of time that the groundwater could be used for the column leachate study.

Tunxis provided the resources to perform the groundwater parameter stability evaluation and column leachate study to evaluate permeable reactive media alternatives. Groundwater was collected from the Site using modified low-flow groundwater sampling techniques and transported to Tunxis Laboratories, LLC (Tunxis) in Plainville, Connecticut for sampling and laboratory analysis.
Tunxis personnel retrieved aliquots from each of the groundwater-filled carboys on the day of receipt and at additional intervals over a seven-day period to assess changes in geochemical characteristics.

Tunxis supported the project ensuring the flow rates were correct, collections were performed on time, and analysis was performed and reported on an expedited turn around time for the duration of the project, which lasted over two months.

Results

Apatite II was selected as the preferred media, based on its effectiveness at removing zinc from site groundwater. From the study, it was determined that the primary mechanism for zinc removal is from bacterial reduction of sulfate to hydrogen sulfide, which led to zinc sulfide precipitates (sphalerite). The Apatite II provided optimal conditions (phosphate, carbon and other essential nutrients) for sulfate reducing bacteria to grow. Rapid breakthrough of zinc was observed in the effluent from the Zeolite columns and; therefore, use of this material was not considered a viable option for the Site. The results from the column study were used to develop an on-site pilot study that was successfully implemented, and the information gathered from both studies were used to conduct a remedial alternatives analysis. Subsequently, the client was able to develop conceptual designs and preliminary cost estimates for the groundwater treatment system.